Novel hyperpolarization-activated K+ current mediates anomalous rectification in crayfish muscle

The ionic current underlying anomalous rectification in opener muscle fibers of crayfish was studied under two-electrode voltage clamp. Opener muscle fibers showed a mean resting potential (RP) of -64.8 mV and an input resistance of 0.4 M omega. Hyperpolarizing voltage command pulses from a holding potential (H) of -60 mV evoked an instantaneous voltage-independent linear current (IL) followed by a time- and voltage- dependent inward current (IAB) that reached a steady state within 500 msec. The reversal potential of IAB (EAB) was estimated from tail current amplitudes. At an extracellular K+ concentration ([K+]o) of 5.4 mm the mean EAB was -61.8 mV. EAB shifted toward positive potentials by 50.8 mV for a 10-fold increase in [K+]o. The conductance underlying IAB (GAB) increased sigmoidally with hyperpolarization, starting close to the RP, saturating at a GAB,max of about -140 mV, and showing a mean half-activation at -94.4 mV. The activation curve of GAB shifted 53.6 mV toward positive potentials with a 10-fold increase in [K+]o. GAB,max did not increase in raised [K+]o. The activation and deactivation kinetics of IAB were accurately described by single exponentials with similar time constants (< 100 msec). Time constants changed as an exponential function of the membrane potential. IAB, its time course, GAB, and EAB were not modified in the following conditions: (1) Na(+)- and Ca(2+)-free solutions, (2) intracellular EGTA, (3) extracellular (100 mM) or intracellular tetraethylammonium, (4) extracellular Cs+ (up to 50 mM), Rb+ (up to 10 mM), Ba2+ (13.5 mM), or Mn2+ (13.5 mM). However, low extracellular concentrations of Cd2+ or Zn2+ strongly and reversibly reduced both IL and IAB. Therefore, we conclude that anomalous rectification in crayfish muscle is generated by a voltage- and time-dependent K+ current IAB. This current displayed many electrophysiological and pharmacological characteristics that distinguished it from all others mediating anomalous rectification described previously.

The ionic current underlying anomalous rectification in opener muscle fibers of crayfish was studied under two-electrode voltage clamp. Opener muscle fibers showed a mean resting potential (RP) of -64.6 mV and an input resistance of 0.4 Mg. Hyperpolarizing voltage command pulses from a holding potential (/+) of -60 mV evoked an instantaneous voltageindependent linear current (I,) followed by a time-and voltage-dependent inward current (I,,,) that reached a steady state within 500 msec. The reversal potential of I,,. (I$,) was estimated from tail current amplitudes. At an extracellular K+ concentration ([K+],) of 5.4 mM the mean E,, was -61.6 mV. E,,, shifted toward positive potentials by 50.6 mV for a lo-fold increase in [K+],. The conductance underlying IAB (GAB) increased sigmoidally with hyperpolarization, starting close to the RP, saturating at a GAB,,,*= of about -140 mV, and showing a mean half-activation at -94.4 mV. The activation curve of G,, shifted 53.6 mV toward positive potentials with a lo-fold increase in [K+],. GAB,maX did not increase in raised [K+],. The activation and deactivation kinetics of IA8 were accurately described by single exponentials with similar time constants (< 100 msec). Time constants changed as an exponential function of the membrane potential. IAB, its time course, G,,, and E,,, were not modified in the following conditions: (1) Na+-and Ca2+-free solutions, (2) intracellular EGTA, (3) extracellular (100 mM) or intracellular tetraethylammonium, (4) extracellular Cs+ (up to 50 mM), Rb+ (up to 10 mM), Ba*+ (13.5 mM), or Mn2+ (13.5 mM). However, low extracellular concentrations of Cd*+ or Zn*+ strongly and reversibly reduced both IL and IAB' Therefore, we conclude that anomalous rectification in crayfish muscle is generated by a voltage-and time-dependent K+ current I,,. This current displayed many electrophysiological and pharmacological characteristics that distinguished it from all others mediating anomalous rectification described previously. [Key words: anomalous rectification, inward rectifier, hyperpolarization-activated current, K+ current, Cff+ blockade, crayfish muscle] Katz (1949) reported a higher membrane conductance for inward than for outward current injection in frog skeletal muscle fibers. This nonlinear membrane behavior. termed anomalous or inward rectification, was later found with different characteristics in other systems. Two main varieties of conductances accounting for anomalous rectification have been described. (1) In frog skeletal muscle (Katz, 1949) cardiac muscle (Hall et al., 1963;Sakmann and Trube, 1984) metacerebral giant cells (Kan-de1 and Taut, 1966), marine eggs (Hagiwara and Takahashi, 1974), tunicate embryos (Miyazaki et al., 1974), and vertebrate neurons (Constanti and Galvan, 1983;Kaneko and Tachibana, 1985;Stanfield et al., 1985;Williams et al., 1988) anomalous rectification is mediated by K+, and depends on the extracellular K+ concentration ([K'],), and its voltage dependence is a function of the difference between the membrane potential (I',,,) and the potassium equilibrium potential (E,) (Hagiwara and Takahashi, 1974;Hagiwara and Yoshii, 1979;Leech and Stanfield, 198 1). It shows instantaneous voltage-dependent activation followed by a rapid time-and voltage-dependent component, and has been termed inward rectifying current. (2) Another type of inward rectification carried by Na+ and K+ was found in cardiac muscle (Brown and DiFrancesco, 1980;Yanagihara and Irisawa, 1980), hippocampal pyramidal neurons (Halliwell and Adams, 1982), and lobster slowly adapting stretch receptor neurons (Edman et al., 1987). This current, termed I,,, I,, or I,,, was later found in several vertebrate neurons (Mayer and Westbrook, 1983;Spain et al., 1987;Takahashi, 1990) and was also called IAR or Z,R. To differentiate it from the inward-rectifying current, it has been more generally called hyperpolarization-activated current. It has no instantaneous voltage-dependent component, shows considerably slower activation kinetics, and its activation is independent of [K'],,.
A third type of inward rectification carried by Cll has been reported in Aplysia neurons (Chesnoy-Marchais, 1983) amphibian oocytes (Parker and Miledi,l988), and mammalian hippocampal neurons (e.g., Madison et al., 1986). Fatt and Ginsborg (1958) described a delayed decrease in input resistance evoked by hyperpolarizing current pulses in crayfish muscle, but the underlying conductance had not been characterized under voltage clamp. Therefore, the aim of the present investigation was to characterize the current responsible for the anomalous rectification in crayfish muscle using the twomicroelectrode voltage-clamp technique. This voltage-and timedependent K+ current shows no instantaneous component, thus we will call it hyperpolarization-activated current. However, it displayed many electrophysiological and pharmacological characteristics that distinguished it from all others described previously. Since this new hyperpolarization-activated current mediates anomalous rectification and probably acts as a K+ balance by reestablishing the transmembrane K+ concentration difference after prolonged activation of crayfish muscle, it will be named IA". ; current-clamp condition. Schematic stimulus protocol with pulses to +200 nA and -400 nA, in 40 nA increments; resting potential (RP) was -65 mV. B, Currents (lower) evoked by two-pulse voltage command protocol (upper) with initial pulse (V) from the holding potential (H) and final pulse (F); voltage-clamp situation, as in all other figures. The currents evoked by H, V, and F were I,,,, I,,, and Jo, respectively, and were measured from the zero current level (upper broken line). For each current, the superscripts 0 and S indicate the initial and steady state levels, respectively. I,,,, was the IO,,, -p,, difference.
A preliminary report of part of the present results has been published as a short communication (Araque and Buiio,199 1).

Materials and Methods
Preparation. Opener muscles from the propodite of the first walking leg of crayfish (Procambarus clurkii) were isolated and transferred to a superfusion chamber (2 ml). Small crayfish (< 5 cm) with short muscle fibers (~400 pm) were used for better space-clamp characteristics. Microelectrodes and recordings. Fibers were impaled with two micropipettes usually filled with 1 M KC1 (l-5 MR). When measuring the Cl-equilibrium potential (E,,), micropipettes were filled with 3 M K-acetate (l-5 MB). In some experiments, the current electrode (filled with 1 M KCl) was substituted, after a control recording, by a new electrode filled either with 700 mM EGTA [ethyleneglycol-bis-@-aminoethyl ether) N,N,N'N'-tetraacetic acid], neutralized with KOH (pH 7.2) or with 1 M tetraethylammonium chloride (TEA). EGTA and TEA were ionophoretically injected with 100 nA negative and positive current pulses, respectively, during 15 min. A 1 M KCl-agar Ag-AgCl electrode was used as the indifferent electrode. An Axoclamp-2A amplifier (Axon Instruments, Foster City, CA) was employed for two-electrode currentand voltage-clamp recordings. Probe gains in the voltage-clamp configuration were x 1 and x 10 for voltage and current electrodes, respectively. To decrease the coupling capacitance a grounded shield was constructed by coating one microelectrode with conductive paint. Voltage command pulses and injected currents were continuously monitored on a storage oscilloscope and stored on FM tape (O-l 250 Hz bandwidth; Hewlett-Packard, model 3964a). The time required to reach the command pulse potential was 50.5 msec and there were no variations throughout the pulse. Capacitive currents ended within about 5 msec. Recordings that did not meet these criteria were rejected. The selected data were also sampled (Lab Master TM-40, Scientific Solutions Inc.), stored, and analyzed on an IBM PC/AT personal computer. Current records were sometimes filtered above 1 kHz with an active filter (Ithaca, model 42 12).
Solutions. The normal solution, modified from Van Harreveld (1936), had the following composition (in mM): NaCI, 210; KCl, 5.4; CaCl,, 13.5; MgCl,, 2.6; Tris buffer, 10; pH adjusted to 7.2 with HCl. Test solutions were made with chloride salts of monovalent (Cs+ and Rb+ ) and divalent (Ba*+, Mg2+, Mn2+, Cd2+, and Zn2+ ) cations, added in equimolar exchange with NaCl and CaCl,, respectively. Ca2+-free solutions were obtained by equimolar replacement of Ca*+ by Mgz+ and addition of 5 mM EGTA. TEA and Ca*+-free TEA solutions were obtained by equimolar exchange of NaCl by 100 mM TEA in normal and Ca'+-free solutions, respectively. In Na+-free solutions, NaCl was replaced equimolarly by either choline chloride or Tris base (titrated with HCl). In experiments with increased [K'],,, KC1 was either replaced to NaCl (or to Tris in Na+-free solution) or was added without osmolarity compensation.
Each opener muscle fiber is innervated by a single inhibitory axon which releases y-aminobutyric acid (GABA), evoking an increase in the postsynaptic membrane Cl--conductance mediated by GABA, receptors Takeuchi, 1965, 1967). To test Ec,, short (up to 250 msec) air pressure pulses delivered by a Picospritzer II (General Valve Corporation) were used to eject small quantities of 0.1 M GABA from a micropipette (tip diameter of 2-5 pm) placed over the fiber surface. Thus, E,, was estimated from the reversal potential of the GABAinduced postsynaptic currents. Experiments were performed at room temperature (21-23°C). All chemicals were purchased from Sigma.
Protocols and parameter measurements. Two voltage command protocols were used. They consisted of a pulse to an initial voltage (V) from a holding potential (H), followed by a pulse to a final voltage (F). In protocol 1, V was variable in amplitude and F was fixed. In protocol 2, V was constant and F was variable. A low-intensity, brief, final pulse was usually added to test for undue conductance changes (e.g., see Fig.  2B). A typical command pulse protocol and the evoked currents are illustrated in Figure 1B. The following conventions and current measurements were used throughout the text. (I) All currents were measured from the zero level (upper broken line). (2) The steady state current at the holding potential H was the holding current (J,,,E in Figure IB it was artificially increased for illustration purposes. (3) The initial oulse from H to Vevoked an instantaneous current component (IO,,) followed by a time-dependent current that reached a steady state (rt,,). (4) The final pulse from V to F evoked an instantaneous current (IO,,) and then a time-dependent current eventually reaching a steady state (I;,). (5) At any voltage, the hyperpolarization-activated inward current (I,,) corresponded to the difference between the current at any time and the instantaneous current. Thus, the l;ll -IO,, and IO,, -P,, differences corresponded to the steady state activation and deactivation of IAB at V and F, respectively. All data are expressed as mean L SD.

Results
This study is based on data from 127 opener muscle fibers. A representative sample of 3 1 fibers showed a resting membrane potential (RP) of -64.8 + 5.8 mV and an input resistance of 0.4 f 0.2 MO. Typical current-clamp responses evoked by hyperpolarizing and depolarizing pulses are shown in Figure 1.4. Both small depolarizing and hyperpolarizing current pulses evoked passive responses. Higher-intensity depolarizing pulses elicited active graded responses. With higher-intensity hyperpolarizing currents, the membrane potential (I',,) attained an initial peak value at about 50 msec and then gradually decayed, reaching a steady state within about 500 msec. The steady state current versus V,,, relationship showed a clear nonlinearity in the hyperpolarizing direction, indicating inward or anomalous rectification.
The ionic conductance responsible for the anomalous rectification was investigated under voltage clamp. In order to reduce CONTROL +EGTA +TEA The Journal of Neuroscience, January 1994, 74(i) 401 the holding current lo0 and to avoid activation of voltage-gated currents, fibers were clamped at H = -60 mV, near the RP. In solutions with raised [K+],, H was set close to the attained RP. Figure 2A shows typical currents evoked by 11 msec duration depolarizing pulses from -60 to -40, -15, and 10 mV (the instantaneous linear currents were subtracted) in normal (control), Ca2+-free (+EGTA), and Ca2+-free TEA (+TEA) solutions. There is evidence indicating that the depolarization-activated currents shown in Figure 2A are an early inward Ca2+ current (I,,), an early Ca-dependent K+ current (Zk(e+), and a late outward K+ current (I,) (Mounier and Vassort, 1975a,b;Hencek and Zachar, 1977;Hencek et al., 1978). Both I,, and I K(Ca) were suppressed in Ca2+-free solution.
In Figure 2B, the 500-msec-duration hyperpolarizing command pulses from H = -60 mV to different V between -70 and -160 mV evoked an instantaneous current IO,, followed by a gradually developing inward current that reached a steady state I',. within about 500 msec. Although both IO,, and IScM increased with hyperpolarization, the latter increased more than the former. Since at any given voltage pulse P,, was higher than IO,, (except the smallest pulse that did not activate the current), the time-and voltage-dependent inward current was associated with an increase in the membrane chord conductance (see Spain et al., 1987). Figure 2C shows the hyperpolarization-activated current (I,,) in isolation, obtained by subtraction of the instantaneous linear components from the total inward current.  (control), Ca*+-free (+EGTA), and Ca*+ -free TEA (+TEA) solutions. A, Depolarization-activated currents evoked by brief pulses from -60 to -40, -15, and 10 mV; linear currents were subtracted. B, Total inward currents activated by protocol 1 fromH= -6OmVto V= -160mV, in -10 mV increments (top, as in all figures). A brief, final, -5 mV pulse was added (as in most other cases) to test for unwanted conductance modilications. C, Same as B, but after subtraction of linear currents, showing voltageand time-dependent current (I,,). Current records were filtered at 1 kHz. C currents shown in Figure 2. Whereas Zca (solid triangles) was suppressed, I, was unaffected in Ca2+-free solution. The remaining I, (solid circles above -60 mV) was strongly reduced when TEA was added. Both the steady state total inward current (open circles) and ZAB (i.e., ZAB = PC, -IO,,: solid circles below -60 mV) were unchanged in Caz+-free and Caz+-free TEA solution. Similar results were obtained by intracellular injection of TEA (not shown), indicating that the ZAB channel has no extracellular or intracellular TEA sensitivity.
Instantaneous current-voltage relationships Figure 4, A and B, exhibits voltage-clamp responses in Ca2+free TEA solution evoked by the protocols 1 and 2 used to measure the instantaneous on and off Z-V relationships, respectively. IO,, and IO,, were measured 7 msec after the pulse transients, when the capacitive currents had ended, and the activation or deactivation of I,, was negligible. Pulses > -100 mV were not used to measure IO,,, because they evoked a significant amount of ZAB (see below). However, it is conceivable that IO,, remained linear beyond that value, as occurred in other fibers. The Z-V relation of Zot,? obtained with pulses from H = -60 mV to V between -25 and -100 mV (Fig. 4C, triangles), was linear (r > 0.99). Therefore, the membrane did not exhibit instantaneous inward rectification, the instantaneous linear current obeyed Ohm's law, and it could be written as where G, and E, are the chord conductance and the reversal potential of the voltage-independent linear current I,, respectively. Similarly, a linear relation was obtained for IO,, (Y > 0.99) by stepping V,, from the V pulse at -130 mV to F from -30 to -100 mV (Fig. 4C, solid circles), indicating that the behavior of the total ionic current was also ohmic. Since both IO,,, and I",,, obeyed Ohm's law, I,, did so too, verifying the equation where G,, and E,, were the chord conductance and the reversal potential of IA", respectively. Hence, the total ionic current (I,,,,) may be written as The slopes of the linear regressions of the I-V relationships of IO,, and IO,, represented the resting chord conductance and the resting chord conductance plus G,, at -130 mV, respectively. They were 5.5 and 13.5 pS, respectively, confirming that an increase in the membrane chord conductance underlies IAB. The IO,, and IO,,, I-V relationships intersect at E,,, as can be deduced from Equation 3, verifying the value of E,, estimated from the tail currents evoked by protocol 2 (see below). Indeed, the intersection point of the I-V curves in Figure 4C (-56.5 mV) agrees well with E,, estimated by tail currents shown in Figure 6C (-53.2 mV), which were obtained from the same fiber.
Voltage dependence of G,, The voltage dependence of G,, was characterized by a dimensionless activation parameter (NJ, similar to the n, and m, parameters defined by Hodgkin and Huxley (1952b), which varied from 0 to 1 as G,, varied from zero to its maximum value deduced from the Boltzmann equation, where N-is the limiting activation parameter (usually equal to l), V, the voltage at which G,, is half-activated, and S a slope parameter. Results obtained from 17 muscle fibers indicate that the activation of G,, increased sigmoidally with hyperpolarization, started close to the RP, and could be fitted (r > 0.99) with the Boltzmann equation. The voltage dependence showed a mean half-activation at -94.4 f 7.1 mV, a slope factor of 12.4 + 2.7, and a GAB,,,, = 7.8 + 3.6 pS. N, approached unity at -130 mV and usually saturated at V, > -140 mV. Hyperpolarization beyond about -140 mV sometimes evoked a large, slow, longlasting inward current, probably due to membrane breakdown. Thus, strong hyperpolarizations were employed only when this current was not present.

Kinetic behavior of I,,
The time course of both IAB activation and deactivation (i.e., during the F/and F pulses, Fig. 6, A and B, respectively) could be fitted (r > 0.95) by single-exponential functions of the form where a, b, c, and d were constants and r,, and T,~ were the activation and deactivation time constants, respectively. While T,, decreased with increasing hyperpolarization (Fig. 60, circles), declining e-fold for 23.6 mV, T,,, decreased e-fold every 3 1.7 mV with depolarization (Fig. 60, triangles), both being exponential functions of Y,,,. The similarity of run and roR. magnitudes indicates that both the I,, activation and deactivation kinetics may be described by a single time constant (T,,). For convenience, 7AB corresponded to r,, for relatively large hyperpolarizations, whereas it corresponded to T,~ for the remaining voltages. According to what is deduced from Hodgkin and Huxley (1952a), the time constant of voltage-gated currents with an activation curve like that of I,, should be bell-shaped functions of V,, with a peak at the half-activation voltage V, (see also Mayer and Westbrook, 1983). Figure 60 shows that TAB was a bell-shaped function of I',,, with a peak around V, (compare with Fig. 5, same fiber).
Ionic nature of I,, The reversal potential of Z,, was estimated from the reversal potential of tail currents evoked by protocol 2. Figure 6B shows the expanded tail currents in Figure 4B. Since the fully activated I,, did not exhibit rectification and its deactivation was time dependent, then LI = 10,, -p,, = 4, (F -Em), where AC,, is the difference between the conductance at the beginning and at the end of the F pulse. Figure 6C shows the 1-V relation of I,,,, from Figure 6B and the corresponding firstorder regression fit (r > 0.99). The reversal potential of I,, estimated by linear interpolation was -53.2 mV. The mean value of EAo was -6 1.8 * 6.3 mV (n = 28), which did not differ substantially from the average RP. E@cts ofCa2+. The effects of Ca*+-free solutions and of in- temal injection of EGTA were tested. ZAB, its kinetics, E,,, and G,, were not significantly modified in Ca2+-free solution or by intracellular EGTA (Figs. 2, 3, 7), suggesting that neither extracellular nor intracellular Ca*+ contributes to ZAB.
Eficfs of Nu+ substitutions. Total replacement of NaCl by either Tris or choline chloride did not evoke any modification of I,,, rAB, E,,, or GAB, indicating that Na+ did not contribute to Z,, and that its activation was unaffected by Na+ (data not shown).
The magnitude of rALI was not modified by increasing [K+],,, but its voltage dependence was changed, shifting the relation between ~~~ and I',, toward positive potentials, as expected by the change in the activation curve. Indeed, the peak of the bellshaped curve was displaced 18 mV to the right in raised [K+], (Fig. 7F).
Although it has been firmly established that alterations of [K+], result in a rapid redistribution of Cll across the cell membrane in many systems, Boistel and Fatt (1958) showed no important E,, changes in raised [K+], and Cl--free solution in crayfish opener muscle fibers. Their observations were based on inhibitory postsynaptic potential amplitude measurements made within the initial 15 min after the solution change and therefore do not preclude slower changes. Figure 8 shows the effects of raised [K+], on E,,, the I, reversal potential, and E,,. While the reversal potential of I, and E,, shifted from -60.1 and -63.5 to -46.5 and -49.4 mV, respectively, E,, did not change much (from -53.3 to -55.8 mV), in 5.4 and 10.8 mM [K+],, respectively. Zc, was the difference between IO,, with and without GABA (see Materials and Methods), and E,, was the reversal potential of Zc,.
Our results are in close agreement with Boistel and Fatt (1958) since they indicate either little Cll redistribution or an extremely slow effect following [K+], changes.
Therefore, we can conclude that the actions of raised [K+], were not due to redistribution ofCl-, and that I,,@ was selectively carried by K+.
Efects of divalent cations Ba2+, A4g2+, and A4nz+. Low concentrations of BaZ+ are known to block the inward-rectifying currents (IO-100 PM; Hagiwara et al., 1978) but not the hyperpolarization-activated currents (Yanagihara and Irisawa, 1980;Halliwell and Adams, 1982;Mayer and Westbrook, 1983;Takahashi, 1990). When extracellular CaCl, was equimolarly replaced by BaCl,, IAB was not modified. I,, was also unchanged when CaCl, was replaced either by MgCl, or MnCl,, while the depolarization-activated inward Ca*+ current was totally suppressed in those conditions. Action of Cd2+ and Zn2+. The effects of 5 mM extracellular Cd*+ are shown in Figure 9A. Low extracellular Cd*+ concentrations (~5 mM) strongly and reversibly reduced both IAB and the instantaneous linear current (Fig. 9, B and C, respectively), A Control Cd" while the holding current IcM was not significantly affected. Similar results were obtained with millimolar extracellular Znz+ concentrations (Fig. 10). The effects of Cd2+ and ZrP+ on Z,, were dose dependent, both with an IC,, of about 300 FM (A. Araque, D. Cattaert, and W. Butio, unpublished observations).
It is noteworthy that the anomalous rectification observed with hyperpolarizing pulses in current-clamp conditions was also suppressed by Cd2+ and ZtP+ (not shown).

Discussion
The above-described experiments indicate that a hyperpolarization-activated time-dependent inward K+ current I,, operates in opener muscle fibers of crayfish. This current underlies the inward (i.e., anomalous) rectification existing in currentclamp conditions (Fig. 1A). Although the effects of IA8 activation were similar to those of other inward-rectifying and hyperpolarization-activated currents, ZAB displayed many electrophysiological and pharmacological characteristics that distinguished it from all others previously described.
There instantaneous voltage-dependent component followed by a rapid voltage-and time-dependent element-its conductance is a function of [K+],,, and has an activation curve that is contingent on the V,,, -E, difference (e.g., Hagiwara and Takahashi, 1974;Hagiwara and Yoshii, 1979;Leech and Stanfield, 198 1); and second, a slower time-dependent hyperpolarization-activated current termed I,,, I,, or I,, carried by Na+ and K+. It lacks the instantaneous voltage-dependent component, and its activation is independent of [K+], (Brown and DiFrancesco, 1980;Yanagihara and Irisawa, 1980;Halliwell and Adams, 1982;Edman et al., 1987).
Two different mechanisms have been proposed to explain both current types. The hyperpolarization-activated currents are pictured as due to the intrinsic voltage-sensitive gating mechanisms of channels that when open are ohmic. On the other hand, the instantaneous voltage-dependent component of the inward-rectifying current is represented as generated by the rectifying quality of the open channels themselves (Sakmann and Trube, 1984) without the participation ofintrinsic gating mechanisms. Evidence exists indicating that intracellular MgZ+, at physiological concentrations, acts as the voltage-dependent blocker that grants instantaneous rectification to the otherwise ohmic open channels (Matsuda et al., 1987;Matsuda, 199 1). However, the time-dependent component of the inward rectifier can also be explained by a gating mechanism (Hagiwara et al., 1976;Hagiwara and Yoshii, 1979;Leech and Stanfield, 198 1). If such a gating mechanism also accounts for the instantaneous current component is still questionable (Ishihara et al., 1989;Silver and DeCoursey, 1990;Matsuda, 199 1;Mitra and Morad, 1991).
In crayfish opener muscle fibers, the instantaneous current was voltage independent and displayed a linear I-Vrelationship; hence it could be the leak current I,. Therefore, opener muscle fibers lack the instantaneous voltage-dependent component that is characteristic of inward-rectifying currents (see the references above). The activation of Z,, was time dependent, suggesting that rectification was due to the channel's kinetic characteristics. At a fixed degree of activation I,, itself obeyed Ohm's law; therefore, rectification was not due to the attributes ofthe ohmic open channel. The time dependence of I,, also implies that a gating mechanism is responsible for its activation. Since the activation and the deactivation kinetics of I,, were the same, the simplest representation to explain the above described phenomena-ifan ohmic open channel with an intrinsic gating mechanism is assumed-is a two-state model like the one proposed by Chesnoy-Marchais (1983) to explain inward rectification carried by Cl-in Aplysia neurons. Interestingly, this seems to be the only electrophysiological similarity between I,, and the hyperpolarization-activated Cl-current described by Chesnoy-Marchais (1983). Indeed, this current is carried by Cl-, its activation curve depends on [Cl-],, and its time course is relatively slow (with time constant in the second range). Contrastingly, I,, is carried by K+, its activation curve depends on [K+], and does not depend on [Cl 1, (not shown), and its time course is faster (with a time constant at least two orders of magnitude lower).
Approximating the inward-rectifying K+ currents (Hagiwara and Takahashi, 1974;Hagiwara and Yoshii, 1979;Leech and Stanfield, 198 l), IA" was specifically mediated by K+ , and the voltage dependence of its activation curve was a function of the V,,, -E, difference. On the other hand, the conductance of the inward rectifier of marine oocytes is proportional to the square root of [K+], (Hagiwara and Yoshii, 1979). However, the conductance underlying ZAB was independent of [K+],,. Contrastingly, the leak conductance increased 1.4 ? 0.1 times in doubled [K+],. Although the conductance of other hyperpolarizationactivated currents (e.g., Mayer and Westbrook, 1983;Spain et al., 1987) increased in raised [K+],, the hyperpolarization-activated current in rat spinal motoneurons did not display a similar behavior (Takahashi, 1990).
Although the electrophysiological characteristics of I,, partially overlap with those of the other types of cationic inward rectification (see Discussion above), the prominent dissimilarity was the pharmacological sensitivity to different extracellular ions. Indeed, while the inward-rectifying currents are blocked by low concentrations of Ba2+ (0.01-5 mM, Standen and Stanfield, 1978; 1 O-l 00 PM, Hagiwara et al., 1978;500 PM, Constanti and Galvan, 1983), I,, was not altered in the presence of 13.5 mM BaZ+. On the other hand, low concentrations of extracellular Cs+ are known to block both the inward-rectifying (0.5-l mM, Hagiwara et al., 1976; 2.5 mM, Gay and Stanfield, 1977) and the hyperpolarization-activated currents (20 mM, DiFrancesco and Ojeda, 1980; 0.5-3 mM, Halliwell and Adams, 1982;l-10 mM, Mayer and Westbrook, 1983;2 mM, Takahashi, 1990); however, concentrations of Cs+ up to 50 mM did not modify I,,,. Furthermore, I,, was not modified by extracellular addition of 10 mM Rb + , although this ion blocks the other cationic inward rectifications at concentrations under 10 mM (Hagiwara and Takahashi, 1974;Standen and Stanfield, 1980;DiFrancesco, 1982). Contrastingly, I,, was blocked by the application of low concentrations of extracellular Cd*+ and Zn'+.
In conclusion, our results provide evidence of the existence of a new type of voltage-and time-dependent inward rectification, selectively mediated by K+ , whose activation curve depends on V,,, -E, (assuming a fixed [K+],) but whose conductance is unaffected by [K+],. It is blocked by low concentrations of Cd*+ and Znz+ but is insensitive to other ions that are potent blockers of other inward rectifications; that is, Cs+, BaZ+, and Rb'.
Although we can only speculate about the functional meaning of ~A,, the following possibilities are likely besides the K+ balance function described above. E,, is close to the RP; therefore, I,, could stabilize the membrane potential preventing large hyperpolarizations. This function may be important in this muscle where hyperpolarizations due to postsynaptic inhibition are present Takeuchi, 1965, 1967). It is conceivable that owing to its voltage sensitivity and kinetics, ZAB could also modulate synaptic efficacy. The synaptic transmitters or modulators operative in this system could control its excitable behavior and the synaptic integration at the postsynaptic membrane through the activation or suppression of I,,. Interestingly, we have found that I,,, was markedly increased by the crustacean neuromodulator octopamine (A. Araque and W. Buiio, unpublished observations).