RT Journal Article
SR Electronic
T1 Effects of proctolin on contractions, membrane resistance, and non- voltage-dependent sarcolemmal ion channels in crustacean muscle fibers
JF The Journal of Neuroscience
JO J. Neurosci.
FD Society for Neuroscience
SP 4356
OP 4369
DO 10.1523/JNEUROSCI.15-06-04356.1995
VO 15
IS 6
A1 CF Erxleben
A1 A deSantis
A1 W Rathmayer
YR 1995
UL http://www.jneurosci.org/content/15/6/4356.abstract
AB The neuropeptide proctolin in nanomolar concentrations enhances the contraction of crustacean muscle fibers manyfold. The cellular mechanisms underlying this potentiation were investigated in single, isolated, fast-contracting abdominal extensor muscle fibers of a small crustacean, the marine isopod Idotea baltica. Force measurements and current-clamp experiments revealed two actions of proctolin on the muscle fibers. In half of the preparations, proctolin (10(-9)-10(-6) M) increased the fiber's input resistance by up to 25%. In about one- fourth of the preparations, proctolin induced all-or-none action potentials in response to depolarizing current pulses in muscle fibers that showed graded electric responses under control conditions. In both cases, proctolin potentiated the peak force of muscle contractions (between 1.5- and 18-fold for 5 x 10(-9) M proctolin). Proctolin affected neither the membrane resting potential nor the threshold for excitation-contraction coupling. Using cell-attached patches on the sarcolemmal membrane, we identified non-voltage-dependent ion channels which contribute to the passive membrane properties of the muscle fibers. A 53 +/- 6 pS channel had its reversal potential near rest and carried outward current at depolarized potentials with physiological saline in the recording pipette. With isotonic K+ saline in the patch pipette, the reversal potential was +85 +/- 12 mV depolarized from the resting potential and single-channel conductances ranged from 36 to 166 pS. Proctolin modulated the activity of all these putative K+ channels by reducing the number of functionally active channels. The effects of proctolin on force of contraction, input resistance, and single-channel activity were mimicked by a membrane-permeating analog of cAMP. Conversely, a monothio analog of cAMP (Rp-cAMPS), a blocker of protein kinase A activity, substantially decreased the membrane input resistance of the muscle fibers. The results suggest that activation of the cAMP signal pathway and phosphorylation of non-voltage-dependent K+ channels by protein kinase A are involved in the potentiation of contractions by proctolin in the muscle fibers of this crustacean.