Journal of Neuroscience, Vol 15, 4356-4369, Copyright © 1995 by Society for Neuroscience
Effects of proctolin on contractions, membrane resistance, and non- voltage-dependent sarcolemmal ion channels in crustacean muscle fibers
CF Erxleben, A deSantis and W Rathmayer
Department of Biology, University of Konstanz, Germany.
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.
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