EFFECTS OF PROCTOLIN ON CONTRACTIONS, MEMBRANE RESISTANCE, AND NON-VOLTAGE-DEPENDENT SARCOLEMMAL ION CHANNELS IN CRUSTACEAN MUSCLE-FIBERS

The neuropeptide proctolin in nanomolar concentrations enhances the co ntraction of crustacean muscle fibers manyfold. The cellular mechanism s underlying this potentiation were investigated in single, isolated, fast-contracting abdominal extensor muscle fibers of a small crustacea n, the marine isopod Idotea baltica. Force measurements and current-cl amp experiments revealed two actions of proctolin on the muscle fibers . In half of the preparations, proctolin (10(-9)-10(-6) M) increased t he fiber's input resistance by up to 25%. In about one-fourth of the p reparations, proctolin induced all-or-none action potentials in respon se 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 memb rane resting potential nor the threshold for excitation-contraction co upling. Using cell-attached patches on the sarcolemmal membrane, we id entified non-voltage-dependent ion channels which contribute to the pa ssive membrane properties of the muscle fibers, A 53 +/- 6 pS channel had its reversal potential near rest and carried outward current at de polarized potentials with physiological saline in the recording pipett e. With isotonic K+ saline in the patch pipette, the reversal potentia l 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 con traction, 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, substanti ally decreased the membrane input resistance of the muscle fibers. The results suggest that activation of the cAMP signal pathway and phosph orylation of non-voltage-dependent K+ channels by protein kinase A are involved in the potentiation of contractions by proctolin in the musc le fibers of this crustacean.