Voltage-Gated and Ca-Activated Conductances Mediating and Controlling Graded Electrical Activity in Crayfish Muscle

Araque, Alfonso, Alain Marchand, and Washington Buño. and Vassort 1975a). Therefore opener fibers provide a Voltage-gated and Ca-activated conductances mediating and unique preparation to quantitatively evaluate the relationcontrolling graded electrical activity in crayfish muscle. J. Neuroships between the waveforms and magnitudes of the ICa and physiol. 79: 2338–2344, 1998. Crayfish opener muscle fibers proIK(Ca) underlying the GAPs that typify these fibers. vide a unique preparation to quantitatively evaluate the relationWe have taken advantage of this preparation to evaluate ships between the voltage-gated Ca (ICa) and Ca-activated K the contribution of ICa , of the delayed rectifier (IK), and (IK(Ca) ) currents underlying the graded action potentials (GAPs) especially of IK(Ca) to the GAP of opener fibers. Therefore that typify these fibers. ICa , IK(Ca) , and the voltage-gated K current we designed voltage-clamp experiments in which simulated (IK) were studied using two-electrode voltage-clamp applying voltvoltage commands (i.e., ‘‘synthetic’’) imitated both the age commands that simulated the GAPs evoked in current-clamp conditions by 60-ms current pulses. This methodology, unlike traGAP evoked under current clamp in control conditions or ditional voltage-clamp step commands, provides a description of the all-or-none AP (ANAP) elicited when IK(Ca) is blocked the dynamic aspects of the interaction between different conducby intracellular bis-(o-aminophenoxy)-N,N,N *,N *-tetraacetances participating in the generation of the natural GAP. The tic acid (BAPTA) loading or extracellular application of initial depolarizing phase of the GAP was due to activation of the charybdotoxin (CTX). This approach provides us with inICa on depolarization above approximately 040 mV. The resulting formation regarding the contribution of the ionic currents Ca inflow induced the activation of the fast IK(Ca) (õ3 ms), to the electrical activity of opener fibers in physiological which rapidly repolarized the fiber (õ6 ms). Because of its relaconditions of operation, permitting a quantitative description tively slow activation, the contribution of IK to the GAP repolarizaof the interactions between ionic conductances during the tion was delayed. During the final steady GAP depolarization ICa generation of the GAP, which is impossible using traditional and IK(Ca) were simultaneously activated with similar magnitudes, whereas IK aided in the control of the delayed sustained response. voltage-clamp pulse commands. The larger GAPs evoked by higher intensity stimulations were due We demonstrate that the initial ICa activation induced the to the increase in ICa . The resulting larger Ca inflow increased depolarizing phase of the GAP and that the resulting Ca IK(Ca) , which acted as a negative feedback that precisely controlled inflow activated IK(Ca) , which rapidly repolarized the fiber. the fiber’s depolarization. Hence IK(Ca) regulated the Ca-inflow IK was activated late, contributing to the final repolarization. needed for the contraction and controlled the depolarization that In this muscle excitation-contraction, coupling is controlled this Ca inflow would otherwise elicit. by a Ca-induced Ca-release mechanism (Gyorke and