Voltage-gated and Ca²⁺-activated conductances mediating and controlling graded electrical activity in crayfish muscle

Crayfish opener muscle fibers provide a unique preparation to quantitatively evaluate the relationships between the voltage-gated Ca²⁺ (I(Ca)) and Ca²⁺ -activated K⁺ (I(K((Ca)) currents underlying the graded action potentials (GAPs) that typify these fibers. I(Ca), I(K((Ca)), and the voltage-gated K⁺ current (I(K)) were studied using two-electrode voltage- clamp applying voltage commands that simulated the GAPs evoked in current- clamp conditions by 60-ms current pulses. This methodology, unlike traditional voltage-clamp step commands, provides a description of the dynamic aspects of the interaction between different conductances participating in the generation of the natural GAP. The initial depolarizing phase of the GAP was due to activation of the I(Ca) on depolarization above approximately -40 mV. The resulting Ca²⁺ inflow induced the activation of the fast I(K((Ca)) (<3 ms), which rapidly repolarized the fiber (<6 ms). Because of its relatively slow activation, the contribution of I(K) to the GAP repolarization was delayed. During the final steady GAP depolarization I(Ca) and I(K)((Ca)) were simultaneously activated with similar magnitudes, whereas I(K) aided in the control of the delayed sustained response. The larger GAPs evoked by higher intensity stimulations were due to the increase in I(Ca). The resulting larger Ca²⁺ inflow increased I(K((Ca)), which acted as a negative feedback that precisely controlled the fiber's depolarization. Hence I(K((Ca)) regulated the Ca²⁺-inflow needed for the contraction and controlled the depolarization that this Ca²⁺ inflow would otherwise elicit.