VOLTAGE-GATED AND CA2-ACTIVATED CONDUCTANCES MEDIATING AND CONTROLLING GRADED ELECTRICAL-ACTIVITY IN CRAYFISH MUSCLE()

Crayfish opener muscle fibers provide a unique preparation to quantita tively evaluate the relationships between the voltage-gated Ca2+ (I-Ca ) and Ca2+-activated K+ (I-K(Ca)) currents underlying the graded actio n potentials (GAPs) that typify these fibers. I-Ca, I-K(Ca), and the v oltage-gated K+ current (I-K) were studied using two-electrode voltage -clamp applying voltage commands that simulated the GAPs evoked in cur rent-clamp conditions by 60-ms current pulses. This methodology, unlik e traditional voltage-clamp step commands, provides a description of t he dynamic aspects of the interaction between different conductances p articipating in the generation of the natural GAP. The initial depolar izing phase of the GAP was due to activation of the I-Ca on depolariza tion 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 contrib ution of I-K to the GAP repolarization was delayed. During the final s teady GAP depolarization I-Ca and I-K(Ca) were simultaneously activate d with similar magnitudes, whereas I-K aided in the control of the del ayed sustained response. The larger GAPs evoked by higher intensity st imulations were due to the increase in I-Ca. The resulting larger Ca2 inflow increased I-K(Ca) which acted as a negative feedback that prec isely controlled the fiber's depolarization. Hence IK(ca) regulated th e Ca2+-inflow needed for the contraction and controlled the depolariza tion that this Ca2+ inflow would otherwise elicit.