Temperature dependence of electro-mechanical coupling in crayfish muscle fibers

The role of the Ca2+-activated K+ current ( I K(Ca)) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca2+, K+, and Ca2+-dependent K+ currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I K(Ca). These voltage- and Ca2+-activated channels had a mean single-channel conductance of ∼ 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca2+concentration. Intracellular loading with the Ca2+ chelator bis(2-aminophenoxy) ethane- N, N,N′,N′-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (≤560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I K(Ca). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I K(Ca), which probably reflects temporal Ca2+ variations in the whole muscle fiber. We conclude that the channels mediating I K(Ca) in crayfish muscle are voltage- and Ca2+-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca2+ sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.

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Effects of Caffeine on Crayfish Muscle Fibers

The Journal of General Physiology ◽

10.1085/jgp.55.5.665 ◽

1970 ◽

Vol 55 (5) ◽

pp. 665-687 ◽

Cited By ~ 15

Author(s):

Dante J. Chiarandini ◽

John P. Reuben ◽

Lucien Girardier ◽

George M. Katz ◽

Harry Grundfest

Keyword(s):

Muscle Fibers ◽

Bathing Medium ◽

Contractile Responses ◽

Crayfish Muscle ◽

Contractile System ◽

Mn Recovery

When caffeine evokes a contraction, and only then, crayfish muscle fibers become refractory to a second challenge with caffeine for up to 20 min in the standard saline (5 mM Ko). However, the fibers still respond with contraction to an increase in Ko, though with diminished tension. Addition of Mn slows recovery, but the latter is greatly accelerated during exposure of the fiber to high Ko, or after a brief challenge with high Ko. Neither the depolarization induced by the K, nor the repolarization after its removal accounts for the acceleration, which occurs only if the challenge with K had itself activated the contractile system; acceleration is blocked when contractile responses to K are blocked by reducing the Ca in the bath or by adding Mn. Recovery is accelerated by redistribution of intracellular Cl and by trains of intracellularly applied depolarizing pulses, but not by hyperpolarization. The findings indicate that two sources of Ca can be mobilized to activate the contractile system. Caffeine mobilizes principally the Ca store of the SR. Depolarizations that are induced by high Ko, by transient efflux of Cl, or by intracellularly applied currents mobilize another source of Ca which is strongly dependent upon the entry of Ca from the bathing medium. The sequestering mechanism of the SR apparently can utilize this second source of Ca to replenish its own store so as to accelerate recovery of responsiveness to a new challenge with caffeine.

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