Fast gating kinetics of the slow Ca2+ current in cut skeletal muscle fibres of the frog.

Abstract

SUMMARY1. Calcium currents and intramembrane charge movements were measured in cut twitch muscle fibres of the frog and the time course of activation of the current was studied using various conditioning pulse protocols.2. When a conditioning activation was produced by a depolarizing pulse which ended before inactivation occurred, a subsequent depolarization led to a faster onset of activation, indicating that the system had not completely returned to the initial state during the interval between the two pulses.3. The interval between conditioning and test pulse was varied at different subthreshold potentials to study the time course of restoring the steady-state conditions. Complete restoration required a waiting period of about 1 min at the holding potential of -80 mV due to a very slow process but partial recovery was reached within 100 ms. This initial recovery process was strongly voltage dependent and became considerably slower when the interval potential approached the threshold for current activation.4. Stepping to a roughly 10 mV subthreshold potential without applying a conditioning activation caused no change in the time course of the current produced by a subsequent test depolarization. Depolarizing just to the current threshold caused a slowly progressing acceleration of test current activation.5. The peak current-voltage relation in the fast gating regime caused by a conditioning activation coincided with the current-voltage relation measured under steady-state conditions, indicating not that a new channel population had become activated but that the same channels showed a different gating behaviour.6. Intramembrane charge movements measured in 2 mM-Cd2+ and tested at potentials between -40 and + 40 mV showed negligible changes when preceded by a strong depolarization. 7. We discuss several possible models which can explain the fact that the current is speeded up by a conditioning activation while the charge movements remain unchanged. It is possible that the fast voltage-dependent transition which becomes visible after conditioning pulses reflects a rapid conformational change of the Ca2+ channel molecule which also occurs during its normal gating mode but remains undetectable in terms of conductance. In view of the hypothesis that the Ca2+ channel molecule forms a voltage sensor for excitation-contraction coupling this fast MS 7918 348

Kinetics of inactivation and restoration from inactivation of the L‐type calcium current in human myotubes

“…In addition they permit an extremely slow calcium inward current (L_type current) whose physiological role is still unclear (for a review see Melzer et al 1995). L_type current characteristics have been extensively studied in adult amphibian and mammalian skeletal muscle (for references see Delbono, 1992;Feldmeyer et al 1990Feldmeyer et al , 1992Feldmeyer et al , 1995Garcia et al 1992;Francini et al 1996). On the other hand, important data on structure-function relationships of the channel have been obtained by heterologous expression of recombinant DHP receptors in cultured immature muscle cells (myotubes) (Tanabe et al 1988;Garcia et al 1994).…”

Molecular Cloning and Functional Expression of a Skeletal Muscle Dihydropyridine Receptor from Rana catesbeiana

“…Fg␣1S might encode the "fast" Ca 2ϩ channel. On the other hand, slowly activating Ca 2ϩ currents of frog muscle can be switched to a fast activation mode by prior depolarization (42,43). Therefore, the fast phase seen in the normal unconditioned current (and the currents induced here by Fg␣1S) may not correspond to a separate channel population but to a fraction of the slow channels showing fast gating kinetics in the steady state.…”

Modulation of calcium current gating in frog skeletal muscle by conditioning depolarization.

“…After activation and subsequent deactivation of the current this channel can remain in a condition that allows more rapid opening showing that the channel exhibits at least two closed states with very different kinetics of voltage-dependent transition to the open state, i.e. a slow and a fast gating mode (Feldmeyer et al 1990).…”

“…However, it has been shown that the activation kinetics of the skeletal muscle channel is accelerated following a conditioning prepulse (Feldmeyer, Melzer, Pohl & Z6llner, 1990;Ma, Hosey & Rios, 1992). After activation and subsequent deactivation of the current this channel can remain in a condition that allows more rapid opening showing that the channel exhibits at least two closed states with very different kinetics of voltage-dependent transition to the open state, i.e.…”

“…The experimental approach and data analysis followed procedures described in a previous paper (Feldmeyer et al 1990). Briefly, cut segments of single fibres of the m. semitendinosus dissected from frogs (Rana esCUlenta), which had been killed by decapitation, were voltage clamped at slack length in a double-Vaseline-gap system (Kovacs, Rios & Schneider, 1983).…”