The Voltage Sensor of Skeletal Muscle Excitation-Contraction Coupling: A Comparison with Ca2+ Channels. In: Morad M, Nayler WG, Kazda S, Schramm M, editors. The Calcium Channel: Structure, Function and Implications. Berlin: Springer Berlin Heidelberg; 1988. pp. 138-56. [DOI: 10.1007/978-3-642-73914-9_13]

The auxiliary subunit gamma 1 of the skeletal muscle L-type Ca2+ channel is an endogenous Ca2+ antagonist

Ca2+ channels play crucial roles in cellular signal transduction and are important targets of pharmacological agents. They are also associated with auxiliary subunits exhibiting functions that are still incompletely resolved. Skeletal muscle L-type Ca2+ channels (dihydropyridine receptors, DHPRs) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored Ca2+. The skeletal muscle-specific gamma subunit of the DHPR (gamma 1) down-modulates availability by altering its steady state voltage dependence. The effect resembles the action of certain Ca2+ antagonistic drugs that are thought to stabilize inactivated states of the DHPR. In the present study we investigated the cross influence of gamma 1 and Ca2+ antagonists by using wild-type (gamma+/+) and gamma 1 knockout (gamma-/-) mice. We studied voltage-dependent gating of both L-type Ca2+ current and Ca2+ release and the allosteric modulation of drug binding. We found that 10 microM diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by gamma 1 elimination; 5 muM devapamil [(-)D888], a phenylalkylamine Ca2+ antagonist, approximately reversed the right-shifted voltage dependence of availability and the accelerated recovery kinetics of Ca2+ current and Ca2+ release. Moreover, the presence of gamma 1 altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that gamma 1 and the drug do not act independently of each other. We propose that the gamma 1 subunit of the DHPR functions as an endogenous Ca2+ antagonist whose task may be to minimize Ca2+ entry and Ca2+ release under stress-induced conditions favoring plasmalemma depolarization.

Functional roles of the gamma subunit of the skeletal muscle DHP-receptor

In excitation-contraction coupling (EC coupling) of skeletal muscle, large and rapid changes of the myoplasmic Ca2+ concentration mediate the activation and termination of force. The L-type Ca2+ channel (dihydropyridine receptor, DHP receptor) is a central component of the EC coupling process. Its predominant role is to provide the Ca2+ release channels of the sarcoplasmic reticulum (SR) with the sensitivity to cell membrane voltage. The DHP receptor consists of five different proteins (alpha1S, beta1, gamma1, delta and alpha2) whose tasks and functional characteristics are still incompletely understood. This short review summarizes progress made in studying the physiology of the gamma1 subunit, a membrane polypeptide that is highly specific for skeletal muscle. The focus is on recent results obtained from muscle of gamma1-deficient mice.

Dihydropyridine-induced Ca2+ release from ryanodine-sensitive Ca2+ pools in human skeletal muscle cells

Dihydropyridines (DHPs) are widely used antihypertensive drugs and inhibit excitation-contraction (E-C) coupling in vascular smooth muscle and in myocardial cells by antagonizing L-type Ca2+ channels (DHP receptors). However, contradictory reports exist about the interaction of the DHP with the skeletal muscle isoform of the DHP receptor and E-C coupling in skeletal muscle cells. Using the intracellular fluorescent Ca2+ indicator fura-2, an increase in [Ca2+]i was observed after extracellular application of nifedipine to cultured human skeletal muscle cells. The rise in [Ca2+]i was dose dependent with a calculated EC50 of 614 +/- 96 nM nifedipine and a maximum increment in [Ca2+]i of 80 +/- 3.2 nM. Similar values were obtained with nitrendipine. This effect of DHPs was restricted to differentiated skeletal muscle cells and was not seen in non-differentiated cells or in PC12 cells. In spite of the observed increase in [Ca2+]i, whole-cell patch clamp experiments revealed that 10 microM nifedipine abolished inward Ba2+ currents through L-type Ca2+ channels completely. Similar to nifedipine, (+/-)Bay K 8644, an agonist of the L-type Ca2+ channel, also increased [Ca2+]i. This effect could not be enhanced by further addition of nifedipine, suggesting that both DHPs act via a common signalling pathway. Based on the specific mechanism of the skeletal muscle E-C coupling, we propose the stabilization of a conformational state of the DHP receptor by DHPs, which is sufficient to activate the ryanodine receptor.

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

1. Inactivation and recovery kinetics of L-type calcium currents were measured in myotubes derived from satellite cells of human skeletal muscle using the whole cell patch clamp technique. 2. The time course of inactivation at potentials above the activation threshold was obtained from the decay of the current during 15 s depolarizing pulses. At subthreshold potentials, prepulses of different durations, followed by +20 mV test pulses, were used. The time course could be well described by single exponential functions of time. The time constant decreased from 17.8 +/- 7.5 s at -30 mV to 1.78 +/- 0.15 s at +50 mV. 3. Restoration from inactivation caused by 15 s depolarization to +20 mV was slowed by depolarization in the restoration interval. The time constant increased from 1.11 +/- 0.17 s at -90 mV to 7.57 +/- 2.54 s at -10 mV. 4. Restoration showed different kinetics depending on the duration of the conditioning depolarization. While the time constant was similar at restoration potentials of -90 and -50 mV after a 1 s conditioning prepulse, it increased with increasing prepulse duration at -50 mV and decreased at -90 mV. 5. The experiments showed that the rates of inactivation and restoration of the L-type calcium current in human myotubes were not identical when observed at the same potential. The results indicate the presence of more than one inactivated state and point to different voltage-dependent pathways for inactivation and restoration.