Search

Malignant hyperthermia (MH) is a life-threatening hypermetabolic condition caused by dysfunctional [Ca.sup.2+] homeostasis in skeletal muscle, which primarily originates from genetic alterations in the [Ca.sup.2+] release channel (ryanodine receptor, RyR1) of the sarcoplasmic reticulum (SR). Owing to its physical interaction with the dihydropyridine receptor (DHPR), RyR1 is controlled by the electrical potential across the transverse tubular (TT) membrane. The DHPR exhibits both voltage-dependent activation and inactivation. Here we determined the impact of an MH mutation in RyR1 (Y522S) on these processes in adult muscle fibers isolated from heterozygous [RyR1.sup.Y522S]-knock-in mice. The voltage dependence of DHPR-triggered [Ca.sup.2+] release flux was left-shifted by [approximately equal to] 8 mV. As a consequence, the voltage window for steady-state [Ca.sup.2+] release extended to more negative holding potentials in muscle fibers of the [RyR1.sup.Y522S]-mice. A rise in temperature from 20[degrees] to 30[degrees]C caused a further shift to more negative potentials of this window (by [approximately equal to]20 mV). The activation of the DHPR-mediated [Ca.sup.2+] current was minimally changed by the mutation. However, surprisingly, the voltage dependence of steady-state inactivation of DHPR-mediated calcium conductance and release were also shifted by [approximately equal to]10 mV to more negative potentials, indicating a retrograde action of the RyR1 mutation on DHPR inactivation that limits window [Ca.sup.2+] release. This effect serves as a compensatory response to the lowered voltage threshold for [Ca.sup.2+] release caused by the Y522S mutation and represents a novel mechanism to counteract excessive [Ca.sup.2+] leak and store depletion in MH-susceptible muscle. dihydropyridine receptor | excitation-contraction coupling | malignant hyperthermia | mouse skeletal muscle | ryanodine receptor

[Ca.sup.2+] 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 [Ca.sup.2+] channels (dihydropyridine receptors, DHPRs) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored [Ca.sup.2+]. The skeletal muscle-specific 1, subunit of the DHPR ([[gamma].sub.1]) down-modulates availability by altering its steady state voltage dependence. The effect resembles the action of certain [Ca.sup.2+] antagonistic drugs that are thought to stabilize inactivated states of the DHPR. In the present study we investigated the cross influence of [[gamma].sub.1] and [Ca.sup.2+] antagonists by using wild-type ([gamma]+/+) and [[gamma].sub.1] knockout ([gamma]-/-) mice. We studied voltage-dependent gating of both L-type [Ca.sup.2+] current and [Ca.sup.2+] release and the allosteric modulation of drug binding. We found that 10 [micro]M diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by [[gamma].sub.1] elimination; 5 [micro]M devapamil [(-)D888], a phenylalkylamine [Ca.sup.2+] antagonist, approximately reversed the right-shifted voltage dependence of availability and the accelerated recovery kinetics of [Ca.sup.2+] current and [Ca.sup.2+] release. Moreover, the presence of [[gamma].sub.1] altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that [[gamma].sub.1] and the drug do not act independently of each other. We propose that the [[gamma].sub.1] subunit of the DHPR functions as an endogenous [Ca.sup.2+] antagonist whose task may be to minimize [Ca.sup.2+] entry and [Ca.sup.2+] release under stress-induced conditions favoring plasmalemma depolarization.

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 (di-hydropyridine receptors, DHPR5) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored Ca2+. The skeletal muscle-specific γ subunit of the DHPR (γ1) 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 γ1 and Ca2+ antagonists by using wild-type (γ+/+) and γ1 knockout (γ-/-) 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 μM diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by 71 elimination; 5 μM devapamil [(-)D8881. 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 γ1 altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that γ1 and the drug do not act independently of each other. We propose that the γ1 subunit of the DHPR functions as an endoge- nous Ca2+ antagonist whose task may be to minimize Ca2+ entry and Ca2+ release under stress-induced conditions favoring plasmalemma depolarization. [ABSTRACT FROM AUTHOR]

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.