Orthograde dihydropyridine receptor signal regulates ryanodine receptor passive leak

Abstract

The skeletal muscle dihydropyridine receptor (DHPR) and ryanodine receptor (RyR1) are known to engage a form of conformation coupling essential for muscle contraction in response to depolarization, referred to as excitation-contraction coupling. Here we use WT and Ca(V)1.1 null (dysgenic) myotubes to provide evidence for an unexplored RyR1-DHPR interaction that regulates the transition of the RyR1 between gating and leak states. Using double-barreled Ca(2+)-selective microelectrodes, we demonstrate that the lack of Ca(V)1.1 expression was associated with an increased myoplasmic resting [Ca(2+)] ([Ca(2+)](rest)), increased resting sarcolemmal Ca(2+) entry, and decreased sarcoplasmic reticulum (SR) Ca(2+) loading. Pharmacological control of the RyR1 leak state, using bastadin 5, reverted the three parameters to WT levels. The fact that Ca(2+) sparks are not more frequent in dysgenic than in WT myotubes adds support to the hypothesis that the leak state is a conformation distinct from gating RyR1s. We conclude from these data that this orthograde DHPR-to-RyR1 signal inhibits the transition of gated RyR1s into the leak state. Further, it suggests that the DHPR-uncoupled RyR1 population in WT muscle has a higher propensity to be in the leak conformation. RyR1 leak functions are to keep [Ca(2+)](rest) and the SR Ca(2+) content in the physiological range and thus maintain normal intracellular Ca(2+) homeostasis.