FPL-64176 alters both charge movement and Ca 2+ release properties in amphibian muscle fibres

Abstract

A number of recent reports have suggested that ryanodine receptor (RyR)-Ca2+ release channels are gated by tubular depolarization in skeletal muscle through their direct coupling to intramembrane dihydropyridine receptor (DHPR)-voltage sensors. The qgama charge movement, which is inhibited by DHPR antagonists, is often regarded as the electrical signature for the voltage sensing process, yet pharmacological modifications of the RyR produce reciprocal upstream kinetic effects on an otherwise conserved qgamma charge. This study investigates the effect of DHPR-specific agonists upon intramembrane charge and the release of intracellularly stored Ca2+. We empirically demonstrate kinetic effects of FPL-64176 upon charge movements that closely resemble the consequences of previous interventions directed instead at the RyR. Increases in extracellular FPL-64176 concentration from 10 to 40 microM converted delayed qgamma transients to monotonic decays indistinguishable from the exponential qbeta current component. Yet total steady-state intramembrane charge and the steepness of its dependence upon test potential closely resembled previous reports from untreated fibres. These changes accompanied an appearance of transient cytosolic [Ca2+] elevations in confocal line-scans in fluo-3-loaded fibres studied in 10mM K+ and 40, but not 10 microM, FPL-64176 that resembled elementary Ca2+ release events ('sparks'). Pharmacological manipulations of the DHPR whose effects on intramembrane charge resembled those from manoeuvres directed at the RyR can thus produce downstream effects upon Ca2+ release.