Altered Inactivation of Ca²⁺ Current and Ca²⁺ Release in Mouse Muscle Fibers Deficient in the DHP receptor γ₁ subunit

Functional impacts of the skeletal muscle-specific Ca²⁺ channel subunit γ₁ have previously been studied using coexpression with the cardiac α_1C polypeptide in nonmuscle cells and primary-cultured myotubes of γ₁-deficient mice. Data from single adult muscle fibers of γ−/− mice are not yet available. In the present study, we performed voltage clamp experiments on enzymatically isolated mature muscle fibers of the m. interosseus obtained from γ+/+ and γ−/− mice. We measured L-type Ca²⁺ inward currents and intracellular Ca²⁺ transients during 100-ms step depolarizations from a holding potential of −80 mV. Ratiometric Ca²⁺ transients were analyzed with a removal model fit approach to calculate the flux of Ca²⁺ from the sarcoplasmic reticulum. Ca²⁺ current density, Ca²⁺ release flux, and the voltage dependence of activation of both Ca²⁺ current and Ca²⁺ release were not significantly different. By varying the holding potential and recording Ca²⁺ current and Ca²⁺ release flux induced by 100-ms test depolarizations to +20 mV, we studied quasi-steady-state properties of slow voltage–dependent inactivation. For the Ca²⁺ current, these experiments showed a right-shifted voltage dependence of inactivation. Importantly, we could demonstrate that a very similar shift occurred also in the inactivation curve of Ca²⁺ release. Voltages of half maximal inactivation were altered by 16 (current) and 14 mV (release), respectively. Muscle fiber bundles, activated by elevated potassium concentration (120 mM), developed about threefold larger contracture force in γ−/− compared with γ+/+. This difference was independent of the presence of extracellular Ca²⁺ and likely results from the lower sensitivity to voltage-dependent inactivation of Ca²⁺ release. These results demonstrate a specific alteration of voltage-dependent inactivation of both Ca²⁺ entry and Ca²⁺ release by the γ₁ subunit of the dihydropyridine receptor in mature muscle fibers of the mouse.