Local Character of Release-Dependent Inactivation of L-Type Calcium Current

Abstract

Proper function of cardiac myocytes requires a reliable mechanism for regulation of calcium influx during the contraction-relaxation cycle. The most effective is negative feedback based on the calcium-dependent inactivation of calcium current. Calcium release activated by L-type Ca-current causes rapid and extensive inactivation of the Ca-current. The aim of this study was to characterize the effect of local Ca-release on the extent of calcium release-dependent inactivation (RDI) using characteristics of calcium spikes.Calcium release was induced in isolated rat ventricular myocytes in the whole-cell patch-clamp configuration, using a two pulse protocol [1]. A short pre-pulse of varying length or voltage was followed by a standard test-pulse (0 mV, 80 ms). The extent of RDI was assessed as the fraction of peak Ca-current remaining in the test-pulse relative to peak current in the absence of the pre-pulse. Local calcium release fluxes (Ca-spikes) were measured by laser scanning fluorescence confocal microscopy using 0.1 mM fluo-3 and 1 mM EGTA in the patch-pipette solution. The latency and the amplitude were determined for individual Ca-spikes. Probability of Ca-release activation (PA) was calculated as the fraction of activated calcium release sites. Synchrony of Ca-release was calculated as the inverse of standard deviation of the latency.The extent of RDI showed linear correlation with PA (R = 0.95) and amplitude (R = 0.89), while latency and synhrony did not affect the extent of RDI. The extent of RDI showed sigmoidal dependence on the product of amplitude and PA that reflects the amount of released calcium. We conclude that RDI of L-type calcium currents results from the local character of calcium release and local calcium-dependent inactivation of L-type calcium channels.Support: VEGA 2/0095/15 and VEGA 2/0147/14[1] Zahradnikova et al., 2004. Am J Physiol Cell Physiol (286): 330-341