ACTIVITY OF CARDIAC L-TYPE CA2+ CHANNELS IS SENSITIVE TO CYTOPLASMIC CALCIUM

Ca2+-induced inactivation of L-type Ca2+ channels is proposed as an important negative feedback mechanism regulating Ca2+ entry. Here, for the first time, evidence for modification of heart L-type Ca2+ channel activity by cytoplasmic calcium is provided from excised inside-out membrane patches. Ba2+ currents through cardiac L-type Ca2+ channels exhibited only modest inactivation in the absence of cytoplasmic Ca2+. Elevation of cytoplasmic Ca2+ to micromolar concentrations strikingly affected L-type Ca2+ channel activity as evaluated from ensemble average Ba2+ currents. Inactivation was markedly increased concomitant with a reduction of peak inward current, which was almost completely eliminated at about 15 muM cytoplasmic Ca2+ concentration. Half maximal suppression of Ba2+ currents was observed at 2.3 muM Ca2+. The observed modifications of L-type Ca2+ channel activity show that cytoplasmic Ca2+ induces channel closure. Below 4 muM Ca2+, channels can be reversibly reactivated during repetitive depolarizations, while at high Ca2+ concentrations (approximately 15 muM) most Ca2+ channels reside in a closed state. This may allow for a delicate regulation of Ca2+ entry, and consequently of heart contraction.