Modulation of calcium current density by intracellular calcium in isolated guinea pig ventricular cardiomyocytes

This study describes quantitatively the relation between L-type calcium current (ICa) density and resting cytosolic free calcium concentration ([Ca2+]i) in isolated guinea pig ventricular cardiomyocytes. ICa was measured simultaneously with [Ca2+]i at 22 and 35 degrees C using the whole-cell patch-clamp method in combination with the Fura-2 ratio-fluorescence technique. At 22 degrees C, the increase in [Ca2+]i upon replacing external Na+ with tetraethylammonium resulted in a monotonic decrease in ICa density. Half-maximal inhibition of ICa density occurred at a [Ca2+]i value (K0.5) of 60 +/- 17 nM (mean +/- S.D., n = 59). At 35 degrees C, with a holding potential of -80 mV, a similar relation between [Ca2+]i and ICa was observed (K0.5 of 62 +/- 12 nM, n = 34). When the cells were depolarized from -40 mV, however, the initial decrease in ICa density with increasing [Ca2+]i was usually followed by transient stimulation (< or = 2-fold), which was again overcome by inhibition. Stimulation was not observed in the presence of broad spectrum protein kinase inhibitors. These data suggest that Ca2+i-dependent facilitation of cardiac ICa is mediated by a temperature-sensitive enzymatic pathway that ends in voltage-dependent phosphorylation of Ca2+ channels.