[Ca²⁺]_i-dependent membrane currents in guinea-pig ventricular cells in the absence of Na/Ca exchange

Abstract

Transient inward currents (I _ti) during oscillations of intracellular [Ca²⁺] ([Ca²⁺]_i) in ventricular myocytes have been ascribed to Na/Ca exchange. We have investigated whether other Ca²⁺-dependent membrane currents contribute to I _ti in single guinea-pig ventricular myocytes, by examining membrane currents during [Ca²⁺]_i oscillations and during caffeine-induced Ca²⁺ release from the sarcoplasmic reticulum in the absence of Na⁺. Membrane currents were recorded during whole-cell voltage clamp and [Ca²⁺]_i measured simultaneously with fura-2. In the absence of Na/Ca exchange, i.e., with Li⁺, Cs⁺ or N-methyl-D-glucamine (NMDG⁺) substituted for Na⁺, the cell could be loaded with Ca²⁺ by repetitive depolarizations to +10 mV, resulting in spontaneous [Ca²⁺]_i oscillations. During these oscillations, no inward currents were seen, but instead spontaneous Ca²⁺ release was accompanied by a shift of the membrane current in the outward direction at potentials between −40 mV and +60 mV. This [Ca²⁺]_i-dependent outward current shift was not abolished when NMDG⁺ was substituted for internal monovalent cations, nor was it sensitive to substitution of external Cl⁻. It was however, sensitive to the blockade of I_Ca by verapamil. These results suggest that the transient outward current shift observed during spontaneous Ca²⁺ release represents [Ca²⁺]_idependent transient inhibition of I _Ca. Similarly, during the [Ca²⁺]_i transients induced by brief caffeine (10 mM) applications, we could not detect membrane currents attributable to a Ca²⁺-activated nonselective cation channel, or to a Ca²⁺-activated Cl⁻ channel; however, transient Ca²⁺-dependent inhibition of I _Ca was again observed. We conclude that neither the Ca²⁺-activated nonselective cation channel nor the Ca²⁺-activated Cl⁻ channel contribute significantly to the membrane currents during spontaneous [Ca²⁺]_i oscillations in guineapig ventricular myocytes. However, in the voltage range between −40 mV and +60 mV Ca²⁺-dependent transient inhibition of I _Ca will contribute to the oscillations of the membrane current.