CA2-DEPENDENT INACTIVATION OF CARDIAC L-TYPE CA2+ CHANNELS DOES NOT AFFECT THEIR VOLTAGE SENSOR()

Inactivation of currents carried by Ba2+ and Ca2+, as well as intramem brane charge movement from L-type Ca2+ channels were studied in guinea pig ventricular myocytes using the whole-cell patch clamp technique. Prolonged (2 s) conditioning depolarization caused substantial reducti on of charge movement between -70 and 10 mV (charge 1, or charge from noninactivated channels). In parallel, the charge mobile between -70 a nd -150 mV (charge 2, or charge from inactivated channels) was increas ed. The availability of charge 2 depended on the conditioning pulse vo ltage as the sum of two Boltzmann components. One component had a cent ral voltage of -75 mV and a magnitude of 1.7 nC/muF. It presumably is the charge movement (charge 2) from Na+ channels. The other component, with a central voltage of approximately -30 mV and a magnitude of 3.5 nC/muF, is the charge 2 of L-type Ca2+ channels. The sum of charge 1 and charge 2 was conserved after different conditioning pulses. The di fference between the voltage dependence of the activation Of L-type Ca 2+ channels (half-activation voltage, VBAR of approximately -20 mV) an d that of charge 2 (VBAR of -100 mV) made it possible to record the io nic currents through Ca2+ channels and charge 2 in the same solution. In an external solution with Ba2+ as sole metal the maximum available charge 2 of L-type Ca2+ channels was 10-15% greater than that in a Ca2 +-containing solution. External Cd2+ caused 20-30% reduction of charge 2 both from Na+ and L-type Ca2+ channels. Voltage- and Ca2+-dependent inactivation phenomena were compared with a double pulse protocol in cells perfused with an internal solution of low calcium buffering capa city. As the conditioning pulse voltage increased, inactivation monito red with the second pulse went through a minimum at about 0 mV, the vo ltage at which conditioning current had its maximum. Charge 2, recorde d in parallel, did not show any increase associated with calcium entry . Two alternative interpretations of these observations are: (a) that Ca2+-dependent inactivation does not alter the voltage sensor, and (b) that inactivation affects the voltage sensor, but only in the small f raction of channels that open, and the effect goes undetected. A model of channel gating that assumes the first possibility is shown to acco unt fully for the experimental results. Thus, extracellular divalent c ations modulate voltage-dependent inactivation of the Ca2+ channel. In tracellular Ca2+ instead, appears to cause inactivation of the channel without affecting its voltage sensor.