Differences in the characterization of the Ca2+ current in ventricular myocytes between spontaneously hypertensive rats and normotensive rats.

1. The slow inward Ca2+ current elicited at the membrane potential -30 mV in SHR and normotensive Wistar rats (WR) with voltage-clamp recording was examined for this study. 2. The maximal Ca2+ current of WR in the current-voltage relationship showed the unified membrane potential of +10 mV and the amplitude was -1.2 +/- 0.2 pA. In the case of SHR, however, the maximal Ca2+ current showed a lower and variable membrane potential between 0 and -20 mV. The amplitudes were -1.7 +/- 0.9 pA at 0 mV, -2.1 +/- 0.8 pA at -10 mV and -4.4 +/- 0.3 pA at -20 mV. 3. From the cell-attached patch-clamp recording, the conductance of unitary Ca2+ current and the slope value were the same in SHR and in WR. From the open-time histogram of the Ca2+ channel, the open state probability in SHR increased and the time constant from the exponential curve became slightly extended in SHR. 4. L-isoproterenol at 10(-6) mol/L increased the Ca2+ current in SHR and WR. The increased ratio of Ca2+ current by l-isoproterenol was smaller in SHR than that in WR. 5. The aspect of 1-4 suggests that the increase in Ca2+ current in SHR obtained by the voltage-clamp was explained partly by the increase of open-state probability of unitary Ca2+ channel activity, and that a possibility of Ca2+ channel being activated by phosphorylation through cAMP did not eventuate.