The process of inactivation of the Ca2+ current of acutely dissociated pyramidal cells from the CA1 subfield of mature guinea-pig hippocampus was characterized. The decline of the current after rapid activation could be approximated well by the sum of two exponentials (time constants approximately 200 ms and 2 s) and a constant offset. 2. The time constants of inactivation exhibited a voltage dependence consistent with a voltage-dependent mechanism. However, under conditions which normally counteract Ca(2+)-dependent inactivation (viz. intracellular bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and external Ba2+) all three showed a U-shaped inactivation curve, characteristic of Ca(2+)-dependent inactivation. 3. The rate of inactivation was found to increase with current at a given voltage; however, increasing external divalent ion concentrations did not accelerate inactivation. 4. Calcium imaging experiments, using the Ca(2+)-sensitive probe, Fura-2, were performed to estimate the accumulation of Ca2+ in the presence of 10 mM-intracellular BAPTA. Under these conditions voltage steps which induced maximal Ca2+ currents lead to free Ca2+ concentrations of less than 500 nM in the bulk of the cytoplasm. 5. Elevation of the intracellular free Ca2+ concentration to above 1 microM suppressed all the components of the Ca2+ current. However, even at a concentration of 3 microM-Ca2+ the U-shaped inactivation curve persisted. 6. Substitution of Ca2+ for Ba2+ led to an acceleration of inactivation through an increase in the proportion of the fast process of inactivation and an acceleration of both the fast and slow rates of inactivation. 7. During the slow decline of Ca2+ current ('run-down') the proportion of all three components remained approximately constant and there was little change in the rate of inactivation. 8. On the basis of the results I suggest that inactivation results fro a dual process of voltage- and Ca(2+)-dependent inactivation. Ca(2+)-dependent inactivation seems to result from the accumulation of Ca2+ close to the channel mouth. 9. The macroscopic properties of the Ca2+ channel are consistent with the existence of one channel type in the CA1 pyramidal cells.
