INACTIVATION CHARACTERISTICS REVEAL 2 CALCIUM CURRENTS IN ADULT BOVINE CHROMAFFIN CELLS

1. Two calcium currents were identified by differences in their inactivation characteristics in adult chromaffin cells maintained in short-term primary culture (3-5 days). Calcium currents were recorded by means of the whole-cell configuration using an intracellular medium highly buffered for pH and pCa. 2. Calcium current evoked from a holding potential of -90 mV inactivated along two components: an initial transient with a time constant of 250 ms followed by a plateau. 3. Steady-state inactivation followed two processes which developed at two distinct membrane potentials. One process was half-inactivated at low voltages around -55 mV and affected mainly the initial transient component. The other process, which affected mainly the sustained component of the calcium current, was half-inactivated at voltages around -10 mV. The proportions of these two processes varied greatly from cell to cell. 4. The dihydropyridine antagonists (nicardipine and nifedipine applied at 10(-5) M) and the phenylalkylamine D600 (5 x 10(-6) M) shifted the half-inactivation value towards -55 mV, indicating the suppression of the sustained component. The snail toxin, omega-conotoxin, had the opposite effect; it shifted the half-inactivation value towards -10 mV. 5. The calcium channel agonist Bay K 8644 (10(-5) M) either had no effect or induced only a slight increase of the response, as did its (-)-enantiomer (10(-6) M). To interpret the present results, we suggest that the L-component was maximally activated in our recording conditions. 6. In chromaffin cells, the calcium current recorded in whole-cell conditions is composed of two components with properties close to those of N- and L-type currents described in sympathetic neurons.