Inactivation characteristics reveal two calcium currents in adult bovine chromaffin cells

Abstract

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-activation 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.