3 TYPES OF BOVINE CHROMAFFIN CELL CA-2+ CHANNELS - FACILITATION INCREASES THE OPENING PROBABILITY OF A 27-PS CHANNEL

1. Cell-attached patch recordings from bovine chromaffin cells were performed with 90 mM-Ba2+ in the patch pipette and with isotonic potassium aspartate in the bathing solution to zero the membrane potential. Three different types of unitary Ca2+ channel activity could be distinguished in these recordings. 2. A 27 pS Ca2+ channel was distinguished by constructing amplitude histograms and measuring slope conductance. This channel activated over a broad range of potentials (depolarizations > - 10 mV). 3. A second Ca2+ channel with a slope conductance of 14 pS could also be detected with amplitude histograms. This channel activated with depolarizations > - 20 mV. 4. An 18 pS Ca2+ channel was observed infrequently indicating that this channel may carry only a small amount of the whole-cell current. This 18 pS channel was sensitive to changes in holding potential. Depolarizing the patch to + 10 mV from a holding potential of -80 mV elicited robust unitary activity. Changing the patch holding potential to -40 mV while maintaining test depolarizations to +10 mV completely inactivated the 18 pS channel. Neither the 25 pS nor the 14 pS Ca2+ channels were affected by changes in holding potential in the range from -80 mV to -40 mV, indicating the 18 pS channel was a different type of channel. As the 18 pS channel was observed so infrequently, no detailed studies of it were possible. 5. Chromaffin cell Ca2+ currents exhibited facilitation. Large pre-depolarizations greatly augmented whole-cell currents observed in these cells. Whole-cell currents could double or triple after recruiting facilitation. The application of large pre-depolarizations altered the gating behaviour of the 27 pS Ca2+ channel manifested as dramatically increased channel opening probabilities measured during subsequent test pulses. Large pre-depolarizations induced unitary activity in the 27 pS Ca2+ channel similar to the long-lived openings exhibited by L-type Ca2+ channels in the presence of Bay K 8644. Large pre-depolarizations did not change the gating behaviour of the 14 pS Ca2+ channel. 6. Repetitive depolarizations in the physiological range could also induce facilitation. At the single-channel level facilitation was manifested as a striking increase in opening probability of the 27 pS Ca2+ channel. No effect of repetitive activity was observed on 14 pS channel gating. At the whole-cell level, repetitive depolarizations dramatically increased the current observed. 7. Facilitation of 27 pS Ca2+ channel activity could be induced by changing the holding potential to a depolarized level (greater-than-or-equal-to -10 mV). 8. Of the two relatively common kinds of Ca2+ channels in chromaffin cells (27 and 14 pS), only the 27 pS channel was sensitive to dihydropyridines. The dihydropyridine Ca2+ channel agonist Bay K 8644 promoted long-lived openings in the 27 pS Ca2+ channel but not the 14 pS Ca2+ channel. The dihydropyridine Ca2+ channel antagonist nisoldipine suppressed the unitary activity of the 27 pS Ca2+ channel but not the 14 pS Ca2+ channel. 9. These results suggest that facilitation of whole-cell currents observed in chromaffin cells is due to alterations in the gating behaviour of the 27 pS Ca2+ channel. Interestingly, in patches that were depolarized infrequently, the 27 pS channel type showed almost no activity of any sort, indicating that it was not a typical L-type Ca2+ channel. 10. The 14 pS Ca2+ channel is mostly likely responsible for the whole-cell Ca2+ current observed in the absence of facilitation. As the 14 pS Ca2+ channel was not affected by changes in membrane holding potential in the range -80 to -40 mV, it is unlike a classic N-type Ca2+ channel and may represent a new type of Ca2+ channel.