Three types of bovine chromaffin cell Ca2+ channels: facilitation increases the opening probability of a 27 pS channel.

Abstract

SUMMARY1. 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 Ca2l 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 +1O 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 predepolarizations 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 Ca21 channel similar to the long-lived openings exhibited by L-type Ca2+ channels in the § To whom correspondence should be addressed. 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 (>-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 dihy-

Short-Term Changes in the Ca2+-Exocytosis Relationship during Repetitive Pulse Protocols in Bovine Adrenal Chromaffin Cells

“…In all cells, the voltage-gated Ca 2ϩ current showed typical inactivation during a train (Fig. 2 B; comparison of first and last current), with no evidence for activation of additional "facilitation" channels as described for calf chromaffin cells (Artalejo et al, 1991(Artalejo et al, , 1994.…”

“…The early rapidly decaying inward component is a Na ϩ current. The Ca 2ϩ current inactivates during all three stimulus trains, and there is no evidence for recruitment of a "facilitation" current (Artalejo et al, 1991). C, Percentage of cells that responded to a given stimulus protocol with a Ca 2ϩ efficacy that was enhanced relative to the standard transfer f unction.…”

Selective potentiation of a novel calcium channel in rat hippocampal neurones.

“…In the hippocampus, dihydropyridine-sensitive calcium channels have been observed that are active at very negative potentials following maintained depolarization (Fisher, Gray & Johnston, 1990). Whether this activity results from a specific kind of calcium channel, as in chromaffin cells (Artalejo et al 1991), or is due to potentiation of the standard L-type calcium channel (Fisher et al 1990) remains unclear.…”

“…Although such changes are often associated with the NMDA receptor channel (Malenka, Kauer, Perkel & Nicoll, 1989), there is also evidence suggesting that voltage-gated calcium channels are involved (Pitler & Landfield, 1990;Moyer, Thompson, Black & Disterhoft, 1992;Johnston, Williams, Jaffe & Gray, 1992). For example, there are 'facilitation' calcium channels in adrenal chromaffin cells that show greatly increased activity subsequent to large depolarizations (Artalejo, Mogul, Perlman & Fox, 1991).…”

Kinetic analysis of prepulse facilitation of calcium currents in hamster submandibular ganglion neurons.

“…Both are affected by VDCCs ␤ subunits but in opposite ways. One type, observed in several neuronal cells 1,23,26) , is displayed by L-type VDCCs and has been recapitulated in Xenopus oocytes injected with cDNAs encoding the ␣ 1C and a ␤ subunit 6) . Another type of prepulse facilitation, also referred to as prepulse potentiation, is seen primarily in non-L-type VDCCs of neurons and is caused by a reversal or attenuation of the inhibition of channel activity imposed by agonists known to act via Gi/Go-type-Gprotein-coupled receptors and formation of a free G-protein ␤␥ dimmer 10,12,19,37) .…”