PERTUSSIS TOXIN STIMULATION OF CATECHOLAMINE RELEASE FROM ADRENAL-MEDULLARY CHROMAFFIN CELLS - MECHANISM MAY BE BY DIRECT ACTIVATION OF L-TYPE AND G-TYPE CALCIUM CHANNELS

We have previously shown that pertussis toxin (PTX) stimulates delayed-onset, [Ca2+]o-dependent catecholamine (CA) release from bovine chromaffin cells. We now show that this effect of PTX is inhibited in part (50%) by dihydropyridine Ca2+-channel antagonists niludipine and nifedipine, and is potentiated by the dihydropyridine Ca2+-channel agonist Bay K-8644. We and others have shown that pretreatment of chromaffin cells with PTX results in enhanced catecholamine secretion in response to high [K+]o, nicotine and muscarine, and here we extend these observations by showing that toxin pretreatment also enhances the secretory response to [Ba2+]o. All these data are consistent with the concept that PTX may act on Ca2+ channels. To examine the possibility of a direct action of the toxin on the voltage-gated L-type Ca2+ channel known to be present in these cells, we studied the effects of the toxin on whole cell Ca2+ currents. We found and report here that spontaneous electrical activity was considerably increased in PTX-treated cells. Our measurements of whole cell inward Ca2+ currents indicate that the underlying mechanism is a marked shift of the activation curve of the L-type Ca2+ current along the voltage axis towards more negative potentials. While treatment of the cells with PTX had no effect on L-type Ca2+-channel conductance (6 nS/cell at 2.6 mM [Ca2+]o), PTX evoked the activation of a new class of Ca2+-selective channels (5 pS in 25 mM [Ca2+]pipet), which are rather insensitive to membrane potential. We have termed these G-type calcium channels. These data suggest that treatment with PTX not only increases the probability of L-type Ca2+-channel activation at more negative potentials, but also increases the probability of opening of an entirely new, voltage-independent, Ca2+ channel. These actions of PTX should promote Ca2+ entry and might explain the stimulation by the toxin of CA secretion from medullary chromaffin cells in culture.