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, [Ca^2-]_a-dependent catecholamine (CA) release from bovine chromaffin cells. We now show that this effect of PTX is inhibited in part (50%) by dihydropyridine Ca^2--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^-]_a, nicotine and muscarine, and here we extend these observations by showing that toxin pretreatment also enhances the secretory response to [Ba²⁺]_a. All these data are consistent with the concept that PTX may act on Ca^2- 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⁺]_a). 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.