Effects of Bay K 8644 on Cat Adrenal Catecholamine Secretory Responses to A23187 or Ouabain

Calcium ionophore A23187 increases the rate of spontaneous catecholamine release from cat adrenal glands perfused at 37 degrees C with oxygenated Krebs bicarbonate solution, in a time- and Ca-concentration-dependent manner. The secretory profile obtained with the ionophore was not modified in the presence of the Ca channel activator Bay K 8644. Ouabain also enhanced the rate of spontaneous catecholamine outputs in a time- and concentration-dependent manner. The threshold ouabain concentration capable of producing a clear, yet delayed secretory response was 10(-6) M. Increasing ouabain concentrations up to 10(-4) M enhanced catecholamine release and shortened the time to peak release. The dihydropyridine Ca channel activator Bay K 8644 (10(-6) M) markedly potentiated the secretory effects of all ouabain concentrations used (10(-7)-10(-4) M). However, the most impressive potentiations were seen at 10(-5)M ouabain; while at this concentration ouabain alone released 2.6 +/- 0.07 micrograms catecholamines per 30 min, in the presence of Bay K 8644 the release was 73.4 +/- 5.7 micrograms per 30 min. Conversely, at a fixed ouabain concentration (10(-5) M), the potentiation was also dependent on the Bay K 8644 concentration (10(-8)-10(-5) M). Although K deprivation inhibits Na pumping as does ouabain, Bay K 8644 did not modify the rate of catecholamine release evoked by K removal from the perfusion medium. Potassium deletion, nimodipine or high Mg all reversed the fully developed secretory response evoked by ouabain plus Bay K 8644. In glands depolarized by continuous perfusion with high K solutions, once the secretory response was inactivated, the introduction of ouabain caused an enhancement of the catecholamine secretory rate. This increase was dependent on the extracellular Na concentration and was not affected by Bay K 8644. In the presence of 6 mm Na the secretory effects of Bay K 8644 plus ouabain were abolished. 7 These results are compatible with the following conclusions: (i) Bay K 8644 potentiates only those catecholamine secretory responses that are known to be mediated through the activation of voltagesensitive Ca channels; the drug does not seem to affect secretory responses by acting on the membrane Na/Ca exchange system or at some intracellular Ca-dependent component of the secretory machinery of Ca buffering systems. (ii) It is likely that ouabain enhances the rates of adrenal catecholamine release by a dual mechanism: chromaffin cell depolarization and activation of a membrane Na/Ca exchange system.