Involvement of Sodium Channels and Two Types of Calcium Channels in the Regulation of Adrenocorticotropin Release*

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Abstract
Recent electrophysiological studies from this laboratory demonstrated that anterior lobe corticotropes exhibited a tetrodotoxin-sensitive sodium current and two types of voltage-dependent calcium currents, consisting of low threshold (transient) and high threshold (long lasting) components. The present report describes cytophysiological and cytochemical studies that used specific blockers of each of these currents to assess their role in the regulation of CRF binding and ACTH secretion and storage. Two dihydropyridines, nimodipine and the pure antagonist enantiomer (–)R202-791, which block high threshold Ca2+ channels, decreased 1 h basal release by 54–74% and CRF-mediated (5 min or 3 h) release completely. Percentages of CRF-bound cells were reduced as much as 74%; however, the inhibitory effect on percentages of CRF-bound cells could be reversed by adding 10 nM Bay K 8644, (a pure dihydropyridine agonist) with the antagonists. CdCl2, which blocks both high and low threshold calcium currents, inhibited basal and CRFstimulated ACTH release, but only the highest concentration (0.1 DIM) reduced percentages of CRF-bound cells. Involvement of the low threshold Ca2+ channels could not be proved by adding dihydropyridine antagonists with 0.1 mM CdCl2. Basal and CRFmediated ACTH release were blocked by the potent sodium channel blocker tetrodotoxin, and the highest concentration (3 μM) reduced percentages of CRF-bound cells. Basal (1 h) and CRF-stimulated (5 min) ACTH release were also inhibited in medium containing 1 mM EGTA and no Ca2+; however, percentages of CRF-bound cells were within the normal range. Densitometric analysis of stains for ACTH showed an increase in the concentration of stain per cell after a 1-h exposure to the highest concentrations of the inhibitors or to no Ca2+ and 1 mM EGTA coupled with a significant (10%) decrease in corticotrope cell area. Finally, in the last series of tests, the Bay K 8644 agonist or arginine vasopressin were used to study mechanisms of augmentation of basal or CRF-mediated ACTH release. Bay K 8644 augmented basal release in a concentration of 1 μM and CRF-mediated release in a concentration of 100 nM or 1 μM. After pretreatment with either Bay K 8644 or arginine vasopressin (10 nM) there was a significant (30%) increase in the percentage of CRF-bound cells. In summary, these studies have demonstrated that the voltage-dependent Na+ channel and the high threshold Ca2+ channel are involved in both basal and CRFstimulated ACTH release. In addition, we have shown a link between the state of inhibition or activation and the receptivity of the corticotrope population to CRF. (Endocrinology120: 2059–2069, 1987)