ACTH Induces Cav3.2 Current and mRNA by cAMP-dependent and cAMP-independent Mechanisms

Bovine adrenal zona fasciculata (AZF) cells express Ca_v3.2 T-type Ca²⁺ channels that function pivotally in adrenocorticotropic hormone (ACTH)-stimulated cortisol secretion. The regulation of Ca_v3.2 expression in AZF cells by ACTH, cAMP analogs, and their metabolites was studied using Northern blot and patch clamp recording. Exposing AZF cells to ACTH for 3–6 days markedly enhanced the expression of Ca_v3.2 current. The increase in Ca_v3.2 current was preceded by an increase in corresponding CACNA1H mRNA. O-Nitrophenyl,sulfenyl-adrenocorticotropin, which produces a minimal increase in cAMP, also enhanced Ca_v3.2 current. cAMP analogs, including 8-bromoadenosine cAMP (600 μm) and 6-benzoyladenosine cAMP (300 μm) induced CACNA1H mRNA, but not Ca_v3.2 current. In contrast, 8-(4-chlorophenylthio) (8CPT)-cAMP (10–50 μm) enhanced CACNA1H mRNA and Ca_v3.2 current, whereas nonhydrolyzable Sp-8CPT-cAMP failed to increase either Ca_v3.2 current or mRNA. Metabolites of 8CPT-cAMP, including 8CPT-adenosine and 8CPT-adenine, increased Ca_v3.2 current and mRNA with a potency and effectiveness similar to the parent compound. The Epac activator 8CPT-2′-O-methyl-cAMP and its metabolites 8CPT-2′-OMe-5′-AMP and 8CPT-2′-O-methyl-adenosine increased CACNA1H mRNA and Ca_v3.2 current; Sp-8CPT-2′-O-methyl-cAMP increased neither Ca_v3.2 current nor mRNA. These results reveal an interesting dichotomy between ACTH and cAMP with regard to regulation of CACNA1H mRNA and Ca²⁺ current. Specifically, ACTH induces expression of CACNA1H mRNA and Ca_v3.2 current in AZF cells by mechanisms that depend at most only partly on cAMP. In contrast, cAMP enhances expression of CACNA1H mRNA but not the corresponding Ca²⁺ current. Surprisingly, chlorophenylthio-cAMP analogs stimulate the expression of Ca_v3.2 current indirectly through metabolites. ACTH and the metabolites may induce Ca_v3.2 expression by the same, unidentified mechanism.