Mechanism underlying corticotropin-releasing hormone (CRH) triggered cytosolic Ca2+ rise in identified rat corticotrophs

1. The patch-clamp technique was used in conjunction with the fluorescent Ca2+ indicator indo-1 to measure simultaneously cytosolic Ca2+ concentration ([Ca2+](i)) and membrane potential in single rat corticotrophs identified with the reverse haemolytic plaque assay. 2. Application of the adrenocorticotropin (ACTH) secretagogue, corticotropin-releasing hormone (CRH), triggered a sustained [Ca2+](i) elevation and membrane depolarization. 3. The CRH action was mediated via the cAMP-dependent protein kinase cascade. Both the CRH-induced depolarization and [Ca2+](i) elevation could be mimicked by extracellular application of the adenylate cyclase activator forskolin or the membrane-permeable cAMP analogue, 8-(4-chlorophenylthio)-adenosine-3',5'-monophosphate (8-CPT-cAMP). Intracellular adenosine cyclic 3',5'-(Rp)-phosphothioate (Rp-cAMPS), a protein kinase A inhibitor, abolished the CRH effects. 4. Voltage-clamp studies suggest that the CRH-triggered depolarization was due to the reduction of background K+ conductances. The CRH-sensitive current was Ca2+ independent and was insensitive to the K+ channel blockers tetraethylammonium (TEA) or 4-aminopyridine (4-AP), but could be partially inhibited by Ba2+. 5. The CRH-triggered steady-state depolarization stimulated extracellular Ca2+ entry via voltage-gated Ca2+ channels and raised [Ca2+](i). CRH failed to stimulate [Ca2+](i) rise in cells that were voltage clamped at their resting potential. Removal of extracellular Ca2+ or inhibition of Ca2+ channels by Ni2+ abolished the [Ca2+](i) rise. 6. Voltage-clamp studies of voltage-gated Ca2+ channels using Ba2+ as charge carrier show that similar to 90% of the channels were available for activation at the resting potential. CRH did not enhance the voltage-gated Ca2+ channels.