Corticotropin-releasing Hormone Stimulation of Ca2+ Entry in Corticotropes is Partially Dependent on Protein Kinase A

The modulation of membrane excitability and cytosolic Ca2+ levels by corticotropin-releasing hormone (CRH), (Bu)2cAMP (dBcAMP), and forskolin was examined in enriched populations of cultured rat anterior pituitary corticotropes. CRH (2 or 20 nM), dBcAMP (1 and 5 mM), and forskolin (10 microM) caused a long lasting membrane depolarization accompanied by the onset of cell firing in quiescent cells or by increased firing frequency in spontaneously active cells. All three substances also increased cytosolic Ca2+ levels by increasing the frequency and amplitude of cytosolic Ca2+ transients. These results are consistent with a previous report on human corticotrope tumor cells demonstrating that CRH-induced action potentials lead to enhancement of Ca2+ uptake through voltage-dependent Ca2+ channels. Preincubation with (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), an inhibitor of cAMP-dependent protein kinase A, did not inhibit the CRH-induced depolarization, but attenuated the CRH-induced increase in action potential frequency. H-89 inhibited CRH-induced changes in cytosolic Ca2+ by 69% in spontaneously active cells and by 83% in quiescent cells. In contrast, H-89 completely abolished the effects of dBcAMP and forskolin on membrane potential and cytosolic Ca2+ levels. It is concluded that activation of protein kinase A mediates all of the response to dBcAMP and forskolin, but only a portion of the response to CRH. The portion of the response to CRH that is resistant to H-89 is mediated by a cAMP-independent mechanism.