cAMP increases Ca2+‐dependent exocytosis through both PKA and Epac2 in mouse melanotrophs from pituitary tissue slices

Cyclic AMP regulates Ca2+‐dependent exocytosis through a classical protein kinase A (PKA)‐dependent and an alternative cAMP–guanine nucleotide exchange factor (GEF)/Epac‐dependent pathway in many secretory cells. Although increased cAMP is believed to double secretory output in isolated pituitary cells, the direct target(s) for cAMP action and a detailed and high‐time resolved analysis of the effect of intracellular cAMP levels on the secretory activity in melanotrophs are still lacking. We investigated the effect of 200 μm cAMP on the kinetics of secretory vesicle depletion in mouse melanotrophs from fresh pituitary tissue slices. The whole‐cell patch‐clamp technique was used to depolarize melanotrophs and increase the cytosolic Ca2+ concentration ([Ca2+]i). Exogenous cAMP elicited an about twofold increase in cumulative membrane capacitance change and ∼34% increase of high‐voltage activated Ca2+ channel amplitude. cAMP‐dependent mechanisms did not affect [Ca2+]i, since the application of forskolin failed to change [Ca2+]i in melanotrophs, a phenomenon readily observed in anterior lobe. Depolarization‐induced secretion resulted in two distinct kinetic components: a linear and a threshold component, both stimulated by cAMP. The linear component (ATP‐independent) probably represented the exocytosis of the release‐ready vesicles, whereas the threshold component was assigned to the exocytosis of secretory vesicles that required ATP‐dependent reaction(s) and > 800 nm[Ca2+]i. The linear component was modulated by 8‐pCPT‐2Me‐cAMP (Epac agonist), while either H‐89 (PKA inhibitor) or Rp‐cAMPS (the competitive antagonist of cAMP binding to PKA) completely prevented the action of cAMP on the threshold component. In line with this, 6‐Phe‐cAMP, (PKA agonist), increased the threshold component. From our study, we suggest that the stimulation of cAMP production by application of oestrogen, as found in pregnant mice, increases the efficacy of the hormonal output through both PKA and cAMP–GEFII/Epac2‐dependent mechanisms.

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