Activation of PKA and Epac proteins by cyclic AMP depletes intracellular calcium stores and reduces calcium availability for vasoconstriction

We investigated the implication of PKA and Epac proteins in the endothelium-independent vasorelaxant effects of cyclic AMP (cAMP). Cytosolic Ca2+ concentration ([Ca2+]c) was measured by fura-2 imaging in rat aortic smooth muscle cells (RASMC). Contraction-relaxation experiments were performed in rat aortic rings deprived of endothelium. In extracellular Ca2+-free solution, cAMP-elevating agents induced an increase in [Ca2+]c in RASMC that was reproduced by PKA and Epac activation and reduced after depletion of intracellular Ca2+ reservoirs. Arginine-vasopressin (AVP)-evoked increase of [Ca2+]c and store-operated Ca2+ entry (SOCE) were inhibited by cAMP-elevating agents, PKA or Epac activation in these cells. In aortic rings, the contractions induced by phenylephrine in absence of extracellular Ca2+ were inhibited by cAMP-elevating agents, PKA or Epac activation. In these conditions, reintroduction of Ca2+ induced a contraction that was inhibited by cAMP-elevating agents, an effect reduced by PKA inhibition and reproduced by PKA or Epac activators. Our results suggest that increased cAMP depletes intracellular, thapsigargin-sensitive Ca2+ stores through activation of PKA and Epac in RASMC, thus reducing the amount of Ca2+ released by IP3-generating agonists during the contraction of rat aorta. cAMP rise also inhibits the contraction induced by depletion of intracellular Ca2+, an effect mediated by reduction of SOCE after PKA or Epac activation. Both effects participate in the cAMP-induced endothelium-independent vasorelaxation. [Display omitted]