Urocortin induces positive inotropic effect in rat heart

The aim of this study is to evaluate the positive inotropic effect of urocortin (Ucn) and to characterize its signalling pathways. Contractility was measured in ex vivo Langendorff-perfused hearts isolated from Wistar rats. Isolated ventricular cardiomyocytes were used to analyse intracellular calcium ([Ca2+](i)) transients evoked by electrical stimulation and L-type Ca2+ current by confocal microscopy and whole-cell patch-clamping, respectively. The application of Ucn to perfused hearts induced progressive, sustained, and potent inotropic and lusitropic effects that were dose-dependent with an EC50 of approximately 8 nM. Ucn effects were independent of protein kinase A (PKA) activation but were significantly reduced by protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) inhibitors and by brefeldin A, an antagonist of guanine nucleotide exchange factor, suggested to be an inhibitor of exchange protein activated by cAMP (Epac). These whole-organ effects were correlated with the inotropic effects observed in isolated cells: Ucn increased I-CaL density, [Ca2+](i) transients, cell shortening and Ca2+ content of sarcoplasmic reticulum. Our results show that Ucn evokes potent positive inotropic and lusitropic effects mediated, at least in part, by an increase in I-CaL and [Ca2+](i) transient amplitude. These effects may involve the activation of Epac, PKC, and MAPK signalling pathways.