Aims: The aim of this study is to evaluate the positive inotropic effect of urocortin (Ucn) and to characterize its signalling pathways.
Methods and results: Contractility was measured in ex vivo Langendorff-perfused hearts isolated from Wistar rats. Isolated ventricular cardiomyocytes were used to analyse intracellular calcium ([Ca(2+)](i)) transients evoked by electrical stimulation and L-type Ca(2+) 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 EC(50) 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, [Ca(2+)](i) transients, cell shortening and Ca(2+) content of sarcoplasmic reticulum.
Conclusion: Our results show that Ucn evokes potent positive inotropic and lusitropic effects mediated, at least in part, by an increase in I(CaL) and [Ca(2+)](i) transient amplitude. These effects may involve the activation of Epac, PKC, and MAPK signalling pathways.