. Cellular distribution of GPR14 and the positive inotropic role of urotensin II in the myocardium in adult rat. J Appl Physiol 97: 2228 -2235 doi:10.1152/japplphysiol.00540.2004.-Urotensin II is a cyclic neuropeptide recently shown to play a role via its receptor GPR14 in regulating vascular tone in the mammalian cardiovascular system. The existence of GPR14 in rat heart has been validated by ligand binding assay and RT-PCR. In the present study, we investigated the cellular distribution of GPR14 protein in rat heart by using immunohistochemistry and confocal microscopic immunofluorescence double staining with antipeptide polyclonal antibodies against GPR14 and cell type markers for myocytes and endothelial cells. The direct effect of urotensin II on left ventricular contractility was further evaluated in isolated left ventricular papillary muscles of the rat. In paraffin-embedded heart sections, positive immunohistochemical staining was observed in the left ventricle but not in the right ventricle and atria. Immunofluorescence double staining revealed the cardiac myocyte as the only cell type expressing GPR14 protein in frozen heart sections as well as in isolated cardiac myocytes. There was no visible signal for GPR14 in intramyocardial coronary arteries and capillaries. The existence of GPR14 protein in rat heart was further validated by immunoprecipitation and Western blot analysis. In isolated rat left ventricular papillary muscle preparations, urotensin II induced an increase in active contractile force. GPR14 mRNA was also detected in rat heart by RT-PCR. These data provide the first direct evidence for the cellular localization of GPR14 receptor protein and a positive inotropic effect of urotensin II in normal rat heart. urotensin II; GPR14; cardiac myocyte; double staining; inotropic effect UROTENSIN II, A CYCLIC NEUROPEPTIDE initially isolated from the urophysis of teleost fish (3), has recently been cloned in several mammalian species including human. Gibson et al. (15) first reported a role of urotensin II in regulating cardiovascular function in the rat. More recently, urotensin II was identified in both the vascular and cardiac tissues and shown to effectively constrict the isolated arteries from rats (14), rabbits (10), dogs (10), pigs (10), nonhuman primates (1), and humans (20). In several recent reports, urotensin II has been recognized as the most potent vasoconstrictor with 8-to 110-fold potent than endothelin 1 (9). On the other hand, however, the reported depressor and regionally selective vasodilator effects (4, 13, 27) of urotensin II endow its vasoactive effects with complexity. Systemic administration of human urotensin II to anesthetized monkeys resulted in a complex dose-dependent hemodynamic response including a decrease in cardiac contractility and stroke volume with a concomitant increase in total peripheral resistance, leading eventually to fatal cardiovascular collapse (1). Intravenous bolus injection of human urotensin II into the rat produced a dose-dependent decrease in mean ...