Our findings suggest that GHRH promotes cardiac myocyte survival through multiple signalling mechanisms and protects against I/R injury in isolated rat heart, indicating a novel cardioprotective role of this hormone.

G. Endothelium-derived nitric oxide mediates the antiadrenergic effect of human vasostatin-1 in rat ventricular myocardium. Am J Physiol Heart Circ Physiol 292: H2906 -H2912, 2007. First published February 9, 2007 doi:10.1152/ajpheart.01253.2006 are vasoactive peptides derived from chromogranin A (CgA), a protein contained in secretory granules of chromaffin and other cells. The negative inotropic effect and the reduction of isoproterenol (Iso)-dependent inotropism induced by VSs in the heart suggest that they have an antiadrenergic function. However, further investigation of the mechanisms of action of VSs is needed. The aim of the present study was to define the signaling pathways activated by VS-1 in mammalian ventricular myocardium and cultured endothelial cells that lead to the modulation of cardiac contractility. Ca 2ϩ and nitric oxide (NO) fluorometric confocal imaging was used to study the effects induced by recombinant human VS-1 [STA-CgA-(1-76)] on contractile force, L-type Ca 2ϩ current, and Ca 2ϩ transients under basal conditions and after ␤-adrenergic stimulation in rat papillary muscles and ventricular cells and the effects on intracellular Ca 2ϩ concentration and NO production in cultured bovine aortic endothelial (BAE-1) cells. VS-1 had no effect on basal contractility of papillary muscle, but the effect of Iso stimulation was reduced by 27%. Removal of endocardial endothelium and inhibition of NO synthesis and phosphatidylinositol 3-kinase (PI3K) activity abolished the antiadrenergic effect of VS-1 on papillary muscle. In cardiomyocytes, 10 nM VS-1 was ineffective on basal and Iso (1 M)-stimulated L-type Ca 2ϩ current and Ca 2ϩ transients. In BAE-1 cells, VS-1 induced a Ca 2ϩ -independent increase in NO production that was blocked by the PI3K inhibitor wortmannin. Our results suggest that the antiadrenergic effect of VS-1 is mainly due to a PI3K-dependent NO release by endothelial cells, rather than a direct action on cardiomyocytes. calcium channel; myocardial contractility; peptide hormones; endothelial cell CHROMOGRANIN A (CgA) and chromogranin B have long been proposed to control the physiological process of secretory granule formation because of their pH-, Ca 2ϩ -, and catecholamine-dependent aggregation properties (18).

Chromogranin A (CGA), produced by human and rat myocardium, generates several biologically active peptides processed at specific proteolytic cleavage sites. A highly conserved cleavage N-terminal site is the bond 64-65 that reproduces the native rat CGA sequence (rCGA1-64), corresponding to human N-terminal CGA-derived vasostatin-1. rCGA1-64 cardiotropic activity has been explored in rat cardiac preparations. In Langendorff perfused rat heart, rCGA1-64 (from 33 nM) induced negative inotropism and lusitropism as well as coronary dilation, counteracting isoproterenol (Iso) - and endothelin-1 (ET-1) -induced positive inotropic effects and ET-1-dependent coronary constriction. rCGA1-64 also depressed basal and Iso-induced contractility on rat papillary muscles, without affecting calcium transients on isolated ventricular cells. Structure-function analysis using three modified peptides on both rat heart and papillary muscles revealed the disulfide bridge requirement for the cardiotropic action. A decline in Iso intrinsic activity in the presence of the peptides indicates a noncompetitive antagonistic action. Experiments on rat isolated cardiomyocytes and bovine aortic endothelial cells indicate that the negative inotropism observed in rat papillary muscle is probably due to an endothelial phosphatidylinositol 3-kinase-dependent nitric oxide release, rather than to a direct action on cardiomyocytes. Taken together, our data strongly suggest that in the rat heart the homologous rCGA1-64 fragment exerts an autocrine/paracrine modulation of myocardial and coronary performance acting as stabilizer against intense excitatory stimuli.

Rapid activation of PI3K, endothelial nitric oxide synthase and protein kinase G and a consequent improvement in diastolic calcium can be added to established Nrg1 protective roles.