The orphan receptor APJ and its recently identified endogenous ligand, apelin, exhibit high levels of mRNA expression in the heart. However, the functional importance of apelin in the cardiovascular system is not known. In isolated perfused rat hearts, infusion of apelin (0.01 to 10 nmol/L) induced a dose-dependent positive inotropic effect (EC50: 33.1+/-1.5 pmol/L). Moreover, preload-induced increase in dP/dt(max) was significantly augmented (P<0.05) in the presence of apelin. Inhibition of phospholipase C (PLC) with U-73122 and suppression of protein kinase C (PKC) with staurosporine and GF-109203X markedly attenuated the apelin-induced inotropic effect (P<0.001). In addition, zoniporide, a selective inhibitor of Na+-H+ exchange (NHE) isoform-1, and KB-R7943, a potent inhibitor of the reverse mode Na+-Ca2+ exchange (NCX), significantly suppressed the response to apelin (P<0.001). Perforated patch-clamp recordings showed that apelin did not modulate L-type Ca2+ current or voltage-activated K+ currents in isolated adult rat ventricular myocytes. Apelin mRNA was markedly downregulated in cultured neonatal rat ventricular myocytes subjected to mechanical stretch and in vivo in two models of chronic ventricular pressure overload. The present study provides the first evidence for the physiological significance of apelin in the heart. Our results show that apelin is one of the most potent endogenous positive inotropic substances yet identified and that the inotropic response to apelin may involve activation of PLC, PKC, and sarcolemmal NHE and NCX.

During the past decade, emerging evidence has accumulated of different nuclear transcription factors in regulation of cardiac development and growth as well as in cardiac hypertrophy and heart failure. GATA-4, -5 and -6 are zinc finger transcription factors that are expressed in the developing heart and GATA-4 and -6 continue expression in the adult cardiac myocytes. GATA-4 and -6 regulate expression of several cardiac-specific genes, and during murine embryonic development, GATA-4 is essential for proper cardiac morphogenesis. In support of this, mutations of gene for GATA-4 or for its cofactors have been associated with human congenital heart disease. Pressure overload of the heart in vivo as well as hypertrophic stimulation of cardiac myocytes in vitro provide adequate stimulus for activation of GATA-4. Activity of GATA-4 transcription factor is subject to regulation at the level of gene expression and through post-translational modifications of GATA-4 protein. A number of genes induced during cardiac hypertrophy possess functional GATA sites in their promoter region and cardiac-specific overexpression of GATA-4 or -6 leads to cardiac hypertrophy. In addition, a pattern of interactions between GATA-4 and its numerous cofactors have been identified, showing an increasing complexity in regulatory mechanisms. The present review discusses current evidence of the role and regulation of GATA transcription factors in the heart, with an emphasis in the GATA-4 and development of cardiac hypertrophy.

In overloaded heart the cardiomyocytes adapt to increased mechanical and neurohumoral stress by activation of hypertrophic program, resulting in morphological changes of individual cells and specific changes in gene expression. Accumulating evidence suggests an important role for the zinc finger transcription factor GATA-4 in hypertrophic agonist-induced cardiac hypertrophy. However, its role in stretch-induced cardiomyocyte hypertrophy is not known. We employed an in vitro mechanical stretch model of cultured cardiomyocytes and used rat B-type natriuretic peptide promoter as stretch-sensitive reporter gene. Stretch transiently increased GATA-4 DNA binding activity and transcript levels, which was followed by increases in the expression of B-type natriuretic peptide as well as atrial natriuretic peptide and skeletal ␣-actin genes. The stretch inducibility mapped primarily to the proximal 520 bp of the B-type natriuretic peptide promoter. Mutational studies showed that the tandem GATA consensus sites of the proximal promoter in combination with an Nkx-2.5 binding element are critical for stretch-activated B-type natriuretic peptide transcription. Inhibition of GATA-4 protein production by adenovirus-mediated transfer of GATA-4 antisense cDNA blocked stretch-induced increases in B-type natriuretic peptide transcript levels and the sarcomere reorganization. The proportion of myocytes with assembled sarcomeres in control adenovirus-infected cultures increased from 14 to 59% in response to stretch, whereas the values for GATA-4 antisense-treated cells were 6 and 13%, respectively. These results show that activation of GATA-4, in cooperation with a factor binding on Nkx-2.5 binding element, is essential for mechanical stretchinduced cardiomyocyte hypertrophy.