Sodium Hydrogen Exchange 1 (NHE-1) Regulates Connexin 43 Expression in Cardiomyocytes via Reverse Mode Sodium Calcium Exchange and c-Jun NH2-Terminal Kinase-Dependent Pathways

Connexin 43, the major connexin isoform in gap junctions of cardiac ventricular myocytes, undergoes changes in distribution and expression in cardiac diseases. The Na(+)-H(+) exchanger (NHE-1), a key mediator of hypertrophy and heart failure, has been shown to be localized in the cardiomyocyte gap junctional regions; however, whether NHE-1 regulates gap junction proteins in the hypertrophied cardiomyocyte is not known. To address this question, neonatal rat ventricular myocytes were treated with phenylephrine (PE) for 24 h to induce hypertrophy. Increased Cx43 expression observed with PE treatment (132.4 +/- 6.3% compared to control; P < 0.05) was further significantly augmented by the specific NHE-1 inhibitor EMD87580 [N-[2-methyl-4,5-bis(methylsulfonyl)-benzoyl]-guanidine hydrochloride] (173.2 +/- 8.7% increase compared to control; P < 0.05 versus PE), an effect that was mimicked by another NHE-1 inhibitor cariporide [4-isopropyl-3-(methylsulfonyl)benzoyl-guanidine methanesulfonate]. PE-induced hypertrophy was associated with mitogen-activated protein kinase c-Jun NH(2)-terminal kinase (JNK) 1/2 activation, whereas inhibition of JNK1/2 with either SP600125 [anthra(1,9-cd)pyrazol-6(2H)-one 1,9-pyrazoloanthrone] or small interfering RNA significantly increased PE-induced up-regulation of Cx43 protein levels. Inhibition of reverse mode Na(+)-Ca(2+) exchange (NCX) with KB-R7943 [2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate] partially reversed JNK1/2 activation (195.2 +/- 21.4 versus 143.7 +/- 14.4% with KB-R7943; P < 0.05) and augmented up-regulation of Cx43 protein (121.1 +/- 8.3 versus 215.9 +/- 25.6% with KB-R7943; P < 0.05) in the presence of PE. Our results demonstrate that NHE-1 negatively regulates Cx43 protein expression in PE-induced cardiomyocyte hypertrophy via a JNK1/2-dependent pathway, which is probably activated by reverse mode NCX activity.