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CARDIOVASCULAR

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

Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada (S.S., D.L.J., M.K.); and Institute of Cardiovascular Sciences, St Boniface General Hospital Research Center, Winnipeg, Manitoba, Canada (L.A.K.)

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 NH₂-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⁺-Ca²⁺ 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.

Address correspondence to: Dr. Morris Karmazyn, Department of Physiology and Pharmacology, University of Western Ontario, Medical Sciences Building, London, ON N6A 5C1, Canada. E-mail: morris.karmazyn{at}schulich.uwo.ca