Calcium and osmotic regulation of the Na+/H+ exchanger in neonatal ventricular myocytes

Intracellular pH regulation in primary cultures of neonatal cardiac myocyte s has been characterized. Myocytes were exposed to hyperosmolar solutions t o examine the effects on pH regulation by the Na+/H+ exchanger. Exposure to 100 mM NaCl, sorbitol, N-methyl-D-glucamine, or choline chloride all cause d significant increases in steady state pH(i) in myocytes. Omission of extr acellular calcium or administration of calmodulin antagonists reduced the o smotic activation of the exchanger. The myosin light-chain inhibitor ML-7 c ompletely blocked osmotic activation of the exchanger suggesting that myosi n light-chain kinase is involved in osmotic activation of the exchanger in the myocardium. The calmodulin-dependent protein kinase II inhibitor KN-93 inhibited the rate of recovery from an acute acid load as did trifluoperazi ne (TFP) and the calmodulin blocker W7, [N-(6-aminohexyl)-5-chloro-1-naphth alenesulfonamide]. Addition of the calcium ionophore ionomycin caused a lar ge increase in resting pH(i) in isolated myocytes. However, this effect was largely resistant to HMA (5-(N,N-hexamethylene)-amiloride) indicating that an alternative mechanism of pH(i) regulation is responsible. The results d emonstrate that the Na+/H+ exchanger of the neonatal myocardium is responsi ve to calcium and osmotically responsive pathways and that myosin light-cha in kinase is a key protein involved in mediating the osmotic response. (C) 2000 Academic Press.