Mechanisms of low Na(+)-induced increase in intracellular calcium in KCl-depolarized rat cardiomyocytes

Although low Na(+) is known to increase the intracellular Ca(2+) concentration ([Ca(2+)]i) in cardiac muscle, the exact mechanisms of low Na(+)-induced increases in [Ca(2+)]i are not completely defined. To gain information in this regard, we examined the effects of low Na(+) (35 mM) on freshly isolated cardiomyocytes from rat heart in the absence and presence of different interventions. The [Ca(2+)]i in cardiomyocytes was measured fluorometrically with Fura-2 AM. Following a 10 min incubation, the low Na(+)-induced increase in [Ca(2+)]i was only observed in cardiomyocytes depolarized with 30 mM KCl, but not in quiescent cardiomyocytes. In contrast, low Na(+) did not alter the ATP-induced increase in [Ca(2+)]i in the cardiomyocytes. This increase in [Ca(2+)]i due to low Na(+) and elevated KCl was dependent on the extracellular concentration of Ca(2+) (0.25-2.0 mM). The L-type Ca(2+)-channel blockers, verapamil and diltiazem, at low concentrations (1 μM) depressed the low Na(+), KCl-induced increase in [Ca(2+)]i without significantly affecting the response to low Na(+) alone. The low Na(+), high KCl-induced increase in [Ca(2+)]i was attenuated by treatments of cardiomyocytes with high concentrations of both verapamil (5 and 10 μM), and diltiazem (5 and 10 μM) as well as with amiloride (5-20 μM), nickel (1.25-5.0 mM), cyclopiazonic acid (25 and 50 μM) and thapsigargin (10 and 20 μM). On the other hand, this response was augmented by ouabain (1 and 2 mM) and unaltered by 5-(N-methyl-N-isobutyl) amiloride (5 and 10 μM). These data suggest that in addition to the sarcolemmal Na(+)-Ca(2+) exchanger, both sarcolemmal Na(+)-K(+)ATPase, as well as the sarcoplasmic reticulum Ca(2+)-pump play prominent roles in the low Na(+)-induced increase in [Ca(2+)]i. (Mol Cell Biochem 263: 151-162, 2004).