The increase in intracellular
sodium (Nai), resulting from inhibition of the Na/K
ATPase by
cardiac glycosides, is known to increase
calcium influx via Na(+)-Ca2+ exchange, and thereby increase contractility. This increase in intracellular Ca2+ has been related to the development of intracellular acidification and enhanced activity of the Na(+)-H+ exchanger as a measure by the cell to prevent further acidification. Thus, the efflux of the H+
ions results in an additional increase in Nai. This may subsequently lead to an increased rate of Ca2+ influx and therefore to the potentiation of the effects of
cardiac glycosides. To assess the role of Na(+)-H+ exchange in the mechanism of
ouabain action in the beating heart we used
amiloride, a known inhibitor of Na(+)-H+ exchange. Isolated rat hearts were perfused with either
ouabain (50 microM) alone (n = 8, Group I),
amiloride (1.0 mM) +
ouabain (50 microM) (n = 8, Group II), or
amiloride (1.0 mM) alone as a control group (n = 4, Group III). 23Na and 31P NMR spectroscopy were used to assess the changes in Nai and intracellular pH (pHi), respectively, while simultaneous and continuous monitoring of left ventricular pressure was carried out. Perfusion with both
ouabain alone (Group I) or
ouabain +
amiloride (Group II), resulted in a time dependent increase in Nai levels, reaching (within 25 mins) a maximum of 200 +/- 7% of control in Group I, and 170 +/- 10% of control in Group II. Concurrently, a mild but significant decrease in pHi was observed in both groups. This decrease, however, was significantly higher in Group II compared to Group I (0.34 pH units vs. 0.19 pH units, respectively; P less than 0.05), suggesting that inhibition of Na(+)-H+ exchange by
amiloride limits the recovery from
ouabain-induced intracellular acidification. While developed pressure gradually increased in Group I to a maximum of 268 +/- 52% of control, the addition of
amiloride in Group II substantially reduced the positive inotropic effect.
Ventricular fibrillation (VF) developed in three of the eight hearts in Group I within 10-13 mins after the addition of
ouabain. Interestingly, the rate of Nai increase in hearts that sustained VF was significantly higher compared to those without VF (mean slope 10.1 +/- 2.11 vs. 3.9 +/- 1.0, respectively; P less than 0.0001).
Ventricular fibrillation did not develop in Group II or III.(ABSTRACT TRUNCATED AT 400 WORDS)