DIFFERENTIAL-EFFECTS OF AMILORIDE ON THE BASAL RATE AND THE PRESSURE OVERLOAD-INDUCED INCREASE IN PROTEIN-SYNTHESIS IN PERFUSED RAT-HEART

The purposes of the present study were to determine the contribution of Na+/H+ exchange to pressure overload-induced cardiac hypertrophy and to examine its potential interaction with cAMP-dependent signaling pathway. Isolated rat hearts were perfused as Langendorff preparations with aortic pressure of 60 mmHg. In pressure overload group, aortic pressure was increased to 120 mmHg. cAMP contents in the heart perfused at 2 min were examined by RIA. Rates of protein synthesis were examined by C-14-phenylalanine incorporation into myocardial protein during the second hour of perfusion. Expression of c-fos mRNA in the heart perfused at 1 hour was analyzed by Northern blotting. Elevation of aortic pressure from 60 mmHg to 120 mmHg in perfused rat hearts increased cAMP contents from 4.89+/-0.09 to 6.30+/-0.28 pmol/mg protein and accelerated rates of protein synthesis from 644+/-13 to 860+/-49 nmol Phe/g dry heart/hr. Expression of c-fos mRNA was induced by elevated aortic pressure. Amiloride, an inhibitor of Na+/H+ exchange, decreased rates of protein synthesis in a concentration-dependent manner (12.5, 25, 50, 100 mu M) but did not change cAMP content (5.25+/-0.11 pmol/mg protein) or expression of c-fos mRNA. Furthermore, amiloride did not prevent the increases in cAMP (6.99+/-0.34 pmol/mg protein), protein synthesis rates (476+/-18 to 689+/-31 nmolPhe/g dry heart/hr) and expressions of c-fos mRNA that were induced by elevation of aortic pressure. These results indicate that amiloride, an inhibitor of Na+/H+ exchange system, while influencing rates of protein synthesis, does not play an important role in pressure overload-induced cardiac hypertrophy. The mechanism by which amiloride influences cardiac protein synthesis is independent of the cAMP-dependent mechanism by which pressure overload induces cardiac hypertrophy.