The mechanism underlying the positive inotropic effect of angiotensin II in the isolated perfused rabbit heart: a 31P NMR study.

Activation of the Na(+)/H(+) exchanger may play an important role in the development of cardiac hypertrophy. Isolated ventricular myocyte studies have suggested that angiotensin II (AII) has direct positive inotropic effect caused by intracellular alkalinization due to increased Na(+)/H(+) exchange, but whether this occurs in the whole heart is unknown. Consequently, we have used non-invasive 31P NMR spectroscopy to determine whether AII stimulation alters energetics or intracellular pH (pH(i)) in the intact beating rabbit heart. Heart rate (HR) and developed pressure (DP) were recorded continuously in isolated perfused rabbit hearts, simultaneously with pH(i) and high energy phosphate metabolite levels measured using 31P NMR spectroscopy. AII (11 nM) increased developed pressure by 14+/-2 mmHg (P<0.05) and increased pH(i) by 0.08+/-0.03 pH units (P<0.05, n=6). There were no significant changes in myocardial phosphocreatine (PCr), ATP or Pi concentrations throughout the protocol. Inhibition of Na(+)/H(+) exchange with 1 microM Hoe642 (n=7) abolished the increase in pH(i), but did not prevent the increase in developed pressure, caused by AII. Inhibition of protein kinase C (PKC) using 25 microM chelerythrine chloride prevented the positive inotropic and alkalinizing effects of AII (n=5). We conclude that the positive inotropic effect of AII is associated with, but not caused by, a decreased proton concentration due to stimulation of Na(+)/H(+) exchange in the whole rabbit heart.