Dissociation between positive inotropic and alkalinizing effects of angiotensin II in feline myocardium

The present study examines the intracellular pH (pH(i)) dependence of angiotensin (ANG) II-induced positive inotropic effect in cat papillary muscles contracting isometrically (0.2 Hz, 30°C). Muscles were loaded with the fluorescent dye 2'-7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester for simultaneous measurement of pH(i) and contractility. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer (n = 4), there was a temporal dissociation between the positive inotropic and the alkalinizing effects of ANG II (0.5 μM). The positive inotropic effect of ANG II peaked at 9.7 ± 0.8 min (240 ± 57% above control) without significant changes in pH(i). The increase in pH(i) became significant (0.05 ± 0.01 pH units) only after 16 min of exposure to the drug, when the positive inotropic effect of ANG II was already fading. In HCO₃/^- buffer (n = 7), the ANG II-induced positive inotropic effect occurred without significant phi changes. In the presence of 5 μM ethyl isopropyl amiloride (EIPA, to specifically inhibit the Na⁺/H⁺ exchanger), the alkalinizing effect of ANG II was changed to a significant decrease in ph(i), despite which ANG II still increased contractility by 87 ± 16% (n = 6). The results indicate that in HEPES buffer only a fraction of the ANG II-induced positive inotropic effect can be attributed to a phi change, whereas in a physiological CO₂-HCO₃ medium the positive inotropic effect of ANG II is independent of phi changes. Furthermore, an ANG II-induced increase in myocardial contractility was observed even when ANG II administration elicited a decrease in pH(i), as occurred after Na⁺/H⁺ exchanger blockade. The results show that in feline myocardium, the increase in contractility evoked by ANG II in a physiological CO₂-HCO₃ medium is not due to an increase in Ca²⁺ myofilament sensitivity secondary to an increase in myocardial pH(i).