Effect of ANG II on pHi, [Ca²⁺]i, and contraction in rabbit ventricular myocytes from infarcted hearts

In this study we examined Na+/H+ exchange activity, Ca2+ transients, and contractility in rabbit ventricular myocytes isolated from normal and chronically (8-12 wk) infarcted left ventricles. Myocytes from infarcted hearts (post-MI myocytes) were isolated from the peri-infarcted region of the left ventricle. Intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i) were measured with the fluorescent pH indicators seminaphthorhodafluor 1 and fluo 3, respectively, and contractility was assessed from changes in cell shortening during field stimulation. Experiments were performed at extracellular pH 7. 4 in the presence and absence (HEPES buffer) of CO2 and HCO-3. Our findings demonstrate that 1) myocytes after myocardial infarction (post-MI) were significantly larger than normal, 2) post-MI hypertrophy was not accompanied by changes in non-CO2 intracellular buffering power, 3) post-MI hypertrophy did not significantly affect the ability of Na+/H+ exchange to mediate pHi recovery from intracellular acidosis, 4) the stimulatory effect of ANG II (100 nM) on Na+/H+ exchange was significantly reduced in post-MI myocytes, 5) in HCO-3-buffered solutions, ANG II did not significantly stimulate pHi recovery from acidosis in post-MI myocytes, 6) the angiotensin AT1 receptor mediates the stimulatory action of ANG II on Na+/H+ exchange in normal and post-MI myocytes, and 7) the stimulatory effect of ANG II on the Ca2+ transient and contraction was blunted in post-MI myocytes bathed in HEPES-buffered solution. A suppressed ventricular responsiveness to ANG II may be beneficial in the intact myocardium by attenuating ATP consumption and by reducing intracellular Na+ accumulation during ischemia-reperfusion.