Intracellular H+ mobility was estimated in the rabbit isolated ventricular myocyte by diffusing HCl into the cell from a patch pipette, while imaging pHi confocally using intracellular ratiometric SNARF fluorescence. The delay for acid diffusion between two downstream regions ≈40 μm apart was reduced from ≈25 s to ≈6 s by replacing Hepes buffer in the extracellular superfusate with a 5 % CO2/HCO3− buffer system (at constant pHo of 7.40). Thus CO2/HCO3− (carbonic) buffer facilitates apparent H+i mobility. The delay with carbonic buffer was increased again by adding acetazolamide (ATZ), a membrane permeant carbonic anhydrase (CA) inhibitor. Thus facilitation of apparent H+i mobility by CO2/HCO3− relies on the activity of intracellular CA. By using a mathematical model of diffusion, the apparent intracellular H+ equivalent diffusion coefficient (DHapp) in CO2/HCO3−‐buffered conditions was estimated to be 21.9 × 10−7 cm2 s−1, 5.8 times faster than in the absence of carbonic buffer. Facilitation of H+i mobility is discussed in terms of an intracellular carbonic buffer shuttle, catalysed by intracellular CA. Turnover of this shuttle is postulated to be faster than that of the intrinsic buffer shuttle. By regulating the carbonic shuttle, CA regulates effective H+i mobility which, in turn, regulates the spatiotemporal uniformity of pHi. This is postulated to be a major function of CA in heart.

This study describes the use of a microperfusion system to create rapid, large regional changes in intracellular pH (pH(i)) within single ventricular myocytes. The spatial distribution of pH(i) in single myocytes was measured with seminaphthorhodafluor-1 fluorescence using confocal imaging. Changes in pH(i) were induced by local external application of NH(4)Cl, CO(2), or sodium propionate. Local application was achieved by simultaneously directing two parallel square microstreams, each 275 microm wide, over a single myocyte oriented perpendicular to the direction of flow. One stream contained the control solution, and the other contained a weak acid or base. End-to-end, stable pH(i) gradients as large as 1 pH unit were readily created with this technique. This result indicates that pH within a single cardiac cell may not always be spatially uniform, particularly when weak acid or base gradients are present, which can occur, for example, in regional myocardial ischemia. The microperfusion method should be useful for studying the effects of localized acidosis on myocyte function, estimating intracellular ion diffusion rates, and, possibly, inducing regional changes in other important intracellular ions.

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[Formula: see text]. 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-CO2intracellular 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[Formula: see text]-buffered solutions, ANG II did not significantly stimulate pHirecovery 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.