CARBONIC-ANHYDRASE AND CARDIAC PH REGULATION

Membrane-bound carbonic anhydrase (CA) has recently been identified in mammalian cardiac tissue. In this study, we have investigated the his tochemical location and functional role of CA in the ferret heart. Hea rt sections stained by a modified Hansson's technique showed CA to be located on capillary endothelial membranes as well as on sarcolemmal m embranes. In the Langendorff-perfused heart, washout of CO2 brought ab out by switching perfusion between 25 mM HCO3-5% CO2-buffered solution and nominally HCO3--CO2-free solution caused a transient rise in intr acellular pH (pH(i)) measured by the chemical shift of 2-deoxy-D-gluco se 6-phosphate with P-31 nuclear magnetic resonance spectroscopy. The initial rate of change of pH(i), measured over the first 60-75 s of CO 2 efflux, was significantly reduced from 0.41 +/- 0.03 pH units/min (n = 9) in control hearts to 0.28 +/- 0.02 pH units/min (n = 5) in the p resence of the membrane-permeable CA inhibitor 6-ethoxzolamide (P < 0. 05 compared with control) and to 0.22 +/- 0.04 pH units/min (n = 5) in the presence of the membrane-impermeable CA inhibitor CL-11,366 (P < 0.01 compared with control). After reperfusion of the ischemic myocard ium, both CA inhibitors caused a significant slowing of initial rate o f change in pH (and initial rate of recovery of contractile function) compared with control hearts. These results suggest that CA, by facili tating the hydration-dehydration of CO2-H2CO3, alters the relative con centrations of CO2 inside and outside the cells, thus enhancing the ra te of CO2 transfer from the intracellular to extracellular compartment s, which contributes significantly to pH(i) recovery after reperfusion of the ischemic myocardium.