Carbonic anhydrase inhibitors modify intracellular pH transients and contractions of rat middle cerebral arteries during CO 2 /HCO 3– fluctuations

Abstract

The CO 2 /HCO 3– buffer minimizes pH changes in response to acid–base loads, HCO 3– provides substrate for Na + ,HCO 3– -cotransporters and Cl – /HCO 3– -exchangers, and H + and HCO 3– modify vasomotor responses during acid–base disturbances. We show here that rat middle cerebral arteries express cytosolic, mitochondrial, extracellular, and secreted carbonic anhydrase isoforms that catalyze equilibration of the CO 2 /HCO 3– buffer. Switching from CO 2 /HCO 3– -free to CO 2 /HCO 3– -containing extracellular solution results in initial intracellular acidification due to hydration of CO 2 followed by gradual alkalinization due to cellular HCO 3– uptake. Carbonic anhydrase inhibition decelerates the initial acidification and attenuates the associated transient vasoconstriction without affecting intracellular pH or artery tone at steady-state. Na + ,HCO 3– -cotransport and Na + /H + -exchange activity after NH 4+ -prepulse-induced intracellular acidification are unaffected by carbonic anhydrase inhibition. Extracellular surface pH transients induced by transmembrane NH 3 flux are evident under CO 2 /HCO 3– -free conditions but absent when the buffer capacity and apparent H + mobility increase in the presence of CO 2 /HCO 3– even after the inhibition of carbonic anhydrases. We conclude that (a) intracellular carbonic anhydrase activity accentuates pH transients and vasoconstriction in response to acute elevations of pCO 2 , (b) CO 2 /HCO 3– minimizes extracellular surface pH transients without requiring carbonic anhydrase activity, and (c) carbonic anhydrases are not rate limiting for acid–base transport across cell membranes during recovery from intracellular acidification.