Carbonic anhydrase inhibitors modify intracellular pH transients and contractions of rat middle cerebral arteries during CO2/HCO3– fluctuations

摘要

The CO2/HCO3^– buffer minimizes pH changes in response to acid–base loads, HCO3^– provides substrate for Na⁺,HCO3^–-cotransporters and Cl^–/HCO3^–-exchangers, and H⁺ and HCO3^– 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 CO2/HCO3^– buffer. Switching from CO2/HCO3^–-free to CO2/HCO3^–-containing extracellular solution results in initial intracellular acidification due to hydration of CO2 followed by gradual alkalinization due to cellular HCO3^– 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⁺,HCO3^–-cotransport and Na⁺/H⁺-exchange activity after NH4⁺-prepulse-induced intracellular acidification are unaffected by carbonic anhydrase inhibition. Extracellular surface pH transients induced by transmembrane NH₃ flux are evident under CO₂/HCO₃^–-free conditions but absent when the buffer capacity and apparent H⁺ mobility increase in the presence of CO₂/HCO₃^– 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 pCO₂, (b) CO₂/HCO₃^– 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.