Different rate-limiting activities of intracellular pH regulators for HCO 3 − secretion stimulated by forskolin and carbachol in rat parotid intralobular ducts

Intracellular pH (pH i ) regulation fundamentally participates in maintaining HCO 3 − release from HCO 3 − -secreting epithelia. We used parotid intralobular ducts loaded with BCECF to investigate the contributions of a carbonic anhydrase (CA), anion channels and a Na + –H + exchanger (NHE) to pH i regulation for HCO 3 − secretion by cAMP and Ca 2+ signals. Resting pH i was dispersed between 7.4 and 7.9. Forskolin consistently decreased pH i showing the dominance of pH i -lowering activities, but carbachol gathered pH i around 7.6. CA inhibition suppressed the forskolin-induced decrease in pH i , while it allowed carbachol to consistently increase pH i by revealing that carbachol prominently activated NHE via Ca 2+ -calmodulin. Under NHE inhibition, forskolin and carbachol induced the remarkable decreases in pH i , which were slowed predominantly by CA inhibition and by CA or anion channel inhibition, respectively. Our results suggest that forskolin and carbachol primarily activate the pH i -lowering CA and pH i -raising NHE, respectively, to regulate pH i for HCO 3 − secretion.