Kinetics and regulation of a polarized Na(+)-H+ exchanger from Caco-2 cells, a human intestinal cell line.
The kinetics and regulation of Na(+)-H+ exchange were studied using BCECF to measure pHi in Caco-2 cells grown on membrane filters. Na(+)-H+ exchange was defined as a Na(+)-dependent H+ efflux in response to an acid load imposed by an NH4Cl prepulse in the absence of added CO2. Na(+)-H+ exchange was present exclusively on the basolateral membrane, had a Kt (Na+) of 21 +/- 2 mM, and an ID50 for amiloride dependent on medium [Na+] with an apparent Ki for amiloride of 3 microM. Na(+)-H+ exchange rates had a greater than first-order dependence on intracellular [H+], suggesting the presence of an internal proton modifier site. Results also suggest that Na(+)-H+ exchange is kinetically inactivated at resting pHi (7.35 +/- 0.02), since neither removal of Na+ nor addition of amiloride affected resting pHi, although monensin alkalinized cells to pHi 7.6. To evaluate regulation of Na(+)-H+ exchange, cells were exposed to either forskolin, 1,9-dideoxyforskolin (a noncyclase-activating forskolin derivative), 8-BrcAMP, E. coli STa toxin, ionomycin, phorbol dibutyrate, or cellular shrinkage in hypertonic medium. Only forskolin and 1,9-dideoxyforskolin caused a significant change (inhibition) in Na(+)-H+ exchange rate. Experiments performed with the Ussing chamber-voltage clamp technique verified that forskolin, 8-BrcAMP, E. coli STa toxin, ionomycin, and phorbol dibutyrate increased transepithelial Isc, verifying that all the regulatory pathways tested were functional and responsive to agonists. Results suggested that the Isc was due to Cl- secretion, since no net transcellular Na+ or Cl- flux was detected in basal conditions, and the Isc response to forskolin was abolished by omission of serosal Cl-. Because forskolin, but not 1,9-dideoxyforskolin, increased both cellular cAMP and Isc, the inhibition of Na(+)-H+ exchange by forskolin derivatives was mediated by a mechanism not involving activation of adenylyl cyclase. In conclusion, Caco-2 cells use a basolateral Na(+)-H+ exchanger to regulate pHi, but this exchanger is not affected by cell shrinkage or second messenger pathways that regulate Na(+)-H+ exchangers in other cell systems.