Two pathways for electrogenic bicarbonate ion movement across the rabbit corneal endothelium

Amiloride (0.5 mM) inhibited the rate of entry of Na+ into corneal endothelial cells by more than half ((0.76 +/- 0.10) to (0.21 +/- 0.10) microEq cm(-2)h(-1)). The same concentration of amiloride caused only minimal disturbance to corneal hydration control by the endothelium (range 0-12%). Amiloride (0.5 mM) and acetazolamide (1 mM) reversibly inhibited trans-endothelial short circuit current by about a half. Their combined effect was not additive. Acetazolamide (1 mM) reduced net HCO3- flux across the short-circuited endothelium by about the same amount ((0.50 +/- 0.11) microEq cm(-2)h(-1)) that amiloride (0.5 mM) reduced Na+ entry into the cells ((0.55 +/- 0.14) microEq cm(-2)h(-1)). Low concentrations of amiloride (10 microM) had little effect on the transport characteristics of the endothelium, indicating that Na+ entry into the endothelial cells under physiological conditions is not primarily through Na+ channels. The data are consistent with an Na+/H+ exchanger acting in tandem with carbonic anhydrase through a pathway which could have a regulatory role on endothelial transport via its effect on Na+ re-entry. Residual trans-endothelial HCO3- transport, apparently unaffected by amiloride or acetazolamide inhibition, is calculated to be of sufficient magnitude to maintain corneal hydration.