Energetic requirements of active transepithelial Na and Cl transport in the isolated bullfrog cornea

Abstract

In the isolated bullfrog cornea, the energetic requirements were estimated of anaerobic active transepithelial Na and Cl transport (i.e. Δ Na Δ ATP and Δ Cl Δ ATP ). These estimates were done by measuring the corresponding effects of various stimulators and inhibitors of active transepithelial Na and Cl transport on anaerobic lactate efflux and the short-circuit current (s.c.c.). In NaCl Ringer's solution, 10 −5 m-amphotericin B stimulated the calculated rate of active transepithelial Na transport by between 0·86 and 1·12 μEq/hr.cm 2. The corresponding increase in lactate efflux was 0·29 μmol/hr.cm 2. Assuming that glucose is the sole substrate for anaerobic glycolysis, the energetic requirement as estimated from these stimulatory effects on active transepithelial Na transport is between 3·0 and 3·9. The energetic requirement of active transepithelial Cl transport was studied by considering the corresponding effects on lactate efflux and the Cl-originated s.c.c. of various known selective inhibitors and stimulators of the Cl-originated s.c.c. The Cl energetic requirement could not be estimated from either the inhibitory effects of 10 −5 m-bumetanide or 3 × 10 −4 m-furosemide since both of these drugs only inhibited the Cl-originated s.c.c. without affecting lactate efflux. Only 10 −3 m-ouabain had a slight inhibitory effect on lactate efflux. The selective stimulants of active transepithelial Cl transport, 10 −4 m-epinephrine and 2 × 10 −3 m-theophylline had large stimulatory effects on the s.c.c. but did not affect lactate efflux. Only the stimulant 10 −5 m-A23187 (calcium ionophore), which had the largest stimulatory effect on the Cl-originated s.c.c, significantly stimulated the lactate efflux. From the inhibitory effects of 10 −3 m-ouabain on the A23187 stimulated s.c.c. and lactate efflux, the energetic requirement of active transepithelial Cl transport is equivalent to a movement of 4·0 mol of Cl per mol of ATP.