Cyclic AMP stimulation of electrogenic uptake of Na+ and Cl− across the gill epithelium of the chinese crab Eriocheir sinensis

Summary Split gill lamellae (epithelium plus cuticle) of hyperregulating Chinese crabs acclimated to fresh water were mounted in a modified Ussing chamber. Active and electrogenic absorption of sodium and chloride were measured as positive amiloridesensitive and negative Cl 2 -dependent short-circuit currents ( INa, ICl), respectively. Both currents were characterized before and after treatment of the tissue with theophylline or dibutyryl cyclic AMP. Both drugs increased INa and ICl. A simple circuit analysis showed that INa stimulation reflected a marked increase in the transcellular Na + conductance, whereas the respective electromotive force was unchanged. The Michaelis constant ( KNa) for Na + current saturation was decreased after INa stimulation, indicating an increased affinity of the transport mechanism for its substrate. Consequently, the affinity for the Na + channel blocker amiloride decreased as expected for a competitive interaction between substrate and inhibitor. Analysis of the amiloride-induced current-noise revealed a marked increase in the number of apical Na + channels after INa stimulation with theophylline, whereas there was little change in the single-channel current. Stimulation of Cl 2 absorption was accompanied by a substantial increase in both transcellular conductance and electromotive force, indicating an activation of the apical H + pump that provides the driving force for active Cl 2 uptake via apical Cl 2 /HCO3 2 exchange and basolateral Cl 2 channels.