Active NaCl absorption across split lamellae of posterior gills of the Chinese crab Eriocheir sinensis: Stimulation by eyestalk extract

Split lamellae of the posterior gills of freshwater-adapted Chinese crabs ( Eriocheir sinensis) were mounted in a modified Ussing-type chamber, and act ive and electrogenic absorption of Na+ and Cl- were measured as positive (I -Na) or negative (I-Cl) short-circuit currents. Haemolymph-side addition of eyestalk extract stimulated Icl by increasing both the transcellular Cl- c onductance and the electromotive force for Cl- absorption, The effect was d ose-dependent. Boiling the eyestalk extract did not change its effectivenes s. The stimulating factor passed through dialysis tubing, indicating that i t has a molecular mass of less than 2 kDa, R(p)cAMPS, a blocker of protein kinase A, reduced the stimulated I-Cl. Eyestalk extract stimulated I-Na by increasing the transcellular Na+ conductance at constant electromotive forc e. Amiloride-induced current-noise analysis revealed that stimulation of I- Na was accompanied by an increase in the apparent number of open apical Na channels at a slightly reduced single-channel current. In addition to the electrophysiological experiments, whole gills were perfused in the presence and in the absence of putative transport stimulators, and the specific act ivities of the V-ATPase and the Na+/K+-ATPase were measured. Eyestalk extra ct, theophylline or dibutyryl-cyclic AMP stimulated the activity of the V-A TPase, whereas the activity of the Na+/K+-ATPase was unaffected. The simult aneous presence of R(p)cAMPS prevented the stimulation of V-ATPase by eyest alk extract or theophylline.