ACTIVE ABSORPTION OF NA- ACROSS THE GILL EPITHELIUM OF THE SHORE CRABCARCINUS-MAENAS - VOLTAGE-CLAMP AND ION-FLUX STUDIES( AND CL)

Mechanisms of active NaCl uptake across the posterior gills of the sho re crab Carcinus maenas were examined using radiochemical and electrop hysiological techniques. In order to measure short-circuit current (I- sc), transepithelial conductance (G(te)) and area-related unidirection al fluxes of Na+ and Cl-, single split gill lamellae (epithelium plus cuticle) of hyperregulating shore crabs were mounted in a modified Uss ing chamber, The negative short-circuit current measured with haemolym ph-like NaCl saline on both sides of the epithelium could be inhibited by application of basolateral ouabain (ouabain inhibitor constant K-O ua=56+/-10 mu mol l(-1)), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB; K-NPPB=7.5+/-2.5 mu mol l(-1)) or Cs+ (10 mmol l(-1)), From th e apical side, I-sc was nearly completely blocked by Cs+ (10 mmol l(-1 )) or Ba2+ (15 mmol l(-1)), whereas apical addition of furosemide (15 mmol l(-1)) resulted in only a small current decrease, Cl- influxes we re linearly related to negative I-sc. The ratio between net influxes o f Cl- and Na+ was found to be approximately 2:1. With a single membran e preparation, achieved by permeabilizing the basolateral membrane wit h amphotericin B, Cl- influxes which were driven by a concentration gr adient were shown to depend on the presence of apical Na+ and K+, On t he basis of these observations, we propose that active and electrogeni c absorption of NaCl across the gill epithelium of hyperregulating sho re crabs proceeds as in the thick ascending limb of Henle's loop in th e mammalian nephron, Accordingly, branchial NaCl transport is mediated by apical K+ channels in cooperation with apical Na+/K+/2Cl(-) cotran sporters and by the basolateral Na+/K+-ATPase and basolateral Cl- chan nels.