Kinetic analysis and partitioning of sodium and chloride influxes across the gills of sea water adapted trout.

1. Sodium and chloride influxes across the gills of sea water adapted trout were studied with the perfused isolated head technique. 2. The haemodynamic scheme describing anastomoses between efferent gill arteries and the filamentous sinus was confirmed and utilized to determine partitioning of sodium and chloride influxes. 3. Two routes of sodium and chloride entry were demonstrated, one lamellar and the other sinusal. The lamellar sodium and chloride fluxes are respectively 133+/-21-8 and 271+/-43-7 micronequiv/hr. 100 g. The sinus sodium and chloride fluxes are: 50+/-18-3 and 50+/-11-3 micronequiv/hr. 100 g. 4. The relative permeabilities of the apical and basal barriers of the lamellar epithelium were determined with influx experiments coupled to radioactive unloading experiments. The basal membrane is 30 times less permeable than the apical to sodium, while this ratio is 45 for chloride. Thus, the basal membrane represents the limiting factor in salt movement across the branchial epithelium. 5. The intracellular pools of the lamellar compartment were calculated for sodium: 1-1+/-0-17 micronequiv/100 g of fish weight; and for chloride: 5-1+/-0-83 micronequiv/100 g of fish weight.