Chloride secretion drives urine formation in leech nephridia

The transport mechanisms underlying urine formation in leech nephridia were investigated in situ and in isolated preparations using pharmacological, electrophysiological and micropuncture techniques. Canalicular cells, which secrete the primary urine, function as a Cl(-)-secreting epithelium. An apical Cl- conductance contributes to the lumen-negative potential which drives transcellular K+ transport and paracellular Na+ transport. On the basolateral side, a ouabain-sensitive Na+/K(+)-ATPase contributes substantially to the cellular and transcellular potential and provides the Na+ gradient necessary for a bumetanide-sensitive Na+/K+/2Cl- cotransport. Final urine is formed by subsequent reabsorption of ions along the central canal, where KCl and NaCl are reabsorbed in different portions. The postprandial diuresis is not a consequence of the changes in blood osmolality or ion concentrations. Similar changes in the ionic environment do not promote diuresis in isolated nephridia. Apparently, the composition and volume of the primary urine cannot be separately controlled. Any increase in fluid secretion by leech canalicular cells involves upregulation of the paracellular pathway and stimulation of Cl- entry, which thereby changes the normally K(+)-enriched primary urine to the Na(+)-enriched primary urine characteristic of leeches in diuresis.