To investigate the mechanism of salt secretion in the avian salt gland, we used quantitative electron probe microanalysis to measure the intracellular elemental concentrations in dry cryosections of unspecialized and partially specialized secretory epithelial cells from fresh water- and salt water-adapted ducklings, respectively. In conjunction with this, human and duckling erythrocytes were also analyzed, since these provided the experimental basis for using in situ erythrocytes as standards for determining the local water content of epithelia from the analysis of dried cryosections. The microprobe results from both types of erythrocytes compared favorably with chemical determinations of elemental concentrations. The nucleated avian erythrocytes, whose wet-weight elemental concentrations were determined by a compartmental analysis that required neither a peripheral standard nor a measure of the local mass, revealed a marked accumulation of P and K in the nucleus (388 and 190 mmol/kg wet wt, respectively) relative to the cytoplasm (67 and 85 mmol/kg wet wt). In both developmental states of the epithelial cells, the nucleus and apical cytoplasm had essentially similar and unremarkable concentrations of Na (76 and 83 mmol/kg dry wt, respectively, in the adapted cells vs. 72 and 81 mmol/kg dry wt in the control cells) and K (602 and 423 mmol/kg dry wt vs. 451 and 442 mmol/kg dry wt). Chloride, however, which was in general rather high, was significantly depressed in the apical cytoplasm of adapted cells only (164 and 124 mmol/kg dry wt in the nucleus and cytoplasm, respectively, of adapted cells (P less than 0.05) vs. 138 and 157 mmol/kg dry wt for control cells (P less than 0.05). Cation concentrations (Na + K) were elevated approximately 15% in the basal regions of adapted cells as compared with apical cytoplasm. When tissue water variations are accounted for, the results suggest that: (a) an active, energy-requiring process is responsible for chloride accumulation in this cell; (b) the apical membrane is a regulatory site for secretion; and (c) there are regional distinctions in the distribution of ions and water, particularly in the salt water-adapted cell. These conclusions are consistent with active chloride transport as the basis for salt secretion in this tissue.

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