Isosmotic secretion in the avian nasal salt gland: X-ray microanalysis of luminal and intracellular ion distributions

Summary

The secretory tubule epithelium of the avian nasal salt gland has been thought to secrete a hyperosmotic fluid containing high concentrations of Na^α and Cl⁻. It is shown by X-ray micro-analysis that the average composition of the luminal fluid of secretory tubules in duckling salt glands is Na 80 mmol l⁻¹, Cl. 100 mmol l⁻¹, K 40 mmol l⁻¹, Mg 11 mmol l⁻¹, Ca 6 mmol l⁻¹. It is assumed that the anion deficit is accounted for by HCO ⁻₃ . This fluid is approximately isosmotic or slightly hyposmotic to blood. There were no significant differences between the composition in different regions of the tubule nor between active and inactive glands. The fluid becomes increasingly hyperosmotic in the duct system, most concentration apparently occurring in the main ducts running from gland to external nares. Modification of the primary hyposmotic or isosmotic secretion therefore appears to occur in the duct system.

There were few differences in intracellular composition between the different cell types in the secretory tubule or between active and inactive glands. The average intracellular composition was Na 47 mmol l⁻¹, Cl 54 mmol l⁻¹, K 113 mmol l⁻¹, Mg 14 mmol kg⁻¹, Ca 8 mmol kg⁻¹ wet weight. The high Cl⁻ concentration is suggestive of active Cl⁻ transport. It is suggested that K^α transport into the cells may be a rate limiting process in secretion. It can be calculated that much of the K^α arriving at the gland in the blood must be extracted at high secretion rates. If modification of the primary secretion rccurs in the main duct by water reabsorption, thereby implying a substantially higher secretion rate of primary fluid, then it can be calculated that virtually all of the K^α arriving in the blood may be extracted by the gland.