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Mechanism of hyperosmotic urine formation in the recta of saline-water mosquito larvae Bradley, Timothy Jud

Abstract

The osmoregulatory function of the larval recta of two saline-water mosquitoes, Aedes campestris and A. taeniorhynchus, was examined. In hyperosmotic waters the rectum was shown to be the site of formation of a concentrated urine by secretion of a hyperosmotic fluid into the rectal lumen. In similar concentrations of sea water, both species produced a rectal secretion having an osmotic concentration and ionic composition similar to that of sea water, with the exception that potassium levels are elevated 18- to 21-fold in the secretion. Osmoregulatory strategies in both species involve the rapid ingestion of the external medium. In taeniorhynchus this drinking rate, (100 nl mg⁻¹ h⁻¹ ) did not vary significantly in salinities between 10% and 200% sea water. It is suggested that two purposes are served by a rapid rate of drinking; 1) dissolved nutrients can be taken up when particulate food is limited and 2) when the uptake of external medium is large relative to the loss of water by osmosis across the cuticle, the concentration of urine necessary to maintain osmotic homeostasis is close to that of the external medium. Rectal function is A. campestris was examined in three media, all with an osmotic concentration of 700 mOsm but varying in their ionic ratios. These media contained the ionic ratios found in sea water and two types of ponds in which A. campestris naturally occur. The larvae were able to survive in all three media, suggesting that the larvae found in one saline environment are not physiologically limited to those ionic conditions. I demonstrated that changes in the relative transport rates of ions in the rectum are very significant in the acclimation process. The rectum was found to be the major site of Na⁺ , K⁺ , Mg⁺⁺, CI⁻ and probably HCO⁻₃ excretion, while the Malpighian tubules are the source of some of the magnesium and almost all of the sulphate in the urine. The ion ratios, exclusive of K⁺ found in the rectal secretion matched those in the external medium, with the exception that an unidentified anion (probably HC0~) substituted for SO₄̿ in (Na + Mg) SO₄̿ medium. A model is proposed showing the sites of ion secretion and uptake in the Malpighian tubules, rectum and anal papillae in all three media. Rectal function in A. taeniorhynchus larvae was examined in 10%, 100% and 200% sea water. In hyposmotic media the rectum does not secrete a fluid and it is postulated that salt and water resorption occur in the rectum under these conditions as in strictly freshwater species. In hyperosmotic media the concentrations of Na⁺ , K⁺ , Mg++ and Cl⁻ as well as the osmotic concentration of the secreted fluid increase with increasing external salinity. Due to the high rate of K+ secretion by the rectum, potassium uptake by the anal papillae is postulated. Sodium and chloride may be excreted at this site as well. An examination of the effect of varying hemolymph concentrations of sodium and chloride on the rate of secretion of these ions in the rectum, showed an allosteric relationship rather than the Michaelis-Mentin kinetics observed with most transport processes (e.g. the Mal-pighian tubules). An in vitro preparation (lacking tracheal and neural con* nections) of the larval rectum of A. taeniorhynchus was used to examine the function of the anterior and posterior rectal segments. These two regions, which differ morphologically, were shown to have separate functions in. vitro, the posterior segment secreting a hyperosmotic fluid while the fluid in the lumen of the anterior segment decreased in osmotic concentration and showed no change in volume. Electrical potential differences were measured across the basal and apical membranes as well as across the entire rectal wall in vivo. Based on these observations in artificial hemolymphs of various ionic compositions, a model is presented of ion transport processes occuring in the posterior rectal segments during the secretion of a hyperosmotic fluid. The model accounts for the ion concentrations and ionic ratios observed in rectal secretion from larvae reared in different media.

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