Magnesium secretion by the distal segment of the Malpighian tubules of theblack field cricket Teleogryllus oceanicus

The short distal segment of unstimulated Teleogryllus Malpighian tubules se creted hyperosmotic fluid containing primarily Mg (125 mmol l(-1)), Cl (242 mmol l(-1)) and Na (43 mmol l(-1)). Remarkably, the volume secreted by the distal segment in unit time was independent of segment length, i.e. the vo lume was constant regardless of the length of the segment. Magnesium was se creted at a rate of 75.5 pmol min(-1) mm(-1); the highest rate recorded for any epithelium. Low concentrations of K (20 mmol l(-1)) were present but a lmost no P or S. Ca (2.5 mmol l(-1)) concentration was higher than in the m ain segment. The short distal segment secreted 100% of the Mg, 54% of the C l and 23% of the Na secreted by the whole tubule. The main segment secreted fluid containing primarily K (199 mmol l(-1)), Cl (149 mmol l(-1)), Na (10 4 mmol l(-1)) and P (48 mmol l(-1)) with very low concentrations of Ca (1 m mol l(-1)) and S. The main segment appeared to reabsorb a small fraction of the Mg secreted by the distal segment. The fluid secreted by the whole tub ule was isosmotic and alkaline, approximately pH 8. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.