Renal proximal tubule of flounder II. Transepithelial Mg secretion.

The kinetics of transepithelial Mg secretion were studied in isolated perfused proximal tubule II of the flounder, Pseudopleuronectes americanus. To detect measurable changes of luminal [Mg] as a function of bath [Mg], the lumen had to be perfused at subnanoliter rates. Transepithelial Mg secretion did not obey first-order reaction kinetics; hence values for half saturation of transport (0.22 mM) and transport maximum (1.5 pmol X min-1 X mm-1) are apparent and suggest a high-affinity low-capacity transport system. Because all experiments were done in the absence of bath SO4, the independence of Mg transport from SO4 transport is established. In the absence of perfusion, when tubules secrete fluid spontaneously, secreted fluid contained Cl (156 +/- 3 mM), Na (130 +/- 6 mM), and Mg (27 +/- 5 mM), all significantly different from the bath. Rates of Cl, Na, and Mg secretion were all positively correlated with fluid secretion, but Na and Mg concentrations in secreted fluid were inversely proportional. The results indicate that NaCl secretion provides basal rates of fluid secretion, and when MgCl2 is secreted in addition, fluid secretion increases with the effect of generating inverse relationships between luminal Na and Mg concentrations.