G proteins regulate calcium channels in the luminal membranes of the rabbit nephron

The filtered calcium (Ca2+) is reabsorbed by the luminal membrane of the pr oximal and distal nephron. Ca2+ enters cells across epical plasma membranes along a steep electrochemical gradient, through Ca2+ channels. Regulation by various hormones implies several steps, including binding of these hormo nes to the basolateral membrane, interaction with G proteins, liberation of messengers, activation of kinases and finally opening of the channels at t he opposite pole of the cells. In the present study, we examined whether th e Ca2+ entry through the luminal membranes of proximal and distal tubules i s also regulated by G proteins, by a membrane-limited process. Luminal memb ranes were purified from rabbit proximal and distal tubule suspensions, and their vesicles were loaded with GTP gamma s or the carrier. Then, the Ca-4 5(2+) uptake by these membrane vesicles was measured in the presence and ab sence of 100 mM NaCl. In the absence of Na+, intravesicular GTP gamma s sig nificantly enhanced 0.5 mM Ca2+ uptake by the proximal membrane vesicles fr om 0.53 +/-: 0.06 to 0.72 +/- 0.06 pmol/mu g/10 s (p < 0.05). In the presen ce of Na+, however, this effect disappeared. In the distal tubules, intrave sicular GTP gamma s increased 0.5 mM Ca2+ uptake in the absence (from 0.57 +/- 0.02 to 0.79 +/- 0.02 pmol/mu g/10 s, p < 0.02) and in the presence (fr om 0.36 +/- 0.03 to 0.55 +/- 0.03 pmol/mu g/10 s, p < 0.02) of Na+. The act ion of GTP gamma s, when present, was dose dependent with a half-maximal ef fect at 20 mu M. The distal luminal membrane is the site of two Ca2+ channe ls with different kinetics parameters. GTP gamma s increased the V-max valu e of the low-affinity component exclusively, in the presence as in the abse nce of Na+. Finally, Ca2+ uptake by the membranes of the two segments was d ifferently influenced by toxins: cholera toxin slightly stimulated transpor t by the proximal membrane, but had no influence on the distal membrane, wh ereas pertussis toxin decreased the cation uptake by the distal tubule memb rane exclusively. We conclude that the nature of Ca2+ channels differs in t he proximal and distal luminal membranes: Ca2+ channels present in the prox imal tubule and the low-affinity Ca2+ channels present in the distal tubule membranes are directly regulated by Gs and Gi proteins respectively, where as the high-affinity Ca2+ channel in the distal tubule membrane is insensit ive to any of them. Copyright (C) 2000 S. Karger AG, Basel.