We previously reported a dual kinetics of Ca2+ transport by the distal tubule luminal membrane of the kidney, suggesting the presence of several types of channels. To better characterize these channels, we examined the effects of specific inhibitors (i.e., diltiazem, an L-type channel; omega-conotoxin MVIIC, a P/Q-type channel; and mibefradil, a T-type channel antagonist) on 0.1 and 0.5 mM Ca2+ uptake by rabbit nephron luminal membranes. None of these inhibitors influenced Ca2+ uptake by the proximal tubule membranes. In contrast, in the absence of sodium (Na+), the three channel antagonists decreased Ca2+ transport by the distal membranes, and their action depended on the substrate concentrations: 50 microM diltiazem decreased 0.1 mM Ca2+ uptake from 0.65 +/- 0.07 to 0.48 +/- 0.06 pmol. microg-1.10 s-1 (P < 0.05) without influencing 0.5 mM Ca2+ transport, whereas 100 nM omega-conotoxin MVIIC decreased 0.5 mM Ca2+ uptake from 1.02 +/- 0.05 to 0.90 +/- 0.05 pmol. microg-1.10 s-1 (P < 0.02) and 1 microM mibefradil decreased it from 1.13 +/- 0.09 to 0.94 +/- 0.09 pmol. microg-1.10 s-1 (P < 0.05); the latter two inhibitors left 0.1 mM Ca2+ transport unchanged. Diltiazem decreased the Vmax of the high-affinity channels, whereas omega-conotoxin MVIIC and mibefradil influenced exclusively the Vmax of the low-affinity channels. These results not only confirm that the distal luminal membrane is the site of Ca2+ channels, but they suggest that these channels belong to the L, P/Q, and T types.
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