Newborn rat distal cells express an apical Ca 2+ channel activated by dihydropyridine drugs. Similarly, in Madin-Darby canine kidney (MDCK) cells, nifedipine increased Ca 2+ i in a concentration-dependent manner (IC 50 = 4 µM ) in fura-2-loaded cells. Response to nifedipine was abolished by EGTA, suggesting that it depends on extracellular calcium. Ca 2+ channel antagonist isradipine and agonist BayK8644 increased Ca 2+ i indicating that this effect is related to the dihydropyridine group. Diltiazem (20 µM ) and gadolinium (200 µM ) decreased the nifedipine effect (62 and 43%, respectively). Lanthanum (100 µM ) did not change the response. Valinomycin clamping of the membrane potential did not modify nifedipine-induced increment, indicating that it was unrelated to potassium fluxes. We performed whole cell clamp experiments in MDCK cells maintained at –50 mV with perfusion solution containing 10 m M CaCl 2 . Nifedipine (20 µM ) induced an increase in current (1.2 +/- 0.3 nA), which was partially inhibited by Gd 3+ . No significant current was induced by nifedipine in the presence of 0.5 m M EGTA. To determine the effects of nifedipine on the membrane potential, we performed oxonol fluorescence experiments. The addition of nifedipine or Bay K8644 induced depolarization, highly dependent on external sodium. Nifedipine (20 M) induced depolarization of 6.9 +/- 0.8 mV (n = 21). EC 50 to nifedipine was in the 10 M range. We conclude that MDCK cells exhibit a dihydropyridine-activated cationic channel.