cis-Diamminedichloroplatinum II (CDDP) is an antineoplastic drug against solid malignant tumors. However, its clinical use is limited by nephrotoxicity. CDDP also causes hypokalemia and in vivo microperfusion method have demonstrated that luminal CDDP increases K+ secretion by hyperpolarization of the transepithelial voltage difference through stimulating Na+ transport in the distal segments. However, there is no direct evidence for this mechanism. We therefore examined the effect of luminal CDDP on Na+ and K+ transport in the rabbit cortical collecting duct (CCD) using in vitro isolated tubular microperfusion. Luminal CDDP hyperpolarized the transepithelial voltage difference (V(T)) in a dose-dependent manner at concentrations from 10(-5) M to 10(-3) M and at 10(-3) M CDDP, V(T) was hyperpolarized from -11.6+/-2.3 mV to -16.6+/-3.3 mV (P<0.001). A concentration of 10(-5) M ouabain, 10(-4) M amiloride and 2 mM BaCl2 all completely abolished CDDP-induced hyperpolarization. To confirm the mechanism, Na+ and K+ flux were measured in the presence of 10(-3) M CDDP. CDDP decreased net K+ secretion from -22.2+/-5.7 to -15.2+/-2.9 pmol mm(-1) min(-1) (P<0.01) without any effect on the lumen-to-bath isotope flux of Na+ (52.6+/-10.6 to 52.1+/-10.7 pmol mm(-1) min(-1)). These data suggest that luminal CDDP hyperpolarizes V(T) primarily by inhibiting K+ conductance but did not influence Na+ transport of the luminal membrane. We conclude that the CCD does not play a role in CDDP-induced hypokalemia when CDDP is applied from the luminal side.