Uptake of the noncytotoxic transport probe procainamide in the Chinese hamster ovary model of multidrug resistance.

Many of the cytotoxic substrates of the multidrug transporter are organic cations. Cimetidine, procainamide, and tetraethylammonium bromide were used in a Chinese hamster ovary model of multidrug resistance, to study handling of noncytotoxic cationic transport probes. Cimetidine and procainamide, but not tetraethylammonium, accumulated to a greater extent (5-fold) in the sensitive CHOAUXB1 (AB) cell line than in the resistant CHRC5 (C5) cell line. Accumulation of both cimetidine and procainamide was significantly increased by verapamil in C5 but not AB. Procainamide accumulation in both AB and C5 was temperature dependent and occurred by passive diffusion. Diltiazem, nifedipine, rifampin, tamoxifen, rhodamine, and ethidium also increased procainamide accumulation in C5 but not AB. Azide in glucose-free medium increased procainamide accumulation in C5, and this was reversed when glucose, but not 3-O-methylglucose, was added. Procainamide efflux rates were similar in AB and C5 and not affected by verapamil or azide. The initial rate of procainamide uptake was higher in AB than in C5, and both verapamil and azide increased the initial rate of procainamide uptake in C5. Thus, differences in accumulation of the noncytotoxic transport probe procainamide in the colchicine-sensitive and colchicine-resistant components of the Chinese hamster ovary cell line mimic the accumulation of known cytotoxic substrates for the multidrug transporter, such as colchicine, vinblastine, and doxorubicin. The differential accumulation of procainamide is due to differences in rates of drug influx, rather than efflux. Since procainamide influx is passive and decreased accumulation in the resistant line appears to parallel M(r) 170,000 glycoprotein presence and activity, we would speculate that decreased procainamide accumulation may be due to an indirect effect of the M(r) 170,000 glycoprotein, such as its effect on intracellular pH.