Transport and metabolism of [3H]morphine in isolated, nonperfused proximal tubular segments of the rabbit kidney.

Proximal tubular segments (S1, S2 and S3) of the rabbit kidney were incubated in oxygenated Ringer's solution (30 min, pH 7.4, 37 degrees C) containing bovine serum albumin (10 g/l) and [3H]morphine (0.7 microM). The uptake, expressed as tissue water/medium ratio at equilibrium, for S1 was 42.2 +/- 3.95 (mean +/- S.E.), n = 16 tubules from six animals; for S2, 41.6 +/- 4.17, n = 18 tubules from six animals; and for S3, 29.0 +/- 3.83, n = 15 tubules from six animals, a value significantly lower (P less than .05) than in the S1 and S2 segment. High-performance liquid chromatography analysis of the accumulated tritium revealed metabolism of [3H]morphine. Unchanged morphine represented 35.5 +/- 3.4% of the total radioactivity recovered in the extract from S1 segments, 51.1 +/- 7.7% from S2 and 77.3 +/- 1.8 from S3 segments. After treating the tubular extracts with beta-glucuronidase (5 hr, 25 degrees C), all the recovered radioactivity represented unchanged morphine. The main metabolite, thus, was a glucuronide. KCN (10(-2) M), mepiperphenidol (Darstine, 10(-4) M) and quinine (10(-4) M) inhibited [3H]morphine uptake by 55-70%. Surprisingly, there was no decrease in uptake in the presence of N1-methylnicotinamide (10(-3) M). We conclude that the whole proximal tubule is able to accumulate morphine by a specific transport system for organic cations, but that part of this uptake might be due to cellular metabolism and intracellular binding of the drug.