Renal tubular transport of penicillin G and carbenicillin in the rat.

The pattern of transport of penicillin G and carbenicillin was examined directly in the rat kidney by means of micropuncture studies. Samples of plasma, tubular fluid, and urine were assayed for antibiotic content by an agar diffusion technique. Secretion accounted for 67% of the penicillin G but for only 37% of the carbenicillin present in the proximal tubule. No further net secretion of either agent could be detected in the distal nephron. Net secretion of penicillin G decreased from 67% in the distal tubule to 60% in the urine (P less than 0.05%); this reduction correlated with reabsorption of water from the collecting ducts. Both penicillin G and carbenicillin were secreted by the proximal tubule of the rat nephron, but the latter was secreted at a lower rate than the former. A significant fraction of penicillin G was reabsorbed from the collecting ducts under conditions of maximal antidiuresis.