Renal handling of gentamicin by the Munich-Wistar rat.

Free-flow micropuncture studies were performed in the Munich-Wistar rat to determine the nephron sites of gentamicin transport. The rats were infused with radiolabeled gentamicin and were volume-expanded with either isotonic sodium chloride or sodium bicarbonate. The fractional delivery of ultrafilterable gentamicin (FD) out of the superficial proximal tubule averaged 75% in both groups of animals. FD to the superficial distal tubule averaged 58.1 +/- 2.1% in saline-infused and 91.6 +/- 5.3% in bicarbonate-infused animals. FD to Henle's loop was 118.2 +/- 8.8 and 124.3 +/- 8.1% in saline- and bicarbonate-infused animals, respectively. FD to the base and tip of the papilla was not significantly different between the two groups and averaged 97.3 +/- 2.9 and 96.1 +/- 3.0% in the saline-infused and 100.5 +/- 2.6 and 94.3 +/- 2.8% in the bicarbonate-infused animals. FD to the base of the papilla was significantly lower than that to the loop of Henle in both groups. These studies indicate that gentamicin undergoes net reabsorption in juxtamedullary proximal tubules. Expansion of the extracellular fluid volume with saline or bicarbonate results in differing rates of gentamicin delivery to the superficial distal tubule but not to the base of the papilla. These findings suggest a significant degree of nephron heterogeneity for gentamicin transport in the rat kidney.