Mechanisms of the defect in glomerular ultrafiltration associated with gentamicin administration.

Micropuncture studies were performed in three groups of Munich-Wistar rats: eight normal hydropenic controls (group I) and two groups (eight rats each) which were treated with gentamicin in doses of either 4 or 40 mg/kg/day for ten days (groups II and III, respectively). Following gentamicin administration, values for single nephron (SN) GFR were reduced markedly, from the control mean of 31 +/- 0.7 (SEM) nl/min to 22.4 +/- 1.5 and 20.5 +/- 0.9 for groups II and III, respectively. Declines in whole kidney GFR paralleled these falls in SNGFR. The primary cause of the reduction in SNGFR was a marked decline in glomerular capillary ultrafiltration coefficient, Kf, in both gentamicin treatment groups. None of the other determinants of glomerular ultrafiltration were significantly affected in the low dose group (group II). In the high dose group (group III), however, mean values for initial glomerular plasma flow rate and mean transglomerular hydraulic pressure difference were significantly lower than in the control group, accounting for the somewhat greater decline in SNGFR observed in group III. Electron microscopic examination of kidney tissue from rats treated with both doses of gentamicin revealed no obvious abnormalities of the glomerular capillary wall, whereas the previously described morphologic aberrations of proximal convoluted tubule cells were readily demonstrable.