Growth hormone is reported to increase renal plasma flow (RPF) and glomerular filtration rate (GFR) in some but not all studies. The discrepant results could be due to a delay in the effects of growth hormone on renal function. We therefore examined whether a growth hormone injection does increase RPF and GFR, whether this increase is delayed, and whether elevation in RPF and GFR is associated with increased plasma levels of insulin-like growth factor I (IGF-I). Seven normal adults received a single intramuscular injection of growth hormone, 0.15 mg/kg, and serial PAH and inulin clearances were then monitored for 3 consecutive days. Plasma growth hormone levels peaked an average of 2.25 hours after injection, at 128 ± 12 SEM ng/ml, and then began to decrease; on the second day values were only slightly elevated and on the third day they were not different from baseline. Plasma IGF-I, analyzed by direct radioimmunoassay, did not change on the first day during 5.5 hours of measurements after injection. By the second day, plasma IGF-I was elevated to over twice baseline levels (P < 0.05) and remained elevated on the third day (P < 0.05). RPF and GFR did not change from baseline (546 ± 19 and 100 ± 3 ml/min/1.73 m2, respectively) during the 5.5 hours after injection on the first day. By the second day, RPF had risen to 715 ± 21, P < 0.05, and GFR to 130 ± 5 ml/min/1.73 m2, P < 0.05, and mean values remained elevated on the third day. The grand mean for total renal vascular resistance was decreased on the second and third days. Thus, after a growth hormone injection there was a rise in RPF and GFR which was delayed, was observed after plasma growth hormone levels had fallen to baseline, and which occurred in association with the increased plasma IGF-I levels. These findings suggest that growth hormone increases RPF and GFR indirectly through one or more mediators, one of which could be IGF-I.
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