Kidney IGF-I mRNA in initial renal hypertrophy in experimental diabetes in rats

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Abstract
It has recently been demonstrated that immunoassayable kidney insulin-like growth factor I concentration increases 24–48 h after induction of diabetes, preceding the initial renal hypertrophy. To elucidate whether this increase is due to increased local production we studied rat kidney insulin-like growth factor I gene expression during the first four days after induction of streptozotocin diabetes. Eighteen hours after injection with streptozotocin the diabetic animals were divided into two groups, one of which was treated with insulin, and daily for four days animals from each group were taken out for investigation. After four days the wet kidney weight had increased from baseline by 20% (from 687±23 to 827±6mg (mean±SEM), p < 0.01) in the untreated diabetic group, while no significant increase occurred in the insulin-treated group (687±23 vs 732±21 mg, NS). Kidney insulin-like growth factor I increased rapidly from baseline, the rise amounting to 52% after 48 h (from 271±11 to 411±32 ng/g, p < 0.01) with a decline to control level on day four in the untreated diabetic group. Kidney insulin-like growth factor I remained unchanged in the insulin-treated diabetic group. Insulin-like growth factor I mRNA was measured by solution-hybridization assay. No differences were found in kidney insulin-like growth factor I mRNA between the two diabetic groups over the study period, while in liver, insulin-like growth factor I mRNA tended to be lower on day four in diabetic rats when compared to insulin-treated rats (p=0.07). These results show that the increase in kidney insulin-like growth factor I during initial renal hypertrophy in experimental diabetes is not associated with an elevated level of kidney insulin-like growth factor I mRNA and suggest that other, possibly translational, mechanisms are responsible for insulin-like growth factor accumulation in the kidney.