Effects of somatostatin and captopril on glomerular prostaglandin E2 production in normal and diabetic rats

Comparison between somatostatin analogues and ACE inhibitor in the NOD mouse model of diabetic kidney disease

BACKGROUND

The growth hormone (GH)-insulin-like growth factor (IGF)-SST (SST) axis is involved in diabetic nephropathy (DN). We have recently shown a beneficial effect on diabetic kidney disease markers by the use of a novel somatostatin (SST) analogue (PTR-3173) (S). The purpose of this study is to compare the effects of S with a previously used SST analogue (octreotide) and an ACE inhibitor (ACEi), a standard of care in DN.

METHODS

Non-obese diabetic mice (a model of type I diabetes) were treated with either S (DS), octreotide (DO), enalapril (DA), or PTR-3173 and enalapril (DAS group) for 3 weeks.

RESULTS

Diabetic renal hypertrophy was blunted in the DS and DO groups only. Serum GH and IGF-I were markedly increased and decreased, respectively, in the D group, a change significantly blunted in DO and DS. Diabetic hyperfitration and albuminuria were blunted in all the four treated diabetic groups. The marked deposition of type IV collagen and PAS material were mildly decreased in DA, but more markedly reduced in DS as well as DO. Diabetic renal laminin accumulation was suppressed in all treated animal groups. No synergistic effect was observed for any parameter in the combination group DAS.

CONCLUSION

SST analogues exert beneficial effects in most parameters of diabetic kidney disease to the same extent as the ACEi. Enalapril treatment had no effect on renal hypertrophy and did not cause a significant decrease in mesangial type IV collagen deposition. A synergistic effect of combined SST-ACEi therapy could not be shown in this study.

Somatostatin expression in human renal cortex and mesangial cells

Somatostatin modulates important physiologic functions of the kidney, including mesangial cell contraction, glomerular prostaglandin synthesis, and phosphate, water and sodium excretion. In diabetic nephropathy, somatostatin inhibits renal hypertrophy. High affinity somatostatin receptors are expressed in the kidney. Circulating somatostatin concentrations, however, are generally well below the affinity constants of known somatostatin receptors. Thus, we hypothesized that somatostatin is produced in the kidney and released locally to act in an autocrine/paracrine manner. Using reverse transcriptase and polymerase chain reaction (RT-PCR) analysis, we found that fresh human renal cortex and cultured human mesangial cells express somatostatin mRNA. Restriction enzyme and Southern blot analysis confirmed that RT-PCR cDNA products were derived from somatostatin mRNA. Radioimmunoassay of mesangial cell culture supernatants demonstrated SS-immunoreactive peptide (87 +/- 30 pg/ml compared to 19 +/- 9 pg/ml in medium not exposed to cells; P < 0.05). In contrast, renal cells did not transcribe detectable levels of vasoactive intestinal peptide (VIP) or neuropeptide Y (NPY) mRNA, nor did they synthesize measurable peptide. Our results demonstrate that renal cells produce somatostatin and suggest that kidney-derived somatostatin may regulate renal function in an autocrine/paracrine manner. Characterization of this pathway may lead to novel methods to alter the course of diabetic nephropathy and other renal diseases.

Direct renovascular effect of somatostatin in the dog

Recently, effects of somatostatin on the renal function have been described and the vasoactive properties of the peptide were proposed to contribute to this action. However, the available data on its effect in the renal vascular bed are very controversial. Therefore, we investigated the effect of local intaarterial somatostatin boluses in a wide range of doses (5 x 10(-11) - 5 x 10(-5) g) on the renal blood flow (RBF) in anesthetized dogs. RBF was measured by an electromagnetic flow probe. Somatostatin did not influence blood pressure or heart rate. RBF exhibited a significant, dose-dependent fall (ranging from 11.6 +/- 11.9% to 31.9 +/- 17.3%), with a threshold at a dose of 5 x 10(-10) g. These results offer conclusive evidence for the contribution of somatostatin-induced direct renal vasoconstriction to its renal effects, in addition to the demonstrated modulation of other vasoactive systems and tubular functions.

Inhibition of diabetic nephropathy by a GH antagonist: a molecular analysis

Streptozotocin-treated C57B1/SJL mice developed glomerular hypertrophy and light microscopic lesions mimicking human diabetic glomerulosclerosis. In contrast, there were no glomerular hypertrophy and lesions in diabetic mice transgenic (TG) for a mutated growth hormone (bGH-G119K) that competes with native endogenous GH and results in dwarfism. We examined the molecular events underlying these findings. The non-transgenic (non-TG) diabetic mouse glomeruli had an increase in mRNA coding for alpha 1IV collagen, laminin B1, TGF-beta 1, 72 kDa collagenase, and TIMP-3. In contrast, glomerular type IV collagen and laminin B1 mRNA levels were normal in diabetic TG dwarf mice. However, the 72 kDa gelatinase, TIMP-3, and TGF-beta 1 mRNAs were elevated in the diabetic dwarfs. Type IV collagen and laminin accumulated in the glomeruli of diabetic non-TG, but not of diabetic dwarf mice, by immunofluorescence microscopy, confirming the mRNA data. GH binding protein mRNA levels were comparable in glomeruli from dwarf and non-TG mice, both diabetic and non-diabetic. We did not detect GH receptor mRNA in glomeruli. These data suggest that diabetic glomerulosclerosis is associated with an increase in type IV collagen and laminin synthesis, and that these changes do not occur in mice transgenic for bGH119K, a functional antagonist of GH. The increase of 72 kDa gelatinase, TIMP-3 and TGF-beta 1 mRNAs, independent of GH, suggested that these changes induced by hyperglycemia were not sufficient for the induction of glomerulosclerosis.