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Somatostatin Molecular Variants in the Vitreous Fluid

A comparative study between diabetic patients with proliferative diabetic retinopathy and nondiabetic control subjects

¹ Diabetes Research Unit, Endocrinology Division, Hospital Universitari Vall d’Hebron, Barcelona, Spain
² Department of Hormonology, Hospital Clínic, Barcelona, Spain
³ Department of Biochemistry, Hospital Clínic, Barcelona, Spain
⁴ Department of Ophthalmology, Hospital Universitari Vall d’Hebron, Barcelona, Spain

Address correspondence and reprint requests to Dr. Rafael Simó, Diabetes Research Unit, Endocrinology Division, Hospital Universitari Vall d’Hebron, Pg. Vall d’Hebron 119-129, 08035 Barcelona, Spain. E-mail: rsimo{at}vhebron.net

OBJECTIVE—There is growing evidence to indicate that somatostatin could be added to the list of natural antiangiogenic factors that exist in the vitreous fluid. In addition, a deficit of intravitreous somatostatin-like immunoreactivity (SLI) has been found in diabetic patients with proliferative diabetic retinopathy (PDR). In the present study, we have determined the main molecular variants of somatostatin (somatostatin-14 and somatostatin-28) in the vitreous fluid and plasma of nondiabetic control subjects and diabetic patients with PDR. In addition, the contribution of cortistatin, a neuropeptide with strong structural similarities to somatostatin, to SLI and its levels in vitreous and plasma in both nondiabetic and diabetic patients has also been measured.

RESERCH DESIGN AND METHODS—Plasma and vitreous fluid from 22 diabetic patients with PDR and 22 nondiabetic control subjects were analyzed. Somatostatin-14, somatostatin-28 and cortistatin were measured by radioimmunoassay but separation by high-performance liquid chromatography was required to measure somatostatin-14.

RESULTS—The predominant molecular form of somatostatin within the vitreous fluid was somatostatin-28 (fivefold higher than somatostatin-14 in control subjects and threefold higher in patients with PDR). Cortistatin significantly contributed to SLI and its intravitreous levels were higher than those detected in plasma (nondiabetic control subjects: 147 [102–837] vs. 78 [24–32] pg/ml; patients with PDR: 187 [87–998] vs. 62 [24–472] pg/ml; P = 0.01 for both). Intravitreous somatostatin-14 was similar in both subjects with PDR and the control group (P = 0.87). By contrast, somatostatin-28 concentration was lower in patients with PDR than in nondiabetic control subjects (350 ± 32 vs. 595 ± 66 pg/ml; P = 0.004).

CONCLUSIONS—Somatostatin-28 is the main molecular variant in the vitreous fluid. The intravitreous SLI deficit detected in patients with PDR is mainly due to somatostatin-28. Cortistatin is abundant in the vitreous fluid and significantly contributes to SLI. These findings could open up new strategies for PDR treatment.

Abbreviations: HPLC, high-performance liquid chromatography • PDR, proliferative diabetic retinopathy • RIA, radioimmunoassay • SLI, somatostatin-like immunoreactivity • SSTR, somatostatin receptor subtype

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