In order to clarify the physiological role of somatostatin in insulin release, isolated rat pancreatic islets were treated with anti-somatostatin serum and challenged by glucose and leucine. The pancreatic islets were isolated from male Wistar rats with collagenase by the method of Lacy and Kostianovsky and subject to the following two models of experiments.
1] Ten islets were pre-incubated in 500 μl of Krebs-Henseleit bicarbonate buffer, pH 7.4, (KHBB) containing 3.3 mM glucose and 0.5% bovine serum albumin (BSA) for 20 minutes under a gas phase of 95% O2-5% CO2 at 37°C. They were subsequently transferred to a medium (KHBB) containing various concentrations of somatostatin ranging from O to 10 μg/ml, and incubated for 10 minutes in 3.3 mM and 16.7 mM glucose, under the same conditions as in the first incubation.
Insulin released in the medium at the last incubation was measured by radioimmunoassay using rat insulin as a standard. The results were expressed not only as actual measurements but also as percents of values of insulin release in the control group. Inhibition by somatostatin of 16.7 mM glucose-induced insulin release was observed at its concentration of 1 μg/ml and 10 μg/ml; the value was 59.5 ± 5.9% (n=6) and 37.9 ± 5.3% (n=6) of the control value at the former and the latter concentration of somatostatin, respectively. But no inhibitory effect by somatostatin was observed at 3.3 mM glucose.
2] Antiserum to somatostatin (cyclized form) was produced in rabbits according to the description of Arimura.
Five islets were incubated in 400 pl of KHBB and 100 μl of anti-somatostatin serum containing 0.5% BSA and 3.3 mM glucose for 60 minutes at 37°C under a gas phase of 95% O2-5% COO2. After the incubation they were rinsed three times in KHBB supplemented by 3.3 mM, 8.3 mM or 16.7 mM glucose in addition to 0.5% BSA, followed by another 60 minutes incubation under the same conditions as in the first incubation.
For the control studies, anti-somatostatin serum was replaced by normal rabbit serum, otherwise the procedure was identical with the experimental studies. Insulin released from the islets treated by anti-somatostatin serum was 414.1±54.0% (n=5) and 129.4 ± 4.8% (n=5) at 3.3 rnM and 8.3 mM glucose, respectively; both were significantly elevated compared with the control. However, 132.7 ± 7.0% (n=3) at 16.7 mM glucose was not significantly different from the control, although it was enhanced to a certain degree. Similarly insulin release induced by 10 mM 1-leucine and 10 mM 1-leucine plus 3.3 mM glucose was enhanced by prior treatment by anti-somatostatin serum, i.e; 606.1 ± 73.8% (n=7) and 439.4 ± 38.4% (n=7) respectively, as compared with control values, which were statistically significant.
These observations, besides the generally recognized findings of inhibition of insulin release by somatostatin, suggest that endogenous somatostatin plays a physiological role in the regulatory mechanism of insulin secretion.
