Pancreas as a Major Site of Somatostatin Metabolism

Abstract

The metabolic late of ¹²⁵I-Tyr¹¹-somatostatin was studied using isolated perfused canine pancreas. When 6.7nCi/min of ¹²⁵I-Tyr¹¹-somatostatin (specific activity, 750μCi/μg) was perfused with or without cold cyclic somatostatin (4ng/ml and 400ng/ml), the recovery of total radioactivity in the venous effluent was the same under the three conditions (ca.75%).The recovery of radioactivity precipitated with 10% trichloracetic acid was 43.5%(without cold somatostatin), 41.8%(with 4ng /ml of cold somatostatin) and 49.3%(with 100 ng/ml of cold somatostatin);the values were significantly increased by the addition of cold somatostatin.The ratio of TCA-precipitable radioactivity to the total radioactivity in the venous effluent was 36.9% in the first 1 min, that of pre-perfusion was 56.1%, and gradually decreased during the perfusion.¹²⁵I-Tyr¹¹-sontatostatin was not degraded by the effluent perfusate.These data suggest that somatostatin is very efficiently degraded by the pancreas via two different processes: instantaneous degradation without uptake, and gradual degradation after its uptake.
The high capacity of the pancreas in somatostatin metablism was also confirmed in experiments, where ¹²⁵I-Tyr¹¹-somatostatin was injected into rats front the femoral artery with or without 2 mg of cold cyclic somatostatin. The specific uptake of radioactivity by the pancreas, which. was estimated from the difference in radioactivities in tissues removed 5 min after the injection in the absence and presence of cold somatostatin, was three times higher than that of the stomach, small intestine and liver.