Measurements of somatostatin-like immunoreactivity in plasma

Abstract

The validity of measurements of somatostatin-like immunoreactivity (SLI) in canine plasma has been examined. The radioimmunoassay employed had a minimal sensitivity of 50 pg/ml and a discriminatory sensitivity of 15 pg/ml above this. The coefficient of variation within and between assays was 6% and 18%, respectively. There was no cross-reaction with any of 9 other peptide hormones tested. Significant incubation damage by canine plasma to the [ 125I]-tyrosine 1-somatostatin tracer was effectively prevented by Trasylol and EDTA, and recovery of synthetic somatostatin added to canine plasma and incubated for 3 h at 20°C was 96%. Subcutaneous injections of somatostatin were followed by a prompt rise in plasma SLI that was significantly correlated with decrease in plasma glucagon levels. Serial dilutions of plasma specimens containing high levels of either endogenous SLI or injected synthetic somatostatin gave proportional readings and their slopes paralleled those of synthetic somatostatin in plasma-free buffer. Ninety-five percent of endogenous plasma SLI was removed by passage of plasma specimens through a column of immobilized somatostatin antibodies. Both endogenous SLI in plasma and synthetic somatostatin added to plasma were eluted in the void volume of Biogel P-10 columns, but following isolation by affinity chromatography the molecular size of both was approx. 1600, suggesting that both endogenous SLI and synthetic somatostatin circulate in plasma bound to larger molecules. It is concluded that this radioimmunoassay permits valid measurements of endogenous SLI in canine plasma.