275 zyxwvutsr Clinica Chimica Acta, 87 (1978) 275- 283 @ Elsevier/North-Holland Biomedical Press CCA 9550 MEASUREMENTS OF SOMATOSTATIN-LIKE IMMUNOREACTIVITY IN PLASMA VIRGINIA HARRIS, J. MICHAEL CONLON, COIMBATORE B. SRIKANT, KAY McCORKLE, VOLKER SCHUSDZIARRA, ELI IPP and ROGER H. UNGER * Veterans Administration Hospital and the University of Texas Southwestern M edical School, Department of Internal M edicine, 5323 Harry Hines Blvd., Dallas, Texas 75235 (U.S.A.) (Received January 13th, 1978) Summary 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 hor- mones tested. Significant incubation damage by canine plasma to the [l*‘I]- tyrosinel-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 fol- lowed by a prompt rise in plasma SLI that was significantly correlated with decrease in plasma glucagon levels. Serial dilutions of plasma specimens con- taining high levels of either endogenous SLI or injected synthetic somatostatin gave proportional readings and their slopes paralleled those of synthetic soma- tostatin 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 somato- statin 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. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA * To whom correspondence should be addressed.