A Sensitive Radioimmunoassay for Immunoreactive Somatostatin in Extracted Plasma: Measurement and Characterization of Portal and Peripheral Plasma in the Rat*

Publisher Website
Google Scholar
Abstract
The RIA of immunoreactive somatostatin (IRS) in unextracted rat plasma has proved difficult because of marked tracer degradation. The presence of 0.1 ml plasma in standard curve tubes resulted in more than 80% damage to [Tyr1-125I]-somatostatin (SRIF) and only 2.7% binding compared to 38% in plasma free buffer. Sephadex gel filtration of [Tyr1-125I]SRIF after plasma incubation showed that most of the radioactivity was converted to a lower molecular weight fragment with no immunoreactivity. EDTA and Trasylol did not prevent [Tyr1- 125I]SRIF degradation. Destruction of [Tyr1-125I]SRIF was overcome by extraction of plasma with acid-ethanol in a simple 1-step procedure. With this technique, recovery of synthetic SRIF from plasma averaged 93.8%. Assay sensitivity was 30 pg IRS/ ml plasma. Exogenous SRIF incubated with rat blood was rapidly inactivated with a t/12 of 17 min at 37 C and 76 min at 22 C. The mean portal plasma IRS level was 310 pg/ml (n = 18; range, 106–570 pg/ml), in good agreement with the IRS content of pooled portal plasma of 346 pg/ml found by affinity chromatography. In 18 inferior vena cava (IVC) samples, the IRS level ranged between less than 30 and 44 pg/ml in 16 samples and 90 and 155 pg/ml in the remainder. On Sephadex G-25 columns, IRS in portal plasma extracts eluted as two approximately equal peaks, 1 in the void volume [big somatostatin-like immunoreactivity (SLI)] and 1 corresponding to SRIF. Big SLI accounted for 95% of IVC IRS. It is concluded that in the rat 1) SRIF and tracer SRIF are rapidly inactivated by blood, 2) portal plasma IRS is 7 times higher than IVC IRS, and 3) IRS in portal plasma exists as SRIF and big SLI; in IVC it exists mainly as big SLI.

This publication has 2 references indexed in Scilit: