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A sensitive radioimmunoassay for immunoreactive somatostatin in extracted plasma: measurement and characterization of portal and peripheral plasma in the rat

, : A sensitive radioimmunoassay for immunoreactive somatostatin in extracted plasma: measurement and characterization of portal and peripheral plasma in the rat. Endocrinology 107(1): 306-313

The radioimmunoassay of immunoreactive somatostatin (IRS) in unextracted rat plasma has proved difficult because of marked tracer degradation. 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 [Thy1-125I]SRIF after plasma incubation showed that most of the radioactivity was converted to a lower MW fragment with no immunoreactivity. EDTA and Trasylol did not prevent [Tyr1-125I]SRIF degradation. Destruction of [Tyr-1-125I]SRIF was overcome by extraction of plasma with acid-ethanol in a simple one-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 half-time of 17 min at C and 76 min at 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 2 approximately equal peaks, 1 in the void volume [big SRIF-like immunoreactivity (SLI)] and 1 corresponding to SRIF. Big SLI accounted for 95% of IVC IRS. Apparently SRIF and tracer SRIF in the rat are rapidly inactivated by blood, portal plasma IRS is 7 times higher than IVC IRS, and IRS in portal plasma exists as SRIF and big SLI; in IVC it exists mainly as big SLI.


PMID: 6103804

DOI: 10.1210/endo-107-1-306

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