Sensitive radioimmunoassay for somatostatin using N-[125I]-Tyr-somatostatin as labelled antigen

Effects of different radioligands on the antigen binding specificity of somatostatin antisera

The regions of the somatostatin (SRIF) molecule recognized by five antisera were systematically studied using three radioligands (125I-N-Tyr-SRIF, [125I-Tyr1]-SRIF and [125I-Tyr11]- SRIF) and SRIF analogs containing sequential substitutions with alanine or tyrosine. Antisera SA and moreso SB had N-terminal specificity when used with [125 I-Tyr11]-SRIF but central molecular specificity when studied with the two N-terminal radiolabelled analogs. The N-terminal and central specific populations of antibodies in antiserum SB were separable by immunoaffinity adsorption using immobilized [Tyr1]-SRIF. It is of practical significance that the same antiserum (SB) could be used with different radioligands to perform N-terminal and central specific radioimmunoassays (RIAs). The central specific RIA detected SRIF-14 and SRIF-28 on an approximately equimolar basis whereas the N-terminal specific RIA was selective for SRIF-14.

On the accuracy of radioimmunological determination of somatostatin in plasma

We performed the following experiments to evaluate the accuracy of our newly developed radioimmunoassay for somatostatin: (1) Recovery of synthetic somatostatin added to human, porcine, and canine plasma with or without extraction with 67% acetone or 76% ethanol, using 3 different region-specific antibodies and, where applicable, 125I-labelled Tyr-1- or Tyr-11-substituted somatostatin or 125I-N-Tyr-somatostatin as tracers. The recovery of somatostatin corrected for losses inherent in the extraction procedure was close to 100%, and independent of species, antibody and tracer. Somatostatin 1-28 was extracted slightly less efficiently. Unextracted plasma interfered massively in the assay. (2) Pharmacokinetic experiments with infusion of somatostatin into 14 pigs and determination of metabolic clearance rate (MCR) and T-1/2. MCR was 27-38 ml/kg per min, independent of infusion rate (6.1 or 13 pmol/kg per min), extraction procedure or tracer. T-1/2 was 1.9 min. The infused somatostatin was not measurable in unextracted plasma. (3) Characterization of endogenous and exogenous, labelled and unlabelled somatostatin 1-14 in human plasma, using Sephadex G-50 columns at pH 7.5 and 9.0. Human plasma showed excess immunoreactivity eluting at the void volume whereas synthetic somatostatin was recovered quantitatively at the position of marker somatostatin when added to the plasma. The immunoreactivity of the tracers was decreased (125I-Tyr-11-somatostatin) or abolished (125I-N-Tyr- or 125I-Tyr-1-somatostatin) after incubation with plasma or void volume fractions of plasma subjected to gel filtration. Extracted plasma did not contain void volume immunoreactivity, but like whole plasma, small amounts of components which coeluted with intact somatostatin.

Regional distributions of somatostatin and cholecystokinin-like immunoreactivities in rat and bovine brain

The distribution of somatostatin- and cholecystokinin octapeptide (CCK-8)-like immunoreactivities in the central nervous system of bovine and rat has been studied using a sensitive and specific radioimmunoassay. The most interesting information is the high concentration of CCK-8 in the various cortical areas in rat and bovine. The nuclei of amygdala contain the highest amount of octapeptide in rat while in bovine brain, this structure contains the second highest concentration after polus frontalis.

Immunohistochemical localization of somatostatin in the human hypothalamus

In order to identify clearly the nervous structures containing somatostatin in the human hypothalamus, an immunohistochemical localization of this neurohormone was performed at light-microscopic level. Using a antiserum specific to somatostatin and the unlabeled antibody peroxidase-antiperoxidase technique, we have found somatostatin in neurons with cell bodies in an area in the anterior hypothalamus corresponding to the infundibular nucleus. Somatostatin-containing fibers were also detected in the neurovascular zone of the pituitary stalk, suggesting that somatostatin is released in that region to reach the capillaries in the pituitary portal plexus. A large bundle of somatostatin fibers extending from the anterior part of the paraventricular nucleus up to the posterior portion of the mammillary bodies has also been detected. The role of these fibers still remains to be clarified.