Immunoreactive Somatostatin in Rat Hypophyseal Portal Blood: Effects of Anesthetics*

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Abstract
Immunoreactive somatostatin (IRS) was measured in whole hypophyseal portal blood of rats anesthetized with urethane, pentobarbital, or Althesin. The portal blood was collected directly into chilled 2 N acetic acid from the pituitary stalk stump after hypophysectomy. The acid-blood mixture was then extracted with acetone and washed with organic solvent. The mean (±SE) concentration and secretion rate of IRS in portal blood under urethane were 502.5 ± 52.9 pg/ml and 3.05 ± 0.42 pg/min, respectively, which were significantly higher than those under pentobarbital (113.1 ± 18.1 pg/ml and 1.09 ± 0.13 pg/min, respectively) or Althesin (155.4 ± 28.4 pg/ml and 1.43 ± 0.24 pg/min, respectively). The mean (±SE) IRS level in the jugular blood was 19.1 ± 4.7, 18.1 ± 6.7, and 15.5 ± 2.1 pg/ml under urethane, pentobarbital, and Althesin, respectively. Plasma GH levels were significantly lower in urethane and higher in pentobarbital- or Althesin-anesthetized rats than in decapitated rats. Somatostatin immunoreactivity of portal blood extracts separated into two peaks by gel filtration on Sephadex G-25 and into four peaks by ion exchange chromatography on a CM-Sephadex C-25 column. Each of these six peaks gave an inhibition curve parallel with that of synthetic somatostatin in the RIA system. These results suggest the following. IRS levels in hypophyseal portal blood are higher than those in systemic blood under the three different anesthetics examined and highest under urethane anesthesia. Low levels of plasma GH in urethane-anesthetized rats may be due to increased secretion of somatostatin from the hypothalamus into the portal blood. Somatostatin in hypophyseal portal blood is immunologically indistinguishable from synthetic somatostatin but is heterogeneous when examined by chromatography.