Antiserum to somatostatin-28 augments growth hormone secretion in the rat

To determine directly whether somatostatin-28 (S-28) physiologically regulates GH secretion, and what its contribution is relative to somatostatin-14 (S-14), we have compared the effects of immunoneutralization with a specific S-28 antibody with those of an anti S-14/S-28 serum on GH secretory dynamics in the rat. Plasma samples were obtained every 15 min for 7 h (1000-1700 h) from conscious, chronically cannulated rats after iv administration of 1 ml of one of the following sera: 1) rabbit anti S-28 (reacts with S-28, but not with S-14; maximum binding, 297 pmol/ml), 2) nonimmune rabbit serum, 3) sheep anti-S-14/S-28 serum (maximum binding, 9.4 nmol S-14 or S-28/ml), and 4) nonimmune sheep serum. A comparison of the mean integrated plasma GH levels during peak and trough periods showed significantly higher trough GH levels in both antibody-treated groups compared to those in the corresponding controls. In the anti-S-14/S-28-treated group, the elevation of trough period GH levels (40.5 +/- 3.5 ng/ml) represented a 3.25-fold increase (P less than 0.01) compared to the control value (12.5 +/- 1.5 ng/ml). In the anti-S-28-treated group, trough period GH levels (14 +/- 1.6 ng/ml) showed a 2.3-fold increase (P less than 0.01) compared to the control value (6.1 +/- 0.9 ng/ml). Mean peak period GH levels were 1.35-fold higher (P less than 0.05) than control values in the anti-S-14/S-28-treated group; anti-S-28 serum did not change mean peak GH levels. These data provide strong evidence that circulating S-28 (like S-14) physiologically regulates trough GH secretion and that the contribution of S-28 to GH inhibition is as important as that of S-14.