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The in vivo bioactivity of ectopic growth hormone-releasing factor (GHRF) was examined in estrogen-primed, urethane-anesthetized male rats bearing intracarotid catheters. Ectopic GHRF was isolated from a carcinoid tumor metastatic to the liver from a patient with gigantism and elevated plasma growth hormone (GH) levels and was partially purified by reverse phase high performance liquid chromatography for intracarotid injection. Ectopic GHRF elicited a promt rise in plasma GH which peaked at 5 min. The time course of the response resembled that to prostaglandin E1, a known potent direct stimulator of GH secretion in vivo. The effect of ectopic GHRF was dose related in the range of 10-100 U/rat which represents approximately 30x the effective in vitro dose range/10(5) cells in rat adenohypophyseal cell cultures. The in vivo response to ectopic GHRF was completely blocked by the simultaneous intracarotid administration of the GH release inhibitory peptide, somatostatin. These findings lend further support for a role of GHRF from human tumors as a potent physiological stimulator of GH release.
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