Somatostatin (Growth hormone-inhibiting hormone, Somatotropin release-inhibiting hormone)

• Overview:  somatostatin

  • 14- and 28-amino acid peptide forms, most widely distributed of the hypothalamic releasing hormones

    • 14-amino acid peptide form: more abundant, less bioactive in GH inhibition (compared to 28-amino acid isoform)

  • Localization: hypothalamus and other CNS locations

    • Periventricular and medial pre-optic areas of anterior hypothalamus

    • Neurosecretory granules at nerve terminals in the median eminence

    • Serves as neurotransmitter in the spinal cord, cerebral cortex, brain stem -- in addition to hormonal action

    • Also gastrointestinal and pancreatic location

      •  Pancreatic D cells (somatostatin-secreting) -- insulin and glucagon regulation (paracrine action)

  • Inhibits growth hormone release

    • Also decreases GH responds to secretagogues without altering GH mRNA levels

    • Somatostatin lowers serum TSH in response to TRH

  • Precursor: Prosomatostatin

  • Pharmacokinetics: somatostatin

    • Half-life-- 1-3 minutes

    • Significant renal metabolism

    • Renal excretion

    • Limited clinical usefulness due to short duration of action and multiple effects on many secretory processes

• Overview: octreotide:

  • Somatostatin analog -- longer plasma elimination half-life (80 minutes)

  • > 40X more potent than somatostatin in inhibiting growth hormone release

  • Only 2X more potent in decreasing insulin secretion

• Clinical Use: octreotide

  • Relatively small effect on insulin secretion (compared to somatostatin) allows clinical use without concern for inducing hyperglycemic states

  • Octreotide used to manage:

    • Acromegaly, thyrotropin-secreting pituitary adenomas and carcinoid tumors

    • Acute bleeding control: esophageal varices

•   Adverse Effects: octreotide

  • Gastrointestinal disturbances

  • Biliary sludge; gallstones (frequency: 20-30% ,> 6 months treatment; symptomatic gallstone exhibit a yearly frequency of about 1%)

Primary Reference: Fitzgerald, P.A. and Klonoff, D.C. Hypothalamic and Pituitary Hormones, in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, pp 603-618.

Primary Reference: Biller, Beverly M. K. and Daniels, Gilbert, H. Neuroendocrine Regulation and Diseases of the Anterior Pituitary and Hypothalamus, In Harrison's Principles of Internal Medicine 14th edition, (Isselbacher, K.J., Braunwald, E., Wilson, J.D., Martin, J.B., Fauci, A.S. and Kasper, D.L., eds) McGraw-Hill, Inc (Health Professions Division), 1998, pp 1972-1998

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