Somatostatin (Somatotropin Release-Inhibiting Factor, SRIF)

Clinical Significance:
Somatostatin is a cyclic peptide originally isolated from sheep hypothalami and shown to inhibit the release of Growth Hormone.  Somatostatin is present primarily in three main forms: a 14 amino acid peptide, a 28 amino acid peptide (“Big” Somatostatin), and a 12,000 molecular weight ProSomatostatin.  This assay measures only the 14 amino acid form of Somatostatin. All three forms of Somatostatin have similar biological properties and overall potencies.  Somatostatin is a physiological regulator of Islet Cell and gastrointestinal functions, and is a suppressor of many pituitary hormones including Growth Hormone, Prolactin, and Thyrotropin (TSH).  Somatostatin levels are often elevated in diabetics, but the levels return to normal upon correction of the hormonal and metabolic deficiencies present.  In many cases of APUDomas including VIPoma, Insulinoma, Glucagonoma, and Gastrinoma, elevated levels of Somatostatin are found.

Reference Range:
Up to 25 pg/ml

Procedure:
Somatostatin is measured by direct radioimmunoassay.

Patient Preparation:
Patient should be fasting for 10 – 12 hours prior to collection of specimen.  Patient should not be on any medications that affect Insulin secretion or intestinal motility, if possible, for at least 48 hours prior to collection.

Specimen Collection:
Collect using EDTA “purple top” plasma tube OR 10mL blood in special ISI GI Preservative tube yielding special GI plasma and separate as soon as possible.  Freeze plasma immediately after separation.  Special G.I. Preservative tubes are available from Inter Science.  Minimum specimen size is 1 ml.

Important Precaution:
Somatostatin specimens must be collected using the G.I. Preservative or EDTA.  No other specimens are acceptable.

Special Specimens:
For tumor/tissue and various fluids (i.e. CSF, peritoneal, synovial, etc.) contact the Institute for requirements and special handling.

Shipping Instructions:
Ship specimens frozen in dry ice.

References:
1. SWJ Lamberts.  The Role of Somatostatin in the Regulation of Anterior Pituitary Secretion and Use of Its Analogues in the Treatment of Human Pituitary Tumors.  Endocrine Reviews 9:  417, 1988.

2. SE Shoelson, KS Polonsky, and T Nakabayashi.  Circulating Forms of Somatostatin-Like Immunoreactivity in Human Plasma.  American Journal of Physiology 250:  E428, 1986.