Measurement and characterization of somatostatin-14-like immunoreactivity in human urine

Timed urine collections were obtained from normal men and women for a study of somatostatin in urine. Somatostatin-14-like immunoreactivity (S-14 LI) was extracted from urine by adsorption to C-18 silica cartridges, and the extracts were analyzed by RIA directly or after gel and high pressure liquid chromatography (HPLC). S-14 LI was consistently detected in all urine samples. The mean S-14 LI content in 24-h collections in men [10.96 +/- 0.91 (+/- SE) pmol/24 h; n = 10; total volume, 1.96 +/- 0.09 L] was not significantly different from that in women (9.09 +/- 0.85 pmol/24 h; n = 10; total volume, 1.87 +/- 0.09 L). Gel chromatography of 24-h urine collections revealed three major peaks of S-14 LI coeluting with S-14, S-28, and a 12,000 mol wt species corresponding to prosomatostatin. HPLC analysis confirmed the presence of S-14, S-28, and the 12,000 mol wt form and additionally revealed a major fourth peak of 3,000 mol wt, closely related to S-28. Immunoreactivity corresponding to [Des,Ala1]S-14 (S-13) was identified by HPLC coelution with synthetic S-13 and reactivity in a centrally, but not N-terminally directed, S-14 RIA. The relative proportions of the different HPLC peaks varied considerably during the 24-h period. S-14 and S-28 were excreted preferentially during the day, whereas the 12,000 and 3,000 mol wt forms were excreted preferentially at night. The ingestion of a mixed meal evoked parallel increases in plasma S-14 LI and urinary S-14 LI excretion in six normal subjects. We conclude that the principal circulating forms of S-14 LI (S-14, S-28, S-13, and pro-S) are present in urine and that the measurement of urinary S-14 LI could provide a reliable index of integrated plasma S-14 LI concentrations.