Characterization of somatostatin in peripheral and portal plasma in the duck: in-vivo metabolism of somatostatin-28 and -14.

A sensitive and specific radioimmunoassay without an extraction step was developed for somatostatin in duck plasma. Degradation of Tyr1-125I-labelled somatostatin-14 (S-14) averaged 2% for blood collected with EDTA and zymofren. Recovery of somatostatin-like immunoreactivity (SLI), added to the plasma, averaged 91% for S-14 and 86% for S-28. Chromatographic analysis of portal plasma on Sephadex G-25 showed three peaks: one peak coeluted with cytochrome c (mol. wt 12500) in the void volume and was called 'big' somatostatin; of the two smaller forms, one coeluted with synthetic S-28 and the other with synthetic S-14; these were immunologically and physicochemically indistinguishable from synthetic S-28 and S-14. In peripheral plasma only the large form of somatostatin, 'big' somatostatin, was found. The mean portal plasma concentration of SLI was 4.1 +/- 0.41 microgram/l (n = 11, range 2.8-5.1 micrograms/l). In peripheral plasma the SLI concentration was 1.05 +/- 0.45 microgram/l (n = 11, range 0.84-1.2 microgram/l). The metabolic clearance rate, distribution volume and calculated half-life values were 63.1 +/- 14 ml/kg per min, 40.9 +/- 8.9 ml/kg and 1.06 +/- 0.19 min for S-14 compared with 45.7 +/- 7 ml/kg per min, 14.8 +/- 2.5 ml/kg and 2.14 +/- 0.54 min for S-28. These results indicated that S-28 was cleared from plasma at a slower rate than S-14 in the duck. It was concluded that: (1) portal plasma SLI was four times higher than peripheral SLI; (2) SLI in portal plasma existed as 'big' somatostatin, S-28 and S-14, whereas in peripheral plasma it existed mainly as 'big' somatostatin; (3) in-vivo studies indicated that S-28 was metabolized less rapidly than S-14.