Development and validation of a specific radioimmunoassay for somatostatin in human plasma

Because of technical difficulties in measuring somatostatin in plasma, little is known about factors controlling its secretion in man, or of data on changes in circulating levels in various physiological or pathophysiological states. In the presence of plasma, binding of somatostatin by the addition of antibody was consistently decreased by about 20%, and this could not be abolished by addition of EDTA and aprotinin or by the use of specially prepared somatostatin-free plasma. Also, in the presence of plasma, endogenous somatostatin does not dilute in parallel with synthetic cyclic somatostatin standard. A radioimmunoassay for somatostatin was developed using prior extraction of the peptide on to leached silica glass. Tyrosine-11 somatostatin was iodinated using lactoperoxidase and purified on ODS silica. This method is superior to iodination using chloramine-T with CMC cellulose purification, and gives a highly purified preparation with a shelf-life of at least eight weeks. Using this tracer and a specific antiserum, the limit of sensitivity of the assay was 10 pg/ml, with an intra-assay coefficient of 12% (n = 16) and inter-assay coefficient of variation of 15% (n = 10). Parallelism has been demonstrated between standard synthetic cyclic somatostatin and all extracted plasma samples. The mean recovery of exogenous somatostatin from plasma was 78%. The fasting level of immunoreactive somatostatin at 0900 hours in 40 normal subjects ranged from 17 to 81 pg/ml. Care is needed, however, when comparing these values with those obtained from other laboratories since standard preparations of somatostatin vary considerably in their immunopotency. (author)