Growth hormone-releasing factor-like immunoreactivity (GRF-LI) was detected in partially purified extracts of rat hypothalamus and duodenum with a highly specific RIA for rat GRF (rGRF). The concentration of rGRF-LI in the rat hypothalamus was 32.5 fmol/mg dry weight (455 fmol or 3.18 ng/region), while the rat duodenum contained 2.15 fmol/mg dry weight (111 fmol or 0.78 ng/region). Partially purified hypothalamic and duodenal extracts exhibited displacement of the tracer parallel to rGRF. GRF-LI levels in all other brain regions (brainstem, cortex) or gastrointestinal tract tissues (forestomach, antrum, ileum, colon, pancreas) measured were close to the detection level of the RIA. rGRF-LI in hypothalamic and duodenal extracts behaved like synthetic rGRF in both gel filtration and reverse-phase HPLC systems. Coelution of rGRF-LI contained in both these tissue extracts with synthetic rGRF suggests that these molecules are similar, if not identical.
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Bruhn et al. (1985) studied this question.