Growth hormone releasing hormone (GH RH) activity in Sephadex G 25 fractions of porcine stalk median eminence (SME) extracts was examined in vivo by infusing these samples into a rat hypophyseal portal vessel. The increment of immunoreactive GH levels in the serum was used as the index for GH RH activity. The GH RH activities were found in two different locations: in the early fractions Nos. 3 to 4, and in somewhat retarded fraction No. 7. These GH RH activities were not due to TRH (thyroid releasing hormone), vasopressin, or potassium. The location of luteinizing hormone releasing hormone (LH RH) and prolactin release inhibiting hormone (PR IH) determined in this in vivo system was in agreement with those found in other in vivo and in vitro assay systems for LH RH and PR IH, respectively. These results help validate this assay system.
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