Studies of TSH and GH releasing activities in plasma from pituitary stalk blood have been conducted. TSH and GH released from anterior pituitary halves incubated in vitro with either portal or peripheral plasma were measured by means of specific radioimmunoassays. Relative to peripheral plasma, portal plasma resulted in 161 % increase in TSH secretion and 149 % increment in GH secretion. Electrical stimulation of the anterior hypothalamus further augmented TSH release to 201 %, but no further GH release was observed after electrode stimulation. Prior treatment with triiodothyronine did not abolish the influence of electrical stimulation upon TSH releasing activity. Both TSH and GH were also identified in portal plasma. It was estimated that these tropic hormones, presumed to originate from pars tuberalis, could contribute as much as 5 % of circulating TSH and GH in the periphery of intact rats.
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