Blood oxygen partial pressure affects plasma prolactin concentration in humans

Responses of plasma prolactin (PRL) concentration to acute and repeated changes in blood oxygen partial pressure (Po 2a ) at rest were investigated in two studies (A; B), with special reference to possible effects mediated via serotonin (5-HT) synthesis. In A, nine male subjects inhaled for 105 min gas containing different oxygen fractions for 6 days. Gas concentrations consisted of 14% (A 14 ), 21% (A 21 ), 40% (A 40 ), 60% (A 60 ) and 80% (A 80 ) O 2 mixed with N 2 as well as 100% O 2 (A 100 ). Venous and capillary blood samples were drawn before and every 15 min during gas inhalation for analysis of plasma PRL and Po 2a . In B, two groups of subjects (B I; B II) were exposed to 30 min day -1 of gas inhalation over 14 consecutive days. Gas concentration consisted for B I of 14% O 2 /86% N 2 and for B II of 100% O 2 . During pre- and post examination a baseline blood sample was drawn, followed by a neuroendocrine test of serotonergic function using a partial 5-HT 1A receptor agonist (60 mg of buspirone hydrochloride). In A, each increase of inhaled oxygen fraction also resulted in higher blood Po 2a . In A 14 , A 21 and A 40 , plasma PRL concentrations did not change from basal level. Increases in plasma PRL concentration were found in A 60 after 30 min as well as in A 80 and A 100 after 15 min. A higher blood Po 2a induced a higher plasma PRL secretion but also an earlier decline from peak plasma PRL value despite continued inhalation of the respective oxygen concentration. During post-examination in B, basal plasma PRL concentrations were increased in B I and decreased in B II. Plasma PRL response to stimulation challenge was not affected by treatments. Thus, chronic adaptations of basal plasma PRL concentrations to decreased/increased blood Po 2a were not related to up/down-regulation, respectively, of central serotonergic receptor function.