Autofeedback regulation of prolactin (PRL) secretion: effect of prolactin before suckling on the subsequent nursing-induced release of PRL in the lactating rat.
PRL can have an inhibitory effect upon its own release. The present investigation assessed quantitative and temporal relationships among exogenously raised systemic PRL concentrations, PRL discharged in situ, and PRL negative feedback control of PRL release. Lactating rats were administered PRL in amounts designed to increase systemic plasma PRL concentrations to levels that either exceed (3 mg ovine PRL injected sc) or fall within (five iv injections of 400 or 800 ng rat PRL given at 1-min intervals) the range of PRL concentrations normally produced by suckling stimulation. The 800-ng iv or 3-mg sc dose of PRL did not block the suckling-induced rise of plasma PRL concentration when given 30 min before suckling. When administered 4 h beforehand, however, 3 mg PRL reduced both the suckling- and ether-stimulated rises of plasma PRL. Although the 400- or 800-ng iv dose of PRL increased systemic plasma PRL concentrations to levels (280 and 389 ng/ml, respectively) that approximate those of the suckled rat, neither dose inhibited the suckling-induced release of PRL when given 4 h before nursing. These data indicate that high concentrations of PRL are required before PRL from the systemic circulation can activate PRL autoregulatory mechanisms, and that these mechanisms are not activated as an immediate response to rising plasma PRL titers. The suckling-induced release of PRL was also inhibited by 3 mg PRL given 16 h before nursing. The PRL discharged in situ by 15 min of nursing was not effective, however, in blocking the suckling-induced release of PRL 16 h later. Since large amounts of PRL can presumably reach PRL regulatory mechanisms during in situ release via retrograde pituitary secretion, we conclude that the functional significance of autofeedback control of PRL release may be limited to conditions where PRL secretion is prolonged or at an abnormally high rate.
D000818:Animals; D005246:Feedback; D005260:Female; D007700:Kinetics; D007774:Lactation; D011247:Pregnancy; D011388:Prolactin / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D012756:Sheep; D013045:Species Specificity
D011388:Prolactin