Prolactin release in man: influence of cimetidine, thyrotrophin-releasing hormone and acute hypercalcaemia

The responsiveness of anterior pituitary lactotrophs and thyrotrophs to cimetidine (Cim) was investigated in healthy volunteers. Four-hundred mg Cim, injected iv, raised the serum prolactin level (Prl) from 14 +/- 2 to 58 +/- 9 ng/ml (P less than 0.001), but left the serum thyrotrophin level (TSH) unaffected. Acute hypercalcaemia, induced by iv infusion of calcium, blunted this Cim-elicited Prl response by 35 +/- 4% (P less than 0.01). Iv injection of 25 micrograms thyrotrophin-releasing hormone (TRH) had similar Prl-releasing potency as 400 mg Cim, and raised the Prl level from 14 +/- 1 to 51 +/- 6 ng/ml (P less than 0.001). In contrast to Cim, TRH also increased the TSH level significantly. Although oral pre-treatment with Cim for 3 days (1000 mg/day) failed to affect the Prl response to TRH in this study, iv injection of the drug more than doubled the above mentioned Prl response to TRH. The TSH response to TRH remained unaffected both by oral and by iv administration of Cim. These results imply that acute changes in serum calcium affect the release pattern of Prl, and that iv administration of Cim may add Prl-releasing power to TRH in healthy individuals.