Kallidin stimulates prolactin secretion from individual rat anterior pituitary cells

Kinins are localized within the adenohypophysis where they have been shown to stimulate the release of pituitary hormones. In the present study we have investigated the effect of [Lys]-bradykinin (kallidin) on prolactin secretion at the single cell level from cultured male rat anterior pituitary cells. This was assessed by use of a reverse haemolytic plaque assay which permits quantitative evaluation of the proportion of all pituitary cells which are secreting prolactin, and the amount of prolactin secreted per lactotroph (plaque area). The rate of plaque development was used as an index of the rate of hormone secretion in time-course studies. Kallidin induced a dose-dependent increase in both the percentage of plaque-forming cells and the median plaque area during the first 2 and 3 h of incubation respectively. The threshold concentration of kallidin was 10 nmol/l. After 4 h of kallidin stimulation there was no difference between treated and control monolayers with respect to median plaque area and the total secretion index. Although recruitment of additional cells into the secretory pool cannot be excluded, this seems unlikely since at 3 and 4 h little or no difference was observed in the number of plaque-forming cells. The data suggest that initially kallidin accelerated the rate of prolactin secretion primarily by inducing an increase in the number of cells secreting prolactin, and subsequently by increasing the amount of hormone secreted per lactotroph. The results presented here are consistent with the proposed role of the kallikrein-kinin system in the paracrine or autocrine control of prolactin release from the pituitary gland.