INVITRO AND INVIVO STUDIES OF EQUINE PROLACTIN SECRETION THROUGHOUT THE YEAR

In vitro, the prolactin response of perifused anterior pituitary cells of horses to thyrotrophin-releasing hormone (TRH) (0.1, 1.0, 10 and 100 nM), correlated significantly with hours of daylight (P < 0.01). Baseline concentrations of prolactin also were significantly correlated with daylength (P < 0.01). When response and baseline data were fitted by nonlinear least squares to a cosine function, the circannual phase was -0.06072 (+/- 0.02170) and -0.05560 (+/- 0.0255), respectively, which are not significantly different from that of daylength. In vivo, prolactin secretion was measured in jugular plasma and pituitary venous effluent. Prolactin secretion by the pituitary (20- to 30-sec samples) showed pulses of 2-7 min duration above a varying baseline. Jugular prolactin concentrations correlated significantly with daylength (P < 0.01, n = 20). The fractional plasma clearance rate was 2.50 x 10(-2) and 3.31 x 10(-2)/min in 2 horses. Seasonal changes in prolactin secretion in vivo and in vitro may reflect the amount of prolactin available for release.