The effect of sudden solar exposure on thermophysiological parameters and on plasma prolactin and cortisol concentrations in male Creole goats.

Rectal temperature (RT), respiratory rhythm (RR), plasma cortisol and prolactin (PRL) levels and haematocrit were measured at noon in male Creole goats during their habituation to shade, during sudden exposure to sunlight and then while they were kept outdoors. Data on the microclimatic environment, especially black-globe temperature (Tg), were recorded. On the day (DO) the bucks were put in the sun, the increase of RT and RR, characteristic of circadian rhythm, was amplified (RT: 40.84 +/- 0,23 degrees C vs 39.48 +/- 0.19 degrees C P less than 0.001; RR: 98 +/- 22 vs 35 +/- 3 respirations/min; P less than 0.01). Plasma cortisol also increased (17.7 +/- 8.6 vs 5.57 +/- 1.7 ng/ml; P less than 0.05). After 24 h (D1), plasma prolactin concentration increased dramatically (923 +/- 653 vs 90 +/- 22 ng/ml; P less than 0.05) and haematocrit decreased (28.6 +/- 2.3 vs 33 +/- 2.3%; P less than 0.05). To clarify the simultaneous effects of time (t) and Tg, we developed a model Log-normal on t and linear on Tg for RR and PRL. A model decreasing exponentially with t and linear with Tg was developed for RT; the determination coefficients were R2 = 0.96, 0.75 and 0.59, respectively. About 3 weeks later, after the adaptation period, RT, RR and plasma PRL stabilized; RT and RR returned to shade values while the PRL level remained higher than the shade PRL level (248 +/- 109 vs 130 +/- 50 ng/ml; P less than 0.05). The 24-hour time-lag in prolactin hypersecretion and the role of prolactin in thermoregulation are discussed.