Effect of prolactin inhibition on thermoregulation, water and food intakes in heat-stressed fat-tailed male lambs

Abstract

AbstractTwo groups each of six lambs of the fat-tailed Naeimi breed, well adapted to a semi-arid climate, were housed under semi-controlled temperature (43·2 to 43·8°C) with low relative humidity (<0·8). After a 1-week adaptation period, the first group (control) was injected subcutaneously with 1 ml vehicle solution of 40% ethanol in saline, twice daily at 08.00 and 17.00 hfor the 11-day experimental period. In the other group, each lamb was injected daily for 6 days with 0·18 mg/kg per day of the prolactin inhibitor, 2 bromo-a-ergocryptine (CB154) dissolved in the vehicle solution. Treatment during the next 5 days was as for the control animals. Rectal temperature (RT), respiratory rate (RR), water and food intakes were recorded before injections. Daily average data were used in the statistical analysis, except those of the 1st day of bromocryptine injection. Lambs of the control group were able to maintain their RT (39·5 (s.e. 0·04) °C) during heat exposure, but those treated with the bromocryptine could not (40·3 (s.e. 0·07) °C). However, the latter group continued to respire more rapidly (P < 0·01) after cessation of the CB154 treatment, reducing their body temperature slightly (39·9 (s.e. 0·06) °C). This shows that, following prolactin suppression, heat-stressed lambs were unable to regulate their body temperature despite their RR. Water intake did not change during bromocryptine treatment, but the ratio of water to dry-matter intake did increase due to lower food consumption. These results suggest that prolactin might have an important role in thermoregulation of the fat-tailed male sheep during elevated ambient temperature.