Effects of mild heat exposure and suppression of prolactin secretion on gastro-intestinal tract function and temperature regulation in sheep

Abstract

The effects of mild heat exposure (30°C; 30% relative humidity), and of the suppression of prolactin secretion under such conditions, were studied in anoestrous ewes given daily 791 g dry matter of a pelleted mixture of lucerne hay and oat grain (3: 2). 51Cr-EDTA, 103Ru-phenanthroline and lignin were used to determine mean retention times (MRT) in the gastro-intestinal (01) tract by a continuous infusion-total sampling procedure. Mild heat exposure reduced the digesta-free tissue weight of all 01 tract segments distal to the omasum. Increases in the amounts of digesta in the stomach compartments were largely due to increases in water content, although the solids content of the abomasum (P < O · 1 0) and omasum also increased. The treatment caused an increase in water intake (P < 0 ·10), increased the MRT of 51Cr-EDTA in the reticulo-rumen (P < 0 ·05), and tended to increase the MRT of all three markers in the omasum and abomasum. However, MRT in the whole 01 tract was unaffected because of a compensating decrease (P < 0·01) in digesta MRT in the distal large intestine. Suppression of prolactin secretion with 2-bromo-α-ergocryptine impaired the ewes’ ability to maintain their body temperature under the warm conditions imposed. This was associated with a consistent reduction in the degree to which water intake and its transactions in the 01 tract changed in response to mild heat exposure. The treatment also caused decreases in omasal MRT and increases in abomasal MRT. Increased plasma concentrations of somatostatin and gastrin were associated with reduced plasma concentrations of prolactin, and it was postulated that some of the effects of prolactin on the GI tract may be mediated via somatostatin. © 1986 ASEG.