Diurnal Patterns of Plasma Insulin, Growth Hormone, Corticosteroid and Metabolite Concentrations in Fed and Fasted Sheep

Abstract

To determine the effects of feeding on plasma concentrations of insulin and growth hormone (GH) in sheep during the full 24 h of the normal feeding cycle, two experiments have been conducted on a group of 18 crossbred wethers.The eating once daily of 800 g of a lucerne chaff-oat grain (1 : 1) diet resulted in rapid increases in plasma insulin and rapid decreases in plasma GH concentrations, with maxima and minima respectively 2-4 h after feeding, in groups of six sheep in both experiments. After this, insulin and GH levels returned towards prefeeding values. The eating of larger amounts of the same diet did not greatly alter the magnitude of changes in insulin, but tended to prolong the period when they were high. An extra 800 g of food 12 h after the usual feed resulted in a repetition of the normal changes after feeding, an increase in insulin and a decline in GH concentrations. In wethers fed 1200 g of food daily or fed ad libitum, GH levels tended to be lower during the full 24 h. The withholding of food for 24 h caused a steady decline in insulin levels, but GH levels did not increase further above prefeeding values in sheep fed 800 g daily. There were large oscillations in GH levels of sheep that were fasted or fed 800 g daily.Corticosteroid concentrations in plasma fluctuated during the day, but there was no influence of feeding on them. There was no evidence for a circadian rhythm.Maximum plasma insulin and minimum GH concentrations occurred at about the same time as maximum concentrations of volatile fatty acids and minimum concentrations of free fatty acids in plasma after feeding, but there was no close correspondence between changes in metabolite and hormone concentrations. Maximum glucose concentrations occurred later after feeding than did those of insulin. There were only small changes in a-amino nitrogen concentrations related to feeding.It is doubtful whether changes in plasma metabolite levels after feeding play a major role in determining the changes in insulin or GH release during the day.

Effects of Exogenous Growth Hormone on Milk Production and Nutrient Uptake by Muscle and Mammary Tissues of Dairy Cows in Mid-lactation

“…1 that plasma concentrations of OH were low and stable at c. 4 /-tg/l throughout the second day of each control period. This observation is consistent with the cows being fed adequately and continuously, as others have shown raised plasma OH in underfed animals (Trenkle 1971;Bassett 1974;Oow et al 1981) and marked fluctuations in plasma OH associated with intermittent feeding (Tindal et al 1982). During treatment with OH, plasma concentrations of the hormone were increased significantly for much of each day.…”

Daily variation in blood enzymes and metabolites in ewes under three levels of feed intake

“…1) during the 24-h period showed a relative stability in all groups, except for the period from 03:00 to 07:00 h in groups 2 and 3 where values were higher (P < 0.05). In general, this is in agreement with findings of others (Bassett 1972(Bassett , 1974Borrebaek et al 1990;Amaral-Phillips et al 1993). The comparison between groups indicates a direct relationship between the level of food intake and the plasma concentration of GLU, although in terms of overall means (mean of values from the eight blood collection times in each group; Table 4) significant differences (P < 0.05) were only detected between group 1 and groups 2 and 3.…”

Pattern of Cortisol Release in Sheep following Administration of Synthetic ACTH or Imposition of Various Stressor Agents

“…The response to shearing and yarding in terms of ACTH equivalents would be slightly underestimated in the present study as the level of cortisol at termination of sampling had not returned to pre-treatment values. The lack of response to fasting is similar to results reported by Reid and Mills (1962) and Bassett (1974) in sheep but contrasts with results of other work in sheep (Purchas 1973) and in other species (Slater 1962;Alleyne and Young 1967). It appears that the response will depend on the severity of the fast-24 h probably being only mild in sheep-and previous feeding regime, i.e.…”