Effects of β-casomorphins on metabolism of dairy cows

1. The studies presented in this thesis investigated some metabolic actions of beta-casomorphins in dairy cows. Three beta-casomorphins (beta-casomorphin-4-amide, -5 and -7) were used. Investigations of the actions of the beta-casomorphins in dairy cows require gram quantities of each peptide. This was achieved by synthesising the peptides using solid phase peptide synthesis combined with FMOC chemistry. 2. The first experiment examined possible stimulatory actions of the peptides on amino acid uptake by the mammary gland using explants of lactating rat mammary glands as a model in vitro system. There was no evidence of their actions on amino acid uptake by the mammary gland as judged by the lack of their effects on the uptakes of four amino acids (glutamate, histidine, leucine and lysine). Because of the inability to demonstrate effects of the peptides at the mammary gland level, it was decided to focus on their effects at the level of the gut. 3. As a first step, the extent of ruminal degradation of the beta-casomorphins was determined in incubations in rumen liquor in vitro. All three peptides were degraded rapidly with half-lives of only 15 to 20 minutes. In a subsequent experiment, an attempt was made to chemically protect the peptides from the ruminal degradation using N-terminal acetylation. The half-lives of the N-acetyl beta-casomorphins were markedly increased to 6 1/2, 7 1/2 and 4 hours for beta-casomorphin-4-amide, -5 and -7 respectively. However, it was recognised that, whilst the method would be a simple and effective method for the protection of the peptides, further developments of the method would be necessary to ensure their release in active form at their sites of their action in vivo. 4. In experiments on the actions of the beta-casomorphins in vivo, the peptides were infused direct into the abomasum. The first experiment compared hormonal responses to the abomasal infusion of sodium caseinate, a potential source of beta-casomorphins, and an acid hydrolysate of casein, which was not a potential source of beta-casomorphins. There were clear suggestions of differences in responses of some hormones, notably insulin and GIP. In the second experiment, effects of abomasal infiisions of mixtures of the ?- casomorphins themselves at three different dose levels on changes in concentrations of the hormones were investigated in lactating dairy cows. There was no clear effect of the beta-casomorphin infusions on insulin concentrations except for some tendency towards inhibition at some time points. However, the incremental response of glucagon to the beta-casomorphin infusions was linearly increased (P < 0.05) leading to statistically significant decreases in the insulin / glucagon ratio at 4 hours by all beta-casomorphin infusions compared with control. 5. In the final section of the thesis, attention was focused on the possible inhibitory effects of the beta-casomorphins on insulin secretion. Three experiments were carried out. Experiment 1 was designed to detect the inhibitory action of the beta-casomorphins on the insulin level prestimulated by an abomasal infusion of glucose. The insulin concentration rise was significantly inhibited by the beta-casomorphins (P < 0.05). The inhibitory action of beta-casomorphin was shown to be compatible with the action of SS- 28 as judged from the effects of SS-28 on the insulin secretion when administered intravenously in the same experiment. However, the inhibitory action was not evident in the next experiment in which lactating animals were used, probably because of differences in the sensitivity of insulin secretion in lactating versus non-lactating animals. The final experiment confirmed the insulinopenic effect of the ?- casomorphins by demonstrating their inhibitory action on the insulin concentrations prestimulated by an intravenous infusion of glucose. 6. Taken together, the results of the experiments described show effects of beta-casomorphin infusions on circulating levels of hormones in the ruminant. The most pronounced effect was the modulation of the insulin response to abomasal or intravenous administration of glucose.