抄録
Unlike monogastric animals, ruminants derive most of their energy from shortchain fatty acids, mainly acetate, propionate and butyrate, which are the major end products of ruminal microbial fermentation of dietery carbohydrates in the rumen and which are absorbed from it. Therefore, the pattern of ruminant nutrition and intermediary metabolism differs in many respects from that of ordinary monogastric animals.
It was reported that infusion of several short-chain fatty acids stimulates insulin secretion in sheep and cow, but not in monogastric animals such as rats, rabbits and pigs, and that infusion of glucose was much less effective than that of short-chain fatty acids on plasma insulin secretion in sheep. It is generally accepted that propionic acid has a glycogenic function, whereas acetic acid and butyric acid have a ketogenic function in ruminants. However, it was reported on the other hand that intravenous injection of butyrate or propionate produces similar hyperglycemia, and butyrate gives rise to a greater release of insulin than propionate.
The present experiment was conducted to investigate the effect of infusion of several short-chain fatty acids on plasma insulin and glucose levels, and also to find a clue on the mechanism of insulin secretion in ruminants.
Crossbred ewes, weighing 40 to 50 kg, were used in the experiment. A saline solution of glucose, lactic, acetic, propionic, butyric, valeric, capronic and caprylic acids, respectively, neutralized up to pH 7.0 with sodium hydroxide in a final concentration of 2.5 M, were separately given intravenously within 2 minutes in a dose of 1.25 m moles per kg.
Blood glucose concentrations were not affected significantly after the injection of lactate, acetate or saline. After the injection of propionate, butyrate or caprylate, blood glucose concentration rose from approximately 40mg/dl to 70 mg/dl in all the cases within 20 minutes. Iso-valerate or capronate had a more potent effect on the hyperglycemic response than other short-chain fatty acids the highest glucose concentration was 90 to 95 mg/dl which had been attained within 15 minutes after the injections.
Plasma insulin level changed in a similar pattern to that of blood glucose concentration after administration of each substance. An injection of equimolar quantities of glucose produced a significantly smaller elevation in plasma insulin despite blood glucose load several times larger than seen after the infusion of the short-chain fatty acids. An insulin response by the injection of short-chain fatty acids was particularly striking in the case of iso-valerate or capronate.
As a result, it is indicated that the increase in plasma insulin level was associated with hyperglycemic responses by the short-chain fatty acids and that both responses increased with increasing chain length of the short-chain fatty acids.
Although the mechanism of the insulinogenic effect caused by administration of short-chain fatty acids remains unsolved, it is of significance on ruminant nutrition that short-chain fatty acids which represent the major energy source for ruminants are much more effective on insulin release rather than glucose.
