Quarterly Journal of Experimental Physiology 69–3 (1984) On page 448 the sentence 3 lines from the bottom should read The sheep were fitted with a re‐entrant cannula in the duodenum close to the pylorus and digesta was collected through this cannula for 30 h after dosing, unlabelled donor material being used to replace that removed.

Aberdeen AB2 9SBStudies in recent years have suggested that the absorption of many of the major mineral elements from the gut may be under greater regulatory control than has hitherto been suspected. In this paper we consider the evidence for such control in ruminants with particular reference to calcium and phosphorus absorption. Ca absorptionThere is now clear evidence that both sheep and cattle absorb Ca from their gut according to need and that they can alter the efficiency of absorption to meet a change in requirement. For example, Braithwaite & Riazuddin (1971) have shown that young sheep with a high Ca requirement absorb Ca at a higher rate and with greater efficiency than mature animals with a low requirement. An increase in absorption and an increased efficiency of absorption also occurs in mature sheep when their requirement for Ca is increased through pregnancy or lactation (Braithwaite et al. 1969; 1970) or after a period of Ca deficiency (Braithwaite, Studies in cattle have given similar results. Thus the efficiency of absorption of Ca in the small intestine of the dairy cow has been shown to rise in response to a reduction in dietary Ca intake and to the onset of lactation (Ramberg et al. 1970; van't Klooster, 1976). The amount of Ca absorbed has also been shown to be directly related to milk production (van't Klooster, 1976) though in early lactation when the demand for Ca is greatest the increase in absorption falls short of requirement (Symonds et al. 1966; van't Klooster, 1976)' the deficit being met by increased bone resorption (Braithwaite et al. 1969).The mechanism by which Ca absorption is adjusted in response to requirement has also received much attention and Fig.

SUMMARYMature sheep fitted with rumen and duodenal cannulae and fed either pelleted hay or grass diets were given supplements of Na2HPO4 by continuous infusion into the rumen and the effects on salivary phosphorus secretion, intestinal phosphorus absorption and pathway of excretion were studied. Little phosphorus was excreted in the urine in control periods and little increase was seen in response to phosphorus supplementation, most of the extra phosphorus given being recovered in the faeces. Infusion of phosphorus into the rumen led to an increase in the flow of phosphorus to the intestine, an increase in absorption, a rise in plasma phosphate level and an increase in salivary phosphorus secretion while withdrawal of supplementary phosphorus was seen to have the opposite effects. In separate studies short sections of the upper small intestine temporarily isolated from the normal flow of digesta were perfused with solutions containing NaH2PO4 (5-50 mmol.1-1). Phosphorus absorption increased with increasing concentration though the relationship was curvilinear such that absorptive efficiency fell from around 0-7 at 5 mmol . 1-1 to around 0-35 at a concentration of 50 mmol. 1-1. The significance of these results in relation to the control of phosphorus balance in ruminants is discussed.

SUMMARYMature sheep fitted with rumen and duodenal cannulae and fed either pelleted hay or grass diets were given supplementary phosphorus by continuous intravenous infusion and the effects on salivary phosphorus secretion, intestinal phosphorus absorption and pathway of excretion were studied. In control periods little phosphorus was excreted in the urine, the faeces being the major pathway for excretion. Infusion of phosphorus increased both urine and faecal phosphorus excretion though in sheep fed the hay diet the urine response was much less marked than in those fed the grass diet. This was not due to differences in plasma phosphate levels but rather to differences in renal tubular reabsorptive efficiency, though the reason for this is at present unclear. Intravenous phosphate loading had little effect on salivary phosphorus secretion, most of the increase in faecal phosphorus excretion being the result of reduced reabsorption within the intestine. The significance of these changes in relation to the control of phosphorus balance in ruminants is discussed.

1. The effects of magnesium Supplementation on Mg absorption proximal and distal to the pylorus in ewes maintained on a grass diet were investigated using a combination of balance, digesta flow and electropotential measurements.2. Three mature ewes each received by intrarumenal infusion a supplement of 0, 1,2 and 3 g Mg/d in sequence over four 10-d periods.3. Net Mg absorption distal to the pylorus took place down its electrochemical gradient, although the quantity absorbed remained small during the control and first infusion periods.4. The bulk of Mg absorption occurred before the pylorus and, during the control and first infusion periods, took place against its electrochemical gradient. The net Mg absorption proximal to the pylorus rose with declining efficiency as Mg intake was increased. It is suggested that saturation of the absorption process at this site was occurring.