SUMMARY1. The ontogenic development of the intestinal Na+-glucose co-transporter was measured in lambs as a function of diet. Transport activity was assayed in brushborder membrane vesicles and the expression of transport protein in the brushborder membrane determined by Western analysis.2. Na+-dependent D-glucose transport increased to a maximum (300-700 pmol mg' s') within the first 2 weeks of birth and then declined to negligible amounts (< 10 pmol mg-' s-1) over the next 8 weeks. There was no further change over the next 2-3 years. Early changes were associated with modifications in both the maximum velocity Vma. for transport and expression of carrier protein in the brush-border plasma membrane.3. Maintaining lambs on a milk replacer diet beyond the normal weaning period prevented the normal decline in the expression of Na+-glucose co-transport. At 5 weeks the transport rate was 433 + 150 pmol mg-' s-I in lambs maintained on milk replacer, but only 79 + 40 pmol mg-' s-I in normally reared control lambs.4. Infusing the proximal intestine of 2-to 3-year-old sheep with 30 mM-D-glucose for four days increased the rate of transport 40-to 80-fold above that found in control animals perfused with mannitol. A similar but smaller increase was observed in one animal perfused with the non-metabolizable sugar a-methyl-D-glucopyranoside. The induced increase in glucose transport was correlated with the expression of the co-transporter protein in the brush-border plasma membrane.5. It is concluded that the age-related decline in Na+-glucose co-transport in the sheep intestine is directly due to the decrease in D-glucose (and D-galactose) reaching the small intestine after development of the rumen. These results further suggest that luminal sugar substrates for the co-transporter promote both the maintenance and the up-regulation of the brush-border transport protein and it is the intact sugar itself which controls gene expression during enterocyte maturation. MS 8905 S. P. SHIRAZI-BEECHEY AND OTHERS

We have investigated the mechanisms of regulation of the Na+/glucose co-transporter (SGLT1) in a ruminant animal, which is an exceptional model system for studying intestinal glucose transport. Pre-ruminant lambs absorb glucose, produced by hydrolysis of the milk sugar lactose, in the intestine via apical SGLT1 and basolateral facilitative glucose transporters (GLUT2). Weaning coincides with the development of the rumen, and consequently the amount of hexoses reaching the small intestine of the ruminant sheep is undetectable. During development, SGLT1 activity and abundance in intestinal brush-border membranes decreased by over 200-fold, and either maintaining lambs on a milk replacer diet or infusing sheep intestine with D-glucose restored co-transporter activity and expression. We have measured ovine intestinal SGLT1 mRNA levels during development, with changes in diet and after direct infusion of D-glucose or methyl alpha-D-glucopyranoside into the intestinal lumen, in order to determine the level of regulation. During development, mRNA levels decreased only 4-fold. Lambs maintained on a milk replacer diet showed no change in mRNA levels relative to age-matched controls. Finally, upon infusion of the intestine of the ruminant sheep with sugars, D-glucose infusion increased SGLT1 mRNA, but only by 2-fold, compared with a 60-90-fold increase in co-transporter number and activity. Since the change in Na(+)-dependent glucose transport activity is correlated with SGLT1 protein abundance, and since changes in mRNA levels do not account for the dramatic changes in protein abundance, we conclude that the principal level of SGLT1 regulation by luminal sugar is translational or post-translational.

The infusion of potassium salts into the rumen of sheep led to an increase in both the concentration and amount of potassium in the fluid in the rumen and to a decrease in the concentration and amount of sodium. The amount of potassium absorbed from the rumen increased as the intake of potassium was increased and was related to the concentration of potassium in the rumen fluid. The amount of potassium flowing out of the rumen increased and the amount of sodium flowing out decreased with increase in potassium intake but neither the volume of fluid in the rumen nor the rate at which it flowed on to the omasum was affected. There was no evidence that potassium supplements reduced the amount of sodium entering the rumen in the saliva. These results indicate that the amount of sodium absorbed from the rumen is increased when potassium supplements are given.IN experiments designed to study the absorption of sodium and chloride from the rumen of the sheep, Dobson and Phillipson [1958] demonstrated that the contents of the rumen were 30 to 40 mV electrically negative with respect to the blood. It is well known that large amounts of sodium move from the rumen to the blood [Danielli et al., 1945; Sperber and Hyden, 1952; Parthasarathy and Phillipson, 1953] and Dobson [1959] showed that the process is an 'active' one since absorption occurred against the electrochemical gradient for this ion.On the other hand, rather meagre evidence suggests that potassium may be absorbed from the rumen by passive diffusion. Sperber and Hyden [1952] found that potassium accumulated in a rumen pouch in a conscious goat to a level of about 5 times higher than that in the plasma, but this would only necessitate active transport if the potential difference between the rumen contents and the blood were less than 40 mV. Parthasarathy and Phillipson [1953] studied absorption from the isolated rumen of the sheep and suggested that potassium may be absorbed by passive diffusion.Large amounts of potassium are normally ingested by the ruminant and the purpose of the present experiments was to examine the quantitative aspects of absorption and outflow of potassium from the rumen in conscious sheep receiving a varied potassium intake. METHODSAnimals and Diets. -Two adult Scottish Blackface ewes (Sheep 1 and 2) weighing 38 and 40 kg. were used in the absorption studies. Both sheep were fitted with permanent ebonite cannulas into the rumen several months before observations were begun. During the experiments the sheep were kept in metabolism cages. Four 382

Mature sheep fitted with rumen and duodenal cannulae, and fed a pelleted hay diet, were given supplementary phosphorus by continuous infusion into the rumen and the effects on salivary phosphorus secretion, net intestinal phosphorus absorption and faecal endogenous phosphorus excretion were studied. In control periods little phosphorus was excreted in the urine and little increase was seen in response to supplementation, the faeces being the major pathway for excretion. Increasing phosphorus intake led to an increase in net intestinal phosphorus absorption, a rise in plasma phosphate concentration, an increase in salivary phosphorus secretion and an increase in faecal endogenous phosphorus excretion. Overall net intestinal absorptive efficiency for phosphorus did, however, decrease as intake rose so that changes in faecal endogenous phosphorus excretion were in part due to increased salivary phosphorus secretion and in part to a reduction in overall absorptive efficiency. The significance of these changes in relation to the control of phosphorus balance in ruminants is discussed. INTRODUCTIONMATERIALS AND METHODS

Sheep and calves fed a roughage diet (53% straw+other components) excreted an alkaline urine poor in phosphate whereas when they were fed concentrate diets (80% barley+ other components) the urine was acid and contained large amounts of phosphate of which a high proportion were present as titratable H2PO4. These differences in phosphorus excretion were not related to differences in phosphorus intake nor did they appear to be directly related to acid excretion. Over a wide range in the amount of phosphate filtered at the glomerulus the amount reabsorbed by the renal tubule was consistently less in animals fed the concentrate diets. These differences in tubular reabsorption appeared to be related to differences between diets in the amounts of phosphorus absorbed from the gut but there were no differences between diets in the level of parathyroid hormone in blood.Both sheep and calves fed a concentrate diet composed of barley and fish meal excreted considerably more acid in the urine than when fed a similar diet in which soya bean meal was substituted as the source of protein. Although attempts have been made to account for these affects of diet on acid and phosphorus excretion [Reed et al., 1965;Scott et al., 1971] there is as yet no complete explanation. The experiments described in this paper were designed to examine further renal excretion of acid and phosphate in the urine of sheep and calves fed either roughage or concentrate diets. METHODS Animals and diets. Four castrate Friesian calves and two adult Dorset-Horn ewes were used in these studies. At the beginning of experiments the average weight of calves was 80 kg and of the sheep 53 kg. Both of the sheep and two of the calves (322, 325) were fitted with a rumen cannula (3 cm internal diameter). During experiments the animals were kept in metabolism cages which allowed the separate collection of faeces and urine.Three pelleted diets were used in these studies, a roughage diet and two concentrate 379 diets which are referred to as the soya meal concentrate and fish meal concentrate respectively. These three diets were designed to be about equal in energy and nitrogen content and their composition is given in Table I. Whatever level of feeding was chosen each day's intake was divided into two equal portions, one being given at 09.00 hr and the other at 16.00 hr. Water was freely available at all times. The amounts of food offered in the different experiments are indicated later.