Beechey Rb

The expression of the Na+/glucose cotransporter (SGLT1) gene in lamb small intestine during postnatal development.

We have shown previously that the activity and abundance of the intestinal Na+/glucose cotransporter (SGLT1) declines dramatically during the postnatal development of lambs, and that it can be restored in the intestine of ruminant sheep by intra-luminal infusion of D-glucose. The work presented in this paper has followed the expression of the SGLT1 gene along the vertical and horizontal axes of the ovine small intestine during early development, using quantitative in situ hybridisation histochemistry. Along the vertical axis, SGLT1 mRNA was first detectable just below the crypt-villus junction and rose rapidly to a peak level approx. 150 microns above this point. After reaching a maximum, the amount of message gradually declined towards the villus tip. This pattern of mRNA accumulation along the crypt-villus axis was similar in all intestinal positions and age groups. Along the length of the small intestine (horizontal axis), a decline in the level of SGLT1 mRNA was observed first in the distal intestine. This decrease in SGLT1 mRNA was significant in the intestine (75% of length) of 5-week-old lambs when compared to tissue taken from 25 and 50% of length (P < 0.01 and P < 0.02, respectively). However, the observed fall in the expression of this gene during weaning did not coincide with the fall in activity and amount of SGLT1. In adult animals, where the activity of SGLT1 is very low, the amount of message was greatly reduced. This work supports the finding that the expression of SGLT1 is primarily controlled at the post-transcriptional level during the postnatal development of ovine intestine.

Mechanisms of phosphate transport in sheep intestine and parotid gland: response to variation in dietary phosphate supply

The transport of phosphate in intestinal brush‐border membrane and parotid basolateral membrane vesicles isolated from sheep maintained on high and low phosphate diets have been studied. The mechanism of the transport of phosphate in the intestine is via a proton symporter whilst in the parotid gland it is effected by a Na+ coupled transporter. In sheep fed a low‐P diet there is no change in the capacity of the parotid basolateral membrane to transport phosphate into the parotid end piece cells. This is in marked contrast to the response of the enterocyte brush‐border membrane, where there is a significant enhancement of the capacity of the membrane to transport phosphate. We conclude that in sheep the gut appears to play a major role in response to phosphate deprivation, by increasing the capacity to transport phosphate. This enhancement is not achieved by increases in the levels of circulating 1,25‐dihydroxycholecalciferol.

Phosphate transport via Na+ -Pi cotransport and anion exchange in lactating rat mammary tissue

Inorganic orthophosphate (Pi) transport, using 32P‐labelled orthophosphate as tracer, by lactating rat mammary tissue has been examined using both tissue explants and the intact perfused gland. Pi uptake was predominantly via a Na+ ‐dependent pathway. Li+, however, unlike choline, was able to partially substitute for Na+. In addition, Pi release from tissue explants preloaded with 32Pi was stimulated by reversing the Na+ gradient. Thus transferring mammary explants from a buffer containing Na+ to one which was Na+ free (choline replacement) doubled the Pi efflux rate constant. The uptake of Pi by tissue explants was saturable with respect to external Pi, having apparent K(m) and V(max) values of 1.13 mM and 3.36 mmol (kg cell water)‐1 (15 min)‐1, respectively. The stimulation of Pi uptake by tissue explants by external Na+ was also saturable; the K(m) for Na+ was 9.7 mM. These results, taken together, suggest that the Na+ ‐dependent pathway is a Na+ ‐Pi cotransport mechanism. The transport of Pi by the perfused lactating rat mammary gland was examined using a rapid, paired‐tracer dilution technique. Pi uptake by the perfused gland was found to be Na+ dependent and displayed saturable kinetics. The results suggest that the Na+ ‐Pi cotransporter is situated at the basolateral aspect of the secretory cells. The release of Pi from preloaded tissue explants was trans‐accelerated by external Pi but not by Cl‐ or SO4(2‐). However, external Pi stimulated Pi efflux with low affinity.

Pyruvate Transport by Thermogenic Tissue Mitochondria

1. Mitochondria isolated from the thermogenic spadices of Arum maculatum and Sauromatum guttatum plants oxidized external NADH, succinate, citrate, malate, 2-oxoglutarate and pyruvate without the need to add exogenous cofactors. 2. Oxidation of substrates was virtually all via the alternative oxidase, the cytochrome pathway constituting only 10-20% of the total activity, depending on the stage of spadix development. 3. During later stages of spadix development, pyruvate oxidation was enhanced by the addition of aspartate. This was caused by acetyl-CoA condensing with oxaloacetate, produced from pyruvate/aspartate transamination, and so decreasing feedback inhibition of pyruvate dehydrogenase. 4. Pyruvate oxidation was inhibited by the long-chain acid maleimides AM5-11, but not by those with shorter polymethylene side groups, AM1-4. 5. The alpha-cyanocinnamate derivatives UK5099 [alpha-cyano-beta-(1-phenylindol-3-yl)acrylate] and CHCA [alpha-cyano-4-hydroxycinnamate] inhibited pyruvate-dependent O2 consumption and the carrier-mediated uptake of pyruvate across the mitochondrial inner membrane. Characteristics of non-competitive inhibition were observed for CHCA, whereas for UK5099 the results were more complex, suggesting a very low rate of dissociation of the inhibitor-carrier complex. 6. A comparison of the values of Vmax. and Km for oxidation and transport suggested that it was the latter which controls the overall rate of pyruvate oxidation by mitochondria from both tissues.