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Biomedical subjects

A R Michell

Publications and source records attributed to A R Michell.

At least 91 records · Page 5Linked to original sources
PubMed1982

The optimum pH of renal adenosine triphosphatase in rats: influence of vanadate, noradrenaline and potassium.

In the presence of vanadate, the optimum pH of renal (Na+, K+)-ATPase in rats is reduced and lies in the range of intracellular pH. This explains the difference in optimum pH observed with ATP extracted from equine muscle. Removal of vanadate from such ATP (with noradrenaline) raises the optimum to the accepted range obtained with synthetic ATP. Changes in the sensitivity of the enzyme to potassium concentration contribute to the alterations in optimum pH. The optimum pH of Mg-ATPase is unaffected by vanadate. Since vanadate may be an intracellular regulator of (Na+, K+)-ATPase changes of optimum pH in relation to intracellular pH could well contribute to the regulation of sodium pump activity.

Adenosine Triphosphatases↗
PubMed1981

Fluid and electrolyte excretion in sheep: comparison of the effect of reduced food and water intake with oestrus.

Food and water intake were restricted by 29% for 48 h in ovariectomized ewes, to see whether such changes could cause a pattern of water and electrolyte excretion similar to that observed during the oestrous cycle. The closest resemblance appeared in the effect on water balance and there was also a rise in sodium excretion, as at oestrus. Restriction of intake failed to duplicate the marked sodium retention or, more particularly, the persistent potassium retention which follow natural oestrus. Restriction of food alone produced changes less similar to those seen at oestrus, mainly because this treatment caused a large increase in urine output. The similarity between the effects of restriction and 'fasting natriuresis' is discussed, with particular reference to the mildness of the stimulus in these experiments. Indications that the gut may be as important as the kidney in the regulation of salt and water balance are discussed in relation to other herbivores and to man.

Animals↗
PubMed1980

Sodium appetite and the oestrous cycle in sheep: effect of oestrogen, progesterone and changes in food intake.

Previous studies of voluntary sodium intake and electrolyte excretion in sheep suggested that the pattern of intake associated with the oestrous cycle was not a response to alterations in sodium balance. The pattern thus seemed likely to be dictated by changes in either hormonal levels or food intake. These possibilities were examined in experiments with ovariectomized sheep. Physiological doses of oestrogen or progesterone had no effect on sodium preference, whether given in isolation or succession. Depression of food intake by 25% (as at oestrus) did affect preference slightly (though significantly) but in the opposite direction to the change at oestrus. It is concluded that none of these 3 factors is responsible for the varying sodium preference during the oestrous cycle and that direct hypothalamic control of this pattern is an interesting possibility.

Animals↗
PubMed1979

The pathophysiological basis of fluid therapy in small animals.

The principles underlying fluid therapy and their application to some medical problems in small animals are presented. The renal mechanisms which normally regulate the volume, electrolyte concentration and pH of extracellular fluid are reviewed. The major clinical disturbances are considered together with the specific means of correcting them by fluid therapy.

Acid-Base Equilibrium↗
PubMed1979

Water and electrolyte excretion during the oestrous cycle in sheep.

Electrolyte excretion was observed during 24 oestrous cycles in housed sheep, together with mixed salivary Na/K ratio during 10 additional cycles. 1. The sharp fall in food and fluid intake at oestrus accompanied a peak of sodium excretion which changed to peak retention 3 days later, both in faeces and urine. 2. Potassium excretion declined with food intake at oestrus but subsequently failed to recover to pre-oestrous levels dispite full recovery of dietary intake. 3. Curiously, water intake also recovered completely whereas urinary and faecal water retention continued; faecal loss actually exceeded renal excretion on these liberal water intakes. 4. Changes in salivary, urinary and faecal Na/K indicated an aldosterone peak neither during the luteal phase nor at oestrus but three days later. The data raise questions concerning the regulation of water and electrolyte balance within the normal cycle. They also provide a baseline for the investigation of renal effects of gonadal steroids. Possible roles for aldosterone, ADH and progesterone in maintaining fluid and electrolyte balance are discussed, emphasising problems confronting species which have evolved with heavy obligatory potassium excretion but undependable supplies of sodium and water.

Animals↗
PubMed1977

The optimum pH of renal adenosine triphosphatase and its variation with the type of ATP.

The in vitro optimum pH of renal (Na+, K+)-ATPase, assumed to be 7.8, depends on the type of ATP used. The accepted value is obtained with 'grade II' ATP but with purer ATP ('Sigma grade') it lies close to the intracellular pH in rat medulla and cortex (pH 7.0, 7.2). Values were identical in rats subjected to dietary potassium depletion for 2-4 weeks. The optimal Mg2+ concentration was also influenced by the type of ATP but Mg ATPase activity was unaffected. Both types of ATP were hydrolysed by trichloracetic acid. Where the purer ATP is used the assay conditions need to be modified accordingly.

Adenosine Triphosphatases↗