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Biomedical subjects

A R Michell

Publications and source records attributed to A R Michell.

At least 37 records · Page 2Linked to original sources
PubMed1995

Responses to reduced water intake, including dehydration natriuresis, in sheep excreting sodium predominantly in urine or in faeces.

Sheep which were predominantly urinary excretors (U) or faecal excretors (F) of sodium were exposed to a 75% reduction of water intake for 72 h. The experiment was performed on moderate, low or high sodium intakes (0.4, 0.05 or 1.2 mmol kg-1 day-1) to test the hypothesis that dehydration natriuresis was not a cause of sodium depletion but a defence against hypernatraemia. Dehydration caused elevation of plasma sodium concentration, osmolality, antidiuretic hormone (ADH) and oxytocin but, as in other experiments, a fall in haematocrit. The two higher levels of sodium intake were associated with dehydration natriuresis but also a smaller increase in faecal sodium excretion in both U and F sheep. On low sodium intake, however, neither urinary nor faecal sodium excretion increased in either group of sheep although the rise in plasma sodium concentration caused by dehydration was similar. Thus, when there is a risk of sodium depletion, due to low sodium intake, dehydration natriuresis does not occur, consistent with the hypothesis. Active sodium transport inhibitor (ASTI) and atrial natriuretic peptide (ANP) fell rather than rose during dehydration. Since aldosterone is suppressed by the higher levels of sodium intake, none of these hormones is likely to mediate dehydration natriuresis in sheep. F sheep showed more effective renal and faecal water conservation when dehydrated. During water restriction, the urinary potassium excretion of U sheep was significantly reduced, unlike that of F sheep; moreover, the latter maintained an identical plasma potassium concentration between baseline and restriction period, whereas in U sheep it was 0.3 mmol l-1 higher during water restriction. Increased drinking rather than reduced urine output was the basis of rehydration when ad lib. water intake was restored.

Animals↗
PubMed1995

Electrolyte and total protein changes in nonheat acclimated horses performing treadmill exercise in cool (20 degrees C/40%RH), hot, dry (30 degrees C/40%RH) or hot, humid (30 degrees C/80%RH) conditions.

Four horses (H, J, N and M) undertook a simulated competition exercise test (CET), designed to simulate physiological and metabolic stresses of the Speed and Endurance (S & E) test of a 3-day-event, under 3 different environmental conditions: 20 degrees C/40% relative humidity (RH) (cool, dry [CD] 2 sessions); 30 degrees C/40% RH (hot, dry [HD]) and 30 degrees C/80% RH (hot, humid [HH]) (Marlin et al. 1995a). Venous blood samples for electrolyte and total protein (TP) determinations were collected from indwelling catheters at predetermined time points throughout each CET and initial 30 min recovery period. Venous blood samples were collected by jugular venepuncture at 2 h and 24 h after the end of the final 8 min canter (Phase D). The effects of exercise, environmental condition and horse on venous TP, sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+) and phosphate (PO4(3-)) plasma concentrations were investigated. In addition, the effect of environmental condition on estimated cation loss was evaluated. All horses completed the full CET under the CD and HD conditions, but only one horse completed the full 8 min of the final canter Phase D under HH conditions. Exercise had a significant (P < 0.05) effect on all parameters similar to those reported previously in field competitions. There was a significant (P < 0.05) interaction between time and horse for TP, Na+ and Cl-. Overall, the environmental condition had a significant (P < 0.05) effect on all parameters, but the differences were not considered to be of any physiological relevance, other than for Cl- and TP. There was a significant (P < 0.05) interaction between time and environmental condition for Cl- and TP only. During much of the CET and recovery period, mean Cl- values were higher with the first CD session than the second CD session or under the HH or HD conditions. For TP after 2 min of Phase D and during the initial recovery period, concentrations were higher under the HH conditions and returned to the Pre- values less quickly. There were marked individual variations in the estimated cation losses and no consistent effect of environmental condition was found. Although estimated fluid loss was similar following both HD and HH sessions, restoration of bodyweight was slower following the CET studies under HH conditions.

Acclimatization↗
PubMed1994

Veterinary manpower.

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Education, Veterinary↗
PubMed1994

A comparison of direct and indirect (oscillometric) measurements of arterial blood pressure in anaesthetised dogs, using tail and limb cuffs.

Oscillometric measurements of arterial blood pressure were compared with direct measurements in anaesthetised dogs being monitored during routine surgery. Readings were obtained with tail cuffs and limb cuffs and the reproducibility (precision) of the readings from various cuff sites was also compared in conscious dogs. Tail cuffs gave the best precision in conscious dogs and the closest correlation with direct measurements in anaesthetised dogs, especially for systolic pressure. The proximal hindlimb site gave results with a slightly lower precision.

Anesthesia, General↗
PubMed1994

Salt, hypertension and renal disease: comparative medicine, models and real diseases.

Dogs are well established as experimental animals for the study of both renal disease and hypertension, but most work is based on surgical or pharmacological models and relatively little on spontaneous diseases. This review argues for the latter as an underexploited aspect of comparative medicine. The most important feature of canine hypertension may not be the ease with which models can be produced but the fact that dogs are actually rather resistant to hypertension, and perhaps to its effects, even when they have chronic renal failure. The importance of natural models of chronic renal failure is strengthened by the evidence that self-sustaining progression is a consequence of extreme nephron loss, that is, a late event, rather than the dominant feature of the course of the disease. The role of salt in hypertension is discussed and emphasis given to the importance of understanding the physiological basis of nutritional requirement and recognizing that it is unlikely to exceed 0.6 mmol/kg/day for most healthy adult mammals except during pregnancy or lactation. Such a perspective is essential to the evaluation of experiments, whether in animals or humans, in order to avoid arbitrary definitions of 'high' or 'low' sodium intake, and the serious misinterpretations of data which result. An age-related rise in arterial pressure may well be a warning of excess salt intake, rather than a normal occurrence. Problems of defining hypertension in the face of variability of arterial pressure are also discussed.

Animals↗
PubMed1993

Non-invasive measurement of arterial blood pressure in dogs: a potential indicator for the identification of stress.

The reproducibility of indirect measurements of arterial pressure in dogs was assessed using the Dinamap 1846 SX oscillometric monitor and tail cuffs. Measurements on different days correlated excellently in 20 working dogs, with indistinguishable group means for systolic, diastolic and mean arterial pressure, whereas heart rates differed, though not significantly. The similarity of the means did not simply result from 'cancelling out' of individual variations; the rank correlation was also highly significant (P < 0.01 diastolic, P < 0.002 systolic). Remeasurement in the veterinary clinic, rather than 'neutral territory' showed significant increases in pressure but not heart rate. Anticipation of work raised systolic pressure whereas heart rate and diastolic pressure were unchanged. Exposure of a dog to a known stressor for that individual (sound) raised systolic and diastolic pressure as well as heart rate; diastolic pressure was the fastest to recover, heart rate the slowest. The data are compared with other observations on non-invasive blood pressure measurement in both dogs and humans.

Animals↗
PubMed1992

Sodium preference in sheep excreting sodium predominantly in urine or faeces.

On moderate sodium intakes (0.5 mmol/kg/day approx.) sheep are readily divided into those that are predominantly urinary or faecal excretors of sodium (U or F sheep). Water turnover and obligatory sodium loss are lower in F sheep. In view of these differences, sodium preference was examined in U and F sheep offered water and sodium bicarbonate solutions (40 or 300 mmol/l). The F sheep took significantly more of the bicarbonate solutions and, therefore, had sodium intakes that were 51% higher on the lower concentration, 94% higher on the higher concentration. Thus, F sheep not only conserve sodium more readily when it is scarce, they select it more readily when it is available.

Animals↗
PubMed1992

Plasma and extracellular volume in calves: comparison between isotopic and 'cold' techniques.

While isotopic techniques have largely superseded traditional markers for the determination of the volume of fluid compartments in vivo, they are not always convenient, especially with diarrhoeic animals. A direct comparison was therefore made in week-old calves between Evans blue and radio-iodinated serum albumin as measures of plasma volume and thiocyanate or 24sodium as measures of extracellular fluid space. The correlation coefficients were excellent (1.00, 0.96; P < 0.001) and the calves had plasma and extracellular fluid volumes of 72 +/- 2 and 438 +/- 2 ml kg-1, respectively. The latter value is, though high, typical of young animals and comparable with other data in calves.

Animals↗
PubMed1992

Comparison of cortisol concentrations in saliva and plasma of dogs.

It is increasingly important to have simple, non-invasive indicators of stress in animals. Studies in various species have shown that concentrations of cortisol in saliva relate closely to plasma levels of the free hormone; the aim of the present procedure was to show a similar correlation in the dog. Baseline blood and saliva samples were collected concurrently from six male beagles. Synthetic adrenocorticotrophic hormone was given and further samples were collected at 0.25, 0.5, one, two and 2.5 hours later. The results indicated a statistically significant correlation between the levels of cortisol in blood and saliva. Concentrations in saliva were between 5 and 10 per cent of those in plasma at each collection time. To demonstrate a response to a more natural stimulus, saliva samples were taken from a dog during exposure to a known stressor for that individual. The results showed a marked, delayed increase from baseline which was maintained for at least 0.5 hours after stressing.

Adrenocorticotropic Hormone↗
PubMed1992

Differences between sheep excreting sodium predominantly in their urine or in their faeces: the effect of changes in sodium intake.

Sheep receiving a total of about 31 mmol day-1 (0.5 mmol kg-1) of sodium were classified according to the predominant route of sodium excretion; urinary (U) or faecal (F). U sheep had a greater water turnover than F sheep; their intake was 41% higher and they produced 133% more urine but there was little difference in faecal water loss. Most faecal sodium was readily exchangeable with water in both groups. When sodium intake was reduced by 80% (to 6 mmol day-1; 0.1 mmol kg-1), the reduction in total sodium excretion was equally effective in F sheep and U sheep after 48 h and after 2 weeks the overall losses of sodium were smaller in F sheep. On sodium intakes close to requirement (0.1 mmol kg-1 or less) the majority of the sheep excreted most of their sodium in faeces and did so on intakes up to 0.5 mmol kg-1 day-1. Excess dietary sodium is mainly excreted renally. When sodium intake is increased abruptly (by 20 mmol day-1, 0.3 mmol kg-1), total sodium excretion only increases gradually but after about 3 days it 'overshoots' as in humans.

Animals↗