Water and electrolyte changes in tropical Merino sheep exposed to dehydration during summer

Abstract

During three summers field studies were made of Merino wethers deprived of water while exposed to sun and to maximum air temperatures ranging from 84°F (29°C) to 10S°F (42°C) at Julia Creek, lat. 21° S. Evaporative cooling determined the rate and extent of water and electrolyte changes and produced a different pattern each year. Control of body temperature failed when 31% of body weight was lost by the end of 10 days without water. In hotter weather 5 days without water caused a 25% loss of body weight and in some sheep irreversible circulatory failure. In the course of dehydration, after an initial increase, the plasma and extracellular volume decreased up to 45% while concentrations of haemoglobin and plasma protein increased by 60%. In the plasma, potassium and sodium concentration increased less than that of haemoglobin. When 25% or more of weight was lost, plasma urea concentration rose to 136 mg/100 ml. Plasma osmolarity in some sheep reached 490 m-osmoles/l after 10 days. Urine volume fell after 2 days without water, and in a hot season less than 100 ml/24 hr was passed on the fourth or subsequent days. Concentrations increased over the first 4 or 5 days, reaching a maximum of 3.8 osmoles/l, then declined. Between half and two-thirds of the osmolarity was accountable to sodium and potassium salts. In rapid dehydration, more sodium was excreted than potassium. There was reduced sodium excretion when water was ingested after dehydration. Renal function studies in ewes indicated that filtration, renal plasma flow, and glucose reabsorption were reduced to half after 5 days without water in the heat. The chance of survival in dehydration appears to be increased by low rates of water loss in the first 3 days, together with high sodium and low potassium excretion. A full rumen, containing water up to 13% of body weight, could provide all the extracellular fluid loss. More than half of the weight loss appears, however, to come from intracellular sources. Extracellular fluid was drawn upon to a greater extent when the rate of dehydration was rapid, than in the cooler periods of slow weight loss. Survival in the sun without water depends upon insulation, water conservation, water reserves in rumen and extracellular fluid, the ability to adjust electrolyte concentrations, and the ability to maintain circulation with lowered plasma volume.

The effect of water-restriction on various physiological variables in intensively reared Lacaune ewes

“…The increased haematocrit values are consistent with what was observed also by other authors in Awassi sheep (Laden et al 1987;Ghanem et al 2008), Merino sheep (MacFarlane et al 1961) and goats (Hossainini-Hilali et al 1994), subjected to different regimens of water deprivation. This has also been reported by Hamadeh et al (2006) in Awassi sheep.…”

“…Water deficiency leads to increased water reabsorption at the nephron level; consequently, increased urea reabsorption is expected as it is a highly permeable molecule. In other experiments conducted on Merino (MacFarlane et al 1961) and Yankasa sheep (Aganga et al 1989;Igbokwe 1993) similar results have been obtained. Laden et al (1987) also reported a gradual increase in urea concentrations in response to two and five days of water restriction in Awassi sheep.…”

Hydration and Vasopressin Effects on Glomerular Filtration Rate in Sheep

“…The increased excretion of water through suppression of ADH in these hydrated sheep was undoubtedly enhanced by the increase in GFR. The opposite picture is seen in dehydration of sheep (Macfarlane, Morris, Howard, McDonald and Budtz-Olsen, 1961) and in acute and chronic fluid depletion in man and dog (Wesson, 1969) where GFR and renal plasma flow decrease, a condition which in addition to an increase in ADH activity would favour a reduction in water excretion.…”

The Effects of Water Restriction on Physiological Responses in Fattening Beef Cattle

“…And the study of water restriction have been reported by MACFARLANE et al [5][6][7] and BIANCA et al 8). But in our country, knowledge of the physiological changes occurring in fattening beef cattle in response to water restriction is limited.…”