Colonic absorption and secretion of electrolytes as seen in five species of East African herbivorous mammals

Water and electrolyte homeostasis in sheep without functional colons

The regulatory role of the colon in water and electrolyte balance and the renal compensation which follows impairment of colonic function were assessed using sheep with ileorectal anastomosis (ILRAN sheep) on restricted and free water intake as experimental models. Faecal electrolyte loss sustained by the ILRAN group was eight to nine times greater than that in the control animals. When water was available ad lib., ILRAN sheep lost 2.81 and 0.51 more water per day via the faeces and urine, respectively, than the controls. Urine volume in the ILRAN sheep comprised largely electrolyte-free water and the renal retention of water was entirely secondary to the high degree of sodium reabsorption in these animals compared with the controls. On restricted water intake, the urine volume of the ILRAN sheep declined due to retention of electrolyte-free water and even greater absorption of sodium (and hence water by osmosis) by the kidney tubules. The latter observation was substantiated by a decrease in fractional excretion of sodium from 0.29 to 0.08% when water intake was restricted. Plasma aldosterone concentration was markedly elevated in the ILRAN sheep as a result of the excessive loss of sodium and water via the faeces. Activation of the renin-angiotensin-aldosterone sequence is believed to underlie the increased sensation of thirst (ILRAN sheep drank on average about 2.51 more water per day than the controls), the homeostatic response by the kidneys and the relatively lower plasma potassium in the ILRAN sheep compared to controls.

Effects of dehydration and rehydration on plasma vasopressin and aldosterone in horses

We have investigated the change in plasma vasopressin and aldosterone concentrations in Namib (desert-adapted) and in control horses from a subtropical region, during an acute 12% dehydration and during rehydration, while food was available. During dehydration, vasopressin concentrations increased significantly in both groups of horses, but the increase was significantly greater in Namib horses than in control horses. During rehydration, vasopressin levels fell, but fell significantly less in Namib horses. The change in vasopressin concentration correlated significantly with plasma osmolality (r = 0.88, p < 0.001), and the relationship between these two variables was the same for both groups of horses during the dehydrated and rehydrated states. Aldosterone concentrations fell up to 48-h dehydration in both groups, but decreased significantly more in desert horses. From 48-h dehydration and during rehydration, aldosterone concentrations increased and the increase was sustained longer in Namib than in control horses. Changes in plasma osmolality did not correlate significantly with changes in aldosterone concentration. There were significant correlations between faecal moisture (%) and both vasopressin and aldosterone concentrations (r = -0.72, p < 0.008; r = 0.80, p < 0.002, respectively). During the 12% dehydration, the Namib horses sustained higher plasma osmolalities and consequently vasopressin levels than the control horses. We conclude that plasma osmolality in conjunction with these two hormones plays a significant role in water homeostasis in horses.

Colonic absorption and secretion of fluids, electrolytes and organic acids in East African wild ruminants

In sixteen species of wild ruminants colonic absorption of fluids averaged 48% of the fluids entering the large bowel. Values ranged from 26% (steenbok) to 65% (gerenuk). Absorption of sodium, potassium, chloride and organic acids were variable between species. Colonic absorption of potassium ions were found to be related to diet selection and body weight of the animals. colonic absorption of volatile fatty acids were significantly greater in browsers (28.4 mmol/l) than grazers (9.6 mmol/l). The African buffalo are the only species of wild ruminants not forming a faecal pellet; colonic functions were not different from those species forming faecal pellets.

Molar proportions of volatile fatty acids in the gastrointestinal tract of East African wild ruminants

The molar proportions of seven individual VFA's were determined at select sites along the gastrointestinal tract of sixteen species of East African wild ruminants. The resulting data were statistically analyzed for species effect, and for effects due to major feeding groups (browsers, grazers, fresh grass grazers, etc.) and for body weight groups (5-750 kg animals). Present data suggest that body weight, rather than diet, is the more influential factor in reticulo-rumen fermentation rate, and in the molar proportion of fatty acids present. The molar proportions of VFA's observed in the mid and hindgut of these wild ruminants appeared more responsive to diet and body weight of the animal than did foregut VFA values.