Water and electrolyte homeostasis in sheep without functional colons

Abstract

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.