Plasma oestradiol concentrations have been shown to vary throughout the rat oestrus cycle. The involvement of oestradiol in the regulation of fluid and electrolyte balance has long been recognized. In order to determine how these fluctuations in oestradiol concentrations are related to fluid retention during the rat oestrus cycle; studies have been carried out in the cycling female rats and ovariectomized rats. Treated with either sesame oil vehicle or oestradiol benzoate in higher doses (100 ug/100 g.b.w) and also in physiological doses (50 ug/rat) for fourteen days. The experiment was repeated using a specific inhibitor of ornithine decarboxylase (ODC). Diflouro methylornithine (DFMO; 20 mg/100 g.b.w.) in cycling rats and also studies on fluid balance were carried out in ovariectomized rats treated with oestradiol benzoats (50 ug/rat) and DFMO (20 mg/100 g.b.w.) for seven days.
The animals were housed in individual metabolism cages, under 12 h light/12 h dark regimen. With free access to food and water. Urine samples to determine urine volume, osmolality and electrolytes concentrations were obtained and food and water intake were recorded at 8.00 to 9.00 h and 17.00 to 18.00 h daily. Plasma samples and pituitary for determination of vasopressin by radio-immuno-assay were taken.
A diurnal variation was seen in food and water intake and urinary excretion in both cycline and overiectomized rats. Food intake was significantly reduced on prooestrue/oestrus as compared to dioestrus I and dioestrue II, possibily due to the influence of the elevated circulating oestradiol levels. Water intake showed a tendency to be reduced during the dark phase of pro-oestrue which was reflected in a significant decrease in urine flow and a rise in urine osmolality. The changes in urine output were correlated with vasopressin concentrations. During the light phase of pro-oestrus there was a significant increase in urine flow and electrolytes excretion. This enhanced diuresis appears to be due to reduced plasma vasopressin levels during the afternoon of pro-oestrus. These results in cycling rats show that vasopressin may play a role in fluid retention during the rat oestrus cycle.
In DFMO treated cycling pro-oestrus rats, an opposite pattern of changes were observed to that of cycling rats. Food intake, water intake, and urine output were increased. But water retention in these animals decreased. Electrolytes ingestion and output increased, and their retention decreased. Urine oamolality was also decreased. Plasma oestradiol and vasopressin concentrations were decreased simultaneously. Plasma volume and blood volume decreased while haematoorit increased. The results of this study indicate less water retention in the cycling prooestrus rats treated with DFMO as compared to the controls
In oestradiol benzoate (100 ug/g.b.w) treated overiectomized rats., food and water intake, were reduced during the dark phase and light phase of the cycle as compared to the vehicle treated rats. However, after about nine days there was little difference between the two groups. Body weight, electrolytes intake and output showed similar pattern of changes. Urine osmolality raised. Plasma volume and blood volume increased while haematocrit decreased in oestradiol benzoate treated rats. These result indicate water and electrolytes retention in the oestradiol benzoate treated rats as compared to the vehicle treated rats.
In oestradiol benzoate (50ug/rat) treated ovariectomized rats same pattern of observations were shown in food and water intake as in the previous study, using higher doses of oestradiol. Plasma osmolality was significantly lower after fourteen days in oestradiol benzoate treated animals as compared to the controls, suggesting greater water retention in these animals. Plasma vasopressin in both groups were correlated with urine flow. These observations indicate role for oestradiol in varsopression turn over and water balance.
In DFMO and oestradiol, and oestradiol and progesterone treated ovariectomized rats same pattern of observations were observed as in DFMO treated cycling pro-oestrus rats. In these animals body weight was decreased as compared to the vehicle treated rats. Plasma osmolality was also increased, showing reduced water and electrolytes retention in these animals.
All these observations indicate water retention. Which appears to be related to the oestradiol-induced vasopressin release during the rat oestrus cycle.