Fluid balance over the oestrus cycle of the rat.

Plasma vasopressin concentrations vary throughout the rat oestrus cycle. In order to determine the relationship of these fluctuations to fluid retention, studies were carried out in cycling female albino rats. The animals were housed in individual metabolism cages under 12 hourlight/12 hour dark regimen, with free access to food and water. Urine samples to determine volume, osmolality and eiecroiytes concentrations were obtained and food and water intake recorded between 0800-0900 and 1700-1800 hours. Plasma volume, blood volume and haematocrit were determined Food intake was significantly reduced on pro-oesirul/oestrus compared to dioestrusl and dioestrus II, possibly due to the influence of the elevated circulating oestradiol levels. Water intake also reduced.during the dark phase of pro-oestrus leading to significant decrease in urine flow and a rise in urine osmolality. 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 prooestrus. These results in cycling rats show that vasopressin may play a rolein fluid retention during the rat oestrus cycle (JPMA 46: 231, 1996).