Endocrine Abstracts

Oestrogens affect fluid balance, influencing both ingestive behaviour and renal excretion. The renal effects are partly due to altered release of the neurohypophysial hormones vasopressin and oxytocin and involve oestrogen receptor beta (ERbeta). Normal rat maintenance diet contains soya isoflavones (phytoestrogens) which are also known to act at ERbeta. A study has been performed comparing the fluid balance and neurohypophysial hormone release in rats maintained for two weeks on either a phytoestrogen-free diet or one containing 150 micrograms genistein and diadzin per gram of diet.

The animals were initially maintained on a diet containing approx 75 micrograms phytoestrogen and then transferred to the soya-free or enriched diet. for two weeks during which time food and water intake, body weight and urine flow, osmolality and sodium and potassium concentrations were determined at 0.800h and 18.00h. At the end of the period of observation, blood samples were obtained for the determination of plasma osmolality and sodium, vasopressin and oxytocin concentrations. There was no significant difference in the weight gain of both groups, nor in the renal excretion of sodium and water. The daily patterns of intake and excretion were also the same in the two groups. However plasma vasopressin concentrations were 2.1 plus/minus 0.4 pmol/l in the group on the soy-free diet as compared to 5.7 plus/minus 0.8 pmol/l (n=8, P<0.05, t test) in the animals receiving soya in their diet . The vasopressin concentrations were correlated with plasma osmolality, but the gradients of the omoregulatory line were different, that with the soy free diet being smaller. There was no significant difference in the plasma oxytocin concentrations between the two groups. Thus phytoestrogens specifically affect vasopressin release and since no effect on urinary excretion was observed, renal responsiveness might be affected.