Observations were made of fluid balance and vasopressin concentrations throughout the oestrous cycle of normally cyclic female rats housed under a 12 h light: 12 h darkness regime. Plasma vasopressin concentrations were found to increase progressively during the light period, falling again during the night on all days of the cycle except pro-oestrus. On this day, peak vasopressin concentrations of 3.32 +/- 0.8 pmol/l were seen between 10.00 and 12.00 h, with lower concentrations of 1.74 +/- 0.22 pmol/l being seen between 18.00 and 19.00 h. Urine flow appeared to reflect the changes in plasma vasopressin concentrations, being significantly lower during the light phase, with a small increase being seen over this period on pro-oestrus. Pituitary vasopressin concentrations were highest between 09.00 and 10.00 h and fell progressively over each of the 4 days of the oestrous cycle. The changes in pituitary content were greater than could be accounted for in terms of the alterations in the plasma concentrations of vasopressin.

Plasma vasopressin concentrations have previously been shown to vary during the oestrous cycle of the rat, being highest on the morning of pro-oestrus and lowest on dioestrus day 1. To determine the effect of gonadal steroids on vasopressin secretion and fluid balance, mature rats were ovariectomized and given oestrogen, progesterone or vehicle alone s.c. for periods of up to 16 days. Plasma vasopressin concentrations fell after ovariectomy and this was reflected in an increase in 24-h urine volume. The normal increase in plasma vasopressin concentrations seen over day-light hours was also suppressed. The change in vasopressin concentrations observed on steroid treatment depended upon both the dose and the duration. High doses of oestrogen were associated with a fall in plasma vasopressin, probably as a result of fluid retention. Thus, of an initial group of rats given silicone elastomer implants containing 50, 500 or 1000 micrograms oestradiol in oil, plasma vasopressin concentrations were reduced after 7 days treatment with 1000 micrograms oestradiol implants in association with reduced plasma sodium concentrations. Daily s.c. injections of 100 micrograms oestradiol benzoate/100 g body weight produced an immediate small increase in plasma vasopressin concentrations, but by 14 days the plasma concentrations of 0.7 +/- 0.16 pmol/l (mean +/- S.E.M.) had fallen significantly and were less than those in the vehicle-treated group (1.2 +/- 0.26 pmol/l). However, after treatment for 14 days with implants containing only 50 micrograms oestradiol, plasma vasopressin concentrations were higher compared with the group receiving vehicle alone, despite the fact that the plasma osmolality was lower in the latter group, suggesting a long term resetting of the osmoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Experiments were performed to test the hypothesis that Na retention and Na in the diet are not required to initiate central aldosterone induced hypertension. Rats were fed either standard rat chow or Na-deficient diet and infused intracerebroventricularly (i.c.v.) with aldosterone (28 ng/h) dissolved in artificial cerebrospinal fluid (vehicle) or vehicle alone. In Na-replete rats the central infusion of aldosterone did not promote Na or water retention, prior to increases in systolic blood pressure (SBP). Infusion of aldosterone in Na-deficient rats also initiated a rise in SBP, although the response was delayed. In neither group of rats did aldosterone infusion significantly change plasma Na, K, renin, norepinephrine (NE) or vasopressin (AVP) concentrations. There was no significant increase in plasma aldosterone concentration in Na replete rats centrally infused with aldosterone. Infusion of vehicle had no effect on SBP. We conclude that central aldosterone infusion initiates an increase in blood pressure by a mechanism independent of Na retention. Furthermore, increased concentrations of systemic renin, vasopressin, and activation of the sympathetic nervous system do not appear to be involved in maintaining hypertension.

SummarySalt and water balance and vasopressin secretion were measured in three colonies of SpragueDawley rats. Although sodium and water retention were similar between the groups, there were marked differences in both the rate and diurnal pattern of intake and excretion. Animals housed under semi-barrier conditions showed a lower basal plasma vasopressin concentration but were more sensitive to physiological stimuli.However, since pathogenic status and environmental conditions cannot entirely explain these results, genetic variation is likely to be a contributory factor. Animal science depends to a large extent upon the comparison of data obtained by different research groups on the same strains of animals. The validity of quantitative comparisons may be questioned if inter-colony variations arise from different breeding and housing conditions, or the isolated genetic pools from which the animals were derived. Fluid balance is one parameter affected by such factors. In rats there are marked strain differences in drinking behaviour (Walsh, 1980; Fregly et 01., 1990) and food intake (Walsh, 1980; Rowland & Fregly, 1988) There is some evidence that in man the osmotic threshold for AVP release is affected by genetic or environmental factors (Schrier et 01., 1979). It is therefore conceivable that the differences in salt and water balance, seen between strains, may also occur between separate colonies of the same strain of rat. KeywordsHaving performed water and salt balance studies on Sprague-Dawley rats at 3 research institutions, we report a retrospective comparison of these parameters from the 3 colonies. Since the secretion of AVP is a major factor in the control of body fluid homeostasis, this comparison also includes data for basal and stimulated plasma AVP concentrations. In addition, there is some evidence for a diurnal variation in AVP secretion (Windle et al., 1992) and therefore the patterns of water and electrolyte turnover have been investigated during the light and dark phases. An attempt was made to interpret the findings in relation to the available information on the pathogenic status and the conditions under which the animals were housed. The first group (colony X) were bred and housed under semi-barrier conditions, as described by Lewin and Hansen (1986), the second (colony Y) under open conditions separated only from other species and the third (colony Z) were purchased from a commercial supplier and housed in isolation.