Bmal1 is a core regulator of the molecular circadian clock which allows the kidney to maintain day/night rhythms in physiologic function (e.g. sodium excretion and glomerular filtration rate). Previous studies in male global Bmal1 knockout (KO) rats have found elevated water intake and urine volume compared to their wild-type (WT) littermate controls. This was not observed in female Bmal1 KO rats. Water intake and urine output are controlled at multiple levels. The supraoptic nucleus in the brain acts as the central osmotic sensor detecting changes in plasma osmolality. The kidneys act to reabsorb water and sodium in response to changes in multiple physiologic and hormonal stimuli (e.g. aldosterone) to ultimately concentrate urine. With these mechanisms in mind, we hypothesized that the elevated water intake and urine output in male Bmal1 KO rats was the result of a re-setting of the central osmotic sensor in the supraoptic nucleus rather than from a defect in urine-concentrating ability. To test our hypothesis, male Bmal1 KO and WT rats were housed in metabolic cages to measure food intake, water intake, and urine volume every 12 hours corresponding to the light and dark periods. Animals underwent a hydration protocol consisting of three phases: (1) 48h hydration (5% sucrose water), (2) 24h water deprivation, and (3) 48h rehydration (normal drinking water restored). To determine if the time-of-day when the protocol was initiated impacted the outcomes Cohort 1 rats began the protocol at ZT0 (lights on) and Cohort 2 at ZT12 (lights off). Blood was collected (jugular vein) at baseline (before sucrose hydration) and at the end of each phase. Blood was immediately spun down to collect plasma which was then assayed for osmolarity using a freezing point osmometer. Urine was also collected at the end of each phase and aliquoted for measurement of sodium, potassium, and chloride using an ion-selective electrode system and osmolarity using a freezing point osmometer. In Cohort 1 rats, the hydration-dehydration protocol had an overall effect of genotype on plasma osmolarity (2-way ANOVA, pGenotype = 0.0008, pHydration < 0.0001, pInteraction = 0.0043). Post-hoc analysis showed that at baseline, following dehydration, and after rehydration, Bmal1 KO rats had significantly lower plasma osmolarity compared to WT controls: 281 ± 6 vs. 308 ± 5 mosm/L, p< 0.0001, at baseline, 306 ± 3 vs. 322 ± 5 mosm/L, p = 0.0068, following dehydration, and 286 ± 2 vs. 301 ± 2 mosm/L, P = 0.0116, after rehydration. However, there were no significant genotype differences in urine osmolarity during any phase, indicating that male Bmal1 KO rats did not exhibit a urine-concentrating when the protocol began at ZT0. These findings support our hypothesis that loss of Bmal1 lowers the plasma osmotic set-point in male rats. In contrast, in Cohort 2 (protocol starting at ZT12), Bmal1 KO males showed no significant differences in plasma osmolarity compared to WT across the hydration protocol and maintained normal plasma osmolarities in the range of 295-305 mosm/L. Instead, there was a significant genotype effect on urine osmolarity (2-way ANOVA, pGenotype = 0.0483, pHydration < 0.0001, pInteraction = 0.6423) with KO rats exhibiting a reduced ability to concentrate urine following 24h of hydration (381 ± 57 vs. 825 ± 149 mosm/L; p = 0.0385) and dehydration (1666 ± 120 vs. 2300 ± 139 mosm/L; p = 0.0144). Interestingly, these findings support a time-of-day-dependent difference in the dehydration response of Bmal1 KO male rats, supporting a potential role for the molecular clock in water-balance regulation. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Best et al. (Fri,) studied this question.