Diurnal Rhythms in GFR Involve Both Tryptophan and Testosterone in Male BMAL1 Knockout Rats

Kidney function varies with the time of day, independent of external factors such as light exposure or meal timing. Research has unveiled that this diurnal (day/night) regulation is driven by an intrinsic molecular clock within the kidneys. In our novel rat model where the core clock gene, brain and muscle ARNT-like protein 1 (Bmal1) is knocked out (KO), we observed impaired rhythmic excretion of electrolytes and glomerular filtration rate (GFR). Male and female KO and littermate control (CON) rats aged 12-14 weeks underwent GFR measurements using FITC-labeled sinistrin clearance at two times of the day corresponding to the dark (active) and light (inactive) periods. Rats were placed in metabolic cages to record food, water, and urine excretion over 12h active and inactive periods. At baseline, male KO rats have no significant diurnal changes in both GFR (day:1.3±0.1 vs night:1.2±0.1mL/min/100gBW; p=0.919) and sodium excretion (UNaV) (day:819±58 vs night:756±113 μmol/12h; p=0.439) compared to CON (GFR day:1.1±0.1 vs night:1.6±0.1mL/min/100g; p=0.044; UNaV day: 849±138 vs night:1423±102 μmol/12h; p=0.003). Rhythms in renal function require intact renal metabolic rhythms so we sought to uncover metabolic factors impacted by the loss of Bmal1. We first determined amino acid bioavailability, known to have a profound influence on GFR and UNaV. An analysis of plasma by LC/MS from KO rats found that levels of tryptophan (trp) were significantly (p Interaction =0.847, pGenotype=0.041, p TimeofDay =0.952) lower in KO males during the day (KO:77±6, CON:99±7μM; post-hoc p=0.049) but not night (KO:78±5, CON:99±12μM; post-hoc p=0.084) with no differences in plasma trp seen in KO females (day:110±26,107±14μM vs night:96±8,96±12μM KO and CON respectively). Trp administration can increase GFR. In addition, trp can both regulate and be regulated by the circadian clock. This led us to hypothesize that time of day trp availability may underlie the changes in both UNaV and GFR in male KO rats. Therefore, we gave a single dose of trp (100 mg/kg,PO) at either 7pm (lights off, ZT12) or 7am (lights on, ZT0). After a 1 wk washout period, rats were dosed again in a crossover design. Male KO rats receiving trp at ZT0 had no significant change from baseline in UNaV (818±58 vs 784±61 μmol/12h; baseline and post respectively) over the following 12hrs; however, a significant increase in UNaV (756±114 vs 1152±146 μmol/12h; baseline and post respectively) following ZT12 dosing was observed (p Interaction =0.038, p TrpTime =0.087, p TimeofDay =0.210; Sidak post-hoc: baseline vs ZT12 trp p=0.003) with no significant change seen in female KO or either male or female CON rats. GFR was similarly impacted with male KO rats having significant increases in GFR following ZT12 trp compared to ZT0 (1.3±0.1 vs 0.9±0.1 mL/min/100g; p=0.028). To interrogate the role of sex-steroids in this male specific response we measured levels of testosterone (T) and found that KO males had significantly lower plasma T (122.7±10.8 ng/ml) during the active period compared to WTs (168.8±15.1; p=0.020). To determine whether T impacted diurnal GFR, we performed orchiectomy (ORX) in WT rats to reduce levels of T. ORX in WT rats decreased GFR exclusively in the active period (0.8±0.1 vs 1.5±0.2 mL/min/100g) and abolished time of day rhythms (p Interaction =0.046, p ORX =0.002, p TimeofDay =0.012). Taken together these results suggest that BMAL1 is necessary for maintaining kidney functional rhythms that involve both tryptophan and testosterone. NIH NIDDK This abstract was presented at the American Physiology Summit 2025 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.