Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C54
Regulation of hepatic aquaporin-9 in starved rats is gender specific
J. Lebeck1, P. Gena2, S. Lund3, G. Calamita2, J. Praetorius1
1. Institute of Anatomy, Aarhus University, Aarhus, Denmark. 2. Department of General and Environmental Physiology, University of Bari Aldo Moro, Bari, Italy. 3. Department of Endocrinology and Internal Medicine, Aarhus University Hospital, NBG, Aarhus, Denmark.
Aquaporin-9 (AQP9) is a broad-selectivity neutral solute channel that facilitates the hepatic uptake of glycerol. Once inside the cell, glycerol is phoshorylated by glycerol kinase (GlyK) into glycerol-3-phosphate, a precursor for gluconeogenesis and lipogenesis. In male rats, it is well established that hepatic AQP9 expression is increased in states with low plasma insulin levels such as starvation (1). In addition, in the fed state the hepatic abundance of AQP9 has been shown to be ~20% higher in male rats when compared to females (2). The aim of this study was to evaluate the influence of gender upon regulation of hepatic AQP9 expression in response to starvation. Values are presented as mean ± S.E.M. and compared by unpaired student t-test. Male (n=6+6) and female (n=6+6) rats were starved for 96 hours with free access to water and a more than 2-fold increase in hepatic AQP9 abundance was observed in starved males using immunoblotting and immunohistochemistry. In contrast, starvation had no effect upon hepatic AQP9 expression in female rats. Coordinately, plasma glycerol levels remained unchanged in male rats whereas it was increased from 55±6 in the fed state to 112±15 µmol/l in starved females (p<0.05). In addition, biophysical assessment of hepatocyte membrane glycerol transport in starved rats showed higher permeability in males when compared to females. The hepatic GlyK expression was not affected by starvation, however, when comparing males with females in the fed state the relative abundance was 2.1±0.2 vs. 1.0 ±0.1, respectively (p<0.0001). The effect of ovariectomy was investigated, to evaluate the role of female gender in the observed sexual dimorphism in hepatic AQP9 regulation. The animals were either ovariectomized (ovx) (n=12) or sham (n=6) operated under isoflurane inhalation anaesthesia (induction: 4%, maintenance: 2% in atmospheric air) and treated with buprenorphine preoperatively (0.025 mg/kg i.p.) and postoperatively for 5 days (0.006 mg/ml in drinking water). Starvation of ovx rats (n=7) resulted in an increased expression of AQP9 similar to what was observed in starved males when compared to fed ovx rats (n=5), whereas GlyK abundance remained unchanged. Like in starved males, no increase in plasma glycerol was observed in the starved ovx females. Overall, these results demonstrate that sexual dimorphism exists in the initial hepatic handling of glycerol in starved rats. In addition, our data suggest that female gender is involved in preventing the starvation induced increase in hepatic AQP9 abundance observed in males.
Where applicable, experiments conform with Society ethical requirements