Variations in epithelial Na transport and epithelial sodium channel localisation in the vaginal cul-de-sac of the brushtail possum, Trichosurus vulpecula, during the oestrous cycle

The fluid in the vaginal cul-de-sac of the brushtail possum, Trichosurus vulpecula, is copious at ovulation when it may be involved in sperm transport or maturation, but is rapidly reabsorbed following ovulation. We have used the Ussing short-circuit current (Isc) technique and measurements of transcript and protein expression of the epithelial Na+ channel (ENaC) to determine if variations in electrogenic Na+ transport are associated with this fluid absorption. Spontaneous Isc (<20 µA cm–2 during anoestrus, 60–80 µA cm–2 in cycling animals) was inhibited by serosal ouabain. Mucosal amiloride (10 µmol L–1), an inhibitor of ENaC, had little effect on follicular Isc but reduced luteal Isc by ~35%. This amiloride-sensitive Isc was dependent on mucosal Na+ and the half-maximal inhibitory concentration (IC50)–amiloride (0.95 μmol L–1) was consistent with ENaC-mediated Na+ absorption. Results from polymerase chain reaction with reverse transcription (RT-PCR) indicate that αENaC mRNA is expressed in anoestrous, follicular and luteal phases. However, in follicular animals αENaC immunoreactivity in epithelial cells was distributed throughout the cytoplasm, whereas immunoreactivity was restricted to the apical pole of cells from luteal animals. These data suggest that increased Na+ absorption contributes to fluid absorption during the luteal phase and is regulated by insertion of ENaC into the apical membrane of cul-de-sac epithelial cells.