In-vivo and In-vitro Evidence of a Carrier-mediated Efflux Transport System for Oestrone-3-sulphate across the Blood-Cerebrospinal Fluid Barrier

Abstract

The efflux transport of oestrone-3-sulphate, a steroid hormone sulphate, across the blood-cerebrospinal fluid barrier has been examined following its intracerebroventricular administration. [3H]Oestrone-3-sulphate was eliminated from cerebrospinal fluid (CSF) with an apparent efflux clearance of 205 μL min−1 per rat. There was 25% of unmetabolized [3H]oestrone-3-sulphate in the plasma 5 min after intracerebroventricular administration, indicating that at least a part of [3H]oestrone-3-sulphate is transported from CSF to the circulating blood across the blood-CSF barrier. This efflux transport was inhibited by co-administration of excess oestrone-3-sulphate (25 mm 10 μL = 0.25 μmol) into rat cerebral ventricle.To characterize the oestrone-3-sulphate transport process, an in-vitro uptake experiment was performed using isolated rat choroid plexus. Oestrone-3-sulphate uptake by isolated rat choroid plexus was found to be a saturable process with a Michaelis-Menten constant (Km) of 18.1 ± 6.3 μm, and a maximum uptake rate (Vmax) of 48.0± 15.1 pmol min−1 μL−1 of tissue. The oestrone-3-sulphate transport process was temperature dependent and was inhibited by metabolic inhibitors such as 2,4-dinitrophenol and rotenone, suggesting an energy dependence. This uptake process was also inhibited by steroid hormone sulphates (1 mm dehydroepiandrosterone sulphate and 1 mm oestrone sulphate), bile acids (1 mm taurocholic acid and 1 mm cholic acid) and organic anions (1 mm sulphobromophthalein and 1 mm phenolsulphonphthalein), whereas 1 mm  p-aminohippuric acid, 1 mm  p-nitrophenol sulphate, 0.1 mm methotrexate and the cardiac glycoside, 2.5 μm digoxin, had little effect.In conclusion, these results provide evidence that oestrone-3-sulphate is transported from CSF to the circulating blood across the blood-CSF barrier via a carrier-mediated efflux transport system.