In-vivo and in-vitro evidence of a carrier-mediated efflux transport system for oestrone-3-sulphate across the blood-cerebrospinal fluid barrier

The efflux transport of oestrone-3-sulphate, a steroid hormone sulphate, ac ross the blood-cerebrospinal fluid barrier has been examined following its intracerebroventricular administration. [H-3]Oestrone-3-sulphate was elimin ated from cerebrospinal fluid (CSF) with an apparent efflux clearance of 20 5 mu L min(-1) per rat. There was 25% of unmetabolized [H-3]oestrone-3-sulp hate in the plasma 5 min after intracerebroventricular administration, indi cating that at least a part of [H-3]oestrone-3-sulphate is transported from CSF to the circulating blood across the blood-CSF barrier. This efflux tra nsport was inhibited by co-administration of excess oestrone-3-sulphate (25 mM10 mu L =0.25 mu mol) into rat cerebral ventricle. To characterize the oestrone-3-sulphate transport process, an in-vitro upta ke experiment was performed using isolated rat choroid plexus. Oestrone-3-s ulphate uptake by isolated rat choroid plexus was found to be a saturable p rocess with a Michaelis-Menten constant (K-m) of 18.1 +/- 6.3 mu M, and a m aximum uptake rate (V-max) of 48.0 +/- 15.1 pmol min(-1) mu L-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 oe strone sulphate), bile acids (1 mM taurocholic acid and 1 mM cholic acid) a nd organic anions (1 mM sulphobromophthalein and 1 mM phenolsulphonphthalei n), whereas 1 mM p-aminohippuric acid, 1 mM p-nitrophenol sulphate, 0.1 mM methotrexate and the cardiac glycoside, 2.5 mu M digoxin, had little effect . In conclusion, these results provide evidence that oestrone-3-sulphate is t ransported from CSF to the circulating blood across the blood-CSF barrier v ia a carrier-mediated efflux transport system.