Bicarbonate transport across the frog choroid plexus and its control by cyclic nucleotides.

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Abstract
The effects of ions, hormones, diuretics and cyclic nucleotides on the short-circuit current (Isc) across the frog choroid plexus were studied. In normal HCO3 Ringer solution, the trans-epithelial potential difference (p.d.), Isc and resistance (Rt) were -0.8 mV (ventricular side negative with respect to the blood (serosal) side), 6 .mu.A/cm2 and 170 .OMEGA. cm2, respectively. Removal of Na, Cl or HCO3 from the solution decreased the Isc to 1.9, 2.3 or -4.6 .mu.A/cm2, respectively. Theophylline, dibutyryl cAMP, isoproterenol, prostaglandin E1, ACTH, cholera toxin and forskolin all significantly increased the Isc. The theophylline-induced change in Isc .**GRAPHIC**. was reduced by 50% upon Cl substitution with gluconate, and was abolished to < 12% by Na-free and HCO3-free solutions. pH monitoring of the bathing solutions showed that acidification of the serosal bathing fluid was enhanced by theophylline while that of the ventricular solution was retarded. Ouabain, acetazolamide, SITS [4-acetamido-4''-isothiocyanostilbene-2,2''-disulfonic acid], DIDS [4,4''-diisothiocyanostilbene-2,2''-disulfonic acid] and furosemide inhibited both Isc and .**GRAPHIC**. HCO3 secretion by the choroid plexus into the CSF is controlled by hormones which stimulate the adenylate cyclase system. A model of ion transport across the choroidal epithelium is proposed.