The organic cation transport system in choroid plexus epithelium functions as part of the """"""""blood-cerebral spinal fluid barrier"""""""" in regulating the concentrations of various organic cations in the cerebral spinal fluid (CSF and thus the extracellular fluids of the brain. The overall goals of the proposed studies are to elucidate the molecular events involved in the transport of organic cations across the choroid plexus epithelial cells. T attain these goals, sequential studies will be carried out on isolated plasma membrane vesicles and in cultured bovine choroid plexus epithelial cells. First, the molecular events involved in the transport of organic cations across the individual plasma membranes will be studied using isolated brush border and basolateral membrane vesicles prepared from choroid plexus epithelium. It will be determined whether there are specific, saturable transport mechanisms in each membrane. Afterwards, the electrogenecity of the transport across each membrane will be determined to obtain information about whether organic cations may be accompanied with anions or may exchange with cations during the transport process across the individual membranes. Next, studies will be carried out to identify a possible driving force for organic cation transport. It is hypothesized that active organic cation transport is secondarily active and thus driven by an ion gradient across one membrane. Several models are being proposed. For example, in one model it is proposed that organic cations are transported across the brush border membrane by a passive, facilitated process and accumulate in choroid plexus cells as a result of the favorable potential difference. Transport across the basolateral membrane is necessarily active and involves a sodium exchange mechanism. Finally, studies will be carried out to determine whether the transporter in each membrane functions as a simple pore or a mobile carrier. The studies in cultured choroid plexus epithelium are designed to address questions relate to the active accumulation of organic cations in intact epithelium. First, the studies will determine whether organic cations are accumulated in the cells by active, saturable and structurally specific mechanisms. Then usin monolayers of the cultured epithelial cells placed in flux chambers, the preferred membrane (brush border or basolateral) through which organic cations are transported into and out of the cell will be identified. It is hypothesized that organic cations are actively transported from CSF to blood; thus, the brush border (ventricular) membrane would be the preferred membrane for influx and the basolateral (serosal) membrane would be preferred for efflux. Studies of transepithelial flux across the cultured monolayers will be carried out to elucidate the vectorial direction of net organic cation flux. The relevance of the findings in the vesicles to transport across the intact cell will be elucidated. N1-methylnicotinamide and choline will be used as model organic cations and the methods will involve isotopic techniques with the tritiated compounds. Clinically many important drugs as well as potent endogenous compounds including various neurotransmitters and toxins are transported by the choroid plexus. Transport via this system ultimately controls the concentrations of many basic compounds in the CSF and thus, may influence their biologic effects. For example, the use of lidocaine, a clinically important antiarrhythmic agent, is limited by its neurological toxicities. Transport in the choroid plexus may control the concentrations of lidocaine in the cerebral spinal fluid thereby influencing the neurological toxicities. Recently, it has been hypothesized that the organic cation transport system of the choroid plexus may play a role in the etiology of Parkinson's Disease. These studies will ultimately lead to a more rational use of drugs and to an enhanced awareness of the mechanisms involved in the transport of biologically active organic cations across the """"""""blood-CSF-barrier"""""""".
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