Kinetics of Choline Uptake into Isolated Rat Forebrain Microvessels: Evidence of Endocrine Modulation

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Abstract
The active uptake of [methyl‐3H]choline into isolated rat brain microvessel suspension was studied as a likely guide to the transport of choline across the blood‐brain barrier. The method consisted primarily of incubation of the suspension with a fixed concentration of labeled choline in the presence of increasing concentrations of un‐labeled choline or any other inhibitor (I) of active uptake, denned as the difference in uptake at 37° and 0°C. From the linear regression of (1/V) against [I], the following values of Kmax (nmol g−1 min−1) and Km (μM) were obtained for choline: 2‐month‐old males, 10.6 ± 3.8 and 6.1 ± 0.9; 3‐month old random females, 28.4 ± 5.9 and 12.6 ± 4.0; females at metaestrus, 17.8 ± 10.3 and 8.3 ± 5.0; at diestrus, 31.1 ± 9.3 and 13.0 ± 2.6; at proestrus, 54.9 ± 2.2 and 14.0 ± 1.5; at estrus, 19.2 ± 2.2 and 2.6 ± 1.7. The differences between males and random females (p < 0.018) and between females at proestrus and estrus (p < 0.005) are significant. It is suggested that these inter‐ and intrasex variations in choline uptake reflect a dynamic adjustment of supply in accordance with brain demand for choline at the time of assay. Hemicholinium‐3 was an effective inhibitor of choline uptake, Ki= 14.0 ± 8.5 μM; dimethylaminoethanol was much less effective; and imipramine had no measurable effect.