Vascular Cholinesterases and Choline Uptake in Isolated Rat Forebrain Microvessels: A Possible Link

Abstract: The two parameters of the active [methyl‐³H]choline uptake into isolated rat forebrain microvessels, K_m and V_max, were determined for 1‐, 3‐, l0‐, and 24‐month‐old Charles River male rats and compared with the activities of the enzymes choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and butyrylcholinesterase (BuChE) in these microvessels over the same time course. The value of K_m remained constant over the entire period, but that of V_maxincreased from 8.5 ± 1.0 to 80.6 ± 16.4 nmol g⁻¹ (mean ± SEM) over the first 3 months of life. Over the same period, the increase in ChAT activity, from an initial value of 7.1 ± 1.6 to 10.2 ± 0.3 nmol g⁻¹ min⁻¹, was not proportional to that of choline uptake. Levels of BuChE activity (0.9‐1.3 μmolg⁻¹ min⁻¹) were almost unchanged throughout the entire 24‐month period, but those of AChE showed a steady and significant increase from 1 to 24 months, remaining relatively high at senescence (4.7 μmolg⁻¹ min⁻¹), when choline uptake had decreased to one‐third of its optimal value. Selective inhibition of AChE with l,5‐bis(4‐allyldimethylammonium‐phenyl)pentan‐3‐one dibromide (0.5 μM) in unruptured capillaries from 3‐month‐old rats resulted in a decrease in V_max of choline uptake from ∼81 to 59 nmol g⁻¹ min⁻¹ or with 9‐amino‐l,2,3,4‐tetrahydroacridine (10 μM) in capillaries from 2‐month‐old rats from ∼ 30 to 15 nmol g⁻¹ min⁻¹. Selective inhibition of BuChE with tetraisopropyl pyrophos‐phoramide (100 μM) resulted in an increase in V_max from ∼81 to 96 nmol g⁻¹ min⁻¹. It is possible that the two vascular enzyme systems are coupled to a hypothetical endothelial choline transporter, but with an action opposite to each other.