Choline+: A substrate of the neuronal noradrenaline carrier in the rat vas deferens

Abstract

The effects of choline+ (10-40 mmol/l) on 3H-noradrenaline uptake by, and 3H-noradrenaline efflux from, noradrenergic neurones were studied in vasa deferentia of reserpine-pretreated rats at an external Na+ concentration of 100 mmol/l. Monoamine oxidase and catechol-O-methyltransferase were inhibited. Choline+ (20 and 40 mmol/l) competitively inhibited the neuronal uptake of 3H-noradrenaline. From the choline+-induced changes in the apparent Km for 3H-noradrenaline transport, a Ki of 35 mmol/l was obtained. Choline+ (10, 20 and 40 mmol/l) accelerated the neuronal efflux of 3H-noradrenaline in a concentration-dependent manner. This acceleration of efflux was greatly reduced in the presence of 1 mumol/l desipramine, indicating that choline+ is capable of eliciting "accelerative exchange diffusion". Choline+ (40 mmol/l) and (-)noradrenaline (4.5 mumol/l) (i.e., concentrations about equivalent to the Ki and Km for choline+ and (-)noradrenaline, respectively) produced virtually identical increases in the neuronal efflux of 3H-noradrenaline. Choline+ (3-300 mmol/l) inhibited the specific binding of 3H-desipramine to plasma membranes derived from cultured rat phaeochromocytoma (PC-12) cells. The Ki for this interaction was 48 mmol/l. This results suggest that choline+ acts as alternative substrate of the neuronal noradrenaline transport system and should, therefore, not be used in transport studies with noradrenaline as substitute for Na+ in Na+-deficient media.