Characterization of choline efflux from the perfused heart at rest and after muscarine receptor activation

Summary

The resting efflux of choline from perfused chicken hearts varied from 0.4 to 2.6 nmol/g min, but was constant for at least 80 min in the individual experiments. The rate of choline efflux was found to be equal to the rate of choline formation in the heart, which, from the following reasons, was essentially due to hydrolysis of choline phospholipids. (1) Cardiac content of choline phospholipids (7,200 nmol/g) was much higher than that of acetylcholine (5.5 nmol/g). (2) Resting release of acetylcholine was 0.016 nmol/g min and, after inhibition of cholinesterase, only about 0.1 nmol/g min.

Resting efflux of choline was reduced by mepacrine, a phospholipase A₂ inhibitor, by perfusion with a Ca²⁺-free Tyrode's solution containing EGTA and by the combination mepacrine plus Ca²⁺-free/EGTA solution. In all experiments the reduced choline efflux levelled off within 10 min at about 50%. Omission or elevation of Mg²⁺ from 1.05 to 10.5 mmol/l had no effect. Resting efflux was increased to 150% by oleic acid (as sodium salt; 2×10⁻⁵ mol/l) which is known to activate phospholipase D. Likewise muscarinic agonists (carbachol and acetylcholine) caused facilitation of the efflux of endogenous choline that was blocked by 3×10⁻⁷ mol/l atropine. This effect was not reduced, but even slightly enhanced, by mepacrine and by infusion of EGTA in a modified Tyrode's solution (Ca²⁺-free, 10.5 mmol/l Mg²⁺).

It is concluded that the resting efflux of choline from the heart is essentially due to hydrolysis of choline phospholipids, that half of the efflux is insensitive to mepacrine and is Ca²⁺-independent (excluding an involvement of phospholipase A₂). Moreover, this Ca²⁺-independent efflux is facilitated by muscarinic agonists, possibly through an effect on phospholipase D activity.