Acetylcholine incorporation by cholinergic synaptic vesicles from Torpedo marmorata

Abstract

The ability of cholinergic vesicles to incorporate acetylcholine (ACh) was studied using highly purified synaptic vesicles from Torpedo electric organ. Depleted vesicles were capable of rapidly taking up exogenous ACh. Evidence that this represented true incorporation was that labelled ACh comigrated with vesicular ATP on gel filtration and that vesicle-associated ACh was protected against enzymatic hydrolysis and was releasable under hypoosmotic conditions. The total amount of ACh incorporated depended on the ACh concentration up to 100 mM. A sudden fall in the external ACh concentration did not cause leakage of the ACh incorporated in vitro. Preliminary results indicated that retention of ACh inside the vesicle was pH-dependent. Choline was also taken up by vesicles, but the time pattern strongly suggested that it was not being retained. The magnitude of ach incorporation was estimated with respect to the intravesicular space.