Acetylcholine in human term placenta: tissue levels in intact fragments after inhibition in vitro of choline acetyltransferase and relationship to [14C]alpha-aminoisobutyric acid uptake.

Abstract

Choline acetyltransferase (ChAc), the enzyme catalysing the biosynthesis of acetylcholine (ACh) in the non-innervated human placenta, was rapidly and persistently inhibited by (2-benzoylethyl)trimethylammonium (BETA) when the drug was applied to intact tissue fragments. This inhibition (50 per cent at congruent to 0.4 mmol/1 BETA) was coupled to a concomitant reduction in the active uptake against a concentration gradient of the nonmetabolizable amino acid alpha-aminoisobutyric acid (AIB). The reduction of AIB accumulation (50 per cent at congruent to 0.1 mmol/1 BETA) was temporally related to inhibition of ChAc. These effects suggest that AIB uptake by the human placenta and ACh biosynthesis catalysed by ChAc are related. Measurements of total ACh content in tissue samples treated in parallel with those destined for ChAc and AIB uptake determinations revealed that BETA (3 mmol/1) significantly reduced the ACh levels by 35 to 50 per cent. This drug concentration caused almost complete inhibition of ChAc and blockade of AIB accumulation.