Human placental cholinergic system: stimulation-secretion coupling for release of acetylcholine from isolated placental villus.

Isolated villus from human term placenta contains about 167 nmol/g of acetylcholine (ACh). It was incubated in a muscle bath containing Krebs-Ringer bicarbonate buffer (pH 7.2-7.4) at 37 degrees C and ACh released into the medium was analyzed by pyrolysis-gas chromatography. The spontaneous release of ACh into the medium was linear with time and was 35 pmol/g/min. ACh was not released when Ca++ was absent in the medium. Raising the Ca++ concentration in the Krebs-Ringer bicarbonate buffer from 2.34 to 4.64 mM, or adding L-nicotine (58 muM) to the bath, increased the rate of release of ACh to 53 and 47 pmol/min respectively. Nicotine did not exhibit any effect on ACh release in the absence of Ca++ in the medium. Both the rate of the spontaneous release of ACh and the nicotine-induced increase in the release of ACh was decreased by atropine (152 muM). They were not influenced by d-tubocurarine (30 muM). Depolarizing concentrations of potassium (16-63 mM) in the medium increased the rate of release of ACh. Cocaine, a known Ca antagonist, decreased the rate of spontaneous release of ACh as well as nicotine-induced release of ACh. These observations indicate that 1) Ca++ ions in the external medium are required for release of ACh, 2) Ca++ ions in the external medium are required for release of ACh, 2) Ca++ ions act as a link between the stimulation of ACh release and the final release of ACh and 3) The effect of nicotine on placental release of ACh may be classified as a muscarinic type.