[Intracellular transmission of the cholinergic signal in the chick amnion]

The role of the system of deposited calcium in the mediation of contractile reactions to carbachol in an isolated amnion of 11-13 day old chicken embryo was studied. It was found that thapsigargin (2 microM, 20 min), an inhibitor of the endoplasmic reticulum Ca2+ -ATPases, decreases the tonic reaction to carbachol by 40 +/- 2%. In the presence of U73122 (5-10 microM, 10 min), a phosphoinositide-specific phospholipase C inhibitor, the rhythmic contractile reaction of the amnion to carbachol is blocked, whereas the tonic reactiondecreases to 47 +/- 9% of the initial one. Ryanodine (10 rM, 5 min) inhibits the spontaneous contractile activity of the amnion and decreases the tonic reaction to carbachol to 36 +/- 3% relative to control. In the presense of ryano- dine, nifedipine (0.05 microM) completely blocks the tonic reaction to carbachol. Thus, calcium mobilized from intracellular stores via inositol trisphosphate and ryanodine receptors is involved in realization of contractile reactions, mediated by M3 receptors, in the chick amnion.