Decreased physiological sensitivity mediated by newly synthesized muscarinic acetylcholine receptors in embryonic chicken heart.

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Abstract
Treatment of chicken embryos in ovo for 8 h with the muscarinic agonist carbachol results in an 85% reduction in the number of muscarinic acetylcholine receptors (mAcChoR) present in atrial membrane homogenates. Subsequent treatment of embryos with the muscarinic antagonist atropine results in a gradual increase in mAcChoR number, which returns to control levels after 14 h. This recovery of receptor number is blocked by administration of the protein synthesis inhibitor cycloheximide, consistent with previous results in cell culture, which suggested that de novo protein synthesis is required for the recovery of mAcChoR after agonist-induced decreases. Measurements of the negative chronotropic response to applied carbachol with isolated atria show that even after recovery of receptor number to control levels the response to agonist is diminished. The IC50 [median inhibitory concentration] for carbachol is shifted .apprxeq. 10-fold from controls at 20 h after atropine treatment, but < 3-fold at 28 h, with no further change in receptor number occurring over this time. This increase in physiological sensitivity is not blocked by cycloheximide. Receptors at 20 h have binding constants for agonists and antagonists that are indistinguishable from controls. This implies that there is a defect in coupling of mAcChoR binding to the physiological response when mAcChoR reappear following agonist-mediated decreases in receptor number. Recovery of the ability of mAcChoR to inhibit adenylate cyclase parallels recovery of the beating response, that is, the IC50 is shifted .apprxeq. 11-fold from controls at 20 h after atropine treatment, yet only 3-fold at 28 h. Newly synthesized mAcChoR exhibit decreased physiological and biochemical responses to muscarinic agonists, suggesting that mAcChoR are initially synthesized in a less active form.