Assay of muscarinic acetylcholine receptor function in cultured cardiac cells by stimulation of 86Rb+ efflux

An assay for the increase in potassium permeability mediated by muscarinic acetylcholine receptors (mAChR) in cultured cardiac cells is described, using the K+ ion substitute 86Rb+ as the tracer ion. Cardiac cells accumulate 86Rb+ from the extracellular medium in a Na+/K+ ATPase-dependent manner. Subsequent efflux of 86Rb+ in the absence and presence of muscarinic agonists follows kinetics similar to those previously reported for 42K+. The mAChR agonist carbamylcholine (carbachol) stimulated 86Rb+ efflux with an EC50 of 50 nM. The half-time for efflux is reduced by greater than 40% at maximally effective concentrations of agonist. Stimulation of 86Rb+ efflux by carbachol is blocked by the mAChR antagonist atropine with an IC50 of 15 nM. The stimulation of 86Rb+ efflux by carbachol is not affected by the presence of the Na+/K+ ATPase inhibitor ouabain. This assay provides a method for quantitating the mAChR-mediated increase in K+ permeability in cardiac cells without the use of 42K+.

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

Treatment of chicken embryos in ovo for 8 hr 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 hr. 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 for carbachol is shifted approximately equal to 10-fold from controls at 20 hr after atropine treatment, but less than 3-fold at 28 hr, with no further change in receptor number occurring over this time. This increase in physiological sensitivity is not blocked by cycloheximide. Receptors at 20 hr 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 approximately equal to 11-fold from controls at 20 hr after atropine treatment, yet only 3-fold at 28 hr. Thus, newly synthesized mAcChoR exhibit decreased physiological and biochemical responses to muscarinic agonists, suggesting that mAcChoR are initially synthesized in a less active form.

Partial agonist activity of oxotremorine at muscarinic acetylcholine receptors in the embryonic chick heart

Previous work has demonstrated that prolonged treatment of chick embryos in vivo with muscarinic agonists leads to a decrease in the number of muscarinic acetylcholine binding sites in the heart and a decrease in the sensitivity of isolated atria to the negative chronotropic response to muscarinic stimulation. We show here that treatment with the agonist carbachol leads to greater decreases in receptor number than does treatment with oxotremorine. Simultaneous treatment with oxotremorine and carbachol resulted in partial blockage in the maximal decrease in receptor number seen after treatment with carbachol alone. After treatment of embryos in vivo with carbachol to decrease the reserve of spare receptors in the atria, oxotremorine could no longer elicit a significant negative chronotropic response under conditions in which carbachol was still effective. These results demonstrate that oxotremorine is a partial agonist at the muscarinic acetylcholine receptor in the embryonic chick heart.