Acetylcholine acts at a GPCR on heart muscle to make the heart beat more slowly
It does so by ultimately opening K+ channels in the plasma membrane (as diagrammed in Figure Q16-32), which decreases the cell's excitability by making it harder to depolarize the plasma membrane.
Indicate whether each of the following conditions would increase or decrease the effect of acetylcholine.
A. addition of a drug that stimulates the GTPase activity of the G? subunit
B. mutations in the K+ channel that keep it closed all the time
C. modification of the G? subunit by cholera toxin
D. a mutation that decreases the affinity of the ?? complex of the G protein for the K+ channel
E. a mutation in the acetylcholine receptor that prevents its localization on the cell surface
F. adding acetylcholinesterase to the external environment of the cell
A. Decrease. An increase in the GTPase activity of the G? subunit will decrease the length of time that the G protein is active.
B. Decrease. If the K+ channel remains closed, acetylcholine will not slow the heart.
C. Increase. Cholera toxin inhibits the GTPase activity of the G? subunit, keeping the subunit in an active state for a longer time.
D. Decrease. The activated ?? complex binds to and activates the K+ channel; decreasing their affinity for each other will decrease the time that the K+ channel is open, effectively decreasing the effect of acetylcholine.
E. Decrease. If there is no receptor on the cell surface, cells will be unable to respond to acetylcholine.
F. Decrease. Acetylcholinesterase degrades acetylcholine and thus will decrease the effect of acetylcholine.
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