Practice Test 3

In pancreatic cells, ligand G binds a GPCR coupled to Gi. Activated Gi inhibits adenylyl cyclase, decreasing cAMP. Lower cAMP reduces PKA activity, leading to decreased phosphorylation of a metabolic enzyme; the enzyme’s activity increases when dephosphorylated. A toxin is introduced that prevents Gi from binding GTP, but receptor binding by G is unchanged and phosphodiesterase activity is normal. Cells are exposed to ligand G and enzyme activity is measured within 2 minutes.

Which result is most likely in toxin-treated cells compared with untreated cells?​

In a neuron, neurotransmitter N binds a ligand-gated ion channel that opens to allow Na$^+$ influx, depolarizing the membrane. Depolarization activates a nearby voltage-gated Ca$^{2+}$ channel, increasing cytosolic Ca$^{2+}$. Ca$^{2+}$ binds synaptotagmin on vesicles, promoting vesicle fusion and additional N release. A Ca$^{2+}$-ATPase pump lowers cytosolic Ca$^{2+}$ to terminate the response. In cells treated with a Ca$^{2+}$-ATPase inhibitor, Ca$^{2+}$ stays elevated longer and vesicle fusion persists. Which change would most likely prevent the prolonged vesicle fusion despite Ca$^{2+}$-ATPase inhibition?