Hormone Transport, Receptors, Signal Transduction (3A) Practice Test

In a cell line expressing a GPCR for hormone M, investigators measure second messengers after acute M exposure. M increases cAMP but does not change IP$_3$. A point mutation is introduced into the receptor’s cytosolic region that prevents coupling to heterotrimeric G proteins while leaving ligand binding intact. After mutation, M still binds the receptor at the cell surface, but cAMP no longer increases.

Which outcome would be expected if the receptor is blocked (or uncoupled) as described?

In thyroid follicular cells, investigators track transport of two hormones: TSH (a glycoprotein peptide) and T3 (a thyroid hormone). In serum-like medium containing albumin, T3 is mostly protein-bound, whereas TSH remains largely unbound. Cells are exposed to equal total concentrations of TSH and T3. TSH produces a rapid increase in intracellular cAMP, while T3 produces no immediate change in cAMP but increases transcription of a metabolic gene after several hours.

Which statement best accounts for these observations?