In the epinephrine pathway, an inhibitor of phosphodiesterase activity would have which of the following effects? A. block the activation of protein kinase A B. block the activation of G proteins in response to epinephrine binding to its receptor C. prolong the effect of epinephrine by maintaining elevated cAMP levels in the cytoplasm D. block the response of epinephrine

Answer: C). prolong the effect of epinephrine by maintaining elevated cAMP levels in the cytoplasm

Explanation: In the epinephrine pathway, binding of epinephrine to its receptor triggers a conformational change in the receptor and the interaction of the receptor with its associated Gs protein. This interaction causes the replacement of GDP bound to Gs protein with GTP thus activating the Gs protein. The activation of the Gs protein causes the alpha subunit of the Gs protein to dissociate and move to adenylyl cyclase, another membrane protein in the pathway. The association of the alpha subunit of the Gs protein with adenylyl cyclase activates adenylyl cyclase which in turn catalyzes the synthesis of cyclic AMP (cAMP) a second messenger. cAMP is quickly degraded to 5'-AMP by an enzyme phosphodiesterase. Inhibition of the activity of phosphodiesterase will increase the half life and the cytoplasmic level of cAMP thus potentiating the action of epinephrine.