Various cells respond differentially to a second messenger (such as increased cAMP) because they have different.
Ans. is 'b' i.e. enzymatic composition Hormones or drugs require receptors for their action, some receptors are directly attached to effector molecules and some receptors are coupled to effectors by second messengers.These second messengers are so named because they intervene b/w the original message and the ultimate effect on the cell.One of the many sequence of events which follows, when a hormone or a drug combines with receptors coupled to second messengers is given below.Hormone combines with a receptor|Now receptor changes shape|Combines with Gs protein|Gs protein releases GDP and binds GTPThis activates adenylate cyclase|ATP converted to cAMP||cAMP|Activates protein kinase|Protein kinase phosphorylates serine or threoninePhosphorylated proteins|Inactivates or activates enzymesI think the answer must be obvious by now i.e.Cells respond differentially to a second messenger because the enzymes present in the cell respond differentially to increase or decrease in cAMP for e.g.Increase in cAMP will cause activation of those enzymes which require phosphorylation to get activated where as enzymes which requiring dephosphorylation for their active state are inactivated.Many students believe that the answer should be 'receptors'.Now don't get confused. Note that: second messengers respond differentially to a hormone or drug because of receptors but cells respond to second messengers differentially because of different enzymes.
Receptors
Enzymatic composition
Nuclei
Membrane lipids
Chemical Messengers: Receptors