5. Molecules that act as agonists for a-adrenergic receptors include the physiological ligands epinephrine and norepinephrine, and the synthetic ligand phenylephrine. The dissociation constants (K.) for epinephrine, norepinephrine, and phenylephrine binding to the a,-adrenergic receptor are 63nM, 1961M, and 239nM, respectively. You place cells containing a;-adrenergic receptors in three separate tubes labelled #1, #2, and #3. In tube #1 you add 200NM epinephrine, in tube #2 you add 200NM norepinephrine, and in tube #3 you add 200nM phenylephrine. Which tube of cells will have the highest percentage of a;-adrenergic receptors bound to ligand?

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### Question 5

Molecules that act as agonists for α₁-adrenergic receptors include the physiological ligands epinephrine and norepinephrine, and the synthetic ligand phenylephrine. The dissociation constants (K_d) for epinephrine, norepinephrine, and phenylephrine binding to the α₁-adrenergic receptor are 63nM, 196nM, and 239nM, respectively. You place cells containing α₁-adrenergic receptors in three separate tubes labelled #1, #2, and #3. In tube #1 you add 200nM epinephrine, in tube #2 you add 200nM norepinephrine, and in tube #3 you add 200nM phenylephrine. Which tube of cells will have the highest percentage of α₁-adrenergic receptors bound to ligand?

#### Explanation:
To determine which tube has the highest percentage of α₁-adrenergic receptors bound to ligand, we consider the dissociation constant (K_d) of each ligand. The lower the K_d, the higher the affinity of the ligand to the receptor, implying more receptors will be bound at the same ligand concentration:
- **Tube #1 (epinephrine):**  K_d = 63nM
- **Tube #2 (norepinephrine):** K_d = 196nM
- **Tube #3 (phenylephrine):** K_d = 239nM

Since the K_d value for epinephrine (63nM) is the lowest, tube #1 will have the highest percentage of α₁-adrenergic receptors bound to ligand.
Transcribed Image Text:### Question 5 Molecules that act as agonists for α₁-adrenergic receptors include the physiological ligands epinephrine and norepinephrine, and the synthetic ligand phenylephrine. The dissociation constants (K_d) for epinephrine, norepinephrine, and phenylephrine binding to the α₁-adrenergic receptor are 63nM, 196nM, and 239nM, respectively. You place cells containing α₁-adrenergic receptors in three separate tubes labelled #1, #2, and #3. In tube #1 you add 200nM epinephrine, in tube #2 you add 200nM norepinephrine, and in tube #3 you add 200nM phenylephrine. Which tube of cells will have the highest percentage of α₁-adrenergic receptors bound to ligand? #### Explanation: To determine which tube has the highest percentage of α₁-adrenergic receptors bound to ligand, we consider the dissociation constant (K_d) of each ligand. The lower the K_d, the higher the affinity of the ligand to the receptor, implying more receptors will be bound at the same ligand concentration: - **Tube #1 (epinephrine):** K_d = 63nM - **Tube #2 (norepinephrine):** K_d = 196nM - **Tube #3 (phenylephrine):** K_d = 239nM Since the K_d value for epinephrine (63nM) is the lowest, tube #1 will have the highest percentage of α₁-adrenergic receptors bound to ligand.
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