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(increase) the number of receptors for that ligand. In that way, the cell can be more responsive to lower concentrations of ligand. Alternatively, long periods of high ligand concentration may down regulate that ligand's receptor population, thereby reducing the cell's responsiveness to the ligand. Finally, a cell's responsiveness to a ligand is determined by the affinity of the receptor for the ligand. As the receptor affinity increases, more receptors will be bound at the same ligand concentration. This is especially important when there are several classes of receptors that have different affinities for a ligand such as epinephrine. Two types of receptors are capable of bindingthe catecholamine epinephrine: α and β2adrenergic receptors. Both of these receptors are found on blood vessels and have the opposite effect on those vessels: α adrenoreceptors induce vasoconstriction, whereas β2adrenoreceptors induce vasodilation. However, in the presence of epinephrine, the β2adrenoreceptor-induced vasodilation will predominate because the β2adrenoreceptor has a greater affinity for epinephrine than the α adrenoreceptor.Diff: 7 Page Ref: 1364) Describe the function of the fast channel-linked receptors, including the role of calcium in their response.