-are located on vascular smooth muscle of the skin and splanchnic regions, the gastrointestinal (GI) and bladder sphincters, and the radial muscle of the iris.
- produce excitation (e.g., contraction or constriction).
-are equally sensitive to norepinephrine and epinephrine, but only norepinephrine is present in concentrations that are high enough to activate ai receptors.
- Mechanism of action: formation of inositol 1,4,5-triphosphate (IP3) and increase in intracellular [Ca2+].
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