REVIEW In general, alpha1-adrenergic receptors mediate contraction and hypertrophic growth of smooth muscle cells. alpha1-receptors are 7-transmembrane domain receptors coupled to G proteins, Gq/11. Three alpha1-receptor subtypes, which share approximately 75% homology in their transmembrane domains, have been identified: alpha1A (chromosome 8), alpha1B (chromosome 5), and alpha1D (chromosome 20). Phenylephrine appears to act similarly on all three receptor subtypes. All three receptor subtypes appear to be involved in maintaining vascular tone. The alpha1A-receptor maintains basal vascular tone while the alpha1B-receptor mediates the vasocontrictory effects of exogenous alpha1-agonists. Activation of the alpha1-receptor activates Gq-proteins, which results in intracellular stimulation of phospholipases C, A2, and D. This results in mobilization of Ca2+ from intracellular stores, activation of mitogen-activated kinase and PI3 kinase pathways and subsequent vasoconstriction. Phenylephrine produces its local and systemic actions by acting on alpha1-adrenergic receptors peripheral vascular smooth muscle. Stimulation of the alpha1-adrenergic receptors results in contraction arteriolar smooth muscle in the periphery. Phenylephrine decreases nasal congestion by acting on alpha1-adrenergic receptors in the arterioles of the nasal mucosa to produce constriction; this leads to decreased edema and increased drainage of the sinus cavities.
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