Alpha1 Receptor
| Overview |
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- The Alpha1 Adrenergic Receptor binds catecholamines including norepinephrine and epinephrine. Alpha1 Receptors are mostly found in the vasculature, sphincters of the GI and GU tract, and the iris where they promote constriction of all these elements.
| Mechanism of Action |
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- The Alpha1 Receptor is a G-protein coupled receptor is a generally excitatory receptor connected to the Gq G-protein. Briefly, following receptor stimulation, Gq leads to activation of Phospholipase C that in turn liberates IP3 and diacylglycerol. Liberation of IP3 yields release of intracellular calcium, which together with diacylglycerol, leads to activation of Protein Kinase C (PKC). PKC in turn phosphorylates a variety of proteins that induce the physiological sequelae described below. All of these steps are described in more detail on the G-protein coupled receptor page.
| Physiological Anatomy |
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- Overview
- The Alpha1 Receptor can be thought of as a receptor of contraction. The receptor is activated by the sympathetic nervous system, either by postganglionic neurons or by circulating catecholamines released by the adrenal medulla.
- Vasculature
- Alpha1 Receptors are expressed on the vascular smooth muscle cells of both arterioles and veins. When activated this leads to both vasoconstriction and venoconstriction. Vasoconstriction increases systemic vascular resistance and in the context of preserved cardiac output will yield increased systemic arterial pressure. Venoconstriction increases venous return to the heart and thus boosts preload.
- GI and GU
- Alpha1 Receptors are expressed on the sphincters of the GI and GU tract. Constriction of GI sphincters reduces motility of food through the intestines. Constriction of the internal urethral sphincter promotes urinary retention.
- Eye
- Alpha1 Receptors on the constrictor muscle of the iris leads to constriction of the pupil and thus miosis.