Alpha-1 adrenergic receptor
The alpha-1 (α1) adrenergic receptor is a G protein-coupled receptor (GPCR) associated with the Gq heterotrimeric G-protein. It consists of three highly homologous subtypes, including α1A-, α1B-, and α1D-adrenergic. Catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) signal through the α1-adrenergic receptor in the central and peripheral nervous systems.
The α1-adrenergic receptor has several general functions in common with the α2-adrenergic receptor, but also has specific effects of its own. α1-receptors primarily mediate smooth muscle contraction, but have important functions elsewhere as well. The neurotransmitter noradrenaline has higher affinity for the α1 receptor than does adrenaline (which is a hormone).
In smooth muscle cells of blood vessels the principal effect of activation of these receptors is vasoconstriction. Blood vessels with α1-adrenergic receptors are present in the skin, the sphincters of gastrointestinal system, kidney (renal artery) and brain. During the fight-or-flight response vasoconstriction results in decreased blood flow to these organs. This accounts for the pale appearance of the skin of an individual when frightened.
It also induces contraction of the urinary bladder, although this effect is minor compared to the relaxing effect of β2-adrenergic receptors. In other words, the overall effect of sympathetic stimuli on the bladder is relaxation, in order to inhibit micturition upon anticipation of a stressful event. Other effects on smooth muscle are contraction in:
- Uterus (when pregnant)
- Urethral sphincter
- Bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles)
- Iris dilator muscle
- Seminal tract, resulting in ejaculation
Activation of α1-adrenergic receptors produces anorexia and partially mediates the efficacy of appetite suppressants like phenylpropanolamine and amphetamine in the treatment of obesity. Norepinephrine has been shown to decrease cellular excitability in all layers of the temporal cortex, including the primary auditory cortex. In particular, norepinephrine decreases glutamatergic excitatory postsynaptic potentials by the activation of α1-adrenergic receptors. α1-adrenergic receptor subtypes increase inhibition in the olfactory system, suggesting a synaptic mechanism for noradrenergic modulation of olfactory driven behaviors.
- Both positive and negative inotropic effects on heart muscle
- Secretion from salivary gland
- Increase salivary potassium levels
- Glycogenolysis and gluconeogenesis from adipose tissue and liver.
- Secretion from sweat glands
- Contraction of the urinary bladder urothelium and lamina propria 
- Na+ reabsorption from kidney
- Activate mitogenic responses and regulate growth and proliferation of many cells
- Involved in the detection of mechanical feedback on the hypoglossal motor neurons which allow a long-term facilitation in respiration in response to repeated apneas.
α1-Adrenergic receptors are members of the G protein-coupled receptor superfamily. Upon activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC), which causes an increase in IP3 and calcium. This triggers further effects, primarily through the activation of an enzyme Protein Kinase C. This enzyme, as a kinase, functions by phosphorylation of other enzymes causing their activation, or by phosphorylation of certain channels leading to the increase or decrease of electrolyte transfer in or out of the cell.
Activity during exercise
During exercise, α1-adrenergic receptors in active muscles are attenuated in an exercise intensity-dependent manner, allowing the β2-adrenergic receptors which mediate vasodilation to dominate. In contrast to α2-adrenergic receptors, α1-adrenergic-receptors in the arterial vasculature of skeletal muscle are more resistant to inhibition, and attenuation of α1-adrenergic-receptor-mediated vasoconstriction only occurs during heavy exercise.
Note that only active muscle α1-adrenergic receptors will be blocked. Resting muscle will not have its α1-adrenergic receptors blocked, and hence the overall effect will be α1-adrenergic-mediated vasoconstriction.
Various heterocyclic antidepressants and antipsychotics are α1-adrenergic receptor antagonists as well. This action is generally undesirable in such agents and mediates side effects like orthostatic hypotension, and headaches due to excessive vasodilation.
- Piascik, M. T.; Perez, D. M. (2001). "Alpha1-adrenergic receptors: New insights and directions". The Journal of Pharmacology and Experimental Therapeutics 298 (2): 403–10. PMID 11454900.
- Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4. Page 163
- Schmitz JM, Graham RM, Sagalowsky A, Pettinger WA (1981). "Renal α1 and α2 adrenergic receptors: biochemical and pharmacological correlations". J. Pharmacol. Exp. Ther. 219 (2): 400–6. PMID 6270306.
- Circulation & Lung Physiology I M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
- Fitzpatrick, David; Purves, Dale; Augustine, George (2004). "Table 20:2". Neuroscience (Third ed.). Sunderland, Mass: Sinauer. ISBN 0-87893-725-0.
- Chou EC, Capello SA, Levin RM, Longhurst PA (2003). "Excitatory α1-adrenergic receptors predominate over inhibitory β-receptors in rabbit dorsal detrusor". J. Urol. 170 (6 Pt 1): 2503–7. doi:10.1097/01.ju.0000094184.97133.69. PMID 14634460.
- Cheng JT, Kuo DY (2003). "Both alpha1-adrenergic and D(1)-dopaminergic neurotransmissions are involved in phenylpropanolamine-mediated feeding suppression in mice". Neuroscience Letters 347 (2): 136–8. doi:10.1016/S0304-3940(03)00637-2. PMID 12873745.
- Dinh, L; Nguyen T; Salgado H; Atzori M (2009). "Norepinephrine homogeneously inhibits alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate- (AMPAR-) mediated currents in all layers of the temporal cortex of the rat". Neurochem Res 34 (11): 1896–906. doi:10.1007/s11064-009-9966-z. PMID 19357950.
- Zimnik NC, Treadway T, Smith RS, Araneda RC (2013). "α(1A)-Adrenergic regulation of inhibition in the olfactory bulb". J. Physiol. (Lond.) 591 (Pt 7): 1631–43. doi:10.1113/jphysiol.2012.248591. PMC 3624843. PMID 23266935.
- Wang, G. Y.; McCloskey, D. T.; Turcato, S; Swigart, P. M.; Simpson, P. C.; Baker, A. J. (2006). "Contrasting inotropic responses to alpha1-adrenergic receptor stimulation in left versus right ventricular myocardium". AJP: Heart and Circulatory Physiology 291 (4): H2013–7. doi:10.1152/ajpheart.00167.2006. PMID 16731650.
- Moro C, Tajouri L, Chess-Williams R (Jan 2013). "Adrenoceptor function and expression in bladder urothelium and lamina propria". Urology 81 (1): 211.e1–7. doi:10.1016/j.urology.2012.09.011. PMID 23200975.
- Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 787
- Tadjalli, Arash; Duffin, James; Peever, John (2010). "Identification of a novel form of noradrenergic-dependent respiratory motor plasticity triggered by vagal feedback". The Journal of Neuroscience 30 (50): 16886–16895. doi:10.1523/JNEUROSCI.3394-10.2010. PMID 21159960.
- Fahed S, Grum DF, Papadimos TJ (2008). "Labetalol infusion for refractory hypertension causing severe hypotension and bradycardia: an issue of patient safety". Patient Saf Surg 2: 13. doi:10.1186/1754-9493-2-13. PMC 2429901. PMID 18505576.
- Timmermans, PB; de Jonge, A; Thoolen, MJ; Wilffert, B; Batink, H; van Zwieten, PA (April 1984). "Quantitative relationships between alpha-adrenergic activity and binding affinity of alpha-adrenoceptor agonists and antagonists.". Journal of Medicinal Chemistry 27 (4): 495–503. doi:10.1021/jm00370a011. PMID 6142954.
- "Adrenoceptors". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.