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 norepinephrine has higher affinity for the α1 receptor than does the hormone
adrenaline.
Smooth muscle 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 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 internal urethral sphincter 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: •
Ureter •
Uterus (when pregnant): this is minor compared to the relaxing effects of the β2 receptor, agonists of whichnotably albuterol/salbutamolwere formerly used to inhibit premature labor. •
Urethral sphincter •
Bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles) •
Iris dilator muscle •
Seminal tract, 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. Norepinephrine also stimulates
serotonin release by binding α1-adrenergic receptors located on serotonergic neurons in the raphe. α1-adrenergic receptor subtypes increase inhibition in the olfactory system, suggesting a synaptic mechanism for noradrenergic modulation of olfactory driven behaviors.
Other • Both positive and negative
inotropic effects on
heart muscle • Secretion from
salivary gland •
Na+ reabsorption from
kidney • Stimulate
proximal tubule basolateral
Na-K ATPase ==Signaling cascade==