Adenosine A1 receptor

Biochemistry

A₁ receptors are implicated in sleep promotion by inhibiting wake-promoting cholinergic neurons in the basal forebrain. A₁ receptors are also present in smooth muscle throughout the vascular system.

The adenosine A₁ receptor has been found to be ubiquitous throughout the entire body.

Signaling

Activation of the adenosine A₁ receptor by an agonist causes binding of G_i1/2/3 or G_o protein. Binding of G_i1/2/3 causes an inhibition of adenylate cyclase and, therefore, a decrease in the cAMP concentration. An increase of the inositol triphosphate/diacylglycerol concentration is caused by an activation of phospholipase C, whereas the elevated levels of arachidonic acid are mediated by DAG lipase, which cleaves DAG to form arachidonic acid. Several types of potassium channels are activated but N-, P-, and Q-type calcium channels are inhibited.

Mechanism

This receptor has an inhibitory function on most of the tissues in which it rests. In the brain, it slows metabolic activity by a combination of actions. At the neuron's synapse, it reduces synaptic vesicle release.

Ligands

Caffeine, as well as theophylline, has been found to antagonize both A₁ and A_2A receptors in the brain.

Agonists

PAMs

Antagonists

In heart

The A₁ and A_2A receptors of endogenous adenosine are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow. Stimulation of the A₁ receptor has a myocardial depressant effect by decreasing the conduction of electrical impulses and suppressing pacemaker cell function, resulting in a decrease in heart rate. This makes adenosine a useful medication for treating and diagnosing tachyarrhythmias, or excessively fast heart rates. This effect on the A₁ receptor also explains why there is a brief moment of cardiac standstill when adenosine is administered as a rapid IV push during cardiac resuscitation. The rapid infusion causes a momentary myocardial stunning effect.

In normal physiological states, this serves as protective mechanisms. However, in altered cardiac function, such as hypoperfusion caused by hypotension, heart attack or cardiac arrest caused by nonperfusing bradycardias, adenosine has a negative effect on physiological functioning by preventing necessary compensatory increases in heart rate and blood pressure that attempt to maintain cerebral perfusion.

In neonatal medicine

Adenosine antagonists are widely used in neonatal medicine;

Because a reduction in A₁ expression appears to prevent hypoxia-induced ventriculomegaly and loss of white matter, the pharmacological blockade of A₁ may have clinical utility.

Theophylline and caffeine are nonselective adenosine antagonists that are used to stimulate respiration in premature infants.

However, we are unaware of clinical studies that have examined the incidence of periventricular leukomalacia (PVL) as related to neonatal caffeine use. Caffeine may reduce cerebral blood flow in premature infants, it is presumed by blocking vascular A₂ ARs. Thus, it may prove more advantageous to use selective A₁ antagonists to help reduce adenosine-induced brain injury.