Adenosine A1 receptor
The adenosine A receptor is one member of the adenosine receptor group of G protein-coupled receptors with adenosine as endogenous ligand. Biochemistry. A receptors are implicated in sleep promotion by inhibiting wake-promoting cholinergic neurons in the basal forebrain. A receptors are also present in smooth muscle throughout the vascular system. The adenosine A receptor has been found to be ubiquitous throughout the entire body. Signalling. Activation of the adenosine A receptor by an agonist causes binding of G or G protein. Binding of G 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 A and A receptors in the brain. In heart. The A and A receptors of endogenous adenosine are believed to play a role in regulating myocardial oxygen consumption and coronary blood flow. Stimulation of the A 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 A 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 A expression appears to prevent hypoxia-induced ventriculomegaly and loss of  white matter, the pharmacological blockade of A 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 A ARs. Thus, it may prove more advantageous to use selective A antagonists to help reduce adenosine-induced brain injury.