Purines as mediators::
ATP funcation as a neurotransmitter (or co-transmitter) at peripheral neuroeffector junctions and central nervous system it also functions as an intracellular mediator inhibiting the nervous opening of membrane potassium channels
ATP acts on two types of puninoceptor (p2) one of which (p2x) is a ligand gated ion channel responsible for fast synaptic responses the other (p2y) is coupled to various second messengers suramin blocks the p2x receptor
Adenosine affects many cells and tissues including smooth muscle and nerve cells it is not a conventional transmitter but may be important as local hormone or modulator
ADP acts selectively on platelets causing aggregation this is important in thrombosis
Adenosine acts through A1-A2-and A3- receptors coupled to inhibition or stimulation of adenylate cyclase A1- and A2- receptors are blocked by xanthines such as theophylline
The main effects of adenosine are :
hypotension (A2) and cardiac depression (A1)
inhibition of AV conduction (antidysrhythmic effect A1)
inhibition of platelet aggregation (A2)
bronchoconstriction (probably secondary to mast cell activation A3)
Presynaptic inhibition in CNS (responsible for neuroprotective effect A1)
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