<prompt> Methyl xanthines acts onHistamine receptorsAdenosine receptorsMast cellsCholinergic receptors  <response>: Adenosine receptors 202-3-KDT 220-KDT 6thMechanism of action of Methylxanthines - Three distinct cellular actionsa Release of Ca from sarcoplasmic reticulum especially in skeletal and cardiac muscle.b Inhibition of phosphodiesterase which degrades cyclic nucleotides intracellularly.ATPorGTPAdenylcyclase---guanylcyclasecAMPorcGMPPhosphodiesterase---Inhibited by Theophylline5 AMPor5 GMPc Blockade of adenosine receptors: adenosine acts as a local mediator in CNS, CVS and other organs- contracts smooth muscles, especially bronchial, dilates cerebral blood vessels, depresses cardiac pacemaker and inhibits gastric secretion. Methylxanthines produces opposite effects. 