Processes which occur in response to metabolic changes within the ischaemic myocardium are induced by two fundamentally different signals:(a) Intracellular signals: ie intracellular effectors acting on regulatory enzymes, such as activation of anaerobic glycolysis.(b) Extracellular signals and their transmembrane transduction: ie transmitters activating receptors which are coupled to second messenger systems, such as presynaptically released catecholamines activating y9-adrenergic receptors which stimulate the formation of the second messenger cAMP.