The catecholamines, primarily epinephrine and isopro-terenol, exert a number of important effects on meta bolic processes. Most of these are mediated through an interaction with ( -adrenoceptors. Norepinephrine is usually effective only in large doses. Epinephrine and isoproterenol in therapeutic doses increase oxygen consumption by 20 to 30%. Endogenous epinephrine secreted by the adrenal medulla in response to stress such as exercise increases blood levels of glucose, lactic acid, and free fatty acids.
Epinephrine, the most potent stimulant of hepatic glycogenolysis, gives rise to glucose, which readily enters the circulation; isoproterenol produces relatively weak hyperglycemia. Administration of both a- and ( -adrenoceptor blocking agents is necessary for complete antagonism of glycogenolysis in this tissue.
Isoproterenol is the most potent stimulant of skeletal muscle glycogenolysis, followed by epinephrine and norepinephrine. ( 2-Adrenoceptors mediate muscle glycogenolysis. Stimulation of skeletal muscle glyco-genolysis will raise blood lactic acid levels rather than blood glucose levels because skeletal muscle lacks the enzyme glucose-6-phosphatase, which catalyzes the conversion of glucose-6-phosphate to glucose.
The release of free fatty acids from adipose tissue (lipolysis) is mediated through ( 3-adrenoceptors. Iso-proterenol is the most potent agonist, followed by epi-nephrine and norepinephrine.
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