Catecholamines help to prepare the individual to cope with emergencies but are not essential for life. In general, epinephrine and norepinephrine mimic the effects of sympathetic nervous discharge. The effects broadly comprise the cardiovascular and metabolic ones. They also have central nervous effects. Both increase alertness. Epinephrine evokes anxiety and fear.
Cardiovascular effects:
Epinephrine stimulates both α and β receptors. The α -induced vasoconstriction is more than nullified by the β -induced vasodilatation. Hence, the peripheral resistance and diastolic BP remain unchanged or fall slightly.
The β – induced increase in stroke volume and heart rate results in higher cardiac output, a rise in systolic BP and a widening of pulse pressure. Norepinephrine has much greater effect on a than on P receptors. Hence, it produces vasoconstriction with a rise in peripheral resistance and diastolic BP. Due to weak β -activation, direct cardiac stimulation is insignificant. Rather, there is reflex decrease in heart rate due to the rise in diastolic BP.
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Metabolic effects:
(i) Catecholamines stimulate glycogenolysis. α -adrenergic receptor stimulates glycogenolysis in the liver while β adrenergic receptor stimulates glycogenolysis in muscle. Liver contains glucose-6-phosphatase, and hence hepatic glycogenolysis is associated with release of free glucose into blood stream. Muscle lacks glucose-6-phosphatase, and epinephrine-induced glycogenolysis in muscle does not release free glucose into blood stream. Instead, glycogenolysis ends with the formation of glucose-6-phosphate. However, the glucose-6-phosphate is metabolized to lactate or pyruvate, which is converted to glucose by the liver and released into blood.
(ii) Epinephrine suppresses insulin secretion (α -adrenergic effect) and stimulates glucagon secretion β -adrenergic effect). It also inhibits insulin-mediated facilitated diffusion of glucose by muscle and adipose tissue, (iii) Epinephrine stimulates lipolysis by stimulating hormone-sensitive lipase via β -adrenergic receptor. Free fatty acids mobilized from stores in adipose tissue are converted in the liver to acetoacetate and β -hydroxybutyrate. These are released in the blood stream and reach the peripheral tissues, where they are quantitatively important as energy sources, (iv) Catecholamines cause a biphasic (a large, immediate and a small, delayed) rise in the metabolic rate and body temperature. The immediate rise is possibly due to increased muscular activity, and/or due to cutaneous vasoconstriction, which decreases heat loss leading to a rise in body temperature. The delayed rise that is probably due to oxidation of lactate (released during muscle activity) in the liver.