Metoprolol Adrenaline

Beta-blockers, by inhibiting the effect of adrenaline and noradrenaline on the heart, prevent the normal increase in heart rate, and are very effective in preventing exercise angina. Examples of beta-blockers used for this purpose include acebutolol, atenolol, metoprolol. nadolol, oxprenolol, pindolol, propranolol, sotalol and timolol. See ADRENOCEPTOR ANTAGONISTS. 

Effects on blood pressure and heart rate The hypertensive effects of adrenaline (epinephrine) can be markedly increased in patients taking non-selective beta blockers such as propranolol. A severe and potentially life-threatening hypertensive reaction and/or marked bradycardia can develop. Cardioselective beta blockers such as atenolol and metoprolol interact minimally. An isolated report describes a fatal hypertensive reaction in a patient given propranolol and phenylephrine, but concurrent use normally seems to be uneventful Paradoxically, marked hypotension occurred in one patient given low-dose carvedilol and dobutamine. Anaphylaxis Some evidence su ests that anaphylactic shock in... 

Adrenaline (epinephrine) stimulates alpha- and beta-receptors of the cardiovascular system, the former results in vasoconstriction (mainly alphaj) and the latter in both vasodilatation (mainly beta2) and stimulation of the heart (mainly betaj). The net result is usually a modest increase in heart rate and a small rise in blood pressure. However, if the heta-reeeptors are blocked by a non-selective beta blocker, such as propranolol or nadolol (see Table 22.1 , (p.833) for a list), the unopposed alpha vasoeonstrietion causes a marked rise in blood pressure, followed by reflex bradyeardia. Cardioselective beta blockers such as atenolol and metoprolol, whieh are more selective for betaj receptors, do not prevent the vasodilator aetion of adrenaline at beta2 receptors to the same extent, and therefore the effect of any interaction is relatively small. Consequently, adrenaline has been used to assess the degree of beta blockade produced by propranolol and other beta blockers.Phenylephrine is largely an alpha stimulator, therefore beta blockers should have a minimal effect on its action. 

The lability of benzylic bonds makes them easy targets of drug metabolism. For example, the antihypertensive metoprolol (Lopressor) works by blocking receptors for adrenaline in the heart muscles and other tissues. The first step of its degradation in the body features cytochrome enzymes (see Real Life 8-1 and Section 22-9), which hydroxylate the benzylic positions. 

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