β-blockers are drugs that bind to β-adrenoceptors and thereby block the binding of norepinephrine and circulating epinephrine. Some beta-blockers (eg labetalol, acebutolol) are partial agonists-partially activate the receptor while preventing norepinephrine from binding to the receptor thereby preventing normal and enhanced sympathetic activity.
The selectivity of beta adrenoceptors ranges from non-selective first generation beta-blockers (eg propranolol) to cardioselective -selective second generation beta-blockers ( Atenolol, misoprolol). The third generation beta-blockers (eg carvedilol) also possess vasodilator actions through blockade of vascular alpha-adrenoceptors.
No currently available β adrenergic antagonist is sufficiently specific to avoid β2 receptor blockade. As a result,these drugs should generally be avoided in patients with asthma or with reactive broncho-pulmonary system.
β blockers also impairs hepatic glycogenolysis which is an important system in glucose homeostasis. β blockers therefore may impede recovery from hypoglycemia and thus should be avoided or used with caution in insulin-dependent patients particularly in those subject to frequent hypoglycemic reactions; patients with inadequate glucagon reserve or pancreatectomized patients in whom hypoglycemic control is dependent on sympathetic drive of hepatic glycogenolysis in response to hypoglycemia.
The hypokalemic effects of β adrenergic blockers necessitate a cautious use in patients on long term digoxin and other cardiac glycosides because hypokalimia is a risk factor for dangerous arrhythmias in such patients.
