Also called beta blockers, these drugs interrupt the transmission of neuronal messages via the body’s adrenergic receptor sites. In the HEART these are called beta1 (cardioselective) receptors. Another type – beta2 (non-cardioselective) receptors – is sited in the airways, blood vessels, and organs such as the eye, liver and pancreas. Cardioselective beta blockers act primarily on beta1 receptors, whereas non-cardioselective drugs act on both varieties, beta1 and beta2. (The neurotransmissions interrupted at the beta-receptor sites through the body by the beta blockers are initiated in the ADRENAL GLANDS: this is why these drugs are sometimes described as beta-adrenergic-blocking agents.)
They work by blocking the stimulation of beta adrenergic receptors by the neurotransmitters adrenaline and noradrenaline, which are produced at the nerve endings of that part of the SYMPATHETIC NERVOUS SYSTEM – the autonomous (involuntary) network
– which facilitates the body’s reaction to anxiety, stress and exercise – the ‘fear and ﬂight’ response.
Beta1 blockers reduce the frequency and force of the heartbeat; beta2 blockers prevent vasodilation (increase in the diameter of blood vessels), thus inﬂuencing the patient’s blood pressure. Beta1 blockers also aﬀect blood pressure, but the mechanism of their action is unclear. They can reduce to normal an abnormally fast heart rate so the power of the heart can be concomitantly controlled: this reduces the oxygen requirements of the heart with an advantageous knock-on eﬀect on the respiratory system. These are valuable therapeutic eﬀects in patients with ANGINA or who have had a myocardial infarction (heart attack – see HEART, DISEASES OF), or who suﬀer from HYPERTENSION. Beta2 blockers reduce tremors in muscles elsewhere in the body which are a feature of anxiety or the result of thyrotoxicosis (an overactive thyroid gland – see under THYROID GLAND, DISEASES OF). Noncardioselective blockers also reduce the abnormal pressure caused by the increase in the ﬂuid in the eyeball that characterises GLAUCOMA.
Many beta-blocking drugs are now available; minor therapeutic diﬀerences between them may inﬂuence the choice of a drug for a particular patient. Among the common drugs are:
Primarily cardioselective Non-cardioselective
Acebutolol Labetalol Atenolol Nadolol Betaxolol Oxprenolol Celiprolol Propanolol Metoprolol Timolol
These powerful drugs have various side-eﬀects and should be prescribed and monitored with care. In particular, people who suﬀer from asthma, bronchitis or other respiratory problems may develop breathing diﬃculties. Long-term treatment with beta blockers should not be suddenly stopped, as this may precipitate a severe recurrence of the patient’s symptoms – including, possibly, a sharp rise in blood pressure. Gradual withdrawal of medication should mitigate untoward eﬀects.