Summary:
Beta adrenoceptor blocking drugs are all competitive inhibitors of the beta receptor although they may or may not possess, beta1-(cardio)selectivity, intrinsic sympathomimetic activity (ISA) or partial agonist activity, alphablocking properties, while membrane stabilizing activity is now thought not to be important. Drugs with ISA give less of a reduction in resting and maximal exercising heart rate and consequently in cardiac output, than those without ISA. The possession of alpha-blocking activity also leads to less fall in cardiac output. Overall evidence suggests that peripheral resistance and peripheral blood flow is less reduced by ISA drugs. Post-beta blockade hypersensitivity which may be important in patients with ischemic heart disease if beta blocking drugs are suddenly stopped, is absent after beta-blocking drugs with ISA as they result in down regulation of beta receptors. Beta1-selective drugs may result in less of a rise in blood pressure in response to isometric exercise, insulin hypoglycemia or smoking. There do not appear to be important differences in the effect on coronary flow. While presently available drugs can produce asthma in susceptible subjects there seems little doubt that beta1 selective agents have less effect than other beta-blocking drugs, and give less inhibition of sympathomimetic bronchodilators. Nonselective, non ISA, beta-blocking drugs elevate triglycerides, cardioselective drugs possibly less so. Those with ISA may elevate HDL unlike those beta-blocking agents without this property. Beta adrenergic blocking drugs without ISA do not lower resting plasma noradrenaline, evidence suggests an increase; whereas those agents with marked ISA, e.g., pindolol, reduce it. Renin levels are lowered, but less so with ISA possessing agents. Some agents, e.g. atenolol, nadolol, sotalol, are hydrophilic, show poor brain penetration, are long-acting, are not liver metabolized, but are excreted by the kidneys unchanged. Others are lipophilic, e.g., metoprolol, propranolol, penetrate the brain, and are liver metabolized. Pindolol is metabolized about 50% by each route. Similarities between beta blocking drugs are dominant but differences are often clinically relevant.
