Calcium Channel Types in Cardiac Myocytes

Bisoprolol, (±)1-(4-(2-isopropoxyethoxy)-methyl-phenoxy)-3-isopropyl-amino-2-propanol-hemifumarate, is a new, highly selective β1-adrenoceptor blocking agent without intrinsic sympathomimetic activity and low to moderate local anaesthetic activity. As demonstrated in binding experiments, and in classical pharmacological studies using rats, guinea pigs, cats, and dogs, bisoprolol markedly differentiated between β1-adrenoceptors of the heart, or the renal juxtaglomerular apparatus, and the β2-subtype in arterial blood vessels, bronchi, liver, or skeletal muscle. Up to concentrations nearly 100-fold higher than the therapeutic plasma levels in humans, bisoprolol did not affect the functional refractory period of the heart, and was devoid of a direct suppressive effect on myocardial contractility and of calcium antagonistic properties in heart and vascular muscle. The pattern of haemodynamic effects of bisoprolol was typical of β-blockers and included decreases in blood pressure (BP), heart rate (HR), and cardiac output, concomitant with an increase in calculated total peripheral resistance. In contrast to other β-blockers, bisoprolol increased renal blood flow in anaesthetized dogs. Bisoprolol lowered BP in hypertensive dogs and rats, attenuated the development of spontaneous hypertension in rats, decreased plasma renin activity and protected the heart from the sequelae of transient ischemia. It did not block presynaptic β-adrenoceptors in blood vessels. Serum lipids and the serum lipoprotein profile remained unaltered after bisoprolol. Bisoprolol was devoid of affinity for autonomic receptors other than β-adrenoceptors or for autacoid receptors. This is probably one of the reasons why bisoprolol did not affect the function of the central nervous, respiratory, and gastrointestinal systems in an obvious way. The high β1-selectivity of bisoprolol is linked with extremely favourable pharmacokinetic properties. These include nearly complete enteral absorption and virtual absence of liver first-pass metabolism, both resulting in high bioavailability, long plasma half-life, pharmacokinetics that are linear over a wide dose range and independent of age, food intake and hydroxylator status, low plasma protein binding, and a 1: 1 ratio of hepatic metabolization to renal elimination of the unaltered substance. This sum of favourable pharmacological and pharmacokinetic properties characterize bisoprolol as an optimized β-blocker.