Mechanism Of Action
Beta-blockers are not general blood-pressure-lowering drugs, that is, they do not cause already normal blood pressure to go still lower. Nor do they usually affect the heartbeat of a person at rest, although they do limit the ability of exercise or emotion to make the heart beat more quickly and strongly.
Indeed, exactly how beta-blockers combat elevated blood pressure remains unclear. One important aspect is their ability to relax small arteries, thus allowing blood to flow more easily and with less pressure behind it. No one knows how beta-blockers do this, however, it would not be expected from their known actions. Furthermore, since relaxation occurs only after several days of beta-blocker use, it is very likely to be an indirect effect.
Scientists also know that beta-blockers reduce the kidney's release of renin, an enzyme essential for production of the hormone angiotensin II. Since angiotensin II raises blood pressure in several ways, there can be little doubt that renin plays a significant role in regulating blood pressure. Unfortunately, researchers have found little relationship between blood pressure levels and the amount of renin circulating in the blood. This leaves them uncertain whether beta-blockers' ability to lower blood pressure is tied to their effect on renin release.
By contrast, reasons for beta-blockers' ability to relieve angina are obvious. This condition results from fatty deposits narrowing the arteries that carry blood to the heart muscle. As a result, the heart muscle does not get enough blood to meet its needs—especially when those needs increase because it is beating harder and faster than usual. Since beta-blockers limit the effects of exercise and emotion on the heartbeat, the gap between the amount of blood the heart receives and what it needs will be smaller. As a result, the patient will experience less pain.