Many drugs can have effects on the circulatory system of the human body, such as expanding or contracting blood vessels to some extent. Those medications that cause the blood vessels to contract are known as vasoconstrictors, and their contracting action is called vasoconstriction. Generally, the action of these drugs causes blood pressure to increase. This effect is similar to that of the hormone vasopressin produced naturally in the body; vasoconstricting drugs are sometimes referred to as either pressors or vasopressors.
The blood vessels that are most commonly affected by vasoconstrictors include the veins and the larger arteries, and, to an extent, smaller arteries known as arterioles. Contracting these large blood vessels leads to higher blood pressure, and the effect on the arteries causes less oxygen to reach some parts of the body. These medications usually affect the smooth muscle cells that line the walls of blood vessels. Pressors cause calcium ion levels in the bloodstream to rise, and smooth muscle tissue takes up this calcium, causing them to contract.
Various types of vasoconstricting drugs may use different methods to increase the amount of available calcium ions. Direct vasoconstrictors bind to special adrenergic receptors on nervous system cells. These nerves send messages to the smooth muscle cells and nearby tissue, which in turn release signal molecules of their own. The signals that result both release additional calcium ions into the blood, and open calcium channels in the muscle cells so that they can make more effective use of the available calcium.
Indirect vasoconstrictors, however, trigger a release of a chemical called norepinephrine from the adrenal gland and other glands. Norepinephrine is a signaling molecule that binds to the same receptors that are affected by direct vasoconstrictors. Through the release of norepinephrine, this indirectly operating class of drugs can exert a similar effect to those that bind directly to nerve cells. Some vasoconstrictors can actually exert a combination of direct and indirect effects, and are known as mixed vasoconstricting drugs.
Different vasoconstrictors can promote these responses in the body for distinct, but varying lengths of time, and at different strengths. The duration and potency of the drugs can determine which medical situations they might be appropriate for. Other drugs, like amphetamines, are not normally taken for their vasoconstricting effects, but these effects may be taken into account when an individual is calculating dosage.