Differences in response to dilator agents in blood vessels of different types: physiological bases for selectivity

Differences in the activation systems by which contraction is maintained in blood vessels of different types are an important cause of selectivity in the action of dilator agents. The sympathetic nervous system is the main stimulus for contraction in the limb veins whereas intrinsic (or myogenic) tone makes an important contribution to the contraction of resistance vessels. Drugs that inhibit the effect of the sympathetic nervous system are therefore selective dilators of the veins and drugs that prevent membrane depolarization or block calcium channels are strongly selective for resistance vessels. Substances such as the nitrovasodilators that increase intracellular concentrations of cyclic (c)GMP act preferentially on the veins. Those that act by increasing the concentrations of cyclic (c)AMP show little selectivity unless, as with the beta-adrenoceptor agonists, they act through receptors whose distribution varies between vessels. Differences in the distribution of receptors may also be important in the action of some other vasoactive substances such as the prostaglandins.