The two main types of p-receptors have been given the designations pj and p2. Among the responses mediated by pj-receptors is cardiac stimulation, whereas p2-receptor stimulation mediates bronchodilation and re laxation of vascular and uterine smooth muscle (see Chapters 9 and 62). These findings are significant, since a number of both agonists and antagonists have some degree of selectivity for either pt- or p2-receptors.
A comparison of the effects produced by propranolol, a nonselective p-receptor blocking agent, with those of metoprolol, a relatively selective pj-receptor blocker, illustrates the clinical utility of such drugs. For example, a patient who is a candidate for p-blocker therapy (angina, hypertension), but who also has obstructive airway disease probably should not receive a nonselective p-blocking agent such as propranolol because of the possibility of aggravating bronchospasm. In this instance, metoprolol would be advantageous, since p-receptors of the respiratory system are p2, hence less affected by metoprolol than by propranolol. However, metoprolol's selectivity is only relative, and at high concentrations the drug will also antagonize p2 responses.
Absolute selectivity of drug action does not exist. Any given effector tissue probably contains more than one receptor subtype, and it is likely that the proportion of receptor subtypes varies within that effector. Nevertheless, the designation of a drug as a selective agent for either a pj-receptor or a p2-receptor seems both useful and justified if one keeps in mind that the designation represents a shorthand notation for what is only a predominance of activities.
Molecular genetic techniques have confirmed the existence of multiple subtypes of p-adrenoceptors. pi-Receptors and p2-receptors have been cloned, and recent molecular biological evidence indicates the existence of at least one additional p-receptor subtype, called the p3-receptor. It is suggested that the p3-receptor may mediate some of the metabolic effects of catecholamines, although no available p-blocker has been shown to rely on p3-receptor antagonism for its therapeutic effectiveness.
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...