PAdrenergic Receptors

Initially, adrenergic receptors were classified as either a or p, based upon the tissue response to various sympathomimetic amines. The receptor population that mediated excitatory responses was called a, and the potency ranking was epinephrine > norepinephrine > a-methyl norepinephrine >a-methyl epinephrine > isoproterenol. The receptor population that mediated inhibitory responses was designated as p, and had an agonist order of potency of isoproterenol > epinephrine > a-methyl epinephrine >a-ethyl norepinephrine > norepinephrine. It later became recognized that although generally correct, this classification was overly simplified, and the a- and p -receptors were further subclas-sified. For p-adrenergics it is now recognized that there are three receptor groups: p b p2, and p3. The p radrenergic receptors are primarily located in the brain, the heart, and the pineal gland, the p2-adrenergic receptors are primarily in the lung and prostate, and the p3-adrenergic receptors are found in brown and white adipose tissue. Improved techniques to identify receptors have led to the discovery that although these receptors have preferential primary locations, they may also be present in other tissues. The p2-adren-ergic receptor is the predominant p-receptor in the lung, but it is also found in adipose tissue, the brain, heart, and placenta. This is of particular importance in understanding the side effects of the p2-specific adrenergic drugs.

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