Alpha-adrenoceptors: recent development and some comparative aspects.

On anatomical and functional bases, alpha-adrenoceptors have been divided into pre- and postsynaptic alpha-adrenoceptors. Recently, alpha-adrenoceptors have been classified as alpha 1 and alpha 2 according to their pharmacological responses, irrespective of their anatomical location. The presynaptic alpha-adrenoceptors, which have been recognized as alpha 2, determine the frequency of the nerve impulses travelling along the axon and also the amount of transmitter released per nerve impulse from the varicose terminal. Postsynaptic alpha-adrenoceptors have been recognized in various tissues including smooth muscle, pancreatic islets, fat cells, platelets and other tissues. Both alpha 1- and alpha 2-adrenoceptors have been located postsynaptically. alpha-Adrenoceptors have been found also in the central nervous system. Generally, they fall into the same categories (alpha 1 and alpha 2) as the peripheral alpha-adrenoceptors. A new class of drugs, the so called calcium blockers, inhibit the postsynaptic response to alpha 2 stimulation but not the alpha 1-mediated response, indicating that the alpha 2-adrenoceptors are dependent on Ca2+ ions for their function. In the most primitive group of vertebrates, the fishes, alpha-adrenoceptors seem to be different in as much as they do not respond to many of the classical drugs employed to distinguish between alpha-adrenoceptors in mammals. In reptiles and amphibians alpha 2-adrenoceptors have been shown to exist. These receptors are involved in the regulation of melanocytes. In the most advanced non-mammalian vertebrates (birds) both peripheral and central alpha-adrenoceptors seem to be qualitatively similar to the mammalian types.