Evidence for the alpha 2 nature of the alpha-adrenergic receptor inhibiting lipolysis in human fat cells.

Theophylline-stimulated human subcutaneous adipocytes were incubatged in vitro in the presence of selected alpha-adrenergic agents in order to characterize the alpha-adrenoceptor of human fat cells. Inhibition of theophylline-induced lipolysis occurred with the agents tested; clonidine was the most potent agonist while methoxamine had no effect. The relative order of potency of the various agonists was: clonidine > adrenaline > phenylephrine > methoxamine; this order is consistent with the classification of agonists described for the presynaptic alpha 2-receptor. Moreover, selected antagonists were used in order to antagonise clonidine inhibition of theophylline-induced lipolysis. The order of potency of the alpha-antagonists for the human alpha-adrenoceptor of adipocytes was: yohimbine > piperoxane > phenoxybenzamine > prazosin. This order is consistent with an alpha 2 type receptor. In conclusion, the results demonstrate that the alpha-adrenergic receptor which inhibits lypolysis in human fat cells is of the alpha 2 type. it is noteworthy that although localized postsynaptically this alpha-receptor can be classified as alpha 2 like the commonly known presynaptic alpha-adrenergic receptor which inhibits noradrenaline release from sympathetic neurons.