Identification and regulation of alpha 2-adrenergic receptors in rabbit ileal mucosa

The alpha 2-adrenergic receptors in rabbit ileal mucosal membranes can be identified by using [ 3H]clonidine. [ 3H]Clonidine bound to a homogeneous population of sites (30–120 moles/mg protein) with a K D of 2.2 nM at 25°. Alpha-adrenergic agonists and antagonists competed with [ 3H] clonidine for the binding sites with an order of potency typical for alpha 2-receptors. Mg 2+, Ca 2+, or Mn 2+ (2.4 mM) markedly increased the binding of [ 3H]clonidine. At the maximally effective concentration, Mg 2+ increased both the binding affinity of [ 3H]clonidine and the number of receptor sites. Both NaCl and GppNHp, the guanyl nucleotide, inhibited [ 3H]clonidine binding. NaCl decreased the binding affinity of [ 3H]clonidine as well as the number of receptor sites. In contrast, GppNHp decreased only the binding affinity of [ 3H]clonidine, with no appreciable effect on the number of receptor sites. These findings indicate that ileal mucosal alpha 2-receptors can exist in multiple affinity states, which can be regulated by divalent cations, NaCl, and guanyl nucleotides. It appears that NaCl and GppNHp regulate alpha 2-receptors in ileal mucosa by different mechanisms.