CHARACTERIZATION AND DISTRIBUTION OF ALPHA-2-ADRENERGIC RECEPTORS IN THE HUMAN INTESTINAL-MUCOSA

The subtype and the expression of the alpha2-adrenergic receptor were investigated in the normal mucosa from human intestine by means of rad ioligand binding, RNase mapping, and measurement of adenylate cyclase activity. The study of the binding of the alpha2-adrenergic antagonist , [H-3]RX821002, to epithelial cell membranes indicated the existence of a single class of noninteracting sites displaying a high affinity f or the radioligand (K(d) = 1.1+/-0.5 nM). The rank order of potency of antagonists to inhibit [H-3]RX821002 binding (RX821002 > yohimbine = rauwolscine > phentolamine almost-equal-to idazoxan much greater than chlorpromazine > prazosin) suggested that the receptor is of the alpha 2A subtype. A conclusion which is confirmed by the fact that only alph a2C10 transcripts were found in the human intestine mucosa. Competitio n curves with (-)-norepinephrine demonstrated that 60% of the receptor population exhibited high affinity for agonists. This high-affinity s tate was abolished by the addition of GTP plus Na+ or by prior treatme nt of the membranes with pertussis toxin indicating it corresponded to G protein-coupled receptors. [P-32]ADP-ribosylation and immunoblottin g experiments identified two pertussis toxin-sensitive G proteins corr esponding to Gi2 and Gi3. The study of the distribution of the recepto r indicated that (a) the proximal colon is the intestine segment exhib iting the highest receptor density and (b) the receptor is predominant ly expressed in crypts and is preferentially located in the basolatera l membrane of the polarized cell. The distribution of the receptor alo ng the crypt-surface axis of the colon mucosa can be correlated with a higher level of alpha2C10-specific mRNA and a higher efficiency of UK 14304 to inhibit adenylate cyclase in crypt cells.