CHARACTERIZATION OF A NOVEL IODOCYANOPINDOLOL AND SM-11044 BINDING-PROTEIN, WHICH MAY MEDIATE RELAXATION OF DEPOLARIZED RAT COLON TONUS

Studies under blockade of alpha-, beta 1-, and beta 2-adrenoreceptor t ors revealed a good correlation between the responses of rat colon rel axation of depolarized tonus and of rat adipocyte lipolysis elicited b y catecholamines or BRL-37344, a selective beta 3-adrenoreceptor agoni st, suggesting beta 3-adrenoreceptor stimulation. In contrast, SM-1104 4, a nonselective beta-adrenoreceptor agonist, stimulated colon relaxa tion more efficiently than lipolysis; its effects were differently ant agonized by cyanopindolol with pA(2) values of 8.31 in colon and of 7, 32 in adipocytes, Binding studies in rat colon smooth muscle membranes using [I-125]iodocyanopindolol under blockade of adrenaline and serot onin receptors revealed the existence of a single class of sites (K-d = 11.0 nM, B-max = 716,7 fmol/mg protein). The specific binding was sa turable and reversible and was displaced by SM-11044 but not by BRL-37 344, isoproterenol, noradrenaline, adrenaline, serotonin, nor dopamine . This binding site was photoaffinity labeled using [I-125]iodocyanopi ndolol-diazirine. The labeling was prevented by SIM-11044 but not by B RL-37344, The amino-terminal amino acid sequences of the high performa nce liquid chromatography-purified peptides generated by enzymatic and chemical cleavages of the affinity labeled 34-kDa protein confirmed t hat the novel. iodocyanopindolol or SM-11044 binding protein of rat co lon smooth muscle membranes is different from known adrenaline, seroto nin, or dopamine receptors. Its functional role might include the rela xation of depolarized colon.