SCA-40 - STUDIES OF THE RELAXANT EFFECTS ON CRYOPRESERVED HUMAN AIRWAY AND VASCULAR SMOOTH-MUSCLE

1 8-methylaminoimidazol[1,2-a]pyrazine-2carbonitrile (SCA 40) has been claimed to induce relaxation in guinea-pig trachea by opening high co nductance, calcium-activated potassium (BKCa) channels. The mechanism of action of SCA 40 has now been further investigated in ring preparat ions from cryopreserved human airway and vascular smooth muscle prepar ations in vitro. 2 Human bronchi with spontaneous tone relaxed in resp onse to SCA 40 in a biphasic way. A high affinity component (pD(2) 8.6 1+/-0.21; mean+/-s.e.mean) accounted for 30% of the response and a low affinity component (pD(2) 6.53+/-0.14) for the remaining 70%. In cont rast, in bronchi contracted with carbachol, 1 mu M, the concentration- response curve to SCA 40 was monophasic and yielded a pD(2) of 6.31+/- 0.29. 3 SCA 40 relaxed pulmonary and mesenteric arteries and periphera l veins which had been precontracted by 10 nM U46619 nearly completely and in a monophasic way; the pD(2) values were 6.37+/-0.08, 6.17+/-0. 15 and 5.45+/-0.25, respectively. 4 Lemakalim, an opener of ATP-depend ent potassium (K-ATP) channels, also relaxed human bronchi under spont aneous lone and the vascular tissues. NS 1619, a recognised opener of BKCa channels, was inactive up to 10 mu M on bronchial and vascular ti ssues. 5 The SCA 40-induced relaxation of human bronchi was reduced co ncentration-dependently in the presence of high potassium chloride (20 and 80 mM). However, in the presence of 80 mM KCl and nifedipine, 30 nM, SCA 40 fully relaxed the remaining contractile response with pD(2) values of 8.08+/-0.13 and 5.27+/-0.13 for the high and low affinity c omponent, respectively. 6 Relaxation responses to SCA 40 in human bron chi were resistant to blockade by glibenclamide at concentrations up t o 10 mu M (which blocked the relaxant response to lemakalim), quinine (30 mu M), apamin (100 nM), tetraethylammonium (0.1-1 mM) and charybdo toxin (10-100 nM), thus excluding the involvement of a variety of K+ c hannels including K-ATP and K-Ca channels. 7 In bronchi contracted wit h carbachol, 1 mu M, the nature of the interaction between SCA 40 and the beta(2)-adrenoceptor agonist, salbutamol, was synergistic. 8 These experiments establish that SCA 40 is a potent relaxant of human bronc hial smooth muscle manifesting spontaneous tone. A low affinity relaxa nt component has its counterpart in the relaxation seen in both human arterial and venous smooth muscle. The consensus of the evidence sugge sts that K+ channel opening is not the basis of the relaxant response to SCA 40. Furthermore, BKCa channels appear to be of minor importance in the regulation of human airway smooth muscle tone. Our data sugges t that inhibition of an adenosine 3':5'-cyclic monophosphate phosphodi esterase may contribute, at least to the low affinity relaxant compone nt of SCA 40. However, the exact mechanism mediating the SCA 40-induce d relaxation of human airways remains to be defined.