Contractile effects of histamine in large and small respiratory airways

The described technique allows recording of circular smooth muscle activity in isolated airways approx. 1 mm in diameter (bronchioles). It also allows recording of graded contractile and relaxant responses to drugs. Bronchiolar preparations and spirally cut airways (diameters 3-6 mm) were obtained from cat, dog, guinea-pig and man. Histamine is shown to contract cat bronchioles, large airways in cat being unaffected. The contraction is blocked by brompheniramine, but not by atropine. Other contractile agents, cholinoceptor-stimulants, prostaglandin F2alpha'5-hydroxy-tryptamine, and potassium contract the isolated cat airways irrespective of their size. In preparations from dog, guinea-pig and man,histamine was shown to contract both small and large respiratory airways. The effect was blocked by brompheniramine, which did not change the effect of acetylcholine or pilocarpine. Both large and small airways contracted to cholinoceptor stimulation Within species, small and large airways were similarly sensitive to the contractile agents, except for histamine in cat airways. The present findings show a size-and species dependent effect of histamine in respiratory airways. The effect of histamine in isolated cat airways might partly explain the pulmonary effect of histamine in vivo. The importance of including both small and large airways in studies of contractile and relaxant effects is emphasized.