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We tested the hypothesis that an increase in c-AMP was necessary or sufficient for relaxation of rabbit pulmonary artery. Isoproterenol （INA） partially relaxed serotonin （5-HT） induced contractions. Theophylline （TH） alone relaxed contraction more effectively, but after TH, INA produced no greater relaxation. At the height of relaxation （2 min）, c-AMP was not increased, but after 5 min the c-AMP was decreased and the artery was recontracted. c-AMP was increased after TH, and more when INA was given after TH. Noradrenaline contracted the arteries and increased c-AMP; both effects were prevented by phentolamine. A series of other relaxants （papaverine, nitroglycerine, D600, Ro-7-2956, diazoxide and dipyridamole） also failed to produce relaxations directly related to c-AMP levels. We concluded that c-AMP increase was not necessary or sufficient for relaxation in artery, and that in pulmonary artery phosphodiesterase activity played an important role in determining tissue levels of c-AMP.
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