title: Theophylline and phenylephrine effects on cardiac relaxation.
abstract: In the driven isolated left atrium of the rabbit theophylline shortened relaxation time in a similar manner to isoprenaline and histamine. 2 Phenylephrine lengthened relaxation time in a similar manner to calcium. 3 Theophylline caused phenylephrine to shorten relaxation time, which was inhibited by a beta-adrenoceptor blocking drug, but theophylline did not potentiate the effect of phenylephrine on peak tension. 4 Theophylline separated drug effects on cardiac relaxation and contraction: in the presence of theophylline at a low calcium concentration, phenylephrine shortened relaxation time by beta-adrenoceptor stimulation and increased peak tension by alpha-adrenoceptor stimulation. At a high calcium concentration, theophylline potentiated the effect of isoprenaline, histamine and phenylephrine on relaxation time but inhibited the effect on peak tension.
mesh_terms: D000818:Animals; D016275:Atrial Function; D002118:Calcium / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006325:Heart Atria / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D009200:Myocardial Contraction / Q000187:drug effects*; D010656:Phenylephrine / Q000494:pharmacology*; D011817:Rabbits; D013806:Theophylline / Q000494:pharmacology*
chemical_list: D010656:Phenylephrine; D013806:Theophylline; D007545:Isoproterenol; D002118:Calcium