THEOPHYLLINE AND PHENYLEPHRINE EFFECTS ON CARDIAC RELAXATION

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Abstract
1 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 β-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 β-adrenoceptor stimulation and increased peak tension by α-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.