Theophylline and phenylephrine effects on cardiac relaxation.

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.