Abstract
In the driven isolated left atrium of the rabbit theophylline shortened relaxation time in a similar manner to isoprenaline and histamine.
Phenylephrine lengthened relaxation time in a similar manner to calcium.
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.
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.
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