Which of caffeine's chemical relatives are able to evoke contractures in mammalian heart?

At concentrations between 0.5 mM and 10 mM caffeine, theophylline, theobromine, imidazole, 2-methyl imidazole, 5-amino-4-imidazole carboxamide, naphazoline, antazoline, and tolazoline in normal MacEwen solution induce contractures in rat and guinea pig myocardium. 9-Methylxanthine, 1,3,9-trimethylxanthine, hypoxanthine, or 2-imidazolidone, when similarly applied, do not. These observations, and similar results from frog heart, can be interpreted assuming that an unsaturated position 9 nitrogen in a xanthine molecule or in the equivalent position of an imidazole group is necessary for the release of calcium into the sarcoplasm to be initiated and therefore for a contraction to develop. The site of action of these compounds is likely to be the sarcoplasmic reticulum.