Isolated rat ventricular myocytes were loaded with fluo-3 which did not result in a loss of beating activity, in order to record the changes in the fluorescence and contractile parameters. Changes in the beating activity induced by various reagents in perfusing medium were related to variations in the peak intensity of fluorescence time course and possibly to changes in myofibrillar ATPase activity. Isoproterenol stimulated, whereas 2,3-butanedione monoxime (BDM) and a medium with a low Ca2+ concentration suppressed the contractile activity by increasing and reducing the Ca2+ transient, respectively, and, in the presence of BDM, a decrease in the maximum ATPase activity also contributed the suppressive effect.
