Slowly relaxing caffeine responses in rat ventricle: Relationships of ryanodine and caffeine actions

Contractile responses to 20 mM caffeine were compared in arterially perfused rat right ventricle as tension, in intact rat myocytes as shortening, and in Triton X-100-'skinned' rat myocytes as shortening and stiffness. Responses in intact quiescent ventricle and membrane-disrupted myocytes at -log molar Ca concentration (pCa) of 7.0-6.8 were similar; force and stiffness rose quickly on application of caffeine and declined incompletely toward base line over 20 min. Subsequent caffeine responses were blunted. By contrast, in freshly isolated rat myocytes, rapid caffeine exposures induced large phasic contractures, which relaxed for the most part in 2-5 s. These responses were largely suppressed by 0.2 μM ryanodine pretreatment at 35°C but not by high ryanodine concentrations (10 μM). Ryanodine was without effect on slowly relaxing contractile responses in both intact ventricle and Triton X-100-treated myocytes. The results are discussed in relation to the predicted effects of a calcium store leak in a simple model of cardiac excitation-contraction coupling. The results suggest that slowly relaxing caffeine contractures can be caused entirely by direct actions of caffeine on myofilaments.