The effects of low concentrations of caffeine on spontaneous Ca release inisolated rat ventricular myocytes

We have investigated the effects on spontaneous SR Ca release of modulating the sarcoplasmic reticulum ryanodine receptor (RyR) with low (<0.5 mM) con centrations of caffeine. Experiments were performed on isolated rat ventric ular myocytes. Intracellular Ca concentration was measured with Indo-1 or F luo-3 in voltage-clamped cells. Spontaneous Ca release was produced by elev ating external Ca to 5 mM. Caffeine application increased the frequency of spontaneous release. Both the magnitude of the spontaneous Ca transients an d the integral of the resulting Na-Ca exchange current were decreased by ca ffeine. The combination of increased frequency of spontaneous release and d ecreased Ca efflux per event meant that the Ca efflux per unit time was una ffected by low concentrations of caffeine. The SR Ca content was reduced by caffeine. The extra Ca efflux calculated from the Na-Ca exchange current i ntegrals occurring during the initial burst of spontaneous activity on appl ication of caffeine accounted for this reduction of SR Ca content. In contr ast to these maintained effects on spontaneous release, caffeine had only t ransient effects on stimulated Ca release produced by depolarizing pulses. We conclude that stimulation of the RyR results in spontaneous release at S R Ca contents lower than those at which release would normally occur. There fore, the balance between normal and spontaneous Ca release can be shifted by modulation of the RyR. This will have important consequences for arrhyth mogenesis due to spontaneous Ca release. (C) 2000 Harcourt Publishers Ltd.