A mechanism for the effects of caffeine on Ca2+ release during diastole and systole in isolated rat ventricular myocytes.

The fluorescent indicator Indo-1 was used to measure both Ca2+i and caffeinei in single ventricular myocytes. 2. Caffeine (at concentrations of 1 mM or above) produced a transient increase of resting Ca2+i attributed to the release of Ca2+ ions from the sarcoplasmic reticulum (SR). Simultaneous measurement of caffeinei showed that the Ca2+ release only began when caffeinei had risen to about 1 mM. Subsequently the rate of release was a steep function of caffeinei. It is suggested that this results from a positive feedback as the Ca2+ released activates further release. 3. If external Ca2+ was removed the release of Ca2+ produced by caffeine was delayed such that caffeinei rose to a greater concentration before release was initiated. This suggests that an increase of Ca2+i increases the efficacy of caffeine to release Ca2+ ions from the SR. 4. Lower concentrations of caffeine (50-500 microM) had no effect on diastolic Ca2+i. In contrast they increased systolic Ca2+i and contraction. This increase was most obvious if the systolic contraction had previously been decreased either by reducing Ca2+o from 1 to 0.25 mM or (in voltage-clamped cells) by decreasing the magnitude of the depolarizing pulse. 5. If the exposure to caffeine was prolonged, this increase of systolic Ca2+i and contraction was completely transient. On removal of caffeine, systolic Ca2+i and contraction decreased to below control before recovering. 6. During these transient changes of systolic Ca2+i and contraction there was no change of the sarcolemmal Ca2+ current. 7. It is suggested that the increase of systolic Ca2+i is due to caffeine increasing the fraction of the SR Ca2+ content released during the twitch. 8. The above results concerning both diastolic and systolic Ca2+i can be explained by a model in which caffeine increases the affinity with which Ca2+ ions activate Ca2(+)-induced Ca2+ release. At high enough caffeine, the threshold Ca2+i for regenerative Ca2(+)-induced Ca2+ release will be reduced to below the resting Ca2+i thus producing a diastolic increase of Ca2+i. At lower caffeine the threshold is higher than resting Ca2+i and caffeine only serves to enhance the release produced during systole.