DIASTOLIC, SYSTOLIC AND SARCOPLASMIC-RETICULUM [CA2+] DURING INOTROPIC INTERVENTIONS IN ISOLATED RAT MYOCYTES

1. The fluorescent indicator Fura-2 has been used to monitor intracellular [Ca2+] (Ca(i)2+) in myocytes isolated from the ventricles of rat hearts. 2. The relationships between diastolic Ca(i)2+, systolic Ca(i)2+ and the Ca2+ content of the sarcoplasmic reticulum (SR; assayed using caffeine) have been studied during changes of stimulation rate and bathing [Ca2+] (Ca(o)2+). 3. When stimulation rate was increased, there were increases in diastolic Ca(i)2+, systolic Ca(i)2+ and the Ca2+ content of the SR. 4. The SR inhibitor ryanodine (1-mu-mol l-1) decreased the size of the Ca(i)2+ transient, and abolished the increase of Ca(i)2+ produced by caffeine (10 mmol l-1). In the presence of ryanodine, increasing stimulation rate increased diastolic Ca(i)2+ but not systolic Ca(i)2+. 5. Increasing Ca(o)2+ led to increases of diastolic Ca(i)2+, systolic Ca(i)2+ and SR Ca2+ content similar to those observed during changes in stimulation rate. 6. Ryanodine altered the relationship between systolic and diastolic Ca(i)2+ during changes of Ca(o)2+. 7. These results are consistent with a change of diastolic Ca(i)2+ leading to an increase in the Ca2+ content of the SR, and hence an increase in the size of the Ca(i)2+ transient during changes in stimulation rate and Ca(o)2+.