Repletion of sarcoplasmic reticulum Ca after ryanodine in rat ventricular myocytes

The ryanodine (R)-induced loss of sarcoplasmic reticulum (SR) Ca²⁺ and the abilities of the SR to accumulate Ca²⁺ and participate in contractile activation after R were studied in rat ventricular myocytes. Indo 1 fluorescence (IF) indexed cytosolic Ca²⁺, and caffeine assayed SR Ca²⁺. Before R, there was a negative staircase, and the SR accumulated Ca²⁺ at rest. During stimulation (0.5 Hz), R decreased IF and contraction, converting the negative staircase to positive. When R was pulsed onto resting cells, IF increased and cells shortened, subsequently behaving as if stimulated in R. After R, there was no caffeine-releasable Ca²⁺ at rest, and little accumulated during 0.5-Hz stimulation. At high rates, caffeine-releasable Ca²⁺ and diastolic IF increased. In isoproterenol and R, IF transients and contractions recovered at 0.5 Hz with a marked positive staircase and little diastolic IF increase. Within 10 beats, SR Ca²⁺ accumulated to pre-R levels. R eliminated the positive inotropic effect of paired-pulse stimulation, but isoproterenol temporarily restored it. Twitch contractions in thapsigargin, an SR Ca²⁺ pump blocker, and isoproterenol were slow compared with control or R + isoproterenol. R leaks SR Ca²⁺ into the cytosol. SR Ca²⁺ can be repleted in R by high-rate stimulation or by low- rate stimulation with a β-adrenergic agonist. SR Ca²⁺ release in R can be temporarily restored if Ca²⁺ influx and SR Ca²⁺ pumping are increased enough to overcome the SR Ca²⁺ leak.