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 contrac tile activation after R were studied in rat ventricular myocytes. Indo 1 fluorescence (IF) indexed cytosolic Ca2+, and caffeine assayed SR C a2+. Before R, there was a negative staircase, and the SR accumulated Ca2+ at rest. During stimulation (0.5 Hz), R decreased IF and contract ion, converting the negative staircase to positive. When R was pulsed onto resting cells, IF increased and cells shortened, subsequently beh aving as if stimulated in R. After R, there was no caffeine-releasable Ca2+ at rest, and little accumulated during 0.5-Hz stimulation. At hi gh rates, caffeine-releasable Ca2+ and diastolic IF increased. In isop roterenol and R, IF transients and contractions recovered at 0.5 Hz wi th a marked positive staircase and little diastolic IF increase. Withi n 10 beats, SR Ca2+ accumulated to pre-R levels. R eliminated the posi tive inotropic effect of paired-pulse stimulation, but isoproterenol t emporarily 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 repl eted in R by high-rate stimulation or by low-rate stimulation with a b eta-adrenergic agonist. SR Ca2+ release in R can be temporarily restor ed if Ca2+ influx and SR Ca2+ pumping are increased enough to overcome the SR Ca2+ leak.