August 1, 1993
View Full PaperTwitch-dependent SR Ca accumulation and release in rabbit ventricular myocytes
Key Result
After SR Ca depletion in rabbit ventricular myocytes, four to five twitches are necessary to reload the SR to the steady-state level, and 90 s of 5 microM TG completely prevents SR Ca reuptake.
Structured PICO
P
Population
Rabbit ventricular myocytes
I
Intervention
Caffeine-induced contractures (10 mM) and thapsigargin (5 microM)
O
Outcome
Fraction of sarcoplasmic reticulum (SR) Ca released at one twitch and number of twitches required to reload a Ca-depleted SRsurrogate
In rabbit ventricular myocytes, 4-5 twitches are required to reload a Ca-depleted sarcoplasmic reticulum, and SR Ca-ATPase activity limits the ability of calcium influx to activate contraction when the SR is depleted.
Abstract
Using caffeine-induced contractures (Ccaf) and thapsigargin (TG), we estimated the fraction of sarcoplasmic reticulum (SR) Ca released at one twitch and also the number of twitches required to reload a Ca-depleted SR. Similar results were obtained for twitches or intracellular Ca (Cai) transient with the fluorescent indicator, indo 1. Sustained exposure to 10 mM caffeine completely depletes the SR of Ca in < 5 s (as assessed by a second Ccaf). After such Ca depletion, four to five twitches are necessary to reload the SR to the steady-state level (with a twitch constant, tau = 1.6 twitches). We also determined the time required for complete inhibition of the SR Ca-adenosinetriphosphatase (ATPase) by TG. After SR Ca depletion, 5 microM TG was applied for different periods of time before a train of "reloading" twitches. A TG exposure period of 90 s was sufficient to completely prevent Ccaf after these reloading twitches. When SR is Ca depleted, the twitch is larger in the presence of TG, indicating that the SR Ca-ATPase can limit the ability of Ca influx to activate contraction. To assess SR Ca released at one twitch in cells with normally Ca-loaded SR, 5 microM TG was applied for 90 s to prevent SR Ca reuptake. Then one or several twitches were activated (causing SR Ca release, but with reuptake completely blocked). After the twitch (or train), a Ccaf was used to assess remaining SR Ca.(ABSTRACT TRUNCATED AT 250 WORDS)