1. Contraction and intracellular Ca2+ (Ca2+(i)) transients were measured in isolated rabbit ventricular myocytes during twitches and contractures induced by rapid application of 10 mM-caffeine. 2. The amplitude of caffeine-induced contractures and the accompanying Ca2+(i) transients were larger than during normal twitches and also declined more slowly. This may be because only a fraction of sarcoplasmic reticulum (SR) Ca2+ is released during a normal twitch, or because of a temporal overlap of SR Ca2+ release and uptake during the twitch. 3. When a caffeine contracture was initiated in Na+-free, Ca2+-free medium (to prevent sarcolemmal Na+-Ca2+ exchange) the contracture and Ca2+(i) transient were larger and decreased much more slowly. Thus, Ca2+ extrusion via Na+-Ca2+ exchange may limit the amplitude of caffeine-induced contractures. 4. Relaxation half-time (t( 1/2 )) for the twitch (0.17±0.03 s) was increased to 0.54±0.07 s for caffeine contractures in control solution and 8.8±1 s for caffeine-induced contractures in Na+-free, Ca2+-free solution. These results confirm that the SR Ca2+ pump and Na--Ca2+ exchange are the predominant mechanisms for cytoplasmic Ca2+ removal during relaxation. However slower mechanisms can still reduce intracellular [Ca2+]. 5. Relaxation of caffeine contractures in Na+-free solution was further slowed when (a) mitochondrial Ca2+ uptake was as inhibited with the oxidative phosphorylation uncoupler, FCCP (t( 1/2 ) = 19.7±3.2 s), or (b) the sarcolemmal Ca2+-ATPase pumping ability was as depressed by a large transmembrane [Ca2+] gradient (t( 1/2 ) = 27.5±6.9 s). 6. When the four Ca2+ transport systems were simultaneously inhibited (i.e. SR Ca2+ pump, Na+- Ca2+ exchange, mitochondrial Ca2+ uptake and sarcolemmal Ca2+ pump), relaxation was practically abolished, but the cell could recover quickly when Na+ was reintroduced and caffeine removed. 7. We conclude that, under our experimental conditions, the sarcolemmal Ca2+ pump and mitochondria are ~ 37- and 50-fold slower than the Na+-Ca2+ exchange at removing Ca2+ from the cytoplasm. Additionally, the SR Ca2+ pump is about 3-4 times faster than Na+- Ca2+ exchange.
|Original language||English (US)|
|Number of pages||18|
|Journal||Journal of Physiology|
|State||Published - 1992|
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