We measured intracellular Ca2+ transients during rapid cooling contractures (RCCs) in guinea-pig ventricular myocytes using the fluorescent Ca2+ indicator, Indo-1. Rapid cooling of myocytes from 22 to 0-1°C induced a rapid increase in [Ca2+](i) which preceded the peak of the contraction and was sometimes large enough to saturate Indo-1. This indicates that [Ca2+](i) may reach > 10 μM during an RCC. The [Ca2+](i) can be attributed to slow reaccumulation of Ca2+ by the sarcoplasmic reticulum (SR) in the cold. RCCs induced in the absence of Ca(o)2+, were not different from control, supporting previous conclusions that RCCs depend exclusively on intracellular Ca2+ stores. RCCs are depressed by long rest periods (rest decay) or by exposure to ryanodine or caffeine, which supports conclusions that RCCs are due to Ca2+ release from the SR. The rest decay of RCCs can be almost completely prevented by applying Na(o)+-free solution during the rest period. This implies that the loss of SR Ca2+ during rest depends on the sarcolemmal Na+-Ca2+ exchange (and not the sarcolemmal Ca2+-ATPase pump). Rapid rewarming during an RCC normally leads to an additional transient contraction (or rewarming spike), without any increase in [Ca2+](i). Thus, the rewarming spike might be attributable to an increase in myofilament Ca2+ sensitivity induced by rewarming. A second RCC is used to assess the fraction of Ca2+ which is re-sequestered by the SR during relaxation from the first RCC. In control solution progressive RCCs decline in amplitude, but in Na+-free, Ca2+-free solution they are of constant amplitude. We conclude that the SR Ca2+ pump and Na+-Ca2+ exchange are responsible for relaxation and that the latter may account for 20-50% of relaxation. These results support the use of RCCs as a useful means of assessing SR Ca2+ content in intact cardiac muscle cells.
|Original language||English (US)|
|Number of pages||17|
|Journal||Journal of Physiology|
|State||Published - 1989|
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