Busselen P, Bosteels S, Tunstall J
Interdisciplinary Research Center, University of Leuven, Kortrijk, Belgium.
J Mol Cell Cardiol. 1991 Nov;23(11):1313-22. doi: 10.1016/0022-2828(91)90088-4.
In rat papillary muscle, rapid cooling causes membrane depolarization which initiates action potentials that lead to a contraction. This rapid cooling contraction (RCC) can be blocked by TTX, Mn2+, Ni2+ or high K+ superfusion. In the presence of caffeine (0.5-1 mM), the rapid cooling contracture (caffeine-RCC) has an amplitude similar to that of a twitch elicited by field stimulation at 37 degrees C, but is not inhibited by these agents. As the caffeine-RCC appears to be independent of membrane depolarization and Ca influx but can be inhibited by increasing the bathing caffeine concentration to 20 mM, we consider that the amplitude of this contracture gives a good indication of the calcium content of the sarcoplasmic reticulum (SR). In Tyrode containing 1.8 mM Ca an increased stimulus frequency leads to a negative force staircase which is paralleled by a similar decrease in the amplitude of the caffeine-RCC. These effects are lost if the bathing Ca is reduced (0.18-0.45 mM) in a way which can be reversed by isoproterenol (100 nM). In verapamil (2 microM), however whilst the twitch responses may show a steeper dependence upon stimulus frequency, the negative frequency dependence of the caffeine-RCC is also lost. Low external Na+ also inhibits the frequency dependent reduction of the caffeine-RCC. The results suggest that if the amplitude of the caffeine-RCC is a good indication of the SR calcium content, then this Ca store is related reciprocally to membrane Ca current where activation of the Ca channels leads to a depletion of the store whereas inhibition of membrane Ca channels leads to a filling of the Ca store. We propose that on stimulation the size of the Ca influx determines the fraction of Ca released from the SR. This released Ca may be partially extruded from the cell by way of the Na/Ca exchange which acts in competition with the re-uptake mechanism of the SR to control SR Ca content.
在大鼠乳头肌中，快速冷却会导致膜去极化，进而引发动作电位，导致肌肉收缩。这种快速冷却收缩（RCC）可被河豚毒素（TTX）、锰离子（Mn2+）、镍离子（Ni2+）或高钾灌注所阻断。在咖啡因（0.5 - 1 mM）存在的情况下，快速冷却挛缩（咖啡因 - RCC）的幅度与37摄氏度时场刺激引发的单收缩幅度相似，但不受这些药物的抑制。由于咖啡因 - RCC似乎与膜去极化和钙内流无关，但可通过将浴液中咖啡因浓度增加到20 mM来抑制，我们认为这种挛缩的幅度很好地反映了肌浆网（SR）的钙含量。在含有1.8 mM钙的台氏液中，刺激频率增加会导致负力阶梯现象，同时咖啡因 - RCC的幅度也会有类似程度的下降。如果将浴液中的钙浓度降低（0.18 - 0.45 mM），这些效应就会消失，而异丙肾上腺素（100 nM）可使其逆转。然而，在维拉帕米（2 microM）存在的情况下，虽然单收缩反应可能对刺激频率表现出更强烈的依赖性，但咖啡因 - RCC的负频率依赖性也会消失。低细胞外钠离子也会抑制咖啡因 - RCC的频率依赖性降低。结果表明，如果咖啡因 - RCC的幅度能很好地反映肌浆网钙含量，那么这个钙库与膜钙电流呈反比关系，即钙通道激活会导致钙库耗竭，而膜钙通道抑制则会导致钙库充盈。我们提出，刺激时钙内流的大小决定了从肌浆网释放的钙的比例。这种释放的钙可能会部分通过钠/钙交换排出细胞，钠/钙交换与肌浆网的再摄取机制相互竞争，以控制肌浆网的钙含量。
