Banijamali H S, ter Keurs M H, Paul L C, ter Keurs H E
Department of Medicine, University of Calgary, Alberta, Canada.
Cardiovasc Res. 1993 Oct;27(10):1845-54. doi: 10.1093/cvr/27.10.1845.
The aim was to investigate the steps in the excitation-contraction coupling process by which chronic exposure to cyclosporin A (cyclosporin) affects twitch force development by rat cardiac trabeculae.
The interval dependence and [Ca2+]o dependence of twitch force development by intact trabeculae isolated from myocardium of untreated rats and rats treated with cyclosporin (15 mg.kg-1.d-1 for 21 d) were studied in Krebs-Henseleit solution (K-H; pH 7.4 and 25 degrees C) and the force-pCa relation was examined in all trabeculae.
The force-[Ca2+]o relation of cyclosporin treated trabeculae was shifted leftward compared to that of controls, but these trabeculae generated 35% less stress (force/cross sectional area) at optimal [Ca2+]o. Unlike control trabeculae, cyclosporin treated trabeculae showed spontaneous activity at all diastolic intervals, even at low [Ca2+]o. Treated and control trabeculae generated the same maximum stress [control: 78.1 (SEM 7.7) mN.mm-2, cyclosporin treated: 70.2(7.4) mN.mm-2] in the presence of extracellular Sr2+ ions in the Krebs-Henseleit medium. Maximum stress observed in the presence of Sr2+ was similar to the stress generated by maximum activation of chemically skinned trabeculae in both groups [control: 70(4.6) mN.mm-2; cyclosporin treated: 73(6.2) mN.mm-2). The force-pCa relation of cyclosporin treated muscles and control muscles after skinning were also indistinguishable [control pCa50 = 5.56(0.04); cyclosporin treated pCa50 = 5.58(0.03)]. The twitch force-interval relation at 0.7 mM [Ca2+]o in intact control trabeculae revealed postrest potentiation with a maximum [equivalent to 70% of twitch force at optimal [Ca2+]o -61.0(2.1) mN.mm-2] at 100 s and subsequent rest depression. Under the same conditions, twitch force development by cyclosporin treated trabeculae was closer to optimal force [41.4(7.1) mN.mm-2] at all intervals, and rest potentiation was reduced. Pronounced rest potentiation (as well as postextrasystolic potentiation) was still observed in cyclosporin treated trabeculae at [Ca2+]o < 0.7 mM. Postextrasystolic potentiation was reduced at 0.7 mM [Ca2+]o in these trabeculae, but the rate of decay of postextrasystolic potentiation and the rate of relaxation of the twitch force were unaffected.
These results suggest that the changes in the sensitivity of intact rat myocardium to [Ca2+]o and in maximum force development induced by cyclosporin are not due to changes in myofilament properties. The increased twitch force development as well as the spontaneous activity at low [Ca2+]o may be due to facilitated Ca2+ release from the sarcoplasmic reticulum due to altered properties of the sarcoplasmic reticular Ca2+ release channel, as both are observed when twitch force is submaximal, suggesting that the sarcoplasmic reticulum was not overloaded with Ca2+. The decline in peak stress with cyclosporin at [Ca2+]o > approximately 1.0 mM can be explained on the basis of spontaneous release of Ca2+ during the interval between twitches which leaves less Ca2+ for release from the sarcoplasmic reticulum with each action potential.
研究慢性暴露于环孢素A（环孢素）影响大鼠心脏小梁收缩力发展的兴奋-收缩偶联过程中的步骤。
在Krebs-Henseleit溶液（K-H；pH 7.4，25℃）中研究了从未经处理的大鼠和用环孢素（15mg·kg⁻¹·d⁻¹，持续21天）处理的大鼠心肌中分离出的完整小梁收缩力发展的间隔依赖性和[Ca²⁺]ₒ依赖性，并检测了所有小梁的力-pCa关系。
与对照组相比，环孢素处理的小梁的力-[Ca²⁺]ₒ关系向左移动，但在最佳[Ca²⁺]ₒ时，这些小梁产生的应力（力/横截面积）减少了35%。与对照小梁不同，环孢素处理的小梁在所有舒张间隔均表现出自发活动，即使在低[Ca²⁺]ₒ时也是如此。在Krebs-Henseleit培养基中存在细胞外Sr²⁺离子的情况下，处理组和对照组小梁产生的最大应力相同[对照组：78.1（标准误7.7）mN·mm⁻²，环孢素处理组：70.2（7.4）mN·mm⁻²]。在Sr²⁺存在下观察到的最大应力与两组化学去膜小梁最大激活时产生的应力相似[对照组：70（4.6）mN·mm⁻²；环孢素处理组：73（6.2）mN·mm⁻²]。环孢素处理的肌肉和去膜后的对照肌肉的力-pCa关系也无差异[对照组pCa50 = 5.56（0.04）；环孢素处理组pCa50 = 5.58（0.03）]。完整对照小梁在0.7mM [Ca²⁺]ₒ时的收缩力-间隔关系显示出静息后增强，在100秒时达到最大值[相当于最佳[Ca²⁺]ₒ时收缩力的70% - 61.0（2.1）mN·mm⁻²]，随后出现静息抑制。在相同条件下，环孢素处理的小梁在所有间隔的收缩力发展更接近最佳力[41.4（7.1）mN·mm⁻²]，静息增强降低。在[Ca²⁺]ₒ < 0.7mM时，环孢素处理的小梁仍观察到明显的静息增强（以及早搏后增强）。在这些小梁中，0.7mM [Ca²⁺]ₒ时早搏后增强降低，但早搏后增强的衰减速率和收缩力的松弛速率未受影响。
这些结果表明，环孢素诱导的完整大鼠心肌对[Ca²⁺]ₒ敏感性的变化和最大力发展的变化并非由于肌丝特性的改变。收缩力发展增加以及在低[Ca²⁺]ₒ时的自发活动可能是由于肌浆网Ca²⁺释放通道特性改变导致肌浆网Ca²⁺释放增加，因为当收缩力未达到最大值时两者均被观察到，这表明肌浆网未被Ca²⁺过度负荷。在[Ca²⁺]ₒ > 约1.0mM时，环孢素导致的峰值应力下降可以基于收缩间期Ca²⁺的自发释放来解释，这使得每次动作电位时从肌浆网释放的Ca²⁺减少。
