Lew W Y, Nishikawa Y, Su H
Department of Medicine, University of California, San Diego.
Circulation. 1994 Oct;90(4):1942-50. doi: 10.1161/01.cir.90.4.1942.
After a brief episode of ischemia, myocardial function may be depressed for prolonged periods despite reperfusion. The mechanisms of postischemic dysfunction differ depending on the experimental model. Regional ischemia and reperfusion in the intact animal provide a clinically relevant model, but experimental variables are difficult to control. Experimental conditions can be well controlled in isolated cardiac muscle and myocyte preparations, but these models are limited by the assumptions used to mimic ischemia and reperfusion. This study combines the unique advantages of both preparations. We characterized in vivo alterations in regional two-dimensional finite strains with ischemia and reperfusion produced in the intact animal, then isolated cardiac myocytes from the region with postischemic dysfunction to characterize in vitro function of postischemic myocytes.
In seven anesthetized rabbits, three piezoelectric crystals were inserted in a triangular array to measure two-dimensional finite strains around the large coronary artery in the left ventricular anterior free wall. After 15 minutes of ischemia and reperfusion, strains were depressed at a stable level approximately 30% to 40% below control values between 1 and 6 hours after reperfusion. The direction of maximal shortening deformations was midway between circumferential and longitudinal directions during control and did not shift after reperfusion. In a second group of five rabbits, cardiac myocytes were isolated from the region with postischemic dysfunction after 15 minutes of ischemia and 45 minutes of reperfusion. We compared in vitro function in 45 postischemic myocytes with 48 cardiac myocytes isolated from five normal rabbits. Each rabbit (postischemic and control) contributed 9 +/- 1 (SD) myocytes to the study. All myocytes were studied within 1 hour after myocyte isolation (approximately 3 to 5 hours after reperfusion for postischemic myocytes). Myocytes were stimulated at 0.5 Hz and perfused with 2 mmol/L [Ca2+] Tyrode's solution to measure unloaded cell shortening. There was significantly less shortening in postischemic myocytes (12.4 +/- 2.1%) than control myocytes (16.2 +/- 1.2%). Maximal cell length (Lmax) was significantly longer in postischemic (134 +/- 7 microns) than control myocytes (122 +/- 7 microns), as was minimum cell length (Lmin) (118 +/- 8 versus 103 +/- 9 microns, respectively). The duration of shortening (time from stimulation to Lmin) was significantly shorter in postischemic (279 +/- 56 milliseconds) than control myocytes (405 +/- 44 milliseconds). Peak rates of cell shortening (-dL/dt) and lengthening (+dL/dt) did not differ.
In rabbits, 15 minutes of ischemia produced a stable depression in finite strains for 1 to 6 hours after reperfusion, with shortening deformations reduced by approximately 30% to 40% without a shift in direction. Cardiac myocytes isolated from postischemic myocardium display functional impairments in vitro similar to those measured in vivo, with an approximately 25% reduction in unloaded myocyte shortening and decreased contraction duration. This indicates that ischemia and reperfusion induce intrinsic impairments in contractility independently of external loading conditions. This model may be useful for examining cellular mechanisms of postischemic myocardial dysfunction.
短暂缺血发作后，尽管恢复了灌注，但心肌功能可能会在较长时间内受到抑制。缺血后功能障碍的机制因实验模型而异。完整动物的局部缺血和再灌注提供了一个与临床相关的模型，但实验变量难以控制。在分离的心肌和心肌细胞制剂中，实验条件可以得到很好的控制，但这些模型受到用于模拟缺血和再灌注的假设的限制。本研究结合了两种制剂的独特优势。我们对完整动物中缺血和再灌注引起的局部二维有限应变的体内变化进行了表征，然后从缺血后功能障碍区域分离出心肌细胞，以表征缺血后心肌细胞的体外功能。
在7只麻醉的兔子中，以三角形阵列插入3个压电晶体，以测量左心室前游离壁大冠状动脉周围的二维有限应变。缺血和再灌注15分钟后，再灌注后1至6小时内，应变稳定下降至比对照值低约30%至40%的水平。对照期间最大缩短变形的方向在周向和纵向之间的中间位置，再灌注后未发生偏移。在第二组5只兔子中，缺血15分钟和再灌注45分钟后，从缺血后功能障碍区域分离出心肌细胞。我们将45个缺血后心肌细胞的体外功能与从5只正常兔子分离出的48个心肌细胞进行了比较。每只兔子（缺血后和对照）为研究贡献了9±1（标准差）个心肌细胞。所有心肌细胞在分离后1小时内进行研究（缺血后心肌细胞约在再灌注后3至5小时）。以0.5Hz刺激心肌细胞，并用2mmol/L [Ca2+] Tyrode溶液灌注，以测量无负荷细胞缩短。缺血后心肌细胞的缩短（12.4±2.1%）明显少于对照心肌细胞（16.2±1.2%）。缺血后心肌细胞的最大细胞长度（Lmax）（134±7微米）明显长于对照心肌细胞（122±7微米），最小细胞长度（Lmin）也是如此（分别为118±8微米和103±9微米）。缺血后心肌细胞的缩短持续时间（从刺激到Lmin的时间）（279±56毫秒）明显短于对照心肌细胞（405±44毫秒）。细胞缩短（-dL/dt）和延长（+dL/dt）的峰值速率没有差异。
在兔子中，15分钟的缺血导致再灌注后1至6小时有限应变稳定下降，缩短变形减少约30%至40%，方向无偏移。从缺血后心肌分离出的心肌细胞在体外表现出与体内测量相似的功能损害，无负荷心肌细胞缩短减少约25%，收缩持续时间缩短。这表明缺血和再灌注独立于外部负荷条件诱导收缩力的内在损害。该模型可能有助于研究缺血后心肌功能障碍的细胞机制。
