Biomechanics on Remodeling of Biological Tissues - Role of Collagen Cross-Linking

8周龄和周龄大鼠颈总动脉内胶原和弹性蛋白的面积分数无明显变化，而周龄大鼠颈总动脉壁的弹性模数明显大于青年大鼠。为了弄清原因，我们将流体力学等长张力法应用于胶原蛋白的交联分析。在该方法中，将组织标本拉伸并加热到胶原变性温度以上，并在恒应变条件下保持该温度。此后观察到的应力松弛表示热稳定、成熟的交联量；应力减小是交联性的测量。首先，我们使用8、16、32和周龄的大鼠颈总动脉研究了动脉壁交联性的年龄相关变化。将小的矩形样品连接到专门设计的测试仪器上，并将其浸入恒温浴中的蒸馏水中。当对每个样品施加拉伸应变(体内应变)时，样品温度升高到100℃。当样品保持在这种条件下时，样品中产生的力被连续监测。负荷的半值期，即负荷减少到一半所需的时间，被用来衡量交联度，周龄的动物显著长于年轻的动物。这一结果与管壁弹性模量的结果很好地吻合，表明老年动脉管壁硬度较高的原因是稳定的胶原交联量较大。然后，将HIT方法应用于DOCA-盐性高血压大鼠的颈总动脉，并将结果与年龄匹配的正常血压动物的结果进行比较。成熟交联物的相对数量随高血压的增加而增加，这是高血压动脉弹性模数高于正常血压的原因。

The area fractions of collagen and elastin in the rat common carotid artery did not change between 8 and 64 weeks, whereas the elastic modulus of wall was significantly larger in 64 weeks old animals compared to younger ones. To know the reason, we have applied a hydromechanical isometric tension method to the analysis of cross-linking of collagen. In the method, a tissue specimen is stretched and heated to above collagen denaturation temperature, and kept at this temperature under constant strain condition. Stress relaxation observed thereafter indicates a measure of the amount of thermally stable, matured cross-linking ; stress reduction is a measure of cross-linking.First, we studies age-related changes of cross-linking in arterial wall using rat common carotid arteries aged 8, 16, 32, and 64 weeks. Small rectangular specimens were attached to a specially designed test apparatus, and immersed in distilled water in a thermostatic bath. While a tensile strain (in vivo strain) was applied to each specimen, specimen temperature was elevated to 100C. While the specimen was kept under this condition, force developed in the specimen was continuously monitored. Half-value period of load, i.e. the time elapsed for the reduction of load to a half, was used as a measure of cross-linking, and it was significantly longer in 64 weeks old animals than in younger ones. This result corresponds well to the results of the elastic modulus of wall, indicating that higher wall stiffness in older arteries is attributable to the larger amount of stable collagen cross-linking. Then, the HIT method was applied to common carotid arteries from DOCA-salt hypertensive rats, and the results were compared with those from the age-matched, normotensive animals. Relative amount of matured cross-links showed a tendency of increasing by hypertension, which explains higher elastic modulus in hypertensive arteries than in normotensive ones.