| Abstract: | 1.眾所周知，過度血壓上升可導致心臟的肥大。但發生在心臟細胞中生理、生化及分子之肥大機制，則還不太清楚。再則心臟細胞中，粒腺體是提供心臟能量的關鍵，但其如何應付心臟過度負擔之需求，亦不得而知。在本實驗中,我們應用大白鼠腹動脈完全結紮模式，誘發心臟肥大，我們除了觀察到血壓上升外，並發現心臟肥大在手術之後的1 到2 天即發生，並持續至實驗結束(第20天)。同時心臟粒腺體中COX-Vb蛋白質量及COX-Vb酵素活性，亦隨著心臟的肥大而上升，且在第5-7天達到最高值，但第10天開始下降，手術後第20天則下降得比基礎值低。藉由in situ hybridization 及dot blotting測量，心臟細胞色素c氧化(cytochrome c oxidase-Vb, COX-Vb) mRNA，亦在結紮第一天快速上升，且持續增加，在第5-7天達到最高值，並在第20天降低，與基礎值差不多。此外,採Wistar(WKY)正常鼠與先天性高血壓大白鼠(SHR & SPSHR)作比較，發現SHR及SPSHR心臟中，COX-Vb蛋白質量在生長初期之12週齡內，即比正常血壓(WKY)大白鼠低。綜觀而論，1.COX之基因表現及蛋白活性可能在心肌肥大的過程中，伴演一適應之角色。2.不論是長效性腹動脈完全結紮大白鼠(第20天)或先天性高血壓大白鼠的模式，在肥大之心臟細胞中，低效率粒腺體活性，可能或至少部分是造成心肌衰竭的原因。3.腹動脈完全結紮造成心臟快速肥大的模式中，COX之活性上升，可能是用來供應心臟在過度負擔時所需之額外能量。
2.報導顯示IGF-IR訊息途徑與血壓之調節和心臟組織存活與肥大有關係。但在高血壓大白鼠心臟組織，是否IGF-IR訊息途徑是否正常運作則不得而知。在本研究中, 我們以先天性高血壓(spontaneously hypertensive rats, SHR)及中風傾向(stroke-prone spontaneously hypertensive rats, SPSHR)大白鼠為實驗對象，以Wistar-Kyoto (WKY) 大白鼠為控制組，在4、6、12 週齡之三種動物中，我們做了心臟型態及IGF-IR訊息途徑活性之測定。結果發現在第12週時，SHR及SPSHR大白鼠血壓、心臟全重及左心室體重的比率均較控制組為高，但IGF-IR及其下游已知對心臟據有存活及維持細胞生理大小作用之蛋白濃度皆有下降趨勢，顯示IGF-IR訊息途徑活性有被負調控之現象。類胰島素生長因子I (Insulin-like growth factor I,IGF-I)之mRNA自第4週起，亦較控制組為高。顯示，為因應心臟IGF-IR訊息途徑活性降低， IGF-IR ligan mRNA的上升，提供了一個代償的作用。再者，另一凋亡蛋白cytochrome-c 在SHR及SPSHR皆有升高趨式，但未達顯著水準。這些發現顯示，發生在早期(4週齡) 心臟IGF-IR訊息途徑的缺損，可能導致高血壓之發生與病理性心臟肥大，而造成後來SHR及SPSHR心臟細胞凋亡之心臟衰竭。
1.It is well known that pressure overload leads to cardiac hypertrophy. The physiological, biochemical, or molecular mechanisms, that underlie the myocyte enlargement occurring during cardiac hypertrophy are not completely understood. The contribution of the mitochondrial components, the main source of energy for the hypertrophic growth of the heart, is not well understood. In the present study, using a model system, complete coarctation of the rat abdominal aorta was used to study the rapid development of cardiac hypertrophy. One to two days after surgery, we observed significantly higher blood pressure and cardiac hypertrophy, which remained constantly high afterwards. We found that mitochondrial COX protein level and enzyme activity increased as the heart enlarged and, however, dropped even lower than baseline 20 days following surgery. We also found an early increased level of cytochrome c oxidase (COX) mRNA determined by in situ hybridization and dot blotting assays in the hypertrophied hearts, though the increased level had dropped to the baseline 20 days after surgery. In addition, the COX protein was significantly lower in natural hypertension-induced hypertrophic hearts in spontaneously hypertensive rats (SHR) and stroke-prone SHR (SPSHR) than in normotensive rats (WKY) throughout the whole period of 12 weeks. Taken together, the lower efficiency of mitochondrial activity in the enlarged hearts of rats with long-term complete coarctation or SHR and SPSHR could be, at least partially, the cause of hypertensive cardiac disease in these two models. Additionally, the rapid cardiac hypertrophy induced by experimental complete coarctation was accompanied by a disproportionate increase in COX activity, which was suggested to maintain the cardiac energy-producing capacity in overloaded hearts.
2.Insulin-like growth factor-I (IGF-I) signaling is reported to contribute to the modulation of blood pressure and set in survival and hypertrophic responses in cardiac tissue. However, whether the IGF-I signaling normally acts in cardiac tissues of hypertensive rats is unknown. In this study, using spontaneously hypertensive rats (SHR) and stroke prone spontaneously hypertensive rats (SPSHR), both with early blood pressure increases, and Wistar-Kyoto (WKY) as controls, we measured the hypertrophic and IGF-I signaling activity changes in rat hearts at 4, 6 and 12 weeks of age. Both SHR and SPSHR were found to have significantly increased blood pressures and ratios of heart- and left ventricle- to body weight at 12 weeks of age. However, IGF-IR and its downstream signaling, including the protein levels of PI3K and phosphorylated Akt, known to maintain physiological cardiac hypertrophy and cardiomyocyte survival, were downregulated. The results of dot blotting showed that cardiac mRNA levels of IGF-I in hypertensive rats were higher than those in controls starting from the age of 4 weeks. This difference suggests the increased ligand IGF-I mRNA levels may be a compensatory response caused by the impaired IGF-I signaling. Moreover, enhanced cardiac cytosolic cytochrome-c, a mitochondria-dependent apoptotic pathway component, tended to occur in both hypertensive rats, although it did not reach a significant level. These findings indicate that impaired IGF-IR signaling occurs at early stages, and it may contribute, at least partially, to the development of hypertension and pathological cardiac hypertrophy and to cardiomyocyte apoptosis at later stages in SHR and SPSHR. |
