Kloock Tanja, Jörg David J, Mühlfeld Christian
Hannover Medical School, Institute of Functional and Applied Anatomy, 30625 Hanover, Germany.
Independent Researcher, 60438 Frankfurt am Main, Germany.
J Exp Biol. 2025 Jan 1;228(1). doi: 10.1242/jeb.249489. Epub 2025 Jan 10.
Small mammals have a higher heart rate and, relative to body mass (Mb), a higher metabolic rate than large mammals. In contrast, heart weight and stroke volume scale linearly with Mb. With mitochondria filling approximately 50% of a shrew cardiomyocyte - space unavailable for myofibrils - it is unclear how small mammals generate enough contractile force to pump blood into circulation. Here, we investigated whether the total number or volume of cardiomyocytes in the left ventricle compensates for allometry-related volume shifts of cardiac mitochondria and myofibrils. Through statistical analysis of data from 25 studies with 19 different mammalian species with Mb spanning seven orders of magnitude (2.2 g to 920 kg), we determined how number, volume density and total volume of cardiomyocytes, mitochondria and myofibrils in the left ventricle depend on Mb. We found that these biological variables follow scaling relationships and are proportional to a power b of Mb. The number [b=1.02 (95% CI: 0.89, 1.14); t-test for b=1: P=0.72] and volume [b=0.95 (95% CI: 0.89, 1.03); t-test for b=1: P=0.18] of cardiomyocytes in the left ventricle increases linearly with increasing Mb. In cardiomyocytes, volume density of mitochondria decreases [b=-0.056 (95% CI: -0.08, -0.04); t-test for b=0: P<0.0001] and that of myofibrils increases [b=0.024 (95%CI: 0.01, 0.04); t-test for b=0: P<0.01] with increasing Mb. Thus, the number or volume of left ventricular cardiomyocytes does not compensate for the higher heart rate and specific metabolic rate of small mammals although a higher mitochondrial and lower myofibrillar volume per cardiomyocyte are present.
小型哺乳动物的心率较高，相对于体重（Mb）而言，其代谢率也比大型哺乳动物高。相比之下，心脏重量和每搏输出量与Mb呈线性比例关系。由于线粒体占据了鼩鼱心肌细胞约50%的空间，而这些空间无法用于肌原纤维，因此尚不清楚小型哺乳动物如何产生足够的收缩力将血液泵入循环系统。在这里，我们研究了左心室心肌细胞的总数或体积是否能补偿与异速生长相关的心脏线粒体和肌原纤维的体积变化。通过对来自25项研究的数据进行统计分析，这些研究涉及19种不同的哺乳动物物种，其Mb跨度达7个数量级（2.2克至920千克），我们确定了左心室中心肌细胞、线粒体和肌原纤维的数量、体积密度和总体积如何依赖于Mb。我们发现这些生物学变量遵循比例关系，并且与Mb的幂b成正比。左心室心肌细胞的数量[b = 1.02（95%置信区间：0.89，1.14）；b = 1的t检验：P = 0.72]和体积[b = 0.95（95%置信区间：0.89，1.03）；b = 1的t检验：P = 0.18]随Mb的增加呈线性增加。在心肌细胞中，线粒体的体积密度随Mb的增加而降低[b = -0.056（95%置信区间：-0.08，-0.04）；b = 0的t检验：P < 0.0001]，而肌原纤维的体积密度随Mb的增加而增加[b = 0.024（95%置信区间：0.01，0.04）；b = 0的t检验：P < 0.01]。因此，尽管每个心肌细胞中线粒体体积较高而肌原纤维体积较低，但左心室心肌细胞的数量或体积并不能补偿小型哺乳动物较高的心率和特定代谢率。
