George J C, Ronald K
Acta Anat (Basel). 1975;93(1):88-99.
Three types of fibre, dark (type 1), light (type 2) and intermediate, were distinguished in the caval sphincter muscle of the diaphragm in the harp seal (Pagophilus groenlandicus) using histochemical and electron-microscopic techniques. The dark fibre contained large peripheral aggregations of mitochondria, numerous lipid droplets and dense aggregates of glycogen granules. The same features were observed in the dark fibre of the diaphragm muscle too. In the light of the low oxidative enzyme activity and high lipase activity observed in the diaphragm and other skeletal muscles of the harp seal in previous studies, it is postulated that these mitochondrial aggregations and lipid droplets represent an adaptation for the generation of heat through non-shivering thermogenesis comparable to that in the brown adipose tissue. Such ability to generate and maintain heat by uncoupling the oxidative phosphorylation process should enable the caval sphincter muscle to function efficiently in regulating the cardiac return of blood from the inferior vena cava during a dive when the body temperature as a whole drops. The lack of the regional differences in the fibre composition of the harp seal diaphragm, as were reported in the rat diaphragm, is attributed to the seal's large body size, lower metabolic rate and diving habit.
利用组织化学和电子显微镜技术，在竖琴海豹（Pagophilus groenlandicus）的膈腔静脉括约肌中区分出了三种类型的纤维，即深色（1型）、浅色（2型）和中间型纤维。深色纤维含有大量周边聚集的线粒体、众多脂滴和密集的糖原颗粒聚集体。在膈肌的深色纤维中也观察到了同样的特征。鉴于先前研究中在竖琴海豹的膈肌和其他骨骼肌中观察到的低氧化酶活性和高脂肪酶活性，推测这些线粒体聚集物和脂滴代表了一种通过非颤抖性产热来产生热量的适应性变化，类似于棕色脂肪组织中的情况。通过使氧化磷酸化过程解偶联来产生和维持热量的这种能力，应能使腔静脉括约肌在潜水期间当整体体温下降时，有效地调节来自下腔静脉的心脏回血。竖琴海豹膈肌纤维组成缺乏如在大鼠膈肌中所报道的那种区域差异，这归因于海豹的体型大、代谢率低和潜水习性。
