Department of Biology, University of Eastern Finland, Joensuu, Finland.
Acta Physiol (Oxf). 2013 Feb;207(2):269-79. doi: 10.1111/j.1748-1716.2012.02479.x. Epub 2012 Sep 4.
Cardiac contraction and relaxation are mediated by rapidly changing calcium concentration around the myofibrils. In comparison with endotherms, ectothermic hearts are more strongly dependent on extracellular calcium for contraction suggesting this trait might represent the primitive vertebrate mode of cardiac activation. This study tests the hypothesis that intracellular calcium stores play a minor role in cardiac excitation-contraction coupling of a basal vertebrate, the lamprey (Lampetra fluviatilis).
Contribution of sarcoplasmic reticulum to cardiac calcium management was examined by measuring the ryanodine sensitivity of contraction, determining the number of cardiac ryanodine receptors and their calcium sensitivity, assessing the size of sarcoplasmic reticulum calcium stores in enzymatically isolated cardiac myocytes and qualitative electron microscopic observations of sarcoplasmic reticulum in cardiac muscle.
An extensive network of nonjunctional sarcoplasmic reticulum around myofibrils and numerous junctional sarcoplasmic reticulum couplings at the periphery of the myocytes was present in atrial and ventricular muscle of the lamprey heart. High numbers of ryanodine receptors were present in atrial (0.16 ± 0.04 nmol mg(-1) prot) and ventricular membranes (0.27 ± 0.03) (P < 0.01), and 10 μm ryanodine inhibited large part of contraction force in atrial (37.8 ± 5.7%) and ventricular (49.2 ± 6.5%) muscle.
Sarcoplasmic reticulum is well developed in the lamprey heart and plays a significant role in cardiac calcium management. This suggests that in the common ancestor of vertebrates, cardiac excitation-contraction coupling could have been fairly strongly dependent on sarcoplasmic reticulum calcium stores. Functionally, this trait might be associated with high cardiac output and active predatory lifestyle of the lamprey.
心肌收缩和舒张是通过肌原纤维周围快速变化的钙离子浓度来介导的。与恒温动物相比，变温动物的心脏对细胞外钙离子的收缩依赖性更强，这表明这种特性可能代表了原始脊椎动物心脏激活的模式。本研究检验了这样一个假设，即在基干脊椎动物七鳃鳗的心脏兴奋-收缩耦联中，细胞内钙库的作用较小。
通过测量收缩时肌浆网对ryanodine 的敏感性、确定心脏ryanodine 受体的数量及其钙离子敏感性、评估酶分离的心肌细胞中线粒体钙库的大小以及心肌中线粒体的定性电子显微镜观察，来研究肌浆网对心脏钙管理的贡献。
在七鳃鳗心脏的心房和心室肌中，肌原纤维周围存在广泛的非连接肌浆网网络和大量的心肌细胞边缘连接肌浆网。心房（0.16 ± 0.04 nmol mg(-1) prot）和心室（0.27 ± 0.03）膜中ryanodine 受体的数量较高（P < 0.01），10 μm ryanodine 抑制了心房（37.8 ± 5.7%）和心室（49.2 ± 6.5%）肌肉中大部分收缩力。
肌浆网在七鳃鳗心脏中发育良好，在心脏钙管理中起着重要作用。这表明，在脊椎动物的共同祖先中，心脏兴奋-收缩耦联可能相当依赖于肌浆网钙库。从功能上讲，这种特性可能与七鳃鳗高心输出量和积极的捕食性生活方式有关。
