Studies on the developmental and species difference of the cardiac function: return to chick embryonic myocardium and the integration of the study

1. (1)实验结果表明，Na+、Ca2+交换器在平衡电位的基础上，在动作电位后期产生Ca2+的外排和Na+的内流，进而引出净内向电流，从而促成了后期平台的形成。(2)小鼠心肌对α -肾上腺素能刺激的负性肌力反应主要是由于Na+、Ca2+交换的增加。收缩蛋白对Ca2+敏感性的增加是由细胞内碱化引起的。特别设计的挛缩实验表明，α刺激不会改变SR对Ca2+的处理。(3)对内皮素的正性和负性肌力反应分别是由于细胞内pH升高导致收缩蛋白对Ca2+的敏感性增加，以及通过Na+、Ca2+交换器对Ca2+的挤压增加所致。(1)选择性t型Ca2+通道抑制剂R(-)-efonidipine对鸡胚胎心室有负性肌力作用，提示Ca2+通过t型Ca2+通道内流促进了发育早期心肌的收缩。(2)鱼毒对幼鼠胚胎心脏有负性肌力作用，而iberiotoxin对幼鼠胚胎心脏无负性肌力作用。因此，在胚胎早期心肌中存在中间传导的K+通道，这是血管平滑肌所特有的。

1. Mouse myocardium(1) From the experimental results, it was concluded that Na+, Ca2+-exchanger produces the efflux of Ca2+ and influx of Na+ at the later phase of action potential on the basis of its equilibrium potential, which in turn elicits net inward current, thus contributing the formation of late plateau.(2) Negative inotropic response of mouse myocardium to the alpha-adrenergic stimulation was mainly due to the augmentation of Na+, Ca2+-exchange. The increase in the sensitivity of contractile protein to Ca2+ is caused by the intracellular alkalization. Specially designed contracture experiments suggested that Ca2+ handling by SR was not modified by alpha-stimulation.(3) Positive and negative inotropic response to endothelin is caused by the increase in Ca2+ sensitivity of the contractile protein due to the increase in intracellular pH, and increase in the Ca2+ extrusion through the Na+, Ca2+-exchanger, respectively.2. Chick embryonic myocardium(1) A selective inhibitor of T-type Ca2+ channel, R(-)-efonidipine showed the negative inotropic effects on the chick embryonic ventricle, suggesting that Ca2+ influx through the T-type Ca2+ channels contributes the contraction of the myocardium at early stages of development.(2) Calibdotoxin produced the negative inotropic effects on the young embryonic hearts but iberiotoxin did not. Thus, the intermediate conductance K+ channels are present in the early embryonic myocardium, which is characteristic to vascular smooth muscle.