[The sinus node itself also plays a role in heart rate slowing down during postnatal development].
The slowing down mechanism of heart rate during growth of the body after birth was studied in isolated rabbit heart and sinus node (SN) preparation with Langendorff perfusion method, conventional microelectrode recording and perforated patch for recording pacemaker current I(f). The radioimmunoassay was also used to measure the concentration of cAMP within SN cells. The results indicate that without the influence of nervous and humoral factors, the spontaneous heart rate would also become slower as the rabbit grew older, which is due to the decrease of spontaneous depolarized rate of phase 4 in SN cells. The negative  directed shift of the threshold potential of I(f) and the decrease in cAMP concentration within SN cells may be responsible for the phenomenon. The results obtained suggest that besides the nervous and humoral factors which influence the heart rate, the changes in automaticity of SN cell itself may take part in the slowing down process as the body grows up.
Yang
Zhi-Fang
ZF
Department of Physiology, Shanghai Second Medical University, Shanghai 200025. liuyuanm@public6.sta.net.cn
Li
Ci-Zhen
CZ
Li
Qian
Q
Wang
Xi-Jin
XJ
Liu
Yuan-Mou
YM
D000818:Animals; D000242:Cyclic AMP / Q000502:physiology; D006339:Heart Rate; D066298:In Vitro Techniques; D008839:Microelectrodes; D010477:Perfusion; D011817:Rabbits; D012849:Sinoatrial Node / Q000502:physiology*
D000242:Cyclic AMP