Proceedings of The Physiological Society
University of Manchester (2007) Proc Physiol Soc 8, PC16
Comparative physiology of the sinoatrial pacemaker of cold-blooded vertebrates
V. A. Golovko1
1. Institute of Physiology, Komi Science Centre, Urals Branch, Russian Academy of Sciences, Syktyvkar, Russian Federation.
It is known that the proximal and distal ends of the tubular heart of tunicates have two centres of automatism that work alternately. As a result, the blood advances by peristaltic contracting waves. Active animal living has led to the appearance of a bent tube with various chambers and valves between them. The pacemaker’s cytoarchitecture also has changed. The aim of our study was to analyse the main parameters of the action potential (AP) in true pacemaker cells in the hearts of different poikilotherm species, including inhabitants of water (ammocoete, Lampetra fluviatilis, dace, Leuciscus rutilus) and land (frog, Rana temporaria, tortoise, Testudo horsfieldi). Experiments were carried out on spontaneously beating strips of sinoatrial (SA) tissue (control conditions: 20°C, 0.9 mM Ca2+), using the standard microelectrode technique. In cold-blooded animals, in the evolutionary scale from Cyclostomata to Reptiles, true pacemaker cells are located along the full border between the sinus venosus and atrium. Two SA valves originated at this site and formed a roller called 'the sinoatrial ring'. These modifications changed the working regime of the vertebrate’s electromechanical pump from a peristaltic one to an impulse one. When the isolated SA ring of the dace’s heart was divided into two, the frequency of AP generation in the left and right segments was 69±11 min-1, (n=11 strips) and 72±9 min-1 (n=11, p>0.05), respectively. However, the frequency of AP generation was higher (91±12 min-1, p<0.05) in the non-divided isolated dace’s SA ring. The action potential duration at 90% repolarization (APD90) increased from the ammocoete (0.11±0.02 s, n=39 cells) to the tortoise (1.1±0.15 s, n=72). The rate of change of membrane potential (dV/dt) during phase 3 was highest in the ammocoete (1.2 V s-1) and lowest in the tortoise (0.2 V s-1). Interestingly, the SA pacemaker of different animal species appeared to be heterogeneous in its resistance to Ca2+-free solution. In particular, the ammocoete’s heart continued generating APs for longer than 10 hours in Ca2+-free solution, while complete blockade of AP generation was observed in strips of the tortoise heart after 5 min of perfusion with 0.45 mM Ca2+ solution. We propose that many of the Ca2+-dependent mechanisms of pacemaker function can be found even in more genetically primitive creatures such as Metazoa.
Where applicable, experiments conform with Society ethical requirements