The matter of whether or not similar molecular and cellular mechanisms underlie related physiological functions in various mammalian species is of direct scientific and general interest. Issues range from the usefulness of experimental research as such, and the appropriateness of inter-species extrapolation in fundamental and applied research, to the utility of the Genome information, and the legitimacy of the Physiome project.

One of the specific questions in which our laboratory is interested, is whether or not sino-atrial node (SAN) pacemaking in species as different as human (resting heart rate 60-90 beats min^-1) and mouse (350-600 beats min^-1) could be based on the same set of underlying mechanisms. To address this, we have recently developed a novel method for isolating spindle-shaped, calcium-tolerant and spontaneously beating murine SAN cells (Lei et al. 2001). (The mice were killed humanely by cervical dislocation.) Compared with rabbit or guinea-pig SAN cells (the current ‘standard’ preparations), pacemaker activity of isolated murine SAN cells is characterized by a faster firing rate (350-400 beats min^-1), higher upstroke velocity (¦50 V s^-1), and a pronounced sensitivity to TTX (30 µM TTX reduces beating rate by 18% Lei et al. 2001).

Based on this preliminary experimental information, we investigated theoretically whether the faster murine SAN pacemaking could be a consequence of differences in current density and/or channel gating properties, or whether it would be necessary to postulate novel mechanisms, which have not been previously identified in other species. We used quantitative mathematical models that were originally developed for rabbit SAN (Oxford rabbit SAN model, Garny et al. 2001). Simulations were run using Oxsoft HEART 4.X software (Noble, 1999), and the starting point for this investigation was to vary the parameter g _Na (conductance of the fast sodium current, i _Na) in the equation i _Na = m³ h g _Na (E – E_mh), where m is the voltage-dependent activation gate, h is the voltage-dependent inactivation gate, and (E – E_mh) is the driving force for movement of sodium ions. The relationship between g _Na (values tested ranged from 0.00625 to 0.625 µS) and SAN action potential upstroke velocity is almost linear (ranging from 4.4 to 5.6 V s^-1), whilst that between g _Na and spontaneous frequency can be described as sigmoidal (ranging from 170 to 320 beats min^-1).

In particular, increasing g _Na (from 0.00625 to 0.35 µS) raises beating frequency to 363 beats min^-1 and upstroke velocity to ~52 V s^-1 in the SAN cell model, which mimics experimental observations obtained from isolated murine SAN cells (Fig. 1A). Increasing furthermore the conductances for both fast and slowly activating delayed rectifier potassium currents, i_Kr and i_Ks (respectively), allowed us to approximate maximum diastolic potentials and general appearance of experimentally recorded spontaneous SAN cell activity (cf. Fig. 1A and B).

Interestingly, varying i_Ks in the model had no significant effect on beating rate, which is in keeping with the observations that minK-deficient mice (replacement of minK by LacZ gene, causing lack of i_Ks) show no significantly different heart rates than control mice (Kupferschmidt et al. 1999).

The results of this theoretical study are in keeping with the concept of a common set of mechanisms that give rise to SAN pacemaker activity in different species, where distinct SAN action potential characteristics are brought about by differences in the relative contribution of pacemaking mechanisms, rather than completely uncommon mechanisms. The present results, of course, neither prove the concept, nor do they yet constitute a bona fide ‘murine’ SAN cell model (important parameters like intracellular ion concentration have not even been touched upon). What is clear, though, is that it is feasible to mimic ‘mouse-like’ and ‘rabbit-like’ SAN activity based on a common set of underlying electrophysiological mechanisms. This emphasises the need for further research into murine SAN electrophysiology, which will allow us to open up the vast toolbox of genetically modified murine models for physiological research.

This was supported by the British Heart Foundation, Physiome Sciences Inc., the MRC and Royal Society.

Figure 1. Murine SAN cell pacemaker activity. Top: recording of spontaneous pacemaker activity in a murine SAN cell. Bottom: results of modifying a rabbit SAN cell model to mimic murine SAN potential characteristics, by increasing iNa and altering iKr and iKs (see text for details).

Garny, A., Noble, P.J., Kohl, P. & Noble, D. (2001). Chaos (in the Press).

Kupferschmidt, S., Yang, T., Anderson, M.E., Wessels, A., Niswender, K.D., Magnuson, M.A. & Roden, D.M. (1999). Circ. Res. 84, 146-152.

Lei, M., Cooper, P., Kohl, P. & Noble, D. (2001). Proceedings of the XXXIV World Congress of the IUPS. Christchurch, New Zealand.

Noble, D. (1999). Oxsoft HEART version 4.X manual. Oxsoft, Oxford.