The persistent sodium current could be a pacemaker current In addition to a transient TTX-sensitive sodium current, about half of

the sinus venosus cells examined displayed a clear TTX-sensitive persistent sodium current which has not been reported in pacemaker cells previously. It was mentioned above that the integrity of transient sodium currents was sometimes impaired by the cell isolation procedure. The absence of a persistent sodium current in some preparations may indicate that this current is more labile than the transient sodium current. This may be due to the small amplitude of the persistent current which becomes obvious only when the "difference" current is revealed by subtraction of currents recorded with and without TTX. This current is active in the potential range more negative than the threshold for action potentials and would be expected to contribute to the pacemaker current. It is hardly surprising, therefore, that TTX slows the frequency of action potentials.

The slowing of heart rate caused by TTX can be explained by removal of the persistent current and consequent slowing of the rate of diastolic depolarization. Furthermore, since TTX is as effective as caesium in slowing pacemaking activity in these preparations (Edwards, Hirst & Bramich, 1993), the persistent sodium current must make as significant a contribution to the pacemaking current as the cation channels activated by

hyperpolarization (DiFrancesco, 1985). These experiments give further support to the idea that pacemaking activity results from the coordinated activity of a variety of ion channels (Brown et al. 1984b).

The current underlying pacemaker activity has been considered as a very complex current system. The evidence so far leads to a general conclusion that no one ionic current system is alone responsible for pacemaking (Irisawa, Brown & Giles, 1993). In addition to large voltage- dependent Ca+ and K+ currents and transporter-mediated sarcolemmal currents (INa/K and INa/Ca), hyperpolarization-activated cation current (L) and inward background current (IB) have been considered play an important role in the generation of the spontaneous diastolic depolarization.

The cells of the pacemaking region of the heart have a relative positively resting membrane potential. The quiescent sino-atrial (SA) node cell, for example, has a resting potential near -40 mV and the maximum diastolic potential is approximately -50 to -60 mV. In contrast, resting membrane potentials of ventricular cells are around -70 mV to -80 mV (Irisawa, 1989). The low resting potential in pacemaker cells has been attributed to a high ratio of Na permeability to K permeability (PNa/PK). The absence of inwardly rectifying K channels provides a low PK (Noma et al. 1984);(also see Chapter 6 Fig. 6-2, Fig. 6-3) favouring the high PNa^K ratio; however, a high value of PNa is still suggested in the pacemaker cells. The theoretical necessity of a background Na (IB) is acknowledged in nearly all mathematical models of primary pacemaking activity (Bristow & Clark, 1983; Brown et al. 1984a; Rasmusson et al. 1990). In these models, a large background leak conductance has usually been assumed (Noble, DiFrancesco & Denyer, 1989) in order to simulate the normal pattern of action potentials. A direct experimental observation of IB currents has been reported recently from sino-atrial node cells of rabbit heart (Hagiwara et al. 1992) and from ventricular and atrial cells of the guinea-pig (Kiyosue et al. 1993)

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The persistent sodium current recorded from sinus venosus cells is different from IB. Firstly, IB is insensitive to TTX (Hagiwara et al. 1992), whereas the persistent sodium current is TTX-sensitive (Fig. 3-1 la). Secondly, IB is voltage-independent, its current-voltage relationship is almost linear, whereas the persistent current is activated at depolarizing membrane potentials (Fig3-3; Fig. 3-1 lc). Thirdly, IB has poor cation selectivity, The reversal potential of IB was around -21 mV at physiological concentrations of intracellular K+ and extracellular Na+ , whereas the reversal potential for persistent current is close to the sodium equilibrium potential. Therefore Na+ conductance increases in the period of diastolic depolarization could be due to persistent sodium channels in pacemaker cells, especially at more positive potentials close to the IB current reversal potential.

^currents are activitied by hyperpolarization, and have slow kinetics. Furthermore, ^currents are modulated in opposite ways by B-adrenergic and cholonergic stimuli that allows a fine rhythm control by the autonomic nervous system. Because of these specific properties, I^has been considered as a current designed to initiate a slow diastolic depolarization and to control its rate of rise in response to neurotransmitter-induced modulation (DiFrancesco, 1981a; 1981b; 1993). However, on the basis of its negative threshold potential and its slow time course of activation, Yanagihara and Irisawa concluded that If plays little role in normal spontaneous pacemaker activity. Nathan (1987) has reported that in cultured SA node cells, the threshold for activation of If is approximately -65 mV but that because of its slow time course of activation, it is not measurable during the first 300 ms of a voltage-clamp pulse. At low doses, Cs+ is a selective blocker of If, it causes a substantial slowing of beating rate, indicating a contribution of If to the pacing of these cell. However, several independent laboratories have demonstrated that when Cs+ is applied to SA node cell in concentrations sufficient to completely and selectively block If, the rate of spontaneous

pacemaking is slowed (by approximately 10-30%), but pacemaking is not arrested (Denyer & Brown, 1990; Campbell, Rasmusson & Strauss, 1992; Giles, van Ginneken & Shibata, 1986; Noble, 1984). In addition, in frog sinus venosus, Gile and Shibata (1985) failed to record If. Nonetheless, sinus venosus cells generate normal spontaneous pacemaker activity under control recording conditions. These observations indicate that 1^ is not essential for pacemaking.

As a candidate for pacemaker currents, the persistent sodium current appears superior to If in some respects. The persistent current activates more quickly than If and it inactivates very slowly, which allows a constant current flow during a depolarization. The amplitude of the persistent current recorded from sinus venosus cells is proportionally bigger than that recorded from ventricular cells. Considering that the input resistance in pacemaker cells is very high (Irisawa, Brown & Giles, 1993), a very small fluctuation in transmembrane current can easily elicit a large deflection of the potential in the pacemaker cells.

Hence it is reasonable to assume that persistent sodium current may play a more important role in diastolic depolarization than both IB and If.