Tetrodotoxin-sensitive inactivation-resistant sodium channels in pacemaker cells influence heart rate

Abstract

There is currently some uncertainty about whether cardiac pacemaker cells contain tetrodotoxin (TTX)-sensitive Na+ channels although TTX is known to slow heart rate. We have recorded transient and persistent single-channel currents activated by depolarization in myocytes isolated from the toad sinus venosus. The myocytes were identified as pacemaker cells by their characteristic morphology, spontaneous action potentials that were blocked by cobalt but not by TTX, and lack of an inwardly rectifying K+ current. The voltage dependence of the single-channel currents, their presence in solutions containing no K+ or Ca2+, or in solutions to which Cs+ and Co2+ had been added, their dependence on [Na+] and their sensitivity to TTX indicated that they were Na+ channel currents. The persistent Na+ channel currents were resistant to inactivation and were activated over the range of potentials that occur during diastole in pacemaker cells: they would therefore contribute to the pacemaker current that sets heart rate. It was concluded that TTX slows heart rate by blocking these channels in pacemaker cells.

A slowly inactivating sodium current contributes to spontaneous diastolic depolarization of atrial myocytes

“…The late I Na of atrial myocytes reported here is similar to a slowly inactivating, TTX-sensitive Na ϩ current found in rabbit (2) and toad (17) pacemaker cells and in canine Purkinje cells (30). The Na ϩ current in those cells (2,17,30) was also activated at negative membrane potentials and appeared to play an important role in generating DD and spontaneous AP firing.…”

Intracellular calcium and Na+‐Ca2+ exchange current in isolated toad pacemaker cells

“…We found that the inward current preceded the [Ca¥]é changes by about 100 ms; for this reason the plot of INa,Ca against [Ca¥]é is not a simple almost linear relation as predicted by models of INa,Ca (DiFrancesco & Noble, 1985). Trafford et al (1995) (Ju et al 1996) so that our peak INa,Ca of 154 pA becomes a normalized INa,Ca in the range 3·9-5·1 pA pF¢. (ii) A method to compare the functional importance of the SR against that of the INa,Ca in Ca¥ extrusion is to compare the time course of decline of the voltage-generated Ca¥ transient with that produced by caffeine application.…”

“…The heart was rapidly removed and the sinus venosus was carefully dissected free under a dissection microscope. Single pacemaker cells were isolated using collagenase and elastase as previously described (Ju, Saint, Hirst & Gage, 1995;Ju et al 1996). The cells were routinely superfused with the following solution (mÒ): 110 NaCl, 2·5 KCl, 0·5 MgSOÚ, 2 CaClµ, 10 NaHepes and 10 glucose; pH 7·3; equilibrated with air.…”

“…This slow depolarization requires the presence of net inward current during the pacemaker potential. Several inward currents have been proposed to contribute to the pacemaker current; these include the hyperpolarization-activated cation current (If) (for review see DiFrancesco, 1993), background Na¤ current (Hagiwara, Irisawa, Kasanuki & Hosoda, 1992), persistent Na¤ current (Ju, Saint & Gage, 1996), L-and T-type Ca¥ currents (ICa(L), ICa(T), Hagiwara, Irisawa & Kameyama, 1988), and the sustained inward current (Guo, Ono & Noma, 1995). In addition the delayed rectifier current (IK, an outward potassium current) will decay during diastole and, in combination with a constant or background inward current, could contribute to pacemaker currents.…”

Pharmacological characterization of ionic currents that regulate the pacemaker rhythm in a weakly electric fish

“…In several rhythmically firing neurons and neural circuits, transient sodium currents generate action potentials, while persistent sodium currents provide tonic depolarization that influences the timing between spikes (Opdyke and Calabrese, 1994;Ju et al, 1996;Oka, 1996;Elson and Selverston, 1997;Onimaru et al, 1997;Takakusaki and Kitai, 1997). Although some persistent sodium currents are resistant to TTX (Oka, 1996), many others are TTX-sensitive (Crill, 1996;Ju et al, 1996;Baker and Bostock, 1997;Elson and Selverston, 1997). Indeed, in some neurons, persistent sodium currents are more sensitive to TTX than transient sodium currents (Hammarstrom and Gage, 1998), which is consistent with the fact that TTX hyperpolarized the Pn neurons and slowed their firing rates before reducing spike amplitude.…”