Hypoxia increases persistent sodium current in rat ventricular myocytes.

Abstract

1. A persistent inward current activated by depolarization was recorded using the whole‐cell, tight seal technique in rat isolated cardiac myocytes. The amplitude of the inward current increased when cells were exposed to a solution with low oxygen tension. 2. The persistent inward current had the characteristics of the persistent Na+ current described previously in rat ventricular myocytes: it was activated at negative potentials (‐70 mV), reversed close to the equilibrium potential for Na+ (ENa), was blocked by TTX and was resistant to inactivation. 3. Persistent single Na+ channel currents activated by long (200‐400 ms) depolarizations were recorded in cell‐attached patches on isolated ventricular myocytes. Hypoxia increased the frequency of opening of the persistent Na+ channels. 4. Persistent Na+ channels recorded during hypoxia had characteristics similar to those of persistent Na+ channels recorded at normal oxygen tensions. They had a null potential at ENa, their amplitude varied with [Na+], they were resistant to inactivation and their mean open time increased with increasing depolarization. 5. The persistent Na+ channels in cell‐attached patches were blocked by TTX (50 microM) in the patch pipette and by lidocaine (100 microM). 6. It was concluded that hypoxia increases the open probability of TTX‐sensitive, inactivation‐resistant Na+ channels. The voltage dependence of these channels, and their greatly increased activity during hypoxia, suggest that they may play an important role in the generation of arrhythmias during hypoxia.

Inhibition of oxidative metabolism increases persistent sodium current in rat CA1 hippocampal neurons

“…The situation in cells in vivo would be different in that [Na¤]é may change as extrusion mechanisms fail to cope with Na¤ influx. Cyanide and hypoxia had similar effects, as has been described previously (Bickler & Hansen, 1994;Ju et al 1996), indicating that they had a common mechanism of action.…”

“…In order to block oxidative metabolism, neurons were exposed to 5 mÒ sodium cyanide through the pipette solution (Elliott, Smith & Allen, 1989;Ju et al 1996). Solutions contained 10 mÒ glucose so that glycolysis was not impaired.…”

“…These techniques have been described in more detail elsewhere (Ju et al 1992(Ju et al , 1996. All values are expressed as means ± 1 s.e.m.…”

“…Hypoxia was achieved by perfusing the patched cell through a fine stainless steel tube with a solution vigorously bubbled with 100% Nµ (BOC gases). This technique has already been used in our laboratory and produces a rapid decrease in oxygen tension around the cell (Ju et al 1996). The bath solution contained (mÒ): 135 NaCl, 5 KCl, 1 MgClµ, 1 CaClµ, 5 CoClµ, 5 CsCl and 10 Tes adjusted to a pH of 7·4 with NaOH.…”

“…Whole-cell currents were recorded with an Axopatch-1D amplifier (Axon Instruments) in response to voltage protocols imposed using an IBM-compatible PC and digital-to-analog interface. Currents were analysed using computer techniques and 'in-house' software (Ju, Saint & Gage, 1992;Ju et al 1996). Subtraction of traces recorded before and after exposure to a high concentration of TTX (0·5 ìÒ) was used throughout to isolate TTX-sensitive Na¤ currents (Fig.…”