The Effect of High Sodium Concentration on the Action Potential of the Skate Heart

Abstract

It already has been well documented that the maximum rate of depolarization and amplitude of action potentials are directly dependent on [Na+]o in the vertebrate myocardium. Almost all studies have been carried out at low sodium concentration ranges by substituting NaCl for other substances. Action potentials should be demonstrable in higher sodium concentrations, but cells are inevitably damaged by osmotic changes. The blood of elasmobranchs is nearly isosmotic with sea water, but NaCIl accounts for 54.5 % of the osmotic pressure and 38.7 % of it is maintained by urea molecules. Utilizing this special situation in elasmobranchs, the effect of high sodium concentration was studied up to 170 % of normal sodium concentration, while still retaining isosmotic condition. The rate of depolarization, amplitude, and duration of the myocardial action potential all increased in direct proportion to [Na 4 ],, and no depressant effect on transmembrane action potentials was observed in solutions of high sodium concentration. With regard to depolarization rate, the regression curve fitted by the least squares method passed through zero within two standard errors. At high sodium levels, the overshoot changed as expected theoretically, but at lower ranges it deviated from the theoretical values.[Na+] and [K+]i in this tissue have been determined, and these data are explained on the basis of the Na theory.

Significance of Na+ current in the excitability of atrial and ventricular myocardium of the fish heart

“…I Na and the rate of AP upstroke I Na is the largest inward current in cardiac myocytes and therefore a prime determinant for the rate of AP upstroke and impulse propagation. V max in the trout heart (16-20·V·s -1 at 4°C) was similar to values previously measured in frog ventricular myocytes (26.4·V·s -1 at 15°C; Seyama and Yamaoka, 1988) and in skate (Dasuyatis akajei) heart (9.5·V·s -1 at 20°C; Seyama and Irisawa, 1967) at low temperatures but more than an order of magnitude smaller than in mammalian heart (270·V·s -1 ) at 35°C (Kiyosue et al, 1993). The large difference in V max between mammalian and ectothermic hearts is mostly explained by temperature differences.…”

Extracellular sodium modulates the excitability of cultured hippocampal pyramidal cells

“…The effects of [Na + ] o on AP peak amplitude are consistent with previous studies(Hodgkin and Katz, 1949; Huxley and Stampfli, 1951; Seyama and Irisawa, 1967). The effects of [Na + ] o on AP width have not been consistent in previous studies.…”

“…This relationship predicts that altered extracellular sodium concentration ([Na + ] o ) will change the AP amplitude, a result confirmed experimentally in squid axon(Hodgkin and Katz, 1949; Hodgkin, 1964). Similar results were obtained in myelinated nerve(Huxley and Stampfli, 1951) and skate heart(Seyama and Irisawa, 1967). Furthermore, lower [Na + ] o decreased the height and rising rate of APs in the smooth muscles of the cat ureter(Kobayashi and Irisawa, 1964), and lowered amplitude and increased the width of APs in rat dorsal root ganglion neurons(Amir et al, 1999).…”

“…Lower [Na + ] o increases the width of APs in rat dorsal root ganglion neurons(Amir et al, 1999). Higher [Na + ] o increased the AP width in skate heart(Seyama and Irisawa, 1967), probably because heart cells have larger calcium conductance. AP width is also affected by other factors: Older rats had lower potassium current (including delayed rectifier and A-type currents) than young rats, which might contribute to a decreased AP width(Alshuaib et al, 2001).…”

Effect of external stimulus and ionic concentrations on Hodgkin-Huxley neural model

“…[Na] o 'in artışı ile deneysel bulgulara paralel olarak AP'nin tepe değerlerinde artış olmakta, ancak AP'nin diğer parametrelerinde ciddi bir değişim gözlenmemektedir. Tepe değerindeki bu artış doğal olarak eşik gerilimini ve depolarizasyon ve repolarizasyon sürelerini az da olsa etkilemiştir ki bunun da deneysel sonuçlara uyum sağladığı tespit edilmiştir [2,3,17,[22][23][24][25][26][27][28].…”