Summary
The actions and interactions of sodium and calcium on S-A nodal activity were investigated using the isolated spontaneously beating guinea-pig atrium suspended in a Mg-free, bicarbonate-buffered physiological solution. Osmolarity was maintained by addition of sucrose to solutions of reduced sodium concentration.
Increasing the calcium concentration at a fixed sodium concentration caused a rate increase (ascending limb of the calcium-concentration-response curve) in small concentrations and again a rate decrease with still higher calcium concentrations (descending limb of the calcium-concentration-response curve). Sodium depletion produced a parallel shift of the concentration-response curve of calcium on rate to smaller calcium concentrations, and a depression of the maximal rate of atrial beat. Rates of beat obtained at the same calcium concentration but at different sodium concentrations do not show a significant difference between each other (on ascending limb of the calcium-concentration-response curve only). Identical points on calcium-concentration-response curves obtained in different sodium concentrations are caracterized by a constant quotient [Ca++]/[Na+]2 of these ions in the bath solution. The rate of atrial beat obtained in solution with constant [Ca++]/[Na+]2 depends on the logarithm of the sodium concentration by linear regression, as long as rates on the ascending limb of the calcium response curve are considered.
A competition of sodium and calcium for an anionic site is suggested. Calcium might exert its positive chronotropic effect by carrying a greater fraction of the depolarizing current during slow diastolic depolarization. S-A nodal rate is under these conditions mainly determined by two factors, 1. the electrochemical sodium potential, 2. the relative calcium concentration given by the quotient [Ca++]/[Na+]2.
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Schaer, H., Waser, P.G. Über die Wirkungen von Natrium und Calcium auf die Impulsbildung im Sinusknoten des Meerschweinchen-Herzens. Pflügers Archiv 290, 18–27 (1966). https://doi.org/10.1007/BF00362616
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DOI: https://doi.org/10.1007/BF00362616