Influence of calcium and magnesium ions on the sino-atrial node pacemaker activity of the canine heart.

HASHIMOTO , K., SUZUKI, Y. and CHIBA, S. Influence of Calcium and Magne sium Ions on the Sino-atrial Node Pacemaker Activity of the Canine Heart. Tohoku J. exp. Med., 1974, 113 (2), 187-196 •\ Using constant pressure perfusion technique of the canine sinus node artery, effects of changing concentrations of calcium and magnesium ions were investigated on the sino-atrial (SA) pacemaker activity. An increase in calcium concentration produced a diphasic chronotropie action; positive at 5-25 mEq/liter but negative above 30 mEq/liter. The positive chronotropic action of calcium was not prevented by pretreatment with reserpine, but partially depressed by beta-adrenergic blocking agents. On the contrary, an infusion of GEDTA, more specific Ca++ chelating agent, slowed the sinus rate and further produced sinus arrhythmia at a higher infusion rate, which was restored to regular sinus rhythm by norepinephrine. Changes in magnesium concentration