Influence of calcium and magnesium ions on myocardial surface and intracellular electrogram and contractile force of a dog

Abstract

Changes in the intracellular potential has been explained by the fluxes of the cations, especially sodium and potassium ions, through the cell membrane. Calcium and magnesium ions also affect the intracellular potential. There have remained still many problems about effects of extracellular calcium and magnesium ions on the intracellular electrogram (ICEG). On the other hand, it is well-known that calcium ion has markedly inotropic action and plays an important role in excitation-contraction coupling. On the contrary, effect of magnesium ion on myocardial contractility seems to be a little. In the present experiments, the author investigated changes in ICEG, surface electrogram (SEG), and contractility of dog's heart muscle in situ subjected to the coronary injections of calcium and magnesium ions of the various concentrations. Methods Sixteen mongrel dogs weighing 8 to 15 kg were used. After anesthetized, the chest was opened, followed by venous injection of 5000 units of heparin. Then a major branch of the anterior descending coronary artery was exposed. In eight dogs, a carotid artery was connected to the major branch of the coronary artery through a polyethylene catheter with a three-way stopcock, and ICEG was taken simultaneously with unipolar SEG from a point on the epicardium supplied by this coronary artery. These methods were similar to those reported by PRINZMETAL, TOYOSHIMA and their colleagues. In the other dogs, a major branch of the anterior descending coronary artery was perfused by arterial blood from a femoral artery with a catheter. Thereafter, 'myocardial tension tracing (MTT), SEG, and left ventricular pressure curve (LVP) were recorded. The load transducer divised by HISADA et al. was used for recording MTT. LVP was obtained by a cardiac catheter inserted through a carotid artery into the left ventricle. The exposed epicardium was irrigated with Tyrode's solution through a polyethylene tube. The temperature of the solution was 35 to 37°C but remained constant during any experiment for the purpose of keeping a constant temperature on the cardiac surface.