[Effect of changed extracellular K(+) and Mg(2+)-concentration on intracellular Ca(2+) homeostasis, contraction coupling and force-frequency relations in the human myocardium].

The intracellular Ca(2+)-homeostasis may be affected by changes of the extracellular K(+)- and/or Mg(2+)-concentrations. Mg2+ reduces the Ca(2+)-influx via L-type Ca(2+)-channels, facilitates Ca(2+)-uptake into the sarcoplasmic reticulum, modulates the Ca(2+)-induced Ca(2+)-release and the Ca(2+)-binding to troponin C. The extracellular K+ activates the Na+/K(+)-ATPase and changes the membrane potential thereby affecting the mode of action of the Na+/Ca(2+)-exchanger. Especially when intracellular Ca2+ regulation is altered, for example in heart failure, Mg2+ and K+ exert beneficial effects on the frequency-dependent force-generation in human myocardium. Thus, extracellular Mg2+ and K+ influence contraction coupling in the human myocardium.