[Comments on the kinetics and extracellular effects of potassium and magnesium]

Potassium and magnesium are important electrolytes which have to be ingested in sufficient amounts. They differ in the necessary daily intake (about 100 mmol potassium, about 12 mmol magnesium), the degree of intestinal absorption (potassium almost 100%, magnesium about 30%) and the distribution between the extracellular and intracellular space. If there is a deficiency in potassium or magnesium, it is necessary to substitute these materials. Deficiency of potassium is not rarely combined with deficiency of magnesium. The concentration gradient between intra- and extracellular potassium mainly determines the resting membrane potential of the cell. Lower extracellular potassium may lead to an instable membrane potential because of a decrease in potassium conductance. An increase in extracellular potassium concentration leads to the depolarization of the cell. Extracellular potassium activates the sodium potassium pump and thereby prevents an increased intracellular accumulation of sodium and calcium. Important effects of extracellular magnesium are: calcium antagonism, increase of excitation threshold and inhibition of transmitter release. By increasing the plasma concentration of magnesium, it is possible to exert pharmacodynamic effects. An increase above the normal range usually is only possible by parenteral application. However, a slight elevation can already be achieved by oral application. This increase may lead to limited pharmacodynamic effects. By elevation of the extracellular magnesium concentration, adverse, depolarisation-dependent effects of an increase in extracellular potassium concentration (for example slowing of conduction of excitation) can be compensated. This effect can be explained by a magnesium-dependent decrease of the membrane surface potential.