The mutual actions of mag~aesium, calcium and sodium ions on spontaneous impuls generation of isolated guhaea-pig atria were investigated. Atria suspended in magnesinmfree bicarbonate buffered solution containing only 0.08 mM calcium exhibit a high, sometimes irregular spontaneous activity. A stepwise addition of calciumchloride leads to 3 characteristic changes in steady state beat interval (s.s.b.i.) : increase in s.s.b.i., maximum at 0.81--0.62 mlVi calcium, then a decrease in s.s.b.i., minimum at 5--10 ml~ calcium, and then again an increase in s.s.b.i, up to a concentration of 15 mM calcium. The possible role of neurotransmirrors in mediating this chronotropic effects was excluded by use of specific antagonists, atropin (1 rag/l) and pronethalol (1 and 5 rag/l), magnesium exerts a pure negative chronotropie action which is not influenced by atropin. The increases in s.s.b.i, caused by stepwise addition of magnesiumsulfate to the bathing fluid are linearcly proportional to the magnesium concentration. The magnitude of this negative chronotropic action depends largely on the concentration of both, sodium and calcium ions present in the bathing fluid. Either reduction of the sodium concentration to 50% or reduction of the calcium concentration to 25% each approximately double the effects of magnesium on s.s.b.i. Equieffective magnesium concentrations with respect to its chronotropie action in bath fluids of various sodium-and calcium concentrations could be calculated. The possibility of a competition of sodium, calcium and magnesium ions for common anionic sites is discussed.
Die Abh~ngigkeit der Calciumwirkung auf die Kontraktionskraft des Herzens yon der Natriumkonzentration wurcIe erstmals yon WrLB~DT U. KOLLEI¢ (1948) am isoliertea Froschherzen genauer untersucht, und auf der theoretischen Basis eines Ionengleichgewichtes zwischen Zellmembran und AuBenl6sung erkl/~rt. Lf)TTGAU U. NIEDEI~GEt~KE (1958) sind zu einer sehr/ihnlichen Deuttmg dieses Natrium-Calcium Antagonismus gelangt. Sie nehmen eine Kompetition yon Natrium-und yon Calciumionen entsprechend dem ~¢[assenwirkungsgesetz um anionische