Changes of isolated cardiac muscle function in response to extracellular sodium reduction.
The effects of low extracellular sodium concentration [( Na+]0, 76 mmol/l) on force of contraction, transmembrane action potentials and on calcium, sodium and potassium contents were studied in guinea-pig heart muscle using sucrose or lithium as substitutes for sodium. In papillary muscle, the positive inotropic response to low [Na+]0 was accompanied by shortening in action potential duration, in atrial muscle, prolongation at 90% of repolarization was observed. After 30 min. in low [Na+]0 solution (Ca2+ 1.8 mmol/l), the net uptake of calcium in left atria was larger in sucrose- than in lithium-substituted solution, i.e. 0.45 and 0.2 mmol Ca2+/kg wet weight, respectively. The net sodium content decreased monophasically; but the potassium content did not change consistently. The uptake of lithium by the atria was at least partially compensated for by a loss in potassium and by a transient decrease in sodium content. Increase in stimulation frequency from 0.1 to 1 Hz accelerated the time course of change by a factor of 2. In low [Na+]0, post-rest adaptation of twitch tension appeared abolished in atria because of high amplitude contractions. The recovery pattern of post-rest twitch amplitude was greatly accelerated in papillary muscle. In conclusion, low [Na+]0 elevates the cellular calcium content; this extra calcium is probably located in cellular stores that are involved in the regulation of twitch amplitude.
Lüllmann
H
H
Institute of Pharmacology, University Kiel, FRG.
Ravens
U
U
Stöckel
P
P
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000032:analysis / Q000378:metabolism; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D009200:Myocardial Contraction; D010210:Papillary Muscles / Q000502:physiology; D011188:Potassium / Q000032:analysis; D012964:Sodium / Q000502:physiology*
D012964:Sodium; D011188:Potassium; D002118:Calcium