Negative cardiac inotropic effects of calcium ion and of N 2 hydroxyethylpiperazine N' 2 ethanesulfonic acid (HEPES) buffer at normal and low sodium concentration

Isolated electrically driven left atria from rabbits were suspended in N 2 hydroxyethyl piperazine N' 2 ethanesulfonic acid (HEPES) buffer and exposed to increasing concentration of calcium ions in the presence of different concentrations of sodium. Maximal contractile force occurred at 10 mM Ca and 145 mM Na. Higher concentrations of Ca had a negative inotropic effect. Lowering the Na concentration decreases the concentration of Ca required to observe the negative inotropy, until at 60 to 70 mM Na, it occurred at 5 mM Ca. This last result was confirmed in Krebs Henseleit solution. Contractility at low Na and/or Ca media was less in HEPES than in Krebs buffer.