Absence of calcium paradox in the cardiac ventricle of the Atlantic hagfish (Myxine glutinosa).

Spontaneously beating strips of the cardiac ventricle of the cyclostome Myxine glutinosa were characterized with respect to dependence on extracellular calcium for the cardiac contractility at 8 degrees C. The force developing (Tmax) was reduced by 20-50% when exposed to calcium-free medium (+/- 10(-3)M EDTA) for 15-60 min. Return to normal Myxine Ringer (4.5 mM Ca2+) resulted in 120-130% recovery to Tmax without change in resting tension. These experiments show that the heart of this primitive vertebrate, analogous to that of other poikilotherms, does not display the "Ca-paradox" phenomenon. Being relatively insensitive to changes in extracellular Ca2+ the Myxine myocardium differs from that in most poikilotherms in other respects, e.g. in the mechanisms regulating beat to beat changes in intracellular Ca2+.