Increase of myocardial diastolic tonus at low temperatures: a Ca-phenomenon?

Myocardial diastolic tonus increases at low temperature. The chief aim of the present series of experiments was to decide whether this phenomenon should be interpreted as being caused by thermoelastic alteration or by an impairment of the Ca-pump. All experiments were performed on isolated right ventricular rabbit papillary muscles. In order to impose ramp-like and sinusoidally modulated changes of the temperature on the perfusate, a Peltier-battery was used. In Krebs-Henseleit solution with normal Ca-content (2.52 mmoles/l) changes of diastolic tonus of the driven muscle within a narrow range at low temperature amount approximately to isometric twitch tension (10(-2) up to 3.10(-2) N mm-2). Non-stimulated preparations show tension alterations of 2.10(-4) N mm-2 at best. With increasing stimulation interval the hysteresis-like loop representing changes of diastolic tonus shifts to lower temperatures. Under threefold Ca2+-concentration of the perfusate the diastolic tension varies over the total range of periodically modulated temperature in both stimulated and non-stimulated muscles. From these results it may be concluded that increasing myocardial diastolic tonus under low temperature is caused by an insufficiency of the Ca-sequestration system.