Effects of moderate and deep hypothermia on Ca2+ signaling in rat ventricular myocytes

We investigated whether the degree of hypothermia determines the impairment in cardiac muscle function upon rewarming and whether the sarcoplasmic reticulum Ca2+ release channel, RyR(2), contributes to hypothermia-induced changes in myoplasmic [Ca2+].

Tension measurements using rat papillary muscle and calcium transients (Fluorescent Ca2+ indicator Fura 2-AM) in rat ventricular myocytes were compared during deep (10 degrees C-16 degrees C) and moderate hypothermic (28 degrees C) myocardial temperatures. In a second experiment, myocytes were pretreated with dantrolene, an RyR(2) antagonist; calcium transients were determined at control temperatures (32 degrees C), 16 degrees C, and upon rewarming (32 degrees C).

Papillary muscle contractility and myocyte calcium transients were significantly reduced during and after rewarming from 16 degrees C. At 28 degrees C, papillary muscle isometric tension was potentiated and calcium transients were unaffected. After rewarming from 28 degrees C, excitation-contraction coupling was maintained as isometric tension returned to 90% of control values. After rewarming from 16 degrees C, myocytes pretreated with dantrolene had return of calcium transients to 89% of control values while myocytes not treated with dantrolene recovered to only 50% of their control values.

We conclude that deep hypothermia, as opposed to moderate hypothermia of the myocardium, disrupts excitation-contraction coupling at cellular and tissue levels. Our finding of preserved calcium transients in dantrolene-pretreated myocytes exposed to deep hypothermia suggests a potential role for the RyR(2) channel in post-hypothermia reductions in cardiac function.