Microscopic Heat Pulses Induce Ca2+-Independent Contraction of Cardiomyocytes

Abstract

Laser irradiation has developed into a novel technique of non-invasive stimulation in cardiac and neural tissues. However, physical parameters for the laser irradiation-induced cardiac contractions have not been clarified, because various physicochemical reactions, such as photochemical and photothermal effects, are triggered in this process. Here we studied the effects of laserinduced local temperature changes on the functions of isolated cardiomyocytes. We demonstrated previously that a microscopic heat pulse (DT = 0.2 C for 2 sec) induces a Ca 2þ burst in cancer cells (HeLa cells) at a body temperature (Tseeb et al., HFSP J., 2009), with the mechanism similar to that of rapid cooling contracture in skeletal and cardiac muscles. In the present study, we generated microscopic heat pulses by focusing infrared laser light in extracellular solution near adult rat cardiomyocytes. We found that a microscopic heat pulse (DT = 5 C for 0.5 sec) induces contractions at basal temperature of 36 C. At 25 C, larger DT was required to induce contractions. When 2.5 Hz heat pulses were repeatedly applied, we observed oscillatory contractions of cardiomyocytes. Different from contractions induced by electric stimulation, Ca 2þ transients were not detected during the contraction. Likewise, heat pulses induced contractions of skinned cardiomyocytes in Ca 2þ-free solution in the presence of ATP. These results demonstrate that heat pulses can regulate cardiac contractions without any involvement of Ca 2þ dynamics, by directly activating the actomyosin interaction. Hence, our microheating technique may be useful for stimulating the beating of failing hearts without causing abnormal Ca 2þ dynamics.