Han, Young Soo

AbstractAging is associated with cardiac contractile abnormalities, but the etiology of these contractile deficits is unclear. We hypothesized that cardiac contractile and regulatory protein expression is altered during aging. To investigate this possibility, left ventricular (LV) lysates were prepared from young (6 months) and old (24 months) Fischer344 rats. There are no age‐related changes in SERCA2 expression or phospholamban phosphorylation. Additionally, neither titin isoform expression nor phosphorylation differed. However, there is a significant increase in β‐isoform of the myosin heavy chain (MyHC) expression and phosphorylation of TnI and MyBP‐C during aging. In permeabilized strips of papillary muscle, force and Ca2+ sensitivity are reduced during aging, consistent with the increase in β‐MyHC expression and TnI phosphorylation. However, the increase in MyBP‐C phosphorylation during aging may represent a mechanism to compensate for age‐related contractile deficits. In isolated cardiomyocytes loaded with Fura‐2, the peak of the Ca2+ transient is reduced, but the kinetics of the Ca2+ transient are not altered. Furthermore, the extent of shortening and the rates of both sarcomere shortening and re‐lengthening are reduced. These results demonstrate that aging is associated with changes in contractile and regulatory protein expression and phosphorylation, which affect the mechanical properties of cardiac muscle.

In previous studies, we found that prolonged hypothermia (2 h or greater) followed by rewarming induces cardiac contractile dysfunction (rewarming shock) mimicking the effects of accidental hypothermia followed by rapid rewarming in trauma patients. We found that rewarming shock was also presented in isolated cardiomyocytes in which reduced contractility was attributed to PKA‐mediated TnI phosphorylation and reduced Ca 2+ sensitivity. During hypothermia rewarming in cardiomyocytes there is increased production of reactive oxidant species (ROS ‐ superoxide anion), which may trigger the intracellular mechanisms underlying reduced contractility. We hypothesized that in cardiomyocytes, hypothermia/rewarming‐induced ROS production is due to altered mitochondrial respiratory function. To test this hypothesis, isolated cardiomyocytes from 6 rats underwent the following hypothermia/rewarming protocol in which they were electrically stimulated every 2 s: experimental temperature was cooled from 35°C to 15°C during the initial 30 min, then maintained at 15°C for 2 h, followed by rewarming to 35°C in the final 30 min. Time‐matched control cardiomyocytes were also electrically stimulated every 2 s and maintained at 35°C for 3 h. After the 3‐h protocol, cardiomyocytes were placed into separate chambers of an Oroboros Oxygraph (high‐resolution respirometry) to measure O 2 consumption rate (OCR) under varying conditions. Initially, basal OCR was measured for 5 min followed by serial exposure to oligomycin (1 μM for 5 min) to block ATP synthase activity, FCCP (0.5 μM repeated 3 times for 5 min each) an H + ionophore to dissipate mitochondrial membrane potential; and antimycin‐A (0.5 μM for 5 min) to block the electron transport chain. This procedure allowed assessment of the key parameters of mitochondrial respiratory function: basal and maximal respiration (after FCCP exposure), ATP production and proton leak (after oligomycin), and reserve respiratory capacity (calculated after antimycin‐A). We found that compared to control cardiomyocytes, hypothermia/rewarming decreased key parameters of mitochondrial function, which suggests that altered mitochondrial function may have contributed to increased ROS production. In addition, decreased OCR following rewarming may contribute to contractile dysfunction by limiting ATP supply in response to metabolic demand, as well as by contributing to a decline in cell viability (e.g., triggering apoptosis). Support or Funding Information 5T32HL105355 (NSS); Norwegian Air Ambulance Foundation (YSH)