Han, Young Soo

We hypothesized that 1) hypothyroidism (Hyp) decreases myosin heavy chain (MHC) content per half-sarcomere in diaphragm muscle (Diam) fibers, 2) Hyp decreases the maximum specific force (Fmax) of Diamfibers because of the reduction in MHC content per half-sarcomere, and 3) Hyp affects MHC content per half-sarcomere and Fmaxto a greater extent in fibers expressing MHC type 2X (MHC2X) and/or MHC type 2B (MHC2B). Studies were performed on single Triton X-permeabilized fibers activated at pCa 4.0. MHC content per half-sarcomere was determined by densitometric analysis of SDS-polyacrylamide gels and comparison with a standard curve of known MHC concentrations. After 3 wk of Hyp, MHC content per half-sarcomere was reduced in fibers expressing MHC2Xand/or MHC2B. On the basis of electron-microscopic analysis, this reduction in MHC content was also reflected by a decrease in myofibrillar volume density and thick filament density. Hyp decreased Fmaxacross all MHC isoforms; however, the greatest decrease occurred in fibers expressing fast MHC isoforms (∼40 vs. ∼20% for fibers expressing slow MHC isoforms). When normalized for MHC content per half-sarcomere, force generated by Hyp fibers expressing MHC2Awas reduced compared with control fibers, whereas force per half-sarcomere MHC content was higher for fibers expressing MHC2Xand/or MHC2Bin the Hyp Diamthan for controls. These results indicate that the effect of Hyp is more pronounced on fibers expressing MHC2Xand/or MHC2Band that the reduction of Fmaxwith Hyp may be at least partially attributed to a decrease in MHC content per half-sarcomere but not to changes in force per cross bridge.

Rewarming patients after profound hypothermia may result in acute heart failure and high mortality (50–80%). However, the underlying pathophysiological mechanisms are largely unknown. We characterized cardiac contractile function in the temperature range of 15–30°C by measuring the intracellular Ca2+concentration ([Ca2+]i) and twitch force in intact left ventricular rat papillary muscles. Muscle preparations were loaded with fura-2 AM and electrically stimulated during cooling at 15°C for 1.5 h before being rewarmed to the baseline temperature of 30°C. After hypothermia/rewarming, peak twitch force decreased by 30–40%, but [Ca2+]iwas not significantly altered. In addition, we assessed the maximal Ca2+-activated force (Fmax) and Ca2+sensitivity of force in skinned papillary muscle fibers. Fmaxwas decreased by ∼30%, whereas the pCa required for 50% of Fmaxwas reduced by ∼0.14. In rewarmed papillary muscle, both total cardiac troponin I (cTnI) phosphorylation and PKA-mediated cTnI phosphorylation at Ser23/24 were significantly increased compared with controls. We conclude that after hypothermia/rewarming, myocardial contractility is significantly reduced, as evidenced by reduced twitch force and Fmax. The reduced myocardial contractility is attributed to decreased Ca2+sensitivity of force rather than [Ca2+]iitself, resulting from increased cTnI phosphorylation.