Measurement of relaxation in isolated rat ventricular myocardium during hypoxia and reoxygenation

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Abstract
The effects of hypoxia and subsequent reoxygenation were examined in isolated left ventricular papillary muscles from the rat at 28°C and 33°C. Studies of relaxation were carried out in isometric and physiologically sequenced contractions. In studies of isometric contractions, the following variables were determined: active tension (AT), maximum rate of tension increase (+dT/dt), time to peak tension (TPT), time for tension to fall from peak to 50% of peak tension (RT½), maximum rate of tension decline (-dT/dt), isometric peak (-dT/dt/T), and isometric maximum (-dT/dt per T). Variables measured in physiologically sequenced contractions were the slopes of the relation between -dT/dtmax and end systolic length (SIM) and maximum rate of isotonic muscle lengthening (+dL/dtmax) and end systolic length (Sit). Tau, the exponential time constant for isometric relaxation, was also examined. Pronounced changes in active tension and +dT/dt were seen during hypoxia at both temperatures, whereas changes in measured relaxation variables were less prominent and inconsistent. At neither 28°C nor 33°C did TPT or RTh indicate impaired relaxation during hypoxia. Isometric peak -dT/dt declined with hypoxia at both temperatures but the normalised indices, isometric peak -dT/dt/T, peak (-dT/dt per T), and tau showed consistent impairment of relaxation only at 33°C. In physiologically sequenced contractions, SIM suggested impaired relaxation during hypoxia at 28°C but not at 33°C. SIT showed impaired relaxation at both 28°C and 33°C. These findings are consistent with data suggesting impairment of the cardiac relaxing system during hypoxia. Nevertheless, relaxation appears less affected than contraction, and impairment is best seen late in the cardiac cycle.