Abstract
The effects of hypoxia and reoxygenation on regular contractions and postrest contractions (PRC) of papillary muscle of rats were studied. Isometric tension was measured during two cycles of hypoxia and reoxygenation. Trains of 80 externally continuous stimulations at 40/min were applied. PRCs were evoked by a stimulus train after a 60sec resting interval.
After 90min of hypoxia (the first hypoxia period), regular contractions and PRCs decreased to 5.6±2.0% and 23.4±2.4% of baseline values, respectively (p<0.001; n=18). After 90min of reoxygenation, the recovery of the PRCs (44.4±3.4%) was better than that of the regular contractions (23.3±3.3%) (p<0.01; n=18). After 30min of hypoxia (the second hypoxic period), regular contractions and PRCs decreased to 2.2±0.6% and 13.6±1.6% of baseline values, respectively (p<0.001; n=18). However, the recovery from the second hypoxic injury was not significant for either regular contractions or PRCs. The % diastolic tension, which was normalized to the baseline for regular contractions, increased to 113.2±6.9% and 133.6±8.4% at the end of the first and the second hypoxic periods, respectively. There was statistically significant correlation between the % diastolic tension and the % hypoxic injury of PRCs (p<0.002; n=18). There was no significant relationship between % diastolic tension and % hypoxic injury of regular contractions. There was no statistically significant correlation between % diastolic tension and % recovery from the hypoxic injury of either regular contractions or PRCs.
Since the PRCs are thought to be more dependent on calcium stored in the sarcoplasmic reticulum (SR) than regular contractions, SR function seems to be resistant to transient hypoxia. The marked increase in diastolic tension during repeated hypoxia may indicate SR dysfunction, leading to irreversible myocardial damage. Thus, both hypoxia and reoxygenation may regulate the recovery from hypoxic dysfunction.
