Biphasic changes in maximum relaxation rate during progressive hypoxia in isometric kitten papillary muscle and isovolumic rabbit ventricle.

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Abstract
The effects of graded hypoxia on mechanical performance of cardiac ventricular muscle was studied by producing controlled, stepwise decreases in partial pressure of O2 (PO2) in the medium bathing the kitten papillary muscle preparation and in the perfusate of the Langendorff-prepared rabbit heart. For kitten papillary muscle at 30.degree. C and with stimulation rate at 30/min, maximum rate of contraction (+dT/dtmax) and maximum rate of relaxation (-dT/dtmax) were 184 .+-. 10 mN/mm2 per s and 162 .+-. 12 mN/mm2/s, respectively, during control conditions with PO2 at 634 .+-. 7 mm Hg. Step decreases in PO2 from 634 mm Hg produced decreases in steady state -dT/dtmax that were significantly greater than corresponding decreases in +dT/dtmax, except at the lowest PO2. When PO2 (mm Hg .+-. SE) was 411 .+-. 10, 218 .+-. 4 and 92 .+-. 3 steady state +dT/dtmax vs. -dT/dtmax (expressed as percent of pre-hypoxia control value .+-. SE) were: 97 .+-. 4 vs. 85 .+-. 7, 76 .+-. 5 vs. 59 .+-. 6 and 47 .+-. 5 vs. 28 .+-. 4, respectively. When the lowest PO2 of 34 .+-. 6 mm Hg was achieved, considerable shortening of the duration of the mechanical cycle occurred; values for +dT/dtmax and -dT/dtmax (expressed as percent of pre-hypoxia control value .+-. SE) of 28 .+-. 7 and 21 .+-. 7, respectively, were not significantly different. Graded hypoxia similarly affected left ventricular isovolumic pressure developed by the coronary perfused rabbit heart. In both preparations changes in relaxation relative to changes in contraction during progressive hypoxia were biphasic; decreases in maximum relaxation rate were disproportionately greater than decreases in maximum contraction rate with intermediate hypoxia, but the proportionality was restored when severe hypoxia produced a decrease in cycle duration.

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