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

We studied the effects of graded hypoxia on mechanical performance of cardiac ventricular muscle by producing controlled, stepwise decreases in partial pressure of oxygen (Po 2 ) in the medium bathing the kitten papillary muscle preparation and in the perfusate of the Langendorffprepared rabbit heart. For kitten papillary muscle at 30C and with stimulation rate at 30/min, maximum rate of contraction (+dT/dtm M ) and maximum rate of relaxation (-dT/dtm.*) were 184 10 inN/mm 2 per sec and 162 12 mN/mm 2 per sec, respectively, during control conditions with Po 2 at 634 7 mm Hg. Step decreases in Po 2 from 634 mm Hg produced decreases in steady state -dT/dtmu, that were significantly greater than corresponding decreases in +dT/dt ma , except at the lowest Po 2 . When Po 2 (mm Hg SE) was 411 10, 218 4, and 92 3, steady state +dT/dt mM vs. -dT/dt, (expressed as % 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 Po 2 of 34 6 mm Hg was achieved, considerable shortening of the duration of the mechanical cycle occurred, and values for +dT/dtmx and -dT/dtma, (expressed as % 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.