Improvement in relaxation by nifedipine in hypoxic isometric cat papillary muscle

Hypoxia has been demonstrated to cause impairment of myocardial relaxation. This impairment of relaxation is particularly pronounced during early reoxygenation. This study was undertaken in 24 isometric cat papillary muscles at 38 degrees C to determine if nifedipine can influence the impairment of relaxation produced during reoxygenation following hypoxia. A dose of nifedipine was chosen that produced only a minimal depression of peak systolic tension and no change in the half time to relaxation (RT 1/2) under well-oxygenated conditions. Thirty minutes of hypoxia were produced in 12 muscles, and systolic tension decreased by the same amount in muscles treated or not treated with nifedipine. During early hypoxia in the absence of nifedipine, RT1/2 was significantly prolonged (P less than 0.01) from 104 +/- 7 to 126 +/- 9 ms. After pretreatment with nifedipine, the change in RT1/2 with hypoxia was not significant. More striking was the near abolition of the marked impairment of relaxation seen during early reoxygenation (238 +/- 33 ms without nifedipine and 128 +/- 8 ms with nifedipine, P less than 0.01). These data establish that, although nifedipine only minimally attenuates the relaxation impairment early during hypoxia, this agent can substantially reduce the impairment of relaxation produced by early reoxygenation.