The myocardial energetic active state. I. Oxygen consumption during tetanus of cat papillary muscle.

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Abstract
The potential role of active state maintenance as a determinant of myocardial oxygen consumption (MVO2) has not been defined. Right ventricular papillary muscles from 15 cats were studied in a polarographic myograph at 23 degrees C in a Krebs-Ringer solution containing 7.5 mM Ca2+ and 10 mM caffeine. MVO2 was determined for isometric tetani at Lmax of 1-5 seconds' duration. Increases in tetanus duration related linearly to increments in both active tension time (delta active tension) and MVO2. In order to examine the oxygen cost of active state maintenance not attributable to associated tension generation, both the same isometric and 2.5- to 10.0-second lightly preloaded isotonic tetani were produced in nine muscles. For each tetanus duration the contribution throughout the contraction of developed force (preload) to MVO2 could be subtracted from overall isotonic MV02. In the absence of the MVO2 associated with force development, the active state duration was related linearly to MVO2, with a mean active state MVO2 of 2.42 +/- 0.29 nl O2/mg dry muscle/sec of isotonic tetanus; this MVO2 is 68% of the value of 3.58 +/- 0.42 nl O2/mg dry muscle/sec that was obtained for isometric tetanus at Lmax. This study identifies active state maintenance as the major determinant of MVO2 during myocardial tetanus and, furthermore, suggests the possibility that alterations in ative state intensity and duration may be the biochemical mechanism by which other determinants of MVO2 act in a more physiological setting.

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