Effect of alterations in shortening and external work on oxygen consumption of cat papillary muscle

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Abstract
A polarographic method for determination of myocardial oxygen consumption (MvO2)was utilized to investigate afterloaded contractions in 20 cat papillary muscle preparations. Resting MvO2 averaged 2.58 [plus or minus] .16 SE /il/mg dry tissue/hr at 29C and was not altered in these muscles by changes in initial muscle length. Initiation of contraction from an increased initial muscle length against a constant total load resulted in increments in shortening and external work at a constant intrinsic velocity of contraction. These increments in shortening and external work were associated with an increased MvO2 in each experiment thus implicating external work as a determinant of energy utilization in heart muscle. Contractile element work (CEW) was calculated as the sum of fiber shortening (external) work, internal work done by the contractile element in extending the series elasticity, and work done against the equipment distensibility. CEW increased with progressive increments in afterload but reached a maximal value and declined prior to isometric contraction. The relationship between afterload and MvO2 was curvilinear with the maximal MvO2 occurring at isometric contraction. Thus there was a lack of correlation between calculated CEW and MvO2 at heavy afterloads.

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