Load-insensitive relaxation caused by hypoxia in mammalian cardiac muscle.

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Abstract
The relaxation of isolated cardiac muscles from mammals was sensitive to the loading conditions because the time course of relaxation could be changed by changing the load. This effect apparently is related to the amount and functional status of the sarcoplasmic reticulum. Whether hypoxia affected the load sensitivity of papillary muscles isolated from rats and cats was studied. Mechnical tests (3) were used to establish the presene of load-sensitive relaxation. Records of isotonic contractions were superimposed at increasing afterloads up to isometric contraction. The ratio (tRi) which was the time from the initiation of the contraction to the initial decay of force at each isotonic afterload was measured, divided by the time it took for the force of an isometric contraction to relax to that same afterload. If the tRi was less than 1.0, then the muscle was load sensitive. Hypoxia caused the loss of load-sensitive relaxation in isotonically contracting rat papillary muscles since the tRi ratios were not significantly different from 1.0 at all afterloads. Both hypoxia and caffeine were required to make cat papillary muscles load insensitive. During hypoxia, loads added in midcontraction did not induce early relaxation in rat papillary muscles, but still did so in cat muscles. Hypoxia plus caffeine eliminated this load-induced early relaxation in cat papillary muscles. Physiologically contracting muscles made hypoxic did not lengthen earlier in response to an additional load. This decrease of load sensitivity under hypoxic conditions could contribute to the relaxation abnormalities observed during regionak wall motion studies.