Relation between length, isometric force, and O2 consumption rate in vascular smooth muscle

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Abstract
The length-tension and length-oxygen consumption rate relationships were studied in bovine mesenteric vein at 37 degrees C. The absence of spontaneous mechanical activity permits straightforward interpretation in terms of active (smooth muscle) and passive components of the vein wall. Longitudinal loops, the predominant smooth muscle component being oriented in the longitudinal (axial) direction, were maximally stimulated using epinephrine (2-5 mug-ml-1). An optimum length for isometric tension development was exhibited at which the passive tension was 25% of the total tension. The population regression indicated that tension was developed at lengths which ranged from 0.33 to 1.41 times the length at which maximum tension was developed. Oxygen consumption was measured using a Clark-type polarographic electrode. Basal oxygen consumption was 0.432 plus or minus 0.014 (n equal to 121) mumol-min-1 (g dry wt)-1. The basal rate was found to be independent of the passive tension. Under conditions of maximal stimulation, the oxygen consumption rate at L-o, the resting length at which the tissue maintained 1 g-wt passive tension, was approximately twice the basal rate. The length dependence of the suprabasal oxygen consumption was parallel to that of the active isometric force. This parallel relation reflected a linear relation between active isometric force (deltaP-o) and suprabasal oxygen consumption rate (deltaJ-o2). The slope of the deltaJ-o2-deltaP-o linear regression was 0.142 plus or minus 0.013 nmol O2-MIN-1 (G-WT-CM)-1. DeltaJ-o2 at the minimum contracted length, at which no active force was developed, was 15-20% of the deltaJ-o2 measured when maximum isometric force was developed. This provides an upper bound to the rate of chemical energy utilization required for activation processes. The length dependence of active isometric force and chemical energy utilization is most simply interpreted in terms of a sliding-filament model.

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