Activation of contraction and ATPase activity in intact and chemically skinned smooth muscle of rat portal vein. Dependence on Ca++ and muscle length.

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Abstract
The mechanical manifestations of muscle contraction (force development or shortening) are accompanied by an increased turnover of chemical energy (ATPase activity, JATP). In intact rat portal veins activated by high potassium medium to produce graded contractions at different levels of extracellular calcium, a linear dependence of oxygen consumption on force was found. The slope of the relation (metabolic tension cost) was higher during early stages than during late stages of contraction, possibly reflecting a transient high crossbridge ATP turnover rate. Chemically skinned (Triton X-100) rat portal vein preparations were used to study the energy turnover of the smooth muscle contractile system under constant activation. In these preparations, JATP increased on activation by calcium in the presence of 1 microM calmodulin, remained constant for maintained contractures, and decreased promptly on relaxation. Force declined with each repeated contraction at optimal calcium level (10(-4.5) M), but the relation between force and JATP remained invariant and agreed with that of the intact muscle during the transient of high ATP turnover. Calcium activation in the range 10(-9) to 10(-4.5) M caused a progressively steeper (nonlinear) increase in JATP with force. Length-force relations showed a lower relative force at muscle lengths below L0 in the intact than in the skinned tissue, possibly indicating depression of excitation-contraction coupling. The slope of the relation between JATP and force was lower when length was varied than when calcium was varied. At a length where no external force was produced, the activated muscle had a significantly higher JATP than while relaxed (10(-9) M Ca++).(ABSTRACT TRUNCATED AT 250 WORDS)