Responses of the relaxed and contracted portal vein to imposed stretch and shortening at graded rates

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Abstract
Portal veins were subjected to stretch and shortening at rates varying from 0.2 to 47% of initial muscle length/s. The total length change was 33% of initial length which was about 6 mm at the resting preload of 1 mN. Relaxed portal veins (Ca‐free medium) gave curvilinear stress‐strain relations which were practically independent of the rate of the length change and they were similar in their ascending and descending branches (non‐linear parallel elasticity). In contractures produced by K+ or hyperosmolar solution the stress‐strain curves showed pronounced and rate dependent hysteresis. The stretch response in K+ contracture showed initially a steep increase in force in relation to rate of lengthening (dP/dL) up to an apparent break‐point at 2–3% elongation, after which the rise in force was quite independent of the rate of stretch. The break‐point is blunted and shifted to greater length change in portal vein due to large series compliance, but in analogy with interpretations from other types of muscle it may reflect the limit of deformation of attached cross bridges. In hypertonicity contracture dP/dL increased in proportion to rate of stretch. Earlier isotonic quick release experiments showed that the force‐velocity relation of portal vein smooth muscle is markedly different in K+ and hyperosmolality contractures, respectively. Similar differences were found when force‐velocity relations were constructed from the present experiments by plotting active force versus rate of imposed length changes. This indicates that the viscous properties of contracted blood vessels, as revealed in stress‐strain hysteresis, are greatly determined by the force‐velocity characteristics of the smooth muscle actomyosin system. There are, however, certain differences between the force‐velocity curves from isotonic quick release experiments and from the present isovelocity study which remain to be explained.