The Active Tension-length Curve of Vascular Smooth Muscle Related to Its Cellular Components

The active and passive isometric tension-length (internal circumference) relation of vascular smooth muscle has been investigated using a 100-200-micron lumen diameter artery from the rat mesenteric bed. Conditions were established under which maximal activation was obtained at all lengths. Below L0 (the length at which maximum tension, delta T0, was developed) the active tension fell with decreasing length along a line which extrapolated to 0.38 L0; below 1.1 L0 the relation was reversible regardless of the protocol used. Above L0 the active tension fell linearly with increasing length along a line which extrapolated to zero tension at 1.82 L0. At the longer lengths investigated (up to 1.6 L0) the passive tension upon which the active responses were superimposed was as high as 4.4 delta T0. However, measurements of the dynamic characteristics of the preparation (with a time resolution of 2 ms) suggest that the active tension measured is nevertheless a measure of the active properties of the contractile apparatus. Direct light microscopic observation of the effect of length change on the cells within the walls of the preparation showed that changes in vessel length produced, on average, the same percentage change in cell length. Histological examination showed no signs of cell destruction following large extensions. The results suggest that the decrease in tension with extension above L0 is due to changes in the properties of the contractile apparatus, rather than to cellular damage.