A simple model describing the elastic properties of human umbilical arterial smooth muscle.

Elastic properties of cylindrical segments of 71 normal double and 4 single human umbilical arteries were studied. This specimen is rich in vascular smooth muscle in comparison with other great arteries. Outer diameter versus intraluminal pressure characteristic curves were taken with decreasing pressures in vitro in different contraction states. Tangential force and circumferential incremental elastic modulus were computed. A sum of 222 curves was analysed. The investigations showed that if we take tangential force and outer radius values measured at the same pressure levels but in different contraction states, then a similar proportional change in tangential force will induce a similar proportional passive change in the outer radius, to some extent independently of the degree of active tangential shortening of the segment. For example to induce a 10% passive decrease of the outer radius from values measured at 100 mm Hg intraluminal pressure, tangential force had to be decreased by 76.7 +/- 9.9% in single relaxed arteries, and by 79.6 +/- 0.8% in normal double relaxed segments. These values corresponded to intraluminal pressure levels of 26.4 +/- 4.9 mm Hg and 23.1 +/- 0.9 mm Hg, respectively. In 30% active spontaneous shortening to reach the same 10% passive decrease in outer radius from the value measured at 100 mm Hg, tangential force had to be decreased by 75.2 +/- 1.9% which corresponded to 29.6 +/- 20 mm Hg. The same values in 5-HT induced contraction, 30% active shortening were 76.7 +/- 2.0% and 28.4 +/- 2.6 mm Hg, respectively. In addition to the similarity of relative changes in tangential force, the pressure levels were to some extent also similar. These data suggest that elastic elements in the human umbilical arterial smooth muscle may be organized in such a way as to ensure similar prestretch of similar elastic elements at similar pressures independently of the degree of active shortening of the circumference.