Force-length dependence of arterial lamellar, smooth muscle, and myofilament orientations

Force generation by contractile elements of arterial tissue can be affected by alterations in their alignment with shortening. The orientation and morphometry of smooth muscle (SM) myofilaments, medial lamellae, and elastic laminae were examined as a function of length in intact swine carotid arteries or strips. Intimal-medial tissue strips were fixed during isometric contractions at lengths (L) defined with respect to the optimal length for force generation (Lo). The average orientation of SM cells in two perpendicular planes remained parallel to the long axis of the tissue at all lengths, but the absolute value of angular deviations increased with shortening. Tissue lengthening was associated with decreased folding of the elastic laminae. This decrease in waviness was quantified by a stretch index (SI). Ultrastructural observations indicated that the myofilament absolute angular deviation was greater than that for the cellular alignment. For arteries fixed in situ while constricted, SI was least in the periintimal laminae and increased in the peripheral laminae. The average decrease in force-generating capacity on shortening from Lo to 0.6 Lo attributed to increasing SM and myofilament angular deviations was calculated to be 7%.