DEPENDENCE OF FORCE ON LENGTH AT CONSTANT CROSS-BRIDGE PHOSPHORYLATION IN THE SWINE CAROTID MEDIA

1. The dependence of force (F) on length (L) in smooth muscle remains uncertain since (i) it is influenced by changes in activation (myosin light chain phosphorylation), (ii) no anatomical reference length for the contractile unit is available, (iii) the length at which optimum f orce is generated (L(0)) exhibits a broad, flat optimum, and (iv) the presence of an extensive connective tissue network makes it difficult to stretch tissues without damage. 2. A swine carotid medial ring prep aration prepared by removal of the adventitia and endothelium could be stretched to 1 . 8 L(0) without decreasing active force generation on return to shorter lengths. 3. A highly reproducible mechanically defi ned reference length, L(0), was obtained by fitting force-length data between 0 . 3 and 1 . 6 L(0) with a third-order polynomial where L = L (0) when dF/dL = 0. 4. Activation as assessed by myosin regulatory lig ht chain (MRLC) phosphorylation increased with length in 100 mu M hist amine-stimulated tissues from 0 . 6 to 1 . 8 L(0). 5. Activation was c onstant in K+-depolarized and field-stimulated tissues from 1 . 0 to 1 . 8 L(0) allowing determination of the descending limb of the force-l ength relation to be assessed independently of activation. 6. The slop e of the descending limb of the force-length relation was linear excep t at very long lengths, which often produced tissue damage. The slope was not statistically different from that estimated for sarcomeres in vertebrate skeletal muscle. 7. The medial ring preparation and the pro cedures used to define the reference length provide advantages for the measurement of length-dependent variables.