Regulation of isometric force and isotonic shortening velocity by phosphorylation of the 20,000 dalton myosin light chain of rat uterine smooth muscle.

The regulation of isometric force maintenance and isotonic shortening velocity by phosphorylation of the 20,000 dalton light chain of myosin has been examined for potassium-depolarized rat uterine smooth muscle. Following a transient peak in myosin light chain (LC20) phosphorylation at 20 s of contraction (0.46 mol PO4/mol LC20), phosphorylation declined to a steady-state by 2 min (0.28 mol PO4/mol LC20) with no significant change from 2-90 min of contraction. Isometric force developed more slowly, reaching a maximum at 2 min with no further change out to 90 min. Lightly-loaded (0.1 F0) shortening velocity, like LC20 phosphorylation, increased initially to a peak of 0.034 L0/s at 20 s of contraction and then declined to 0.023 L0/s by 2 min. However, unlike LC20 phosphorylation and isometric force, shortening velocity decreased approximately 4-fold from 0.023 L0/s at 2 min to 0.006 L0/s at 90 min of contraction. Graded activation with reduced extracellular calcium was associated with proportional changes in steady-state isometric force and LC20 phosphorylation. Shortening velocity was also decreased with reduced calcium, however, unlike LC20 phosphorylation, the greatest changes in velocity occurred at low levels of developed force. Moreover, in contrast to the large reductions in shortening velocity observed during 90 min contractions where force and LC20 phosphorylation were unchanged, similar reductions in shortening velocity did not occur with graded activation in spite of significant (greater than 3-fold) decreases in both force and LC20 phosphorylation. These results suggest that factors other than light chain phosphorylation are involved in the regulation of isotonic shortening velocity during extended isometric contractions of uterine smooth muscle.