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.

Injury to the pleura ultimately results in either repair with fibrosis or repair without fibrosis and a reestablishment of the normal mesothelial monolayer. The role of the mesothelial cell, and of local mediators, in these repair processes remains essentially undefined. In order for repair without fibrosis to occur, mesothelial cells, in response to local mediators, must be capable of migration and/or proliferation to cover the injured and denuded mesothelium. We hypothesized that rat pleural mesothelial cells were capable of both chemotaxis and proliferation in response to thrombin. In an in vitro assay, mesothelial cells demonstrated directed migration in response to a known chemoattractant, formylmethionylleucylphenylalanine. In addition, mesothelial cells demonstrated chemotaxis in a dose-dependent manner in response to thrombin, with a maximal response at a concentration of 10(-8) M. Finally, this chemotaxis was blocked by a specific blocker of thrombin, antithrombin 3. Thrombin also stimulated mesothelial cell proliferation, which was measured both in a [3H]thymidine incorporation assay and by direct cell counts. Again, the response was dose dependent, with the maximal response at 10(-8) M causing the same amount of [3H]thymidine incorporation as 10% fetal bovine serum. As before, this response was completely blocked by antithrombin 3. These results demonstrate that mesothelial cells are capable of both chemotaxis and proliferation in response to thrombin. Thrombin may play an important role in the regulation of pleural repair without fibrosis and the re-establishment of the mesothelial monolayer.

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