OXYTOCIN-MEDIATED RECRUITMENT OF SLOWLY CYCLING CROSS-BRIDGES AND ISOMETRIC FORCE IN RAT MYOMETRIUM

The relationship between cross-bridge cycling rate and isometric stres s was investigated in rat myometrium. Stress production by myometrial strips was measured under resting, K+ depolarization, and oxytocin-sti mulated conditions. Cross-bridge cycling rates were determined from me asurements of maximal unloaded shortening velocity, using the quick-re lease method. Force redevelopment after the quick release was used as an index of cross-bridge attachment. With maximal K+ stimulation, stre ss increased with increased cross-bridge cycling (+76%; P < 0.05) and attached cross bridges (+112%; P < 0.05). Addition of oxytocin during KC stimulation further increased stress (+30%; P < 0.05). With this fo rce component, the cross-bridge cycling rate decreased (-60%; P < 0.05 ) similar to that under resting conditions. Attached crossbridges did not increase with this additional stress. The results suggest two dist inct mechanisms mediating myometrial contractions. One requires elevat ed intracellular calcium and rapidly cycling cross bridges. The other mechanism may be independent of calcium and appears to be mediated by slowly cycling cross bridges, supporting greater unit stress.