1. The relations between mechanical parameters and myosin heavy chain isoforms were studied in myometrial smooth muscle from ovariectomized rats (O) and oestrogen‐treated, ovariectomized rats (E). 2. Treatment of the rats for three days with beta‐oestradiol (2 micrograms kg‐1 day‐1) 2‐4 weeks postsurgery, produced maximal changes in uterine mass and myosin content of approximately threefold. 3. Myosin heavy chain isoform SM1 (204 kDa) was increased from 65.5 +/‐ 0.8% to 72.9 +/‐ 0.6% of the total isoform species (P < 0.001, n = 24, O and E respectively) after oestrogen treatment. 4. To avoid complications associated with activation processes, mechanical parameters were measured in permeabilized myometrial fibre bundles activated at a calcium concentration of 12.6 microM. After oestrogen treatment the maximum velocity of shortening (Vmax) measured by the slack test increased from 0.044 +/‐ 0.006 of the reference length (Lo) s‐1 to 0.101 +/‐ 0.006 Lo s‐1, and maximal isometric force (Pmax) increased from 23.3 +/‐ 4.4 mN mm‐2 to 74.1 +/‐ 13.9 mN mm‐2 (P < 0.001, n = 24, respectively). Series elasticity and the half‐time to peak force were not significantly altered. 5. Both Vmax and Pmax correlated significantly with percentage SM1 in O and E fibre bundles (r = 0.61 and 0.56, n = 48 fibres; or r = 0.87 and 0.89, n = 8 grouped data per rat). Vmax, however, was only weakly correlated with Pmax (r = 0.39, n = 48). 6. To assess the relative significance of the correlation between Vmax and the percentage of SM1 and that between Vmax and Pmax, we used a multiple regression analysis with the model Vmax = intercept + beta 1 x % SM1 + beta 2 x Pmax, where intercept, beta 1 and beta 2 are regression parameters. This analysis (n = 48) indicated that Vmax was significantly dependent on the percentage of SM1 (P < 0.0002) but not on Pmax (P < 0.61). 7. There were no significant differences in the levels of myosin light chain phosphorylation between O and E fibre bundles, indicating that light chain phosphorylation is unlikely to be the basis for the differences in mechanical parameters demonstrated by these fibres.
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