Correlations between myosin heavy chain isoforms and mechanical parameters in rat myometrium.

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Abstract
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.