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

Abstract

SUMMARY1. The relations between mechanical parameters and myosin heavy chain isoforms were studied in myometrial smooth muscle from ovariectomized rats (0) and oestrogen-treated, ovariectomized rats (E).2. Treatment of the rats for three days with /l-oestradiol (2 jtg kg-1 day-') 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 729 + 0-6 % of the total isoform species (P < 0-00 1, n = 24, 0 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 #M. After oestrogen treatment the maximum velocity of shortening (Vmax) measured by the slack test increased from 0-044 + 0-006 of the reference length (L.) s-' to 0 101± 0-006 L_ 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 SM I in 0 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 SMI and that between Vmax and Pmax, we used a multiple regression analysis with the model Vmax = intercept + I,A x % SMI + ±2 X Pmax, where intercept, /3, and /82 are regression parameters. This analysis (n = 48) indicated that Vmax was significantly dependent on the percentage of SMI (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 0 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.MS 9474

Smooth Muscle Myosin Heavy Chain Isoform Distribution in the Swine Stomach

“…These latter results bring into question any causal relationship for the MLC17a/b isoforms with shortening velocity. Hewett et al (1993) and Cai et al (1995) reported correlations between SM1/2 and maximal shortening velocity in tissue preparations. Other attempts to correlate SM1/2 MHC isoforms with actin motility and ATP hydrolysis, however, failed to establish such a clear relationship (Packer et al 1991;Kelley et al 1993;Meer and Eddinger 1997).…”

Effects of sex and estrogen on myosin COOH-terminal isoforms and contractility in rat aorta

“…Frozen tissue samples were pulverized under liquid N2 and homogenized in 1% SDS, 50 mM Tris, pH 6.8, and 25 mM dithiothreitol. The COOH-terminal myosin heavy chain isoforms SM1 (204 kDa) and SM2 (200 kDa) were separated by electrophoresis on 6% polyacrylamide gels as previously described (19). The gels were stained with Coomassie Blue R and scanned on a LKB Ultrascan XL Laser Densitometer.…”

“…Using a rat model involving ovariectomy and estrogen replacement, we characterized major changes in uterine smooth muscle function (19). Changes in both maximum isometric force and shortening velocity were associated with estrogen replacement.…”

“…Smooth muscle contains both COOH-and NH 2 -terminal isoforms produced by alternate splicing. With estrogen treatment, no changes in the NH 2 -terminal isoforms were noted (6), whereas the SM1 variant of the COOHterminal isoforms was increased (19).…”

Shortening velocity and myosin heavy- and light-chain isoform mRNA in rabbit arterial smooth muscle cells

“…Alternative splicing near the 3Ј end of the MHC pre-mRNA results in expression of either one of two proteins, SM1 (204 kDa) or SM2 (200 kDa), differing only at their carboxy termini (1,10). In studies performed in chemically permeabilized tissue strips, SM1 expression has been correlated with an increase in unloaded shortening velocity (21), an increase in calcium sensitivity, and also a decrease in shortening velocity (3). However, studies on purified proteins (23a), expressed proteins (33), and single cells (28) suggest that expression of the SM1 and SM2 isoforms results in similar in vitro ATPase activities (23a, 33) and similar maximal unloaded shortening velocities (28).…”