Myosin composition and functional properties of smooth muscle from the uterus of pregnant and non-pregnant rats

Abstract

The myosin heavy chain stoichiometry and the force-velocity relation have been determined in the myometrium of the non-pregnant and pregnant rat. The relative proportions of the slower migrating heavy chain (MHC1) greatly exceeded that of the faster migrating heavy chain (MHC2) as shown by electrophoresis on SDS 4%-polyacrylamide gels. The ratios of MHC1/MHC2 were 2.2/1 in the non-pregnant rats, 2.6/1 in the pregnant rat, and contrasted with 0.8/1 in the rat portal vein. This stoichiometry was unchanged by extracting the myosin from the smooth muscle as native myosin in a salt extract, as dissociated myosin using sodium dodecyl sulphate (SDS) or by isolating the native myosin first by a non-dissociating (pyrophosphate) electrophoresis step and subsequently analysing the protein bands on the SDS 4%-polyacrylamide gel. Although the unequal proportions of the heavy chains suggested the possibility that the native myosin molecule may be arranged as homodimeric heavy chains, no evidence for or against the existence of native myosin isoforms could be obtained by electrophoresing native myosin extracts on pyrophosphate-polyacrylamide gels. The force-velocity relations of the intact electrically stimulated myometrium from the non-pregnant and pregnant rats gave isometric force of 45 and 135 mN/mm2 and Vmax of 0.71 and 0.52 lengths/s (37 degrees C) when measured at 95% of optimal length, whereas in chemically skinned uterine strips at 22 degrees C Vmax was 0.09 and 0.13 lengths/s, respectively. The length-force relationship was of similar shape in the non-gravid and gravid skinned tissues. The energetic tension cost (ATP-turnover/active stress) in skinned fibres was also similar. The mechanical and metabolic characteristics of the gravid and non-gravid uterus found in the present study do not suggest an obvious difference in the intrinsic properties of the myosin, although significant functional alterations in the tissue appear during pregnancy. This corresponds to the lack of a difference in the pattern of the heavy chains.

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

“…In fibres contracted in the presence of calcium, there was a slight increase in myosin light chain phosphorylation with oestrogen treatment; however, this difference was not statistically significant. Sparrow et al (1988) and other previously published reports (Cavaille, Janmot, Ropert & d'Albis, 1986 of Sparrow et al (1988) from non-gravid (68-6% SMI) and gravid (71-9% SMi) rat myometrium. The increase we observed in SMI content with oestrogen treatment (65-5 to 72-9 % SMI) in ovariectomized uterus is similar, though more dramatic, than the increase in percentage SMI observed in the myometrium with pregnancy.…”

“…After mounting, the fibres were slightly stretched beyond their slack length to a passive force of 0 05 mN or approximately 7 mN mm-2. This length was chosen based on experience and published uterine muscle force-length curves (Sparrow et al 1988) to obtain a reference length (L.) in the region for optimal force development. The plateau region of the force-length relation for uterine muscle is broad with only minor changes in active force between 0-8 and 1 1 of the optimal length for force development.…”

Myosin heavy chain isoform expression in human myometrium: Presence of an embryonic nonmuscle isoform in leiomyomas and in cultured cells

“…However, in these studies, the presence of nonmuscle MHC in rat uterus was not reported. Sparrow et al [1988] found an equivalent SM USM2 ratio (2.2-2.6) in pregnant and nonpregnant rat uterus and no modifications in mechanical muscle parameters which could be associated with changes in MHCs. In nonpregnant uterus of mouse and swine, the SMI/SM2 ratio appeared to vary with aging, from 4.8-3.4 for the former [Eddinger and Wolf, 19931 to 4.5-1.2 for the latter [Eddinger and Murphy, 19911, associated with variations in the relative content of nonmuscle MHC(s).…”

Smooth muscle myosin heavy chain isoforms and their role in muscle physiology

“…Mechanical studies performed using uterus smooth muscle tissues have shown a positive correlation between SM1 myosin content and maximal velocity of muscle shortening (V max ) (Hewett et al, 1993;Sparrow et al, 1988). Cai et al (1995) showed that a 13 amino acid peptide from the unique SM1 tail decreases contractile velocity in permeabilized smooth muscle tissue, suggesting that the non-helical tail of SMHC may modulate contractility.…”