Nonmuscle myosin, force maintenance, and the tonic contractile phenotype in smooth muscle

Abstract

Recent studies have demonstrated that nonmuscle (NM) myosin II forms filaments and can generate and maintain force in smooth muscle tissue [Lofgren et al. (2003) J Gen Physiol 121:301-310; Morano et al. (2000) Nat Cell Biol 2:371-375]. To further investigate the mechanical contribution of NM myosin to force maintenance during smooth muscle contraction, we utilized a selective inhibitor of the NM myosin ATPase, blebbistatin [Straight et al. (2003) Science 299:1743-1747]. Force and myosin light chain (MLC(20)) phosphorylation were measured during KCl stimulation of small strips of intact mouse bladder and aorta at 22 degrees C. The bladder strips contracted with a typical phasic force response, characterized by a large, rapid, transient increase in force followed by a decline to a lower, steady-state level. The addition of blebbistatin did not alter the peak force, but decreased force maintenance. KCl depolarization of aortic strips resulted in a tonic contraction; force increased to a sustained steady state. Similar to the bladder tissue, blebbistatin substantially decreased the steady-state force in the aorta. Blebbistatin did not influence the MLC(20) phosphorylation transient in either tissue type. Additionally, blebbistatin did not change the maximum shortening velocity (V (max)) during KCl depolarization of the aorta. Our results also suggest that NMIIA and NMIIB isoforms are differentially expressed. The expression of NMIIA is more prominent in the bladder, while NMIIB expression is predominant in the aorta. These results suggest that NM myosin contributes to the mechanism of force maintenance in smooth muscle, and could suggest that the expression of NMIIB is a factor for determining the tonic contractile phenotype.

Load-dependent mechanism of nonmuscle myosin 2

“…4). NM2 (and especially NM2B) has been implicated for smooth muscle function, namely in the prolonged force maintenance phase (tonic contraction) (1,2). Load sensitivity of NM2 kinetics reported here thus may vitally contribute to its efficient functioning both under assisting and resisting loads.…”

“…It is able to sustain loads because it forms long, bipolar filaments containing Ϸ150 molecules in each half, so load can be sustained when only a small fraction of the heads are attached at any time. By contrast, nonmuscle myosin 2 (NM2), essential for cytokinesis, tension maintenance, and contractility of nonmuscle and smooth muscle cells (1)(2)(3)(4), assembles into short filaments with only Ϸ10 molecules in each half (5,6), so their duty ratios under load need to be higher to avoid slippage. The unloaded kinetic mechanisms of single-headed fragments of the NM2A and NM2B isoforms show that NM2A is the faster, lower duty ratio motor, whereas NM2B is slow, with a higher duty ratio suggesting a role in tension maintenance (7)(8)(9).…”

Blebbistatin inhibits the chemotaxis of vascular smooth muscle cells by disrupting the myosin II-actin interaction

“…However, inhibition of contraction has been reported in smooth muscle at significantly lower concentrations of blebbistatin (7,15,39). Because IC 50 for nonmuscle myosin II is 1-5 M (27), the increased inhibition in these studies might be explained in terms of the nonmuscle myosin II (7,39). The content of nonmuscle myosin II in the primary culture of vascular smooth muscle tissues increased with the subculture (16).…”

Smooth muscle myosin expression, isoform composition, and functional activities in rat corpus cavernosum altered by the streptozotocin-induced type 1 diabetes

“…Recent work also suggested that differences existed in BLEB efficacy for different SMM isoforms and/or SMM vs. nonmuscle myosin (NMM) (39). Therefore, whether DM has an effect on CC SMM isoform composition and whether STZ diabetes influences BLEB efficacy in relaxing CCSM are novel, highly intriguing questions that were in need of investigation.…”

“…Concomitant upregulation of myocardin expression, which is a master regulator of SM expression (46), further confirmed the relative overexpression of SM MHC in STZ animals. Because BLEB has been found not to compete with ATP binding or inhibit MLC kinase (43) or alter SMM regulatory light chain phosphorylation levels (39), it thus appears that BLEB functions via binding to the myosin-ADP-P i complex and blocking of the myosin II in an actin-detached state. Therefore, the threefold upregulation of SM MHC possibly contributed to the increased efficacy of BLEB for CCSM from STZ rats.…”