Regulation of force and shortening velocity in smooth muscle

Sammanfattning: Smooth muscle is characterised by high tension economy, low shortening velocity and regulation mainly by myosin light chain (MLC) phosphorylation. The aim of this study was to investigate regulation of force and maximal shortening velocity (Vmax) in smooth muscle. In intact muscle, activated with depolarisation, inhibition of force and rise in intracellular [Ca2+] ([Ca2+]i) by 2,3-butanedione monoxime (BDM) suggested influx of Ca2+ across the cell membrane as the major source of activator Ca2+ during contraction. Decreasing temperature (37-2 C) reduced peak and plateau (5 minutes after activation) force and [Ca2+]i (measured with fura 2 - technique). At 27 C, plateau force was inhibited by 50% at unchanged MLC-phosphorylation, compared to at 37 C. This suggests inhibition of a force-maintaining temperature sensitive process independent of MLC-phosphorylation. In demembranised ("skinned") preparations, inorganic phosphate (Pi), vanadate (Vi), BDM, temperature and the protein calponin were used to probe specific cross-bridge reactions. Presence of Pi increased rate of contraction (measured with photolytic release of ATP from caged ATP), but inhibited force. This shows that force-generation in smooth muscle is associated with the Pi-release reaction. Force was less sensitive to Pi, BDM and temperature variations than in striated muscle, suggesting differences in distribution of cross-bridge states. BDM reduced force, Vmax and MLC-phosphorylation. Vi reduced force and Vmax similarly to a reduction of [Ca2+]. Calponin reduced Vmax at largely unchanged force, whereas Pi at submaximal activation increased Vmax at reduced force. These results show that Vmax and force in smooth muscle can be regulated independently and that cross-bridge states acting as an internal load may be involved in regulation of Vmax.