Regulation of smooth muscle contraction ca still required, different mechanism leading to contraction no troponin in smooth muscle; troponin replaced w/ caldesmon calcium-calmodulin also involvedboth actin and myosin involved and regulated by cacalcium enters cell from primarily from ecf and pumped back to ecfca influx and removal is much slower match rate of contractionca enters cell via voltage-gated ca channelsin some smooth muscle, depolarization leads to an apin others, no ap generated, tension proportional to [ca]cyto ca binds to and activates: (13, 14)myosin light chain kinase turning on myosin atpase activitycalmodulin (cam) regulates at the level of actinprotein kinase c (pkc) regulates at the level of actinthus, ca interactions required on both actin and myosin ca regulation of actin filaments by caldesmon (13 left)caldesmon acts somewhat like troponin does in skeletal musclebound to actin when muscle at rest preventing interaction w/ myosin 2 ways in which to remove caldesmon from the myosin binding site on actin(1) ca-calmodulin causes caldesmon to move away from actinca binds to calmodulinca-cam binds to caldesmoncauses caldesmon to move out of way of myosin (comes off of actin)(2) via pkc (14 - bottom)phosphorylation of caldesmon by pkc pulls caldesmon off actinpkc is activated when bound to cacaldesmon cant bind to actin filament when phosphorylateda phosphatase dephosphorylates caldesmon w/ decr in [ca]ca regulation of myosin light chain (13 right)myosin is a multimeric protein composed of two heavy and two light chainsmyosin light chains regulatory in action and modulate activity of myosin head direct activation of myosin light chain (mlc) by camyosin light chain part of the myosin protein to myosin head atpaseif no ca on mlc, myosin head cant bind actinmyosin light chain bound to ca allows myosin head to bind actin (if caldesmon is out of the way)indirect activation of mlc by ca-calmodulin (13 right; 14 - topmyosin light chain must be phosphorylated before myosin head can bind to actin phosphorylation done by myosin light chain kinase which is activated by ca-calmodulin depending on the type of smooth muscle, [ca] may increase due to an incoming electric signal (ap), stretch (mechanically gated ca channels) or other non-electric stimulus
