Flashcards in Smooth Deck (21)
What are the general types of actions smooth muscle carries out?
Continuously ongoing maintenance movements with a steady force output.
What is the general type of smooth muscle, and what can it alter to?
General is contractile with a high degree of contractile proteins and receptors. Can be stimulated to become a secretory cell with little of these, however, and a high amount of non muscle myosin, actin, elastin, and collagen.
Describe a unique structural quality of smooth muscle thin filaments.
Actin thin filaments terminate in dense bodies, some of which are on the membrane while others exist in the cytoplasm bound to the membrane via desmin thin filaments (or vimentin).
Contrast smooth muscle and skeletal muscle thick filaments.
Smooth thick filaments' ATPase on the head activates 100x slower, and only about 20% as much exists in the cell.
Contrast smooth muscle and skeletal muscle thin filaments.
Smooth has 2x the amount of actin.
Tropomyosin exists in the same amount, but does not cover the myosin binding site of actin.
Caldesmon exists, which binds calcium, actin, myosin, and calmodulin. It inhibits myosin ATPase activity with low calcium or calmodulin levels, and inactivates via phosphorylation.
Calponin to bind calcium, calmodulin, and actin to inhibit actin-activated myosin ATPase.
NO TROPONIN EXISTS IN SMOOTH
Describe calcium leak channels in smooth muscle.
Leak channels permit a contuous passive entry of calcium, insufficient to cause contraction under normal conditions.
Describe stretch activated calcium channels in smooth muscle.
Changes in membrane tension allow calcium entry, thus contraction, in response to stretching. Called the myogenic response.
Describe ligand gated calcium channels in smooth muscle.
Agonist binding, like NE, cause conformational change and calcium entry. These are ROCs, receptor operated channels.
Describe voltage gated calcium channels in smooth muscle.
Depolarization of smooth muscle DHP channels allows calcium influx, but in this case it can trigger contraction without SR calcium release. Is blocked via dihydropyridines.
Describe CICR calcium entry in smooth muscle.
Elevation of calcium in membrane channels interacts with RyRs of the SR to trigger calcium release.
What is the indirect method of calcium increase in smooth muscle?
Activated GPCRs activatet eh IP3/DAG pathway, which released calcium from the SR (pharmacomechanical coupling, as no depolarization needed).
How do smooth muscle cells rid themselves of calcium?
Both SR ATPase activity and Na/Ca exchanger at the cell surface.
What is the controlling factor on cross bridge formation in smooth muscle?
A myosin light chain must be phosphorylated so that myosin can then interact with actin. This occurs by calcium binding to calmodulin, which then activates MLCK to phosphorylate it. Dephorphosrylation and termination then occurs via MLCP.
Describe the Rho Kinase activity in smooth muscle.
ROCK phosphorylates the MLC too, while also inhibiting the MLCP by phosphorylating its myosin binding subunit.
Describe protein kinase C function in smooth muscle.
It is activated in the IP3/DAG system, and phosphorylates CPI-17 which binds and blocks MLCP.
Describe cGMP function in smooth muscle.
It activates MLCP, causing relaxation.
Describe cAMP function in smooth muscle.
It leads to an increased calcium pumping into the SR, thus increasing MLCP activity. It also inhibits MLCK.
Describe the latch state of smooth muscle.
Is generally the maintenance of high levels of force during conditions of basal calcium and MLC phosphorylation. Is done via slow cycling of cross bridges with reduced ATP consumption.
How is the latch state reached?
1. MLC is phosphorylated, allowing entry to cross bridging.
2. MLC is dephosphorylated while cross bridge is connected, allowing slower cross bridge deformation (cross bridge is called a dephosphorylated cross bridge, or latch bridge).
Describe smooth muscle velocity and force compared to skeletal.
Smooth takes much longer (seconds to minutes) to reach maximal force levels due to slow cross bridge cycling. However more force is generated per cross section due to slower cycling, thus equal amounts of force can be generated, just at a slower rate.