Smooth muscle contraction
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Name some examples of smooth muscles
Arteries Guts Bladder Reproductive organs
What is the name for a nerve terminal
Varicosity - Contains noradrenaline
How is noradrenaline released
Activation of sympathetic nerve Calcium influx Vesicles fuse with membrane (calcium binds to synaptotagmin) Synaptotagmin removes inhibition, binding to phospholipid so v-SNARe binds to T-snare to create Snarepin, bringing membranes closer together to release noradrenaline.
Describe features of smooth muscle
Junction coupling cells Dense body Intermediate filaments Membrane dense area Thick and thin filaments No T tubules Many G coupled receptors Irregular myofilament arrangement
Describe G protein receptor mechanism
Couple to GAQ -> Phospholipase C activation -> Breakdown of PIP2 to IP3 and DAG, which bind to their receptors and release calcium/promote calcium influx, leading to smooth muscle contraction.
What calcium sensor is used in smooth muscle contraction vs skeletal and cardiac?
Troponin - Skeletal/Cardiac Calmodulin - Smooth
A rise in calcium in skeletal and cardiac muscle results in the removal of inhibition. (NO mechanism) What happens instead in smooth muscle contraction?
Myosin is activated (a GO mechanism)
How is smooth muscle contraction different to skeletal/cardiac contraction
No troponin Tropomyosin does not interact with the mysoin binding sites (it blocks in skeletal) Myosin in smooth muscle is a different isoform (MYH11 in smooth vs MYH1) Lower ATPase activity Lower affinity for ATP
What does MLCK (kinase) do
Attach phosphate to a protein - In this case, phosphorylation of MLC at ser 19 This increases the ATPase activity of the myosin head (more hydrolysis), and alters the structure of myosin ATP affinity increased
Describe the contractile mechanism for smooth muscle
Calcium influx, calcium released Calcium binds to calmodulin MLCK activated Phosphorylated myosin interacts with actin, actin-myosin complex rings out the smooth muscle.
How do smooth muscles maintain force for a long time with low levels of calcium
When de-phosphorylated, low affinity for ATP. This means it is harder for the myosin to detach from actin. Consequently, smooth muscles can maintain a degree of interaction for long periods.
What does a rise in cAMP do in smooth muscle contraction
Activates PKA which inhibits MLCK -> Increase in calcium does nothing as MLCK does not work. Muscles relax This is why B-adrenoreceptor activation increases cardiac contraction but relaxes arteries (smooth)