Which of the following is TRUE regarding excitation contraction coupling mechanism in smooth muscles?
Depending on the activating stimulus, Ca2+ increase can be due to influx through voltage- or ligand-gated plasma membrane channels, efflux from intracellular stores through the RyR receptors, efflux from intracellular stores through the inositol trisphosphate receptor (IP3R) Ca2+ channel, or a combination of these channels. The lack of troponin in smooth muscle prevents Ca2+ activation troponin binding. They do not have the troponin system, and the light chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Myosin in smooth muscle must be phosphorylated for activation of the myosin ATPase. Sustained contraction is by latch bridge mechanism by which myosin cross-bridges remain attached to actin for some time after the cytoplasmic Ca2+ concentration falls. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.
Presence of troponin is essential
Sustained contraction occurs with high calcium concentration
Phosphorylation of actin is required for contraction
Presence of cellular calcium is essential to cause muscle contraction