Source of calcium for smooth muscle contraction: (A) Influx into intracellular store through IP3 receptor Ca2+ channel, (B) Efflux through voltage - gated Ca2+ channel, (C) Efflux through ligand - gated Ca2+ channel, (D) Combination of influx of Ca2+ through voltage - gated, ligand gated Ca2+channel
Answer is D. Ca2+ plays a prominent role in the initiation of contraction of smooth muscle. 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, efflux from intracellular stores through the inositol trisphosphate receptor (IP3R) Ca2+channel, or a combination of these channels.