This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Source of calcium for smooth muscle contraction 
Select and explain. 
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
D: Combination of influx of Ca2+ through voltage - gated, ligand gated Ca2+channel
Answer: D: Combination of influx of Ca2+ through voltage - gated, ligand gated Ca2+channel 
Explanation: 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.