Final answer:
The main driver of cytosolic calcium increase in vascular smooth muscle is the influx of extracellular calcium ions through calcium channels in the sarcolemma.
Step-by-step explanation:
The main driver of cytosolic calcium increase in vascular smooth muscle is the influx of extracellular calcium ions (Ca²+) through calcium channels in the sarcolemma. These calcium channels open during the action potential along the sarcolemma, allowing calcium ions to enter the cell. This increase in cytosolic calcium concentration then activates various enzymes, including those with calmodulin or calmodulin-like subunits, leading to smooth muscle contraction.