Final answer:
The calcium release channel opened by increases in phospholipase C activity in the smooth muscle sarcoplasmic reticulum is the inositol trisphosphate (IP₃) receptor channel, which releases Ca²⁺ ions into the cytosol.
Step-by-step explanation:
The calcium release channel in the smooth muscle sarcoplasmic reticulum that is opened by increases in phospholipase C activity is the inositol trisphosphate (IP₃) receptor channel. The mechanism starts with a signal that leads to the activation of the enzyme phospholipase C (PLC), which then leads to the production of IP₃. The IP₃ subsequently binds to its receptors on the sarcoplasmic reticulum, triggering the release of Ca²⁺ ions into the cytosol. This increase in cytosolic Ca²⁺ levels activates various enzymes—some via calmodulin or calmodulin-like subunits—and it is essential for processes such as muscle contraction.