Final answer:
In vascular smooth muscle, ATP-dependent calcium pumps on the sarcoplasmic reticulum are responsible for actively transporting calcium ions into the SR for muscle relaxation, while diffusion allows calcium ions to exit the SR to trigger contraction.
Step-by-step explanation:
The transporters on the sarcoplasmic reticulum (SR) involved in calcium regulation in vascular smooth muscle are primarily the ATP-dependent calcium pumps. These pumps actively transport calcium ions into the SR, a process essential for muscle relaxation. During muscle contraction, calcium ions diffuse out of the SR and combine with calmodulin, which activates myosin light chain kinase leading to muscle contraction. Muscle contraction continues until these calcium pumps actively transport calcium ions back into the SR, thereby lowering calcium levels in the sarcoplasm and facilitating muscle relaxation. The movement of calcium ions into and out of the SR is a critical regulatory mechanism in smooth muscle function, including its role in maintaining vascular tone.