Final answer:
Calcium regulates muscle actions by binding to troponin, which causes a shift in tropomyosin, allowing actin and myosin interaction for muscle contraction.
Step-by-step explanation:
The substance that regulates muscle actions among the options provided is calcium. Calcium ions (Ca2+) play a crucial role in the process of muscle contraction. They are released from the sarcoplasmic reticulum and bind to one of the three troponin molecules, which, along with tropomyosin, help to regulate skeletal muscle contraction. The troponin and tropomyosin proteins work together to control the interaction between the contractile proteins actin and myosin. In the resting state of a muscle, tropomyosin blocks the myosin binding sites on actin filaments, preventing muscle contraction. When calcium ions bind to troponin, this induces a conformational change that moves tropomyosin away from the binding sites, allowing the actin and myosin to interact and the muscle to contract.