Which of the following is NOT a characteristic of the dihydropyridine receptor (DHPR) in skeletal muscle? a) It physically interacts with ryanodine receptors (RyR). b) It is located in the sarcolemma along T-tubules. c) it is essential for excitation-contraction coupling. d) its shape changes in response to depolarization. Once opened, it allows the transport of calcium from the extracellular fluid (ECF) to the sarcoplasm.