An apparatus for measuring plasma electron density precisely measures electron density in plasma even under a low electron density condition or high pressure condition. This system includes a memory controller which is coupled to a set of memory chips, wherein each memory chip includes a phase detector configured to calibrate a phase relationship between a data-strobe signal and a clock signal received at the memory chip from the memory controller during a write operation. The invention further relates to methods of using such dog P-glycoprotein nucleic acids and polypeptides, especially in methods for determining bioavailability of drugs and for screening for inhibitors of dog PGP.