(1) Function of P-glycoprotein in the proximal tubule
To clarify the functional significance of the P-gp in the proximal tubule, we perfused proximal straight tubule (PST) isolated from the control and the P-gp knockout mice (mdr1a/ab(-)(-)). The activity of the transporter was assessed by microfluorometry of rodamine uptake to the kidney epithelia. In the control animals, T1/2 of the rotamine uptake was 34 sec, and verapamil, a blocker of P-gp, increased the value to 434 sec. By contrast, in the KO mice, T1/2 was 407 sec without any treatments, and it was unchanged after verapamil. Repeated administration of digoxin to the KO mice caused to increase the concentration of the drug in the kidney tissue. Subsequent studies using a proteinkinase C (PKC) stimulant (PMA), PKC inhibitors (staurosporine, H-7), and PI3-kinase inhibitors (wortomannin, LY294002) revealed that the function of P-gp is mediated by processes requiring both PKC and PI3-kinase.
(2) Transition from avian to mammalian-type urine concentrating mechanisms during perinatal period
To study functional significance of anatomical changes in the ascending limb from thick segment to thin segment by apoptosis during perinel period, we perfused isotaled medullary nephron segments obtained from varying perinatal periods. Active transport in the medullary thick ascending lim observed in the fetal period rapidly changed to Cl selective passive transport observed in the adult type thin ascending limb. While urea transporter in the inner medullary collecting duct is lacking during fetal period, the vasopressin sensitive transporter becomes apparent soon after the birth. It is concluded that transition from avian to mammalia type kidney is essential for attaining high urine concentration.