Renal Na-myo-inositol cotransporter mRNA expression in Xenopus oocytes: regulation by hypertonicity.
Canine renal cells in culture (MDCK cells) accumulate organic osmolytes, including myo-inositol (MI), in response to hypertonic stress. When medium tonicity is increased, intracellular concentration of MI rises because hypertonicity elicits increased uptake of MI via Na-MI cotransporter(s). To study the mechanism for this increase in cotransporter activity, poly(A)+ RNA isolated from MDCK cells maintained in hypertonic or isotonic medium was injected into Xenopus oocytes, and Na-dependent MI uptake was measured 3-5 days later. Poly(A)+ RNA from hypertonic cells induced clear expression of the cotransporter. In contrast, oocytes injected with poly(A)+ RNA isolated from MDCK cells maintained in isotonic medium exhibited cotransporter activity like oocytes injected with water. Upon size fractionation of RNA, peak activity appeared in a fraction that contained poly(A)+ RNA with median size of approximately 4 kilobases. Na-dependent MI uptake by poly(A)+ RNA-injected oocytes was inhibited by both phlorizin and phloretin. We suggest that hypertonicity-induced upregulation of the Na-MI cotransporter involves an increase in mRNA and synthesis of cotransporter protein(s).
Kwon
H M
HM
Department of Medicine, Johns Hopkins University School of Medicine, Baltimore 21205.
Yamauchi
A
A
Uchida
S
S
Robey
R B
RB
Garcia-Perez
A
A
Burg
M B
MB
Handler
J S
JS
D000818:Animals; D002460:Cell Line; D003470:Culture Media; D006982:Hypertonic Solutions / Q000494:pharmacology*; D007294:Inositol / Q000235:genetics*; D007668:Kidney / Q000166:cytology / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D014981:Xenopus / Q000378:metabolism*
D003470:Culture Media; D006982:Hypertonic Solutions; D012333:RNA, Messenger; D007294:Inositol