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  • Title: Renal Na-myo-inositol cotransporter mRNA expression in Xenopus oocytes: regulation by hypertonicity.
    Author: Kwon HM, Yamauchi A, Uchida S, Robey RB, Garcia-Perez A, Burg MB, Handler JS.
    Journal: Am J Physiol; 1991 Feb; 260(2 Pt 2):F258-63. PubMed ID: 1996676.
    Abstract:
    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).
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