AJP - Renal Physiology, Vol 260, Issue 2 258-F263, Copyright © 1991 by American Physiological Society
Renal Na-myo-inositol cotransporter mRNA expression in Xenopus oocytes: regulation by hypertonicity
H. M. Kwon, A. Yamauchi, S. Uchida, R. B. Robey, A. Garcia-Perez, M. B. Burg and J. S. Handler
Department of Medicine, Johns Hopkins University School of Medicine, Baltimore 21205.
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).
