Transmembrane Na+/H+ exchange in the rat thyroid cell strain FRTL-5: a possible role in insulin-like growth factor-I-mediated proliferation.
Using the fluorescent pH indicator 2'7'-bis(2-carboxyethyl)-5'-(6')-carboxyfluorescein to monitor intracellular pH (pHi), we have investigated whether transmembrane Na+/H+ exchange, as measured by experimental changes in pHi under bicarbonate-free incubation conditions, may be involved in the early growth-promoting actions of insulin-like growth factor-I (IGF-I) on the rat thyroid cell stain FRTL-5. In initial studies to characterize Na+/H+ exchange in FRTL-5 cell suspensions, the recovery of a resting pHi in acid-loaded cells was shown to be dependent upon the presence of extracellular Na+, was enhanced by the presence of the sodium ionophore monensin and was abolished by amiloride, an antagonist of Na+/H+ antiport activity. Unlike TSH, which was without effect on the pHi of FRTL-5 cells for up to 15 min after addition, IGF-I (1000 micrograms/l) caused a rapid and sustained increase within 3 min, which was abolished in medium in which Na+ had been replaced with an iso-osmotic level of choline chloride. The change in pHi in response to IGF-I was mimicked by phorbol 12-myristate 13-acetate (PMA; 100 nmol/l), an activator of thyroid cell proliferation. In the presence of TSH, exposure of cells to IGF-I or PMA had no additional effect on the cytoplasmic alkalinization induced by either of these two agonists alone. However, blockade of transmembrane Na+/H+ exchange with amiloride inhibited both the individual actions of IGF-I and PMA on [methyl-3H]thymidine incorporation, and the synergistic interaction between TSH and IGF-I.(ABSTRACT TRUNCATED AT 250 WORDS)
Woods
D J
DJ
Department of Medicine, University of Manchester.
Soden
J
J
Tomlinson
S
S
Bidey
S P
SP
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002455:Cell Division; D002460:Cell Line; D002794:Choline / Q000378:metabolism; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007474:Ion Exchange; D008985:Monensin / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers; D013002:Somatomedins / Q000494:pharmacology*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013936:Thymidine / Q000378:metabolism; D013961:Thyroid Gland / Q000166:cytology / Q000378:metabolism*; D013972:Thyrotropin / Q000494:pharmacology
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D006859:Hydrogen; D013972:Thyrotropin; D008985:Monensin; D012964:Sodium; D002794:Choline; D013755:Tetradecanoylphorbol Acetate; D013936:Thymidine