Cytoplasmic pH in the action of insulin-like growth factor-I in cultured porcine thyroid cells.

The effects of insulin-like growth factor-I (IGF-I) on cytoplasmic pH (pHi) and [3H]thymidine incorporation were studied in primary cultures of porcine thyroid cells. IGF-I alkalinized thyroid cells and stimulated thymidine incorporation in a dose-dependent manner; the effects of IGF-I on alkalinization (the maximal rates of change of cytoplasmic pHi/min ((dpHi/dt)max)) and thymidine incorporation were observed at 2 ng/ml and were maximal at 100 ng/ml, with half-maximal stimulation at approximately 10 ng/ml. The results indicate that Na+/H+ exchange or cell alkalinization may function as a transmembrane signal transducer in the action of IGF-I on thyroid cell proliferation. Several mitogens and comitogens which activate sodium hydrogen exchange, including epidermal growth factor, platelet-derived growth factor and nerve growth factor, have been listed. Activation with IGF-I has not, however, been presented before. Thus the present study constitutes the first demonstration of IGF-I-stimulated activation of Na+/H+ exchange or cell alkalinization.