Multiplication-stimulating activity-induced alkalinization of canine renal proximal tubular cells

The actions of a variety of polypeptide growth factors on isolated cells are thought to be initiated by stimulation of Na+-H+ exchange across the plasma membranes of the cells resulting in intracellular alkalinization. To determine whether insulin-like growth factors (IGFs) exert actions through such a mechanism, we incubated suspensions of canine renal proximal tubular segments with insulin or IGF I or with multiplication-stimulating activity (MSA)/IGF II. Changes in intracellular pH were detected by measurements of the distribution of [14C]5,5-dimethoxazolidine-2,4-dione. Incubation of segments with 10(-9) M MSA under conditions such that extracellular [Na+] greater than intracellular [Na+] effected intracellular alkalinization detectable within 1-2 min. Alkalinization was not observed under conditions where this gradient was not present. Alkalinization was not prevented by inclusion of 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid or 1 mM ouabain in incubations, but was inhibited by amiloride. Incubation of proximal tubular segments with as little as 10(-11) M MSA effected intracellular alkalinization. Incubation with as much as 10(-6) M insulin or IGF I did not. Our findings are consistent with an action of MSA/IGF II to stimulate Na+-H+ exchange across the plasma membrane of the renal proximal tubular cell. It is possible that the stimulation represents a mechanism by which actions of IGF II are initiated in growth factor-sensitive cells.