The purpose of the proposed studies is to establish an in vitro model of renal tubular hypertrophy and to examine whether a unifying mechanism can be found for the initiation of cell hypertrophy. Prior studies have suggested that a flow-induced increase in luminal Na-H antiport occurs in the proximal tubules of kidneys with reduced nephron populations and it is possible that this signal (either Na influx or intracellular alkalization) initiates the process of cell growth. Many mitogens stimulate cell proliferation by inducing an early increase in Na-H antiport. A primary culture of proximal tubular cells grown in serum-free hormone supplemented medium will be stimulated to hypertrophy by prolonged exposure to insulin, prostaglandin E1 and hypertonic sodium chloride. The existence of a luminal Na-H antiporter will be demonstrated by measuring Na influx kinetics and H+ efflux rates in intact cells. We will study the acute effects of the hypertrophic stimuli on these transport processes and the respective stimulatory and inhibitory effects of a transmembrane H+ gradient and amiloride, a competitive inhibitor, of the Na-H antiport. The steady-state rate of Na-H antiport will be studied in hypertrophied vs. non-hypertrophied cells and the ability of prolonged exposure to amiloride to inhibit the hypertrophic response will be evaluated. Our ability to induce hypertrophy without cell proliferation in this experimental model using a variety of completely different stimuli (i.e., hormonal and ionic) will make it possible to ascertain whether a single initiating event is common to the process of cell growth. If increased Na-H antiport is consistently found, it is likely that this could be the event which initiates hypertrophy of renal tubular cells in vivo.
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