INSULIN-LIKE GROWTH-FACTOR-I STIMULATES APICAL SODIUM HYDROGEN EXCHANGE IN HUMAN PROXIMAL TUBULE CELLS/

To determine whether insulin-like growth factor I (IGF-I) stimulated a pical sodium/hydrogen exchange (NHE), confluent primary human proximal tubule cells (PTC) were incubated for 48 h in serum-free media in the presence or absence of 100 ng/ml IGF-I. Cells incubated in IGF-I demo nstrated significant increases in thymidine incorporation (181.2 +/- 3 0.3% of control values; n = 12, P = 0.01) and in resting intracellular pH (pH(i)) (7.52 +/- 0.08 vs. 7.30 +/- 0.06; n = 20, P < 0.05), as de termined by 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein quantitati ve microspectrofluorometry. Following intracellular acid loading, ethy lisopropylamiloride (EIPA)-inhibitable H+ efflux and Na-22(+) influx a fter 1 min were both significantly enhanced in IGF-I-treated cells com pared with controls (8.78 +/- 1.69 vs. 3.03 +/- 0.72 mM/min and 3.47 /- 0.49 vs. 1.55 +/- 0.35 nmol mg protein(-1).min(-1), respectively). Na-22(+) uptake studies in PTC grown on permeable supports demonstrate d preferential stimulation of apical vs. basolateral NHE. The 50% inhi bitory concentrations (IC50) in IGF-I-treated and, control cells for E IPA (0.5 and 1.1 mu M, respectively) and for HOE-694 (4.0 and 10.0 mu M, respectively) were also consistent with predominant activation of a pical, rather than basolateral, NHE activity. Kinetic analysis reveale d an increase in maximal transport velocity (V-max, 15.50 +/- 1.50 vs. 7.26 +/- 3.07 mM/min; n = 10, P < 0.05), without a significant change in antiporter affinity for extracellular Na+. Incubation of PTC with 100 ng/ml IGF-I produced an acute, reversible, and ETPA-inhibitable pH (i) increase of 0.05 +/- 0.01 pH units (n = 5, P < 0.05). The results suggest that IGF-I may contribute to the metachronous stimulation of a pical NHE and PTC growth observed in many physiological and pathologic al conditions involving the human kidney.