PHYSIOLOGICAL-CHANGES IN EXTRACELLULAR-SODIUM DIRECTLY CONTROL HUMAN PROXIMAL TUBULE GROWTH AND TRANSPORT

In order to examine the nature and potential mechanisms of action of e xtracellular sodium on human proximal tubule growth and transport, qui escent primary cultures of human proximal tubule cells (PTC) were incu bated for 24 h in serum-free, growth-factor-free culture media contain ing low (130 mmol/l), control (140 mmol/l) or high (150 mmol/l) Na+. C ompared to control conditions, cells exposed to a high Na+ concentrati on demonstrated stimulated thymidine incorporation (121.8 +/- 7.6%, P < 0.05) and increased cellular protein content (139.7 +/- 9.9%, P < 0. 05); the latter arising from suppressed protein degradation ([H-3]vali ne release 72.3 +/- 2.5%, P < 0.01) and unchanged protein synthesis ([ H-3]valine incorporation 98.5 +/- 2.6%, P > 0.1). Substitution of chol ine chloride for NaCl did not replicate these effects. Conversely, cel ls incubated in low-Na+ media showed reduced thymidine incorporation ( 77.2 +/- 4.4%, P < 0.05), reduced protein synthesis (60.6 +/- 4.3%, P < 0.01), reduced protein degradation (79.5 +/- 1.8%, P < 0.01) and an unaltered protein content (102.4 +/- 8.8%). A role for apical Na+/H+ e xchange (NHE) activity in mediating Na+-dependent alterations in PTC g rowth was suggested by the findings of increased apical, ethylisopropy lamiloride-(EIPA)-sensitive Na-22(+) uptake in the presence of a high Na+ concentration (159 +/- 19% of control, P < 0.05) and concentration -dependent inhibition of cellular growth by EIPA at levels correspondi ng to those producing inhibition of apical NHE. Conditioned media from low Na+, control or high Na+ PTC contained comparable amounts of plat elet-derived growth factor-AB (1.19 +/- 0.23, 1.14 +/- 0.22 and 1.28 /- 0.20 ng/mg protein, P > 0.1) and transforming growth factor-beta(1) (1.76 +/- 0.32, 1.73 +/- 0.33 and 1.45 +/- 0.28 ng/mg protein, P > 0. 1), and did not exhibit autocrine growth factor activity on separate P TC following adjustment of Na+ concentrations to 140 mmol/l by dialysi s. Similarly, low-Na+ control or high-Na+ media did not modify the mit ogenic responsiveness of PTC to insulin-like growth factor-I (IGF-I) o r alter the affinity or number of PTC IGF-I binding sites. The results confirm that physiological increases in extracellular Na+ concentrati on directly stimulate human proximal tubule growth and Na+ transport. Such stimulation does not appear to be mediated by altered PTC secreti on of, or responsiveness to, cytokines known to affect tubule growth a nd transport.