Inhibition of Na+ influx and DNA synthesis in human fibroblasts and neuroblastoma-glioma hybrid cells by amiloride analogs.

Identification of a Na+ influx inhibitor that is significantly more potent than amiloride and devoid of the nonspecific effects of amiloride would be of great value in determining the validity of the hypothesis that mitogen-stimulated Na+ influx acts as a signal for induction of cell proliferation. In this study, we evaluated a number of amiloride analogs for potency of Na+ influx inhibition in human fibroblasts (HSWP). One analog, benzamil, was found to exhibit a 60-fold enhanced potency relative to amiloride. We also assessed the relative efficacies with which amiloride and benzamil inhibit Na+ influx and DNA synthesis in HSWP cells and neuroblastoma-glioma hybrid cells (NG108-15). Concentrations of benzamil required for 50% inhibition (ID50) of Na+ influx and DNA synthesis of HSWP cells are in excellent agreement (15 and 18 microM, respectively), an observation which, on the surface, is supportive of the hypothesis in question. Benzamil also inhibits Na+ influx of NG108-15 cells with an ID50 comparable to that for HSWP cells (18 microM) and suppresses DNA synthesis with a slightly higher ID50 (38 microM). Although the benzamil concentrations needed to inhibit cell growth and Na+ influx are in reasonable agreement, caution should be exercised in interpreting the effects of benzamil on cell growth with respect to the role of Na+ influx as we also observed that an analog of benzamil with a reduced ability to inhibit Na+ influx gave inhibition of DNA synthesis at concentrations which do not inhibit Na+ influx.