Na(+)-H+ exchange inhibitors decrease neointimal formation after rat carotid injury. Effects on smooth muscle cell migration and proliferation.

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Abstract
The presence of multiple growth stimuli at the sites of vascular injury following angioplasty suggests that therapies targeted toward common growth pathways will be more effective than therapies that inhibit only a single growth factor. We tested this hypothesis using amiloride and ethyl isopropyl amiloride (EIPA), which are inhibitors of the Na(+)-H+ exchanger, whose activity is required in many cells for proliferation and migration. In the rat carotid injury model, EIPA (100 micrograms/h for 15 days) significantly decreased intimal area and the ratio of intimal to medial area, whereas amiloride (25 micrograms/h) showed an inhibitory trend that was similar to that observed for captopril (80 mg/kg per day) and heparin (25 U/h). EIPA and amiloride inhibited rat vascular smooth muscle cell DNA synthesis, with IC50 values of 8.8 and 82.2 microM, respectively. Using platelet-derived growth factor as a chemoattractant, EIPA caused a concentration-dependent inhibition of migration (IC50, approximately 60 microM). Because amiloride and EIPA have nonspecific effects on cellular function (especially inhibition of tyrosine kinases), we sought to characterize the specific role of the Na(+)-H+ exchanger in vascular smooth muscle cell proliferation and migration. We generated a Na(+)-H+ exchanger-deficient mutant cell line [RNHE(-)]. Studies with these cells suggested that the inhibitory effects of EIPA and amiloride were mediated only in part via Na(+)-H+ exchange because (1) RNHE(-) cells grew well at pH 6.8 to 7.5 in bicarbonate-containing medium, and (2) there was no difference in migration in response to platelet-derived growth factor in the RHNE(-) cells. In summary, these data indicate that amiloride and EIPA inhibit neointimal formation in the rat carotid after injury. However, the mechanism of inhibition is likely to involve cellular events other than Na(+)-H+ exchange, such as an effect on tyrosine kinases.

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