Na⁺/H⁺ exchange and vascular smooth muscle proliferation

Abnormal growth of vascular smooth muscle (VSM) is seen in various pathologic conditions such as hypertension and atherosclerosis. Many classic vasoconstrictors have now been shown to be mitogenic, either by themselves or in conjunction with other cofactors, such as insulin. The mitogenic effects of vasoconstrictors may be due, in part, to activation of similar second messenger pathways, including stimulation of the Na⁺/H⁺ antiporter. It has been suggested, therefore, that an enhanced proliferation rate may be, in part, the consequence of elevated Na⁺/H⁺ exchange. This hypothesis is supported by several observations of the close association between Na⁺/H⁺ exchange activity and DNA synthesis in some cell types including fibroblasts and VSM. Stimulation of Na⁺/H⁺ exchange may play a permissive role in optimal growth by preventing H⁺ accumulation (a fall in intracellular pH [pHi]) due to the increased metabolic activity during cell stimulation. Enhancement of Na⁺/H⁺ exchange activity increases Na⁺ influx into the cell, and secondarily increases K⁺ entry through activation of Na⁺/K⁺ ATPase activity. Although the Na⁺/H⁺ antiporter may influence cell proliferation through various ionic mechanisms, it is not clear that enhanced proliferation is the consequence of overactivity of this antiporter. In VSM, there are also differences in the pattern of activation of the Na⁺/H⁺ antiporter by hyperplastic and hypertrophic agents. Although pHi is increased in response to both acute and chronic stimulation by hyperplastic factors, such as platelet-derived growth factor, a hypertrophic agonist such as angiotensin II increases pHi acutely but lowers it chronically. Likewise, hyperplastic factors increase the Na⁺/H⁺ antiporter (NHE-1) mRNA levels, whereas angiotensin II does not. Clearly, much remains to be learned about the role of the Na⁺/H⁺ antiporter in the regulation of normal and exaggerated growth of vascular smooth muscle.