Antiproliferative effects of verapamil enantiomers on vascular smooth muscle cells

There is considerable interest in the role of Ca²⁺ in the genesis of atherosclerosis and vascular remodeling. Calcium channel blockers impede atherosclerosis in rodents and have been shown to attenuate vascular smooth muscle cell (VSMC) Ca²⁺ influx. Accordingly, we studied the roles of Ca²⁺, S- verapamil (an active calcium channel blocker) and R+ verapamil (an inactive channel blocker) on A7R5 VSMC ionic currents and proliferation. During the period of most rapid growth of control cultures (after the first 2-4 days of culture), low-Ca²⁺ (0.12 mM) medium significantly inhibited proliferation by 39.3% on day 6 (day 5 of low-Ca²⁺ medium), and inhibition of proliferation by low Ca²⁺ continued for up to another 6 days (47.9% inhibition after 11 days in low Ca²⁺). S- verapamil (5 μM), but not R+ verapamil, reduced Ba²⁺ current through L-type Ca²⁺ channels, and neither had effects on Ca²⁺ channels at lower concentrations. S- and R+ verapamil both inhibited VSMC proliferation significantly within 6 days after seeding, and significant inhibition continued to day 12. At 5 μM, a concentration at which S- reduces Ca²⁺ currents but R+ does not, significant decreases in proliferation by S- were 33.4% at day 6, 25.3% at day 8, 35.9% at day 10, and 40.5% at day 12; and by R+ were 35.2% at day 6, 27.6% at day 8, 38.4% at day 10, and 36.2% at day 12. In summary, low calcium and both the active and inactive verapamil enantiomers inhibit proliferation. Thus, although Ca²⁺ appears to be important in the proliferation response, the antiproliferative effect of verapamil on VSMC proliferation is independent of the calcium-channel-blocking effect of the drug.