Calcium Channels and Nifedipine Inhibition of Serotonin-Induced [ 3 H]Thymidine Incorporation in Cultured Cerebral Smooth Muscle Cells

Cultures of smooth muscle cells were prepared from the basilar artery of adult guinea pigs. Passaged cultures (10–30 passages) that expressed serotonin receptors were studied using [³H]thymidine incorporation. When tested in quiescent medium, serotonin potently stimulated [³H]thymidine incorporation (EC₅₀ of 31 nM) by as much as 400% at 24 h. The number of cells was not significantly increased at 24 or 48 h. At concentrations of 10⁻⁸–10⁻⁵ M 5-HT, [³H]thymidine uptake was reduced 40–50% by the dihydropyridine Ca²⁺ channel blocker, nifedipine (1 μM). To demonstrate a possible mechanism for the sensitivity to nifedipine, Ca²⁺ currents were measured using the whole cell patch clamp technique. The cells expressed dihydropyridine-sensitive L-type Ca²⁺ channels, but not other subtypes of Ca²⁺ channels, as indicated by the kinetic and voltage-dependent characteristics of the current and by the stimulatory effect of Bay K 8644. The magnitude of the Ca²⁺ currents was related exponentially to the membrane surface area, measured as cell capacitance. These data support the association of dihydropyridine-sensitive Ca²⁺ channels with mitogenesis in vascular smooth muscle, and suggest an alternate mechanism of action for the beneficial effect of dihydropyridines in prophylaxis of cerebral vasospasm.