Nitric oxide suppresses L-type calcium currents in basilar artery smooth muscle cells in rabbits

Objectives: Nitric oxide (NO) is well known to be a vasodilator, and NO donor compounds are currently used for treating vasospasm following subarachnoid hemorrhage. However, the action mechanism of cerebral vascular relaxation is not yet clear. L-type calcium channels have been determined to play an essential role in smooth muscle contraction. To investigate the role of L-type calcium channels in NO-induced relaxation of basilar smooth muscle cells, we examined the effect of the NO donor, sodium nitroprusside (SNP) on calcium (Ca²⁺) currents using smooth muscle cells isolated from a rabbit basilar artery. Method: The smooth muscle cells were isolated from rabbit basilar artery by enzyme treatment. To identify L-type Ca²⁺ currents, we used cesium chloride, a potassium channel blocker and Bay K8644, an activator of L-type Ca²⁺ channel. Results: The L-type calcium currents (91±13.0 pA; n=11) were significantly reduced by SNP (32±5 pA; n=11; P<0.05). 1H-[1,2,4] Oxadiazolo [4,3-a] quinoxalin-1-one, a 3′,5′-cyclic guanosine monophosphate inhibitor, blocked the effect of SNP on L-type Ca²⁺ currents, and similar results were obtained after the application of 7-nitroindazole, a specific NO synthase inhibitor. Furthermore, inward currents were enhanced by Bay K8644 (170±22 pA; n=5) and were suppressed by SNP (54±13 pA; n=5; P<0.05). Discussion: These results demonstrate that NO suppresses the L-type Ca²⁺ currents in rabbit basilar smooth muscle cells, and suggest that L-type Ca²⁺ channels may play a pivotal role in NO-induced vascular relaxation.