Vasodilatory mechanism of levobunolol on vascular smooth muscle cells

Topical application of levobunolol hydrochloride, a beta-adrenergic antagonist used for treatment of glaucoma, is reported to increase ocular blood flow. We studied the mechanism of levobunolol-induced vasodilation in arterial smooth muscle. The effects of levobunolol or Other agents on isolated, pre-contracted rabbit ciliary artery were investigated using an isometric tension recording method. The effects of the same agents on intracellular free calcium ([CA(2+)](i)) in cultured human aortic smooth muscle cells were also studied by fluorophotometry. Levobunolol relaxed ciliary artery rings that were pre-contracted with either high-K solution, 1microM histamine, 10microM phenylephrine, or 100nM endothelin-1. The relaxation induced by levobunolol was concentration-dependent over the range of 10microM to 0.3mM. Inhibition of endothelial nitric oxide synthase or denudation of the endothelium did not affect this relaxation. Histamine-induced contractions were inhibited by the histamine H(1) antagonist pyrilamine. Radioligand binding experiments showed that levobunolol did not bind to the H(1) receptor. Further, histamine induced transient contraction in CA(2+)-free solution, and levobunolol inhibited this contraction by 74.6+/-11.0%. In cultured smooth muscle cells in the presence of extracellular CA(2+), levobunolol significantly inhibited the histamine-induced elevation of [CA(2+)](i). However, it did not inhibit the increase of [CA(2+)](i) in histamine-stimulated cells incubated in CA(2+)-free solution. These results indicate that levobunolol may relax rabbit ciliary artery by two different mechanisms. First, the relaxation could be due to the blockade of CA(2+) entry through non-voltage-dependent CA(2+) channels. Second, levobunolol may change the CA(2+) sensitivity of vascular smooth muscle cells.