Nitric Oxide in the Iris Sphincter Muscle
The aim of this chapter is to describe the effect of nitric oxide (NO) on the iris sphincter muscle. NO is a labile free radical, and it has a role as a neurotransmitter as well (Moncada et al. 1991; Grozdanovic et al. 1994). Studies have demonstrated that NO relaxes various kinds of smooth muscles, including those of the respiratory (Sekizawa et al. 1993), digestive (Stark and Szurszewski 1992; Wiklund et al. 1993), genitourinary (Persson and Andersson 1992; Ehren et al. 1994), and vascular (Moncada et al. 1991) systems. There are few reports in terms of the effect of NO on the iris sphincter muscle. We have already clarified the effect of NO on the rabbit iris sphincter muscle for the first time (Chuman et al. 1996, 1997); here we summarize the effect of NO on the iris sphincter muscle (Fig. 1). We noted in rabbits that, the increased accumulation of cyclic guanosine monophosphate (cGMP) induced by sodium nitroprusside (SNP) (an NO donor) in the iris sphincter muscle inhibits cholinergic muscular contraction but does not affect tachykinergic muscular contraction. These results suggest that the different effects on cGMP are essential for the different responses to NO in cholinergic and tachykinergic muscular contractions. Furthermore, this NO-cGMP pathway is operative in vivo for the modulation of iris sphincter muscle tone, at least when the eyes are infected with bacteria.
KeywordsNitric Oxide Sodium Nitroprusside Myosin Light Chain NADPH Diaphorase cGMP Content
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