Effects of Prostaglandin F2alpha (PGF2alpha) and Latanoprost on Phosphoinositide Turnover, Myosin Light Chain (MLC) Phosphorylation and Contraction in Cat Iris Sphincter

Abstract

Abstract: : Purpose: To investigate in isolated cat iris sphincter the effects of the ocular hypotensive agents, PGF_2α and latanoprost, on inositol phosphates production, MLC phosphorylation and contraction in order to gain information about the mechanism through which these pharmacological agents regulate smooth muscle function and lower IOP. Methods: [³H]myo-inositol phosphates production was measured by ion-exchange chromatography, MLC kinase activity was measured by incorporation of ³²Pi into MLC, and changes in muscle tension were recorded isometrically. Results: In general, we found that the PGs potently stimulated the biochemical and pharmacological responses in a dose-dependent manner, however, these responses are stimulated with a slightly lower concentrations of PGF_2α than latanoprost. These conclusions are supported by the following findings: (1) PGF_2α and latanoprost induced contraction in a dose-dependent manner with EC₅₀ values of 18.6 and 29.9 nM, respectively. (2) PGF_2α and latanoprost increased inositol phosphates production in a dose-dependent manner, at 1 µM they increased the responses by 125 and 102% over the basal, respectively. (3) PGF_2α and latanoprost increased MLC phosphorylation in a dose- and time-dependent manner, at 1 µM and 5 min incubation, they increased the responses by 171 to 181% and 153 to 176% over the basal, respectively. (4) Wortmannin, as well as ML-7 and ML-9, selective MLC kinase inhibitors, potently inhibited PGF_2α- and latanoprost-induced MLC phosphorylation and contraction in a dose-dependent manner. Conclusions: PGF_2α and latanoprost significantly stimulate inositol phosphates accumulation, MLC kinase activity and MLC phosphorylation, and contraction. The MLC kinase inhibitors, wortmannin, ML-7 and ML-9, which have been shown to relax the ocular smooth muscles and lower IOP, inhibited the PGs-induced responses. These findings suggest that regulation of MLC kinase activity in the smooth muscles of the anterior segment could be involved in the mechanism of the IOP-lowering effects of the PGs, and that phosphorylation-dephosphorylation of myosin could serve as a potential target for anti-glaucoma drugs.

Keywords: second messengers: pharmacology/physiology • phosphorylation • ciliary body