FULL PAPER Pharmacology Effects of Various Selective Phosphodiesterase Inhibitors on Relaxation and Cyclic Nucleotide Contents in Porcine Iris Sphincter Takuya YOGO1), Takeharu KANEDA2), Yoshinori NEZU1), Yasuji HARADA1), Yasushi HARA1), Masahiro TAGAWA1), Norimoto URAKAWA2) and Kazumasa SHIMIZU2) 1)Laboratories of Veterinary Surgery and 2)Veterinary Pharmacology, Nippon Veterinary and Life Science University, 7–1 Kyonan-cho 1– chome, Musashino, Tokyo 180–8602, Japan (Received 26 March 2009/Accepted 1 July 2009) ABSTRACT. The effects of various selective phosphodiesterase (PDE) inhibitors on muscle contractility and cyclic nucleotide contents in porcine iris sphincter were investigated. Forskolin and sodium nitroprusside inhibited carbachol (CCh)-induced contraction in a concen- tration-dependent manner. Various selective PDE inhibitors, vinpocetine (type 1), erythro -9-(2-hydroxy-3-nonyl)adenine (EHNA, type 2), milrinone (type 3), Ro20–1724 (type 4) and zaprinast (type 5), also inhibited CCh-induced contraction in a concentration-dependent manner. The rank order of potency of IC50 was zaprinast > Ro20–1724 > EHNA milrinone > vinpocetine. In the presence of CCh (0.3 M), vinpocetine, milrinone and Ro20–1724 increased cAMP, but not cGMP, contents. In contrast, zaprinast and EHNA both increased cGMP, but not cAMP, contents. This indicates that vinpocetine-, milrinone- and Ro20–1724-induced relaxation is correlated with cAMP, while EHNA- and zaprinast- induced relaxation is correlated with cGMP in porcine iris sphincter. KEY WORDS: cAMP, cGMP, iris sphincter, PDE inhibitor, smooth muscle. J. Vet. Med. Sci. 71(11): 1449–1453, 2009 Cyclic nucleotides are important secondary messengers, chol (CCh)-induced contraction of porcine iris sphincter and are associated with smooth muscle relaxation [7]. induced by selective PDE (type1–5) inhibitors. Cyclic nucleotides are synthesized by adenyl cyclase or gua- nylyl cyclase and degraded by phosphodiesterases (PDEs). MATERIALS AND METHODS In the smooth muscles of the iris-ciliary body, it has been suggested that -adrenoceptor agonists [22, 24] and Muscle preparations and tension measurement: Eyes adrenomedullin [26] increase cAMP accumulation by stim- from adult pigs of either sex were obtained from a local ulating adenyl cyclase, and that atrial natriuretic peptide [9] abattoir. Two strips of iris sphincter muscle were cut from and nitric oxide [16] increase cGMP accumulation by stim- each eye (ciliary margin removed). The muscle strips were ulating guanylyl cyclase. incubated with physiological salt solution (PSS) containing Currently, PDEs are classified into 11 families [3, 10, 27] (in mM): NaCl, 136.8; KCl, 5.4; CaCl2, 2.5; MgCl2, 1.0; and selective PDE inhibitors have been identified [4]. It has NaHCO3, 11.9; and glucose, 5.6. PSS was aerated with 95% been reported that M&B 22948 (zaprinast, type5 inhibitor) O2 and 5% CO2 to adjust pH to 7.2 at 37C. Muscle tension inhibited the carbachol-induced contraction in bovine iris was recorded isometrically. One end of each strip was sphincter muscle [2]. However, to our knowledge, there bound to a glass holder and the other end was connected by have been no reports that compared the potency of maximal a silk thread to a strain-gauge transducer (TB-611T; Nihon relaxation of various selective PDE inhibitors-induced- Kohden, Tokyo, Japan) in an organ bath containing PSS relaxation in iris smooth muscle. with a resting tension of 0.5 g. Muscle strips were equili- The angle closure glaucoma is a main pathologic condi- brated for 30 min in order to obtain stable contractility tion with the elevation in intraocular pressure because of iri- induced by hyperosmotic 65 mM KCl. docorneal angle obstraction. When iridocorneal angle Assay of cGMP and cAMP contents: The cGMP and obstraction, the play of iris and ciliary muscle is also cAMP contents of muscle strips were measured by enzyme changed. To study the influence of PDE (type1–5) inhibi- immunoassay. After incubation of the strips with vinpocet- tors in the iris and ciliary muscle contraction may not only ine, EHNA, milrinone, Ro20–1724 or zaprinast for 10 min clarify the mechanism of cAMP or cGMP with PDE in the presence of CCh (0.3 M), the strips were rapidly fro- isozyme, but also contributes to the development of novel zen in liquid nitrogen and stored at –80C until homogeniza- antiglaucoma drug. tion in 6% trichloroacetic acid (0.4 ml). Homogenates were In the present study, we investigated the relationship centrifuged at 3000 g for 15 min and the supernatant was between relaxation and cyclic nucleotide contents in carba- washed with 1.5 ml of water-saturated diethylether 4 times; the cGMP and cAMP contents of the strips were then *CORRESPONDENCE TO: YOGO, T., Laboratory of Veterinary Surgery (e-mail: [email protected]), KANEDA, T., Laboratory of Veteri- assayed using enzyme immunoassay kits (GE Healthcare, nary Pharmacology, (e-mail: [email protected]), Nippon Vet- Buckinghamshire, UK). The cGMP and cAMP contents erinary and Life Science University, 7–1 Kyonan-cho 1-chome, were expressed in terms of picomole per gram wet weight of Musashino, Tokyo 180–8602, Japan. 1450 T. YOGO ET AL. tissue. Chemicals: The chemicals used were forskolin, sodium nitroprusside (SNP), milrinone, zaprinast, CCh (Sigma, St. Louis, MO, U.S.A.), vinpocetine, erythro-9-(2-hydroxy-3- nonyl)adenine•HCl (EHNA) (BIOMOL Research Laborato- ries, Plymouth Meeting, PA, U.S.A.) and Ro20–1724 (LC Laboratories, New Boston, MA, U.S.A.). Vinpocetine crys- tallizes if the concentration exceeds 30 M in PSS; there- fore, we used vinpocetine at below 30 M in the present experiments. Statistics: Concentration-response curve for each inhibi- tor was fitted to a logistic equation using MS Excel; the fit- ting parameter was IC50. Values are expressed as means S.E.M. and statistical analyses were performed by Student’s t-test. RESULTS Fig. 1. Effects of forskolin and sodium nitroprusside on the Effects of forskolin, and SNP on CCh-induced contrac- contraction induced by 0.3 M carbachol (CCh) in porcine iris tion: When the contractile response induced by 0.3 M CCh sphincter. Preparations were precontracted with CCh, and the reached a steady level at about 15–20 min after addition, indicated agents were added cumulatively. The maximum con- forskolin, an adenyl cyclase activator, or SNP, a soluble traction induced by 0.3 M CCh in the absence of these agents was taken as 100%. Each point represents the mean of 6–7 guanylyl cyclase activator, was applied cumulatively. The preparations. Vertical bars indicate S.E.M. threshold concentrations of forskolin and SNP to induce the relaxation of the iris sphincter were 0.1 M and 0.03 M, respectively, and both inhibited CCh-induced contraction in a concentration-dependent manner (Fig. 1). The concentra- tions of forskolin and SNP producing 50% relaxation (IC50) of CCh-induced contraction were 0.68 (0.51–0.89) or 0.053 (0.045–0.061) M, respectively. Based on IC50 and maxi- mum inhibition, SNP is more potent than forskolin in the porcine iris sphincter (P<0.01). Effects of various selective PDE inhibitors on CCh- induced contraction: When the contractile response induced by 0.3 M CCh reached a steady level at about 15–20 min after application, each PDE inhibitor was added cumula- tively. Vinpocetine (type 1), EHNA (type 2), milrinone (type 3), Ro20–1724 (type 4) and zaprinast (type 5) inhib- ited CCh-induced contraction in a concentration-dependent manner (Fig. 2). The IC50 and maximum relaxation concen- trations of these inhibitors are shown in Table 1. The rank orders for the relaxant effects of IC50 were zap- Fig. 2. Effects of vinpocetine, EHNA, milrinone, Ro20–1724 rinast (type 5) > Ro20–1724 (type 4) > EHNA (type 2) and zaprinast on the contraction induced by 0.3 M CCh in milrinone (type 3) > vinpocetine (type 1) (P<0.05). porcine iris sphincter. Preparations were precontracted with Effects of various PDE inhibitors on cGMP and cAMP CCh and the indicated agents were added cumulatively. The contents: In the presence of CCh (0.3 M), vinpocetine, mil- maximum contraction induced by 0.3 M CCh in the absence rinone and Ro20–1724 increased cAMP contents. With of these agents was taken as 100%. Each point represents the these drugs, cAMP contents increased by 1.47 (vinpocet- mean of 5–8 preparations. Vertical bars indicate S.E.M. ine), 1.6 (milrinone) and 1.5 (Ro20–1724) times above rest- ing levels. cAMP contents were not affected by EHNA and DISCUSSION zaprinast (Fig. 3A). In contrast, zaprinast and EHNA both increased cGMP contents by 1.57 (EHNA) and 2.15 (zapri- Physiological iris contractions are mediated by the nona- nast) times above resting levels. cGMP contents were not drenergic, noncholinergic nervous system in addition to the affected by vinpocetine, milrinone or Ro20–1724 (Fig. 3B). adrenergic, cholinergic nervous system. The stimulators of adenyl cyclase, -adrenoceptor agonists [22, 24] and adrenomedullin [26], induce relaxation by increasing cAMP contents, and the stimulators of guanylyl cyclase, atrial PHOSPHODIESTERASE INHIBITORS IN IRIS SPHINCTER 1451 natriuretic peptide [9] and nitric oxide [16], induce relax- Table 1. IC50 and maximum relaxation concentrations for various selective PDE inhibitors in porcine iris sphincter treated with 0.3 ation via cGMP in the iris sphincter muscle. In the present M CCh study, forskolin and SNP inhibited the muscarinic agonist carbachol (CCh)-induced contraction in a concentration- Agent IC50 (M) Maximum relaxation (%) n dependent manner (Fig. 1). These results suggest that the Vinpocetine >30 43.9 4.6 5 cAMP/adenyl cyclase and cGMP/guanylyl cyclase path- EHNA 92.4 (89.4 –98.6) 53.9 1.6 6 ways are both involved in the relaxation of porcine iris Milrinone 99.4 (54.0 – >100) 52.8 4.0 8 Ro20–1724 58.3 (36.9 – 95.0) 58.4 3.4 6 sphincter muscle.