Ghrelin as a novel locally produced relaxing peptide of the iris sphincter and dilator muscles

Abstract

Ghrelin is a recently described acylated peptide, which works as a somatosecretagogue and has described effects on the smooth, skeletal and cardiac muscle. We examined the production and effects of ghrelin on relaxation of the iris muscles. Contractile effects of 1–5 human ghrelin (frGhr, 10 −9−6 × 10 −5 M) and 1–5 human des-octanoyl-ghrelin (d-frGhr; 10 −9−6 × 10 −5 M) were tested on iris rabbit sphincter ( n = 11 frGhr; n = 7 d-frGhr), dilator ( n = 6 frGhr; n = 6 d-frGhr) and rat sphincter ( n = 6 frGhr; n = 8 d-frGhr) precontracted muscles. On rabbit sphincter the effect of frGhr was also tested in presence of: i) L-NA (10 −5 M; n = 7); ii) indomethacin (10 −5 M; n = 7); iii) DLys 3GHRP6 (10 −4 M; n = 6); and iv) apamin + carybdotoxin (10 −6 M; n = 6). Furthermore, on rabbit dilator the effect of frGhr was tested in presence of DLys 3GHRP6 (10 −4 M; n = 7). Finally, ghrelin mRNA production was assessed by “ in situ” hybridization in Wistar rat eyes ( n = 8). In all muscles, frGhr promoted a concentration-dependent relaxation, maximal at 6 × 10 −5 M, 1.5–3 min after its addition, decreasing tension by 34.1 ± 12.1%, 25.8 ± 4.8% and 52.1 ± 10.3% in the rabbit sphincter, dilator and rat sphincter, respectively. In the rabbit sphincter the relaxing effects of frGhr were: (i) enhanced in presence of DLys 3GHRP6 (118.1 ± 21.1%); (ii) blunted by indomethacin; and (iii) not altered by apamin + carybdotoxin (36.4 ± 14.4%) or L-NA (52.4 ± 11.4%). Relaxing effects of d-frGhr in rabbit (43.3 ± 5.2%) and rat (77.1 ± 15.3%) sphincter muscles were similar to those of frGhr. In rabbit dilator muscle, d-frGhr did not significantly alter active tension and the relaxing effect of frGhr was blunted by GHSR-1a blockage. Ghrelin mRNA was identified in iris posterior epithelium. In conclusion, ghrelin is a novel, locally produced, relaxing agent of iris dilator and sphincter muscles, an effect that is mediated by GHSR-1a in the former, but not in the latter. Furthermore, in the sphincter it seems to be mediated by prostaglandins, but not by NO or K Ca channels.