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

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) DLys3GHRP6 (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 DLys3GHRP6 (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 DLys3GHRP6 (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 KCa channels.