Ghrelin: A Potent Relaxator Of Iris Sphincter Muscle. A Morphofunctional Study

Abstract

Abstract: : Purpose: Ghrelin is a 28 aa peptide with important functions in smooth, cardiac and skeletal muscle. Its ocular effects are unknown. The aim of this study was to investigate the production and effects of Ghrelin in the iris muscle. Methods: Rabbit iris sphincter muscles (n=31) were dissected and mounted on a vertical organ bath containing a modified Krebs–Ringer solution (1.8 mM Ca²⁺; 35ºC) and attached to a force transducer. The effects of 1–5,Ghrelin (frGr) were studied in different conditions, always after the muscle being pre–contracted with Charbacol (0.1 µM). The effects of a single doses (60 µM; n=4) were studied during 20 minutes. The effects of increasing doses of frGr were in the absence (n=7) or in the presence of: i) L–NA (60 µM; n=7); ii) Indomethacin (1 µM; n=7); iii) D–Lys GHRP 6, a selective inhibitor of the Ghrelin receptor (GHS–R) (100 µM; n=6). The other eye muscle was used as control. Only significant results (mean ±SE, p<0.05) are given, and expressed as % changes from control. "In situ" hybridisation for Ghrelin mRNA was performed in preparations of the anterior segment of rats eye. Results: When compared to control, frGr promoted a 34.1±12.1 % decrease in isometric tension 1.5 to 3’ after its addition to the bathing solution, for doses higher than 10^–6M. This effect disappeared at 10’ (2.33±6.8 %) and was not affected by the presence of L–NA or Indomethacin. The inhibition of GHS–R increases and prolongs the relaxation elicited by frGr (118.1±21.1 % vs 34.1±12.1 % at 1,5 to 3’ and 76.6±19.8 % vs 2.33±6.8 % at 10’; p<0.05). The Ghrelin mRNA was identified in the posterior epithelium of the iris. Conclusion: Ghrelin is produced in the posterior epithelium of the iris and seams to have a regulatory role in the iris muscular kinetics. It has a potent and rapid relaxing effect of the iris sphincter muscle, independent of NO or Prostaglandins and increased by the GHS–R inhibition witch suggests that this is not the only receptor mediating its effects in smooth muscle tone.  

Keywords: iris • growth factors/growth factor receptors • pupil