Glucagon receptor agonists and antagonists affect the growth of the chick eye: a role for glucagonergic regulation of emmetropization?
OBJECTIVE
In chicks, plus defocus retards eye growth, thickens the choroid, and activates glucagonergic amacrine cells, probably releasing glucagon. Glucagon receptor antagonists (expected to inhibit compensation to plus defocus) and agonists (expected to block myopia induction by form deprivation) were administered to eyes of chicks, to test the hypothesis that glucagon mediates the induction of changes in eye growth by plus defocus.


METHODS
Seven-day-old (P7) chick eyes were injected intravitreally with peptides at concentrations of approximately 10(-9) to 10(-5) M in 20 microL (injection volume). The glucagon-receptor antagonists [des-His(1),des- Phe(6),Glu(9)]-glucagon-NH(2) (des- Phe(6)-antagonist) and [des-His(1),Glu(9)]-glucagon-NH(2) (Phe(6)-antagonist) were administered daily for 4 to 5 days to plus-defocused eyes. Agonists (porcine glucagon-[1-29] and [Lys(17,18),Glu(21)]-glucagon-NH(2)) were monocularly administered daily for 5 days to form-deprived eyes. The contralateral eye remained open and received saline. After treatment, eyes were refracted, measured, and examined for histologic changes.


RESULTS
The Phe(6)-antagonist at 10(-5) M (in the syringe) inhibited changes in both refractive error and axial length compensation induced by +7-D lens wear; however, des-Phe(6)-antagonist (10(-5) M) had weak, inconsistent effects and did not antagonize the action of exogenous glucagon. Glucagon prevented ocular elongation and myopia and induced choroidal thickening in form-deprived eyes. [Lys(17,18),Glu(21)]-glucagon-NH(2) had little effect at 10(-7) M, but at 10(-6) to 10(-5) M altered rod structure and inhibited eye growth.


CONCLUSIONS
Exogenous glucagon inhibited the growth of form-deprived eyes, whereas Phe(6)-antagonist inhibited compensation to plus defocus, as might be expected if glucagon is an endogenous mediator of emmetropization. The reason for the failure of des-Phe(6)-antagonist to counteract the effects of exogenous glucagon requires further investigation.
Vessey
Kirstan A
KA
Department of Anatomy and Cell Biology, Neuroscience Research Group, University of Calgary Faculty of Medicine, Alberta, Canada.
Lencses
Kathy A
KA
Rushforth
David A
DA
Hruby
Victor J
VJ
Stell
William K
WK
D000818:Animals; D002645:Chickens; D002829:Choroid / Q000187:drug effects / Q000473:pathology; D004195:Disease Models, Animal; D005123:Eye / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D019084:Fluorescent Antibody Technique, Indirect; D005934:Glucagon / Q000008:administration & dosage* / Q000031:analogs & derivatives*; D007267:Injections; D008297:Male; D009216:Myopia / Q000378:metabolism / Q000503:physiopathology / Q000517:prevention & control*; D009799:Ocular Physiological Phenomena; D020419:Photoreceptor Cells, Vertebrate / Q000187:drug effects / Q000473:pathology; D018027:Receptors, Glucagon / Q000819:agonists* / Q000037:antagonists & inhibitors*; D012683:Sensory Deprivation; D014822:Vitreous Body
C506341:(des-His(1),des- Phe(6),Glu(9))-glucagon-NH(2); D018027:Receptors, Glucagon; C080071:glucagonamide, desHis(1)-Glu(9)-; D005934:Glucagon