In vivo tyrosine hydroxylation in the diabetic rat retina: effect of tyrosine administration.

The in vivo rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation was measured in light-adapted retinas of normal and diabetic albino rats. DOPA accumulation was significantly below normal in diabetic retinas; tyrosine levels were also reduced. In vitro tyrosine hydroxylase activity was normal. The administration of tyrosine methylester (500 mg/kg i.p.) to diabetic animals significantly raised retinal tyrosine levels and enhanced DOPA accumulation. Reduced tyrosine levels may therefore contribute to the observed reduction in in vivo tyrosine hydroxylation rate in the diabetic retina.