Purpose: To test the proposal that inhibiting monocarboxylate transport in the rat retina results in altered retinal function measured using the electroretinogram (ERG) and to evaluate the efficacy of exogenous metabolic substrates to restore any functional deficit.
Methods: Full-field white-flash ERGs were measured after monocarboxylate transport inhibition with intravitreal injection of alpha-cyano-4-hydroxycinnamic acid (4-CIN, 10 mM), and functional recovery was assessed after the introduction of various exogenous metabolic substrates (10 mM): lactate, pyruvate, alpha-ketoglutarate, alanine, succinate, and glutamine. The efficacy of glutamine as a metabolic substrate was also considered in the presence of phosphate-activated glutaminase inhibition (6-diazo-5-oxo-norleucin, 10 mM) or aminotransferase inhibition (aminooxyacetic acid, 10 mM). Pyruvate and alanine recovery was also assessed after aminooxyacetic acid application.
Results: 4-CIN application resulted in an increased phototransduction amplitude but a mild reduction of gain. A greater reduction of postreceptoral b-wave and oscillatory potential amplitudes (80%) was observed, along with delayed implicit times (35 ms). Partial recovery of b-wave amplitudes was achieved with exogenous lactate (24%), pyruvate (27%), alpha-ketoglutarate (27%), alanine (25%), and succinate (26%), whereas glutamine provided 62% recovery. However, none of the substrates improved phototransduction gain. Both 6-diazo-5-oxo-norleucin and aminooxyacetic acid completely suppressed the glutamine-induced b-wave recovery. Aminooxyacetic acid also abolished the b-wave recovery from 4-CIN afforded by pyruvate and alanine.
Conclusions: The greater loss of the b-wave and oscillatory potentials may reflect preferential routing of amino acid carbon skeletons to oxidative metabolic pathways, which in turn reduces glutamate availability for neurotransmission between photoreceptors and ON-bipolar cells. The reduction in log S provides evidence that inhibition of monocarboxylate transport produced some metabolic dysfunction in the rat.