The effect of strenuous exercise and β-adrenergic blockade on the visual performance of juvenile rainbow trout, Oncorhynchus mykiss

Abstract.

It is hypothesised that the visual performance of rainbow trout, Oncorhynchus mykiss, will be impaired by strenuous exercise as a result of metabolic stress (blood lactacidosis) that activates the Root effect and limits the oxygen-carrying capacity of blood flowing to the eye. The ability to resolve high contrast objects on a moving background, as a measure of visual performance, was quantified pre- and post-exercise using the optomotor response. Strenuous exercise induced a metabolic acidosis (8.0 mmol l^–1 blood lactate) and a significant red cell swelling response but no change in the optomotor response threshold (120 min of arc) was observed. β-adrenergic blockade (propranolol) abolished post-exercise red cell swelling but optomotor response thresholds were still maintained at 120 min of arc despite a significant blood lactate load (7.8 mmol l^–1). The choroid rete mirabile of the trout is extremely well developed (rete area:eye area=0.39) and may maintain visual performance by ensuring a relatively direct supply of oxygen to the central regions of the avascular retina. Exercised fish under β-adrenergic blockade exhibited an enhanced optomotor response at 240–300 min of arc. Assuming that these responses reflect "tunnel vision", adrenergic regulation of red cell function may preserve a high ocular PO₂ gradient that satisfies the oxygen demand of peripheral retinal cells.