Tryptamine in the vertebrate lens

The normal ionic balance and transparency of the lens is regulated by Na⁺-K⁺ pumps in the lens epithelium and lens fibers. Inhibition of the (Na⁺+K⁺)-ATPase and ion transport results in osmotic swelling and opacification. We have previously shown that 10^-4-10^-3 M tryptamine has an inhibitory effect on both ion transport and ATPase activity in the lens. In this paper we show that isolated lenses incubated in Ringer's solution containing tryptamine exhibit Na⁺ and K⁺ concentration reversal. Agents that affect tryptophan metabolic pathways, such as pargyline and p-chlorophenylalanine, were tested for their effect on lens tryptamine levels and Na⁺ and K⁺ concentrations. Toad lenses cultured 10 days in Medium-199 with tryptophan, p-chlorophenylalanine and pargyline, each at a concentration of 1 mM, had increased tryptamine concentrations relative to lenses incubated in control medium. The lenses with the larger increase in tryptamine content also showed a decrease in K⁺ concentration and an increase in Na⁺. The endogenous tryptamine levels in toad, frog, rabbit and cow lenses were measured, and varied from 0.065 μg/g in the rabbit to 0.219 μg/g in the cow. Tryptamine levels were also determined in senile cataractous and in non-cataractous human lenses. The lenses diagnosed as cataractous had a significantly higher tryptamine concentration, with the highest levels found in the cataractous lenses which were dark brown on inspection. Tryptophan, which has recently been shown to be elevated in the plasma of patients with cataracts, is actively transported into the lens where it may be decarboxylated to tryptamine. The accumulation of the amine may be a contributing factor in the development of senile cataracts.