Inhibition of ionic transport and ATPase activities by serotonin analogues in the isolated toad lens

The effects of serotonin and five other indoles were tested on the electrical parameters and ionic transport in the isolated toad lens. Serotonin, tryptophan and 5-hydroxy-L-tryptophan did not affect the electrical parameters of the lens at concentrations as high as 1 mM. Tryptamine, 5-methyltryptamine and 5-methoxytryptamine had dual effects: 1 mM in the posterior bathing solution depressed the potential difference of the posterior face of the lens, which resulted in an increase in the translenticular potential difference and short-circuit current; 1 mM in the anterior solution (in contact with the lens epithelium) produced a quick and pronounced reduction of the potential difference of the anterior face. This resulted in a 90-100% decline of the translenticular short-circuit current. Serotonin and tryptamine were then tested for their effect on the ATPases of lens epithelium. Both amines inhibited the enzymes with tryptamine at 5 mM completely inhibiting all ATPase activity. Since tryptophan is transported from the aqueous humor into the lens and may be converted by lens enzymes to serotonin and tryptamine, these findings may have physiological implications in cataractogenesis.