The effect of catecholamines and background color on the in vitro incorporation of labeled amino acids into the neurointermediate lobe of the pituitary in Xenopus laevis

Abstract

The incorporation in vitro of labeled phenylalanine and leucine into TCA-precipitable material of the neurointermediate lobe of the pituitary in Xenopus has been studied. In the method employed, TCA-precipitable material was filtered onto glass fiber filter disks, and it was found that in addition to incorporation into TCA precipitates, incorporation into at least 50% of the newly synthesized MSH was probably being determined. Although there was little difference between the rates of leucine and phenylalanine incorporation into neurointermediate lobes from pale toads, the rate of incorporation of phenylalanine into lobes of darkened toads was twice as great as the rate of incorporation of leucine: incorporation of phenylalanine into lobes of darkened toads was 3.2 times the rate into lobes from pale toads, and the incorporation of leucine 1.6 times the rate of incorporation into lobes of pale toads. This pattern of incorporation may be related to rapid synthesis of MSH by the pars intermedia of darkened toads and the absence of leucine from any known MSH. Studies were also undertaken into the effect of catecholamines on phenylalanine and leucine incorporation. Especially at low concentrations, catecholamines had an inhibitory effect on phenylalanine incorporation, so that phenylalanine incorporation into neurointermediate lobes from darkened toads was inhibited by 60% during a 1-hr exposure to norepinephrine at 0.01 μg/ml, but were without a similar action on leucine incorporation. These results suggest a role for catecholamines in the regulation of MSH synthesis in the pars intermedia.