Synthesis, storage, and release of MSH in the pars intermedia of the pituitary gland of Xenopus laevis during background adaptation

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Abstract
Pituitary levels of melanophore-stimulating hormone (MSH), release of MSH, and protein synthetic activity in the pars intermedia were determined in Xenopus laevis during background adaptation. MSH was measured using a radioimmunoassay to α-MSH; uptake of [3H]lysine, determined autoradiographically, was used to assess protein synthesis; changes in melanophore index indicated changes in release of MSH. Adaptation to black background led to eventual depletion of MSH and increased protein synthetic activity. Conversely, during adaptation to a white background MSH levels increased and protein synthesis decreased. Changes in synthesis lagged considerably behind changes in release. During the initial stage of black-background adaptation, release of MSH was not accompanied by simultaneous decrease in levels of MSH in the gland from which it is concluded that replenishment of MSH took place. Our results indicated that this replenishment in the gland could not be accounted for by de novo synthesis of the hormone. It is proposed that a stored precursor to MSH exists, conversion of which provides for rapid replenishment of MSH. It is suggested that the factor(s) controlling MSH release affect synthesis of this hormone only indirectly.