Evidence against mass action direct feedback control of melanophore-stimulating hormone (MSH) release.

The results of the in vitro incubation of rat pituitaries and frog neurointermediate lobes under a variety of experimental conditions fail to support a recently proposed theory of mass action direct feedback inhibition of melanophorestinuilating hormone (MSH) secretion. Various methods of varying the MSH concentration in the incubation media were devised; all resulted in hormone secretion independent of ambient MSH concentrations. Release of MSH from either the rat pituitary or the frog neurointermediate lobe was not enhanced under conditions of increased incubation volume. The total amount of MSH secreted was directly related to the number of pituitaries present in the incubation flask. Addition of exogenous MSH to the incubation medium did not inhibit MSH release. The amount of MSH secreted was directly related to the incubation time and was not increased by changing pituitaries to fresh medium during a given incubation period. We do not know why our results differ from those of previous workers, howeve...