Ionic requirements for melanophore-stimulating hormone (MSH) release

Abstract

Listen

Translate article

In vitro studies on ionic requirements for melanophore-stimulating hormone (MSH) release were carried out on a variety of vertebrate species. Frog ( Rana pipiens, Agalychnis dacnicolor), toad ( Bufo marinus), lizard ( Anolis carolinensis), mouse, and rat pituitaries release an MSH-like substance into the incubation medium. incubation of the separated pars distalis or pars intermedia of the frog or rat reveal that most MSH-like activity is confined to the intermediate lobe, suggesting that the hormone is MSH. Removal of the Ca 2+ ion from the medium results in an almost total inhibition of MSH release from the pituitary of each of the species studied. Similarly, removal of the K + ion reduces secretion to about half that of control values, except in the mouse and the toad ( B. marinus), where such an effect could not be demonstrated. High (5×) K + failed to increase the rate of MSH release in either normal or Ca 2+-deficient media. Long-term (4–8 hr) incubation of frog or rat pituitaries in a Ca 2+-deficient medium did not decrease the level of pituitary MSH over that of pituitaries incubated for similar lengths of time in normal media. Excess (10×) Ca 2+ failed to stimulate MSH release from frog or rat pituitaries over that released into normal media. Ouabain reversibly blocked the release of MSH from frog ( R. pipiens) and rat pituitaries. Ouabain failed to block MSH release from the mouse and the toad, B. marinus (the two species which were similarly unresponsive to the absence of K + from the medium). MSH release from the frog was inhibited in a Li +-Ringer or an isotonic sucrose medium. These results suggest that MSH release from pars intermedia cells may be regulated by a Na +K + pump (active transport) mechanism. Although Ca 2+ is ultimately necessary for MSH release (in all species studied), its role may be secondary to that of an initial action of MSH release-inhibiting factors on cellular permeability to the K + and/or Na + ion. The experimental evidence presently suggests that some species may not possess an active transport system linked to the Ca 2+ ion requirement for hormone release.