The characteristics of melanocyte stimulating hormone release from incubated pituitaries of the lizard, Anolis carolinensis

Abstract

A procedure for studying melanocyte stimulating hormone (MSH) release from incubated hemipituitaries of the lizard, Anolis carolinensis, is described. Hemipituitaries incubated in vitro at 30° for 5 hr, released bioassayable MSH into the incubation medium; an initial phase of rapid release lasted .5-1 hr, followed by a lower rate of release during the remainder of the incubation. There was no evidence that MSH released from hemipituitaries and accumulated in the media influenced subsequent MSH release during the incubations. Release of MSH was dependent on the presence of Ca 2+ in the medium. Increase of medium K + to 56 m M did not alter the rate of MSH release at any stage of the incubation. The explanted lizard pars intermedia responded to low temperature in an unusual way. Tissue incubated at 15° released MSH at a lower rate than at 30°, but tissue incubated at 1–2° released MSH at a rate indistinguishable from that of tissue at 30°. MSH release at 1–2° was prevented by omitting Ca 2+ from the medium, suggesting that MSH release in the cold was not the result of cell damage. When tissue was incubated at 1–2° during the first half-hour of incubation and allowed to rewarm to 30° there was no detectable increase in the rate of MSH release; when the same tissue was recooled to 1–2° during the fourth hour and rewarmed during the fifth, a greatly increased release of MSH resulted, indicating that during the initial period of release, but not at later times, the tissue was resistant to the effect of cooling on hormone release. Theophylline (8 m M) significantly increased MSH release at all stages of the incubation except during the first half-hour, suggesting that MSH release may be stimulated by raised intracellular cyclic AMP levels. Imidazole (10 m M), a cyclic AMP phosphodiesterase stimulator, significantly reduced MSH release by 82% during the first half-hour of incubation and again at 1–3 hr by 44%, but had no significant effect on MSH release at other times. These results may indicate that “basal” release at the earliest stage is linked to the adenyl cyclase-cyclic AMP mechanism, but it is not certain that MSH release is tightly linked to cyclic AMP production at later stages of the incubation.