Synthetic tocinamide and tocinoic acid, and ring structures of oxytocin, reversibly inhibit (at nanogram, or less, concentrations) the release of melanophore stimulating hormone (MSH) from the rat and hamster pituitary in vitro. These peptides are less effective (on Bufo marinus and Rana catesbeiana) or totally without effect (on Rana pipiens) on MSH release from the isolated amphibian pars intermedia. Oxytocin, lysine vasopressin, and ring structures of the vasopressins (pressinamide and pressinoic acid) are without effect of MSH release in the animals studied. The synthetic tripeptide side chain of oxytocin (L Pro L Leu Gly NH2) is devoid of in vitro MSH release inhibition in either the mammal or the frog. Crude hypothalamic extracts from either the frog or the rat 'appear' to reversibly inhibit in vitro MSH release. If, however, the extracts are heated to boiling or acid extracted, inhibition of MSH release is not observed, suggesting enzymatic or other reactions may be responsible for the loss of bioassayable MSH. The addition of MSH to crude hypothalamic extracts results in a loss of hormone activity. It is, therefore, unclear whether hypothalamic extracts provide evidence for the existence of a hypothalamic MSH release inhibiting factor (MRIF). It remains for further work to establish whether the ring structures of the neurohypophyseal hormones can be considered as possible candidates for a natural vertebrate MRIF. Both calcium and potassium ions are necessary for MSH release in vitro. Release of the hormone is inhibited by ouabain and related cardiac glycosides suggesting that a Na+-K+ pump (active transport) system is involved in MSH release. Cytochalasin B, but not colchicine (or vinblastine and vincristine) is reversibly inhibitory to hormone release and suggests that a microfilament (rather than a microtubular) system may be involved in the mechanism of acute release of MSH. However, other actions of cytochalasin B on transport mechanisms or ionic fluxes may account for its inhibition of MSH release. The relationships between an active transport system, calcium ions, and a microfilament system in the control of MSH release by hypothalamic substances, neurohypophyseal peptides, or related structures are still unclarified.
|Original language||English (US)|
|Number of pages||15|
|Journal||Yale Journal of Biology and Medicine|
|State||Published - Dec 1 1973|
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)