A novel effect of salmon calcitonin on in vitro Ca-uptake by rat brain hypothalamus: the regional and hormonal specificities.

It was found that salmon calcitonin-I (sCT) inhibited in vitro 45Ca2+-uptake by rat brain hypothalamus blocks in a dose-dependent manner. The minimum effective concentration was estimated to be 10 nM or less. The effect appeared to be specific to the hypothalamus and was not observed with the pons plus medulla oblongata or the cerebral cortex. Two C-terminal fragments of the fish hormone, sCT (10-32) and sCT (22-32), and porcine calcitonin failed to inhibit the ion-uptake though tested in concentrations abolishing 125I-sCT binding to these brain tissues, indicating that the whole structure of sCT is essential for the inhibitory effect but not for the binding. Another finding to be noted was a possible dependency of this effect on the integrity of the cell membrane structure. A crude synaptosomal fraction subsequently prepared from sCT-exposed hypothalamus blocks exhibited a decreased uptake of 45Ca2+, while a corresponding fraction from unexposed tissue did not respond to the hormone. These characteristics of this novel in vitro effect of sCT suggest its possible relevancy to the anorectic effect which also appears to be specific to the fish hormone.