Studies on secretory activity in the pars intermedia of Xenopus laevis 3. The synthesis and release of melanocyte stimulating hormone (MSH) in vitro.

Methods of bioassay and electron microscopy have been employed in the study of secretion of MSH from single pars intermedia/nervosa lobes maintained in vitro. It is shown that in a modified Krebs-Ringer medium the tissue displays an endogenous capacity for the synthesis and release of MSH. The amount of MSH produced is directly related to the development and functioning of the endoplasmic reticulum within the MSH cell, and although a net increase in the amount of MSH is adversely affected by anaerobic conditions and low temperature it is independent of an exogenous amino acid supply. The release of MSH, which is studied both independently and in association with concomitant synthesis, is also significantly reduced by anaerobic conditions and low temperature. Of the inorganic ions present in the medium only Ca(++) is essential for the synthesis and release of MSH. The evidence suggests that more than one step in the secretory process is dependent upon an exogenous supply of Ca(++) since in its absence the inhibition of MSH release is immediate but reversible, whereas although the inhibition of synthesis is delayed it is eventually complete and irreversible.