Molecular forms of alpha-melanocyte-stimulating hormone in the canine pituitary anterior and intermediate lobe

Reverse-phase high pressure liquid chromatography (HPLC) and radioimmunoassay (RIA) were used to determine the distribution of naturally occurring forms of alpha-melanocyte-stimulating hormone (alpha-MSH) in acid extracts of pars intermedia (PI) and anterior lobe (AL) tissue from canine and rat pituitary. Similarly, intracellular and secreted forms of alpha-MSH were determined using cultured canine PI and AL cells. Rat PI tissue contained predominantly diacetyl-alpha-MSH, while monoacetyl-alpha-MSH was the most abundant form in canine PI. In both canine and rat AL tissue extracts desacetyl-alpha-MSH was the major form of alpha-MSH. The profile of alpha-MSH contained in and secreted into culture medium by canine PI cells was found to be very similar to that in PI tissue extracts. The proportion of monoacetyl-alpha-MSH and diacetyl-alpha-MSH secreted by cultured canine AL cells and contained in extracts of AL cells in culture, however, was much higher than that in tissue extracts. These results indicate that in the dog, as in all other mammalian species studied, acetylated forms of alpha-MSH predominate in PI tissue, while nonacetylated alpha-MSH is the major form in AL tissue. It appears, however, that acetylation of alpha-MSH may occur in cultured canine AL cells, possibly as a result of the absence of factors that normally inhibit acetyltransferase in vivo or as a consequence of culture conditions.