INHIBITION OF ALPHA-MSH SECRETION IS ASSOCIATED WITH INCREASED CYCLIC-AMP EGRESS FROM THE NEUROINTERMEDIATE LOBE OF XENOPUS-LAEVIS

Cyclic-AMP is known to be released from cells and tissues and the amou nts released have been reported to reflect intracellular cAMP levels. To measure cAMP release the phosphodiesterase inhibitor IBMX is often used to increase the amount of cAMP to a detectable level. Using this method to follow cAMP dynamics of melanotrope cells in the neurointerm ediate pituitary lobe of the amphibian Xenopus laevis we show that the alpha-MSH secrete-inhibitors baclofen (GAB(A) receptor agonist) and d opamine inhibit cAMP release, confirming the idea that these factors i nhibit alpha-MSH secretion by reducing adenylyl cyclase activity. Usin g a sensitive cAMP radioimmunoassay we were able to measure cAMP relea se from Xenopus neurointermediate lobes in the absence of IBMX. Both b aclofen and dopamine appeared to inhibit alpha-MSH secretion but stron gly stimulated the release of cAMP. This indicates that the extracellu lar cAMP level is not a reliable parameter to measure the intracellula r cAMP level in the absence of IBMX. The data furthermore suggest that cAMP release is a physiologically regulated process, which might be i nvolved in lowering intracellular cAMP levels associated with a cellul ar secretory compartment. No apparent differences could be found in th e lobe content of cAMP at the termination of secrete-inhibitor treatme nt, leading to the idea that the cAMP compartment associated with secr etion is small relative to the total amount of cAMP present in the lob e.