title: Melatonin inhibits insulin secretion and decreases PKA levels without interfering with glucose metabolism in rat pancreatic islets.
abstract: The effect of melatonin (0.1 microM) on freshly isolated islets from adult rats was investigated. Melatonin caused a marked decrease of insulin secretion by islets in response to glucose. The mechanism involved was then examined. Melatonin did not interfere with glucose metabolism as indicated by the measurement of glucose oxidation. However, the content of the protein kinase A (PKA) catalytic alpha-subunit was significantly decreased in islets exposed to melatonin for 1 hr in the presence of 8.3 mM glucose, whereas that of the protein kinase C (PKC) alpha-subunit remained unchanged. Melatonin also inhibited forskolin-induced insulin secretion, a well known activator of adenylate cyclase (AC) activity. This may explain the low content of insulin found in islets incubated in the presence of melatonin for 3 hr. In fact, 3',5' -cyclic adenosine monophosphate (cAMP), a product of AC activity, stimulates insulin synthesis. These findings led us to postulate that a down-regulation of the PKA signaling pathway may be the mechanism involved in the melatonin inhibition of the process of glucose-induced insulin secretion.
authors.affiliation: Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of São Paulo, São Paulo, Brasil.
authors.forename: Maria Cecília
authors.initials: MC
authors.lastname: Picinato
authors.forename: Esther Piltcher
authors.initials: EP
authors.lastname: Haber
authors.forename: José
authors.initials: J
authors.lastname: Cipolla-Neto
authors.forename: Rui
authors.initials: R
authors.lastname: Curi
authors.forename: Carla Roberta
authors.initials: CR
authors.lastname: de Oliveira Carvalho
authors.forename: Angelo Rafael
authors.initials: AR
authors.lastname: Carpinelli
mesh_terms: D000818:Animals; D005576:Colforsin / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000737:chemistry / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000502:physiology*; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D011493:Protein Kinase C / Q000737:chemistry / Q000378:metabolism; D021122:Protein Subunits; D051381:Rats
chemical_list: D007328:Insulin; D021122:Protein Subunits; D005576:Colforsin; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D005947:Glucose; D008550:Melatonin