Localization of pro-opiomelanocortin mRNA transcripts and peptide immunoreactivity in rat heart

Objective: alpha-Melanocyte-stimulating hormone (alpha-MSH), beta-endorphin and other pro-opiomelanocortin- (POMC) derived peptides have been detected in the heart, but it is uncertain whether they are synthesized by cardiomy ocytes or by cardiac nerves innervating the heart. The objective of this st udy was to determine whether POMC peptides are synthesized by cardiomyocyte s. Methods: Pro-opiomelanocortin peptides were localized in rat heart by im munohistochemistry using antisera against alpha-MSH, beta-endorphin and alp ha N-acetyl-beta-endorphin, the predominant POMC peptides found in heart. P ro-opiometanocortin mRNA was investigated by reverse transcription polymera se chain reaction (RT-PCR) using primers that discriminate between full-len gth POMC mRNA and a 5' truncated POMC transcript that is presumed to be non -functional. Results: alpha-Melanocyte-stimulating hormone, beta-endorphin and alpha N-acetyl-beta-endorphin immunoreactivities were localized in atri al myocytes, particularly in the atrial appendages, but not to a significan t extent in ventricular myocytes. Cardiac nerves were not immunostained. At rial natriuretic peptide (ANP) immunoreactivity was similarly distributed i n the adult heart. In neonatal heart, POMC-peptide and ANP immunoreactiviti es were present in both atrial and ventricular myocytes. RT-PCR amplificati on showed that full-length POMC mRNA transcripts were present in both atria l and ventricular tissue and provide evidence that 5' truncated POMC mRNA i s expressed in heart. Conclusions: These results support the hypothesis tha t cardiomyocytes synthesize POMC peptides. (C) 1999 Elsevier Science B.V. A ll rights reserved.