Association of proenkephalin transcripts with polyribosomes in the heart

Previous results have shown that the relative abundance of proenkephalin mRNA in the rat heart is comparable to the levels found in the brain; however, the extractable enkephalin-containing peptide levels are much lower in the heart. This lack of correspondence between the levels of transcript and peptide could arise from either the inefficient translation of proenkephalin transcripts or the translation of proenkephalin transcripts into peptides that are rapidly secreted or degraded. To distinguish between these possibilities, the translational status of proenkephalin mRNA in the rat heart was established by Northern blot analysis of sucrose density gradient-sedimented polysomal fractions and compared to the striatum, which is known to efficiently translate proenkephalin transcripts. In both tissues, we detected 1.5-kilobase transcripts, but an additional larger transcript of approximately 3.6 kilobases was detected in the heart. Both transcripts were associated primarily with polyribosomes, suggesting active translation of proenkephalin mRNA in the rat heart. RIA of the culture media and extracts from primary cultures of neonatal rat cardiomyocytes indicated the presence of immunoreactive Met-enkephalin-Arg6-Phe7, which was stimulated by 8-(4-chlorophenylthio)cAMP. These results suggest that proenkephalin transcripts are translated in the heart and that detectable levels of immunoreactive Met-enkephalin-Arg6-Phe7 are present in the media and cell extracts of primary cultures of neonatal rat cardiomyocytes.