Increased cardiac ppENK mRNA in cardiac hypertrophy and effects on blood pressure of its peptide products

Abstract

The objective of this study was to examine the expression of preproenkephalin (ppENK) in the heart in cardiac hypertrophy and the effects on cardiac contractility and blood pressure regulation of its peptide products. The ppENK-derived peptides Leu5-enkephalin (LE), Met5-enkephalin (ME), Met5-enkephalin-Arg6-Gly7-Leu8 (MEAGL), and Met5-enkephalin-Arg6-Phe7 (MEAP) were administered intravenously to unanesthetized Sprague-Dawley rats and to an isolated heart preparation from the same species. LE, ME, MEAGL, or MEAP (360 nmol iv) produced an immediate decrease in heart rate, reaching its maximum within 10 s and returning to baseline by 30 s. The blood pressure response for each enkephalin was a small initial decrease followed by a marked and significant increase (P < 0.05 for MEAP). In the isolated heart preparation, neither LE, ME, MEAGL, nor MEAP altered left ventricular contractility. Cardiac hypertrophy was produced in the Dahl salt-dependent model of hypertension with a significantly greater heart weight-to-body weight ratio in the Dahl salt-sensitive (S) compared with the Dahl salt-resistant (R) rat on a high-salt diet (P < 0.05). Tissue RNA was extracted, and Northern blot analysis identified and quantitated mRNA with a 0.93-kilobase cDNA of ppENK A. There was more ppENK mRNA in the left than in the right ventricle and much less in the atria than in the ventricles. The amount of ppENK mRNA was markedly and significantly increased in the left ventricle of the Dahl S compared with the Dahl R rat (P < 0.05). In contrast, there were no differences in ppENK mRNA levels in different brain regions between the R and S rats on a high-salt diet. Interestingly, a larger ppENK mRNA of 1.75 kilobases was abundantly expressed in testicular tissue. These data showing increased ppENK expression raise the possibilities of 1) an autocrine/paracrine role for enkephalins in cardiac hypertrophy and 2) an endocrine role for the hypertrophic heart, with an increased production of enkephalins, especially MEAP, that produces vasoconstriction and further increases in blood pressure.