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CARDIOVASCULAR

Obligatory Role for Endogenous Endothelin in Mediating the Hypertrophic Effects of Phenylephrine and Angiotensin II in Neonatal Rat Ventricular Myocytes: Evidence for Two Distinct Mechanisms for Endothelin Regulation

Department of Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada

Various G_q protein-coupled receptor agonists such as the ₁ adrenoceptor agonist phenylephrine, angiotensin II, and endothe-lin-1 are potent hypertrophic factors. There is evidence of potential cross talk between these agents, particularly in terms of endothelin-1 as playing a central role in mediating the actions of other hypertrophic factors. Using cultured rat neonatal ventricular myocytes, we assessed the potential cross talk between these factors and sought to examine the potential underlying mechanisms. Twenty-four-hour exposure to either agent produced significant hypertrophy as determined by cell size and molecular markers. Although the hypertrophic effects of phenylephrine and angiotensin II were expectedly prevented by ₁ and AT₁ receptor antagonists, respectively, these effects were also blocked by the ET_A receptor antagonist BQ123 [cyclo(D-Asp-Pro-D-Val-Leu-D-Trp)] but not by the ET_B antagonist BQ788 (N-cis-2,6-dimethylpiperidinocarbonyl-L--methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine). Both phenylephrine and angiotensin II significantly increased protein expression of both endothelin receptor subtypes. Both phenylephrine and angiotensin II produced significant activation of p38 as well as extracellular signal-regulated protein kinase and c-Jun NH₂-terminal kinase, although this was unaffected by endothelin receptor blockade. Further studies revealed that the effects of phenylephrine and angiotensin II were mediated by stimulated endothelin-1 production occurring via two separate mechanisms: angiotensin II by increasing the levels of the endothelin-1 precursor prepro endothelin-1 and phenylephrine by upregulating endothelin-converting enzyme 1. Our results indicate that the endothelin-1 system plays an obligatory role in the hypertrophic response to both phenylephrine and angiotensin II in cultured myocytes through a mechanism independent of mitogenactivated protein kinase activation.

Address correspondence to: Dr. Morris Karmazyn, Department of Physiology and Pharmacology, University of Western Ontario, Medical Sciences Building, London, Ontario N6A 5C1, Canada. E-mail: morris.karmazyn{at}fmd.uwo.ca

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