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Augmentation of moxonidine-induced increase in ANP release by atrial hypertrophy

¹Department of Physiology, Medical School, Institute for Medical Sciences; and ²Department of Physiology, College of Veterinary Medicine, Chonbuk National University, Jeonju 561-180, Korea

Submitted 15 October 2003 ; accepted in final form 18 February 2004

Imidazoline receptors are divided into I₁ and I₂ subtypes. I₁-imidazoline receptors are distributed in the heart and are upregulated during hypertension or heart failure. The aim of this study was to define the possible role of I₁-imidazoline receptors in the regulation of atrial natriuretic peptide (ANP) release in hypertrophied atria. Experiments were performed on isolated, perfused, hypertrophied atria from remnant-kidney hypertensive rats. The relatively selective I₁-imidazoline receptor agonist moxonidine caused a decrease in pulse pressure. Moxonidine (3, 10, and 30 µmol/l) also caused dose-dependent increases in ANP secretion, but clonidine (an ₂-adrenoceptor agonist) did not. Pretreatment with efaroxan (a selective I₁-imidazoline receptor antagonist) or rauwolscine (a selective ₂-adrenoceptor antagonist) inhibited the moxonidine-induced increases in ANP secretion and interstitial ANP concentration and decrease in pulse pressure. However, the antagonistic effect of efaroxan on moxonidine-induced ANP secretion was greater than that of rauwolscine. Neither efaroxan nor rauwolscine alone has any significant effects on ANP secretion and pulse pressure. In hypertrophied atria, the moxonidine-induced increase in ANP secretion and decrease in pulse pressure were markedly augmented compared with nonhypertrophied atria, and the relative change in ANP secretion by moxonidine was positively correlated to atrial hypertrophy. The accentuation by moxonidine of ANP secretion was attenuated by efaroxan but not by rauwolscine. These results show that moxonidine increases ANP release through (preferentially) the activation of atrial I₁-imidazoline receptors and also via different mechanisms from clonidine, and this effect is augmented in hypertrophied atria. Therefore, we suggest that cardiac I₁-imidazoline receptors play an important role in the regulation of blood pressure.

contractility; imidazoline; hypertension; adrenoceptor