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Endothelin-1 Augments Pressor Response to Angiotensin II Infusion in Rats Kazunori Yoshida, Minoru Yasujima, Masahiro Kohzuki, Masayuki Kanazawa, Kaoru Yoshinaga, and Keishi Abe To assess possible roles of endothelin in the regulation of blood pressure, we studied effects of a subpressor dose of endothelin-1 (3 /ig/kg/day) on chronic blood pressure responses to infusion of angiotensin II and norepinephrine in rats. Rats were infused with angiotensin II at a subpressor dose (400 fig/kg/day i.p.) or with norepinephrine at a subpressor dose (360 jtg/kg/day i.p.) for 6 days. Systolic blood pressure was significantly elevated during combined infusion of endothelin-1 and angiotensin II, whereas endothelin-1 alone or angiotensin II alone failed to induce any significant changes in systolic blood pressure compared with vehicle alone. This effect was sustained for the whole experimental period and was not associated with any significant changes in body weight, fluid intake, urine volume, or urinary electrolyte excretion. In contrast, combined infusion of endothelin-1 and norepinephrine failed to elevate systolic blood pressure, and no significant difference in systolic blood pressure was observed for the whole experimental period among the four groups of rats with endothelin-1 in combination with norepinephrine, endothelin-1 alone, norepinephrine alone, and vehicle alone. The present results indicate that angiotensin II and endotheIin-1, but not norepinephrine and endothelin-1, work synergistlcally to raise the blood pressure and also suggest the possibility that endothelin-1 may modulate blood pressure control. (Hypertension 199220:292-297) KEY WORDS

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blood pressure • angiotensin II • Sprague-Dawley rats • norepinephrine •

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ndothelin (ET), an endothelium-derived 21-residue peptide vasoconstrictor, has been isolated from the culture supernatant of porcine aortic endothelial cells and has also been shown to be one of the most potent vasoconstrictors in a variety of blood vessels from various species.1 Continuous infusion of endothelin-1 (ET-1) at a dose of 60 fig/kg per day for up to 6 days induced a significant increase in systolic blood pressure in conscious rats, 2 - 4 whereas ET-1 at doses of smaller than 6 fig/kg per day did not induce any significant increase in systolic blood pressure." It is also postulated that ET-1-induced smooth muscle contraction is mediated by an increase in intracellular calcium concentration.1-5 The relation of the ET system to the sympathetic nervous system and the renin-angiotensin system in the regulation of blood pressure and sodiumwater metabolism still remains to be determined. Therefore, we assessed the chronic effects of ET-1, angiotensin II (Ang II), and norepinephrine (NE) on systolic blood pressure and sodium-water metabolism in conscious rats and also evaluated the interaction between ET-1 and Ang II and between ET-1 and NE.

From the Second Department of Internal Medicine (K.Yoshida, M. Kohzuki, M. Kanazawa, K. Yoshinaga), the Institute of Rehabilitation Medicine (M.Y.), and the Department of Clinical Biology and Hormonal Regulation (K.A.), Tohoku University School of Medicine, Sendai, Japan. Address for correspondence: Minoru Yasujima, MD, Institute of Rehabilitation Medicine, Tohoku University School of Medicine, Narugo, Miyagi 989-68, Japan. Received June 25,1991; accepted in revised form April 30,1992.

Methods Male Sprague-Dawley rats (Funabashi, Chiba, Japan) weighing approximately 200 g were used. All rats were maintained in a humidity- and temperature-controlled room. Each rat was housed in a metabolic cage designed to prevent feces-urine contact (model ST, Sugiyamagen, Tokyo) during the study. The rats were fed on a regular diet (Oriental CMF, 0.20 meq sodium per gram, 0.27 meq potassium per gram; Oriental Yeast Co., Tokyo) and had free access to tap water. Two study protocols were performed to assess the effect of ET-1 on chronic blood pressure responses to Ang II and NE in rats. Study 1: Effects of Endothelin-1 in Ang H-Infused Rats Rats were divided into four groups. Group 1 rats (n=7) received a subpressor dose of Ang II (400 /tg/kg per day i.p.) only. Group 2 rats (n=7) received a subpressor dose of ET-1 (3 /ig/kg per day i.v.) only. Group 3 rats (n=7) received simultaneously both the subpressor dose of Ang II (400 fig/kg per day i.p.) and the subpressor dose of ET-1 (3 figjkg per day i.v.). Group 4 rats (n=7) received vehicle (physiological saline) only. In a previous study, we assessed the effect of continuous Ang II infusion at doses of 10, 75, 100, and 150 ng/min for up to 6 days in conscious rats.6 Systolic blood pressure increased dose-dependenth/ at Ang II levels of 100 and 150 ng/min, whereas the changes induced by 10 and 75 ng/min did not differ from those induced by vehicle infusion. In a previous study,6 we have shown

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