Nitric oxide synthase inhibition exaggerates the hypotensive response to ghrelin: role of calcium-activated potassium channels Urmila A. Shinde, Kaushik M. Desai, Changhua Yu and Venkat Gopalakrishnan Objective To investigate the mechanism underlying the observation that infusion of the growth hormone secretagogue peptide, ghrelin, produces a decrease in mean arterial pressure (MAP) with no change in heart rate. Method The effect of a single bolus infusion of ghrelin (12 nmol/kg intravenously) on the changes in MAP and heart rate was determined in 12-week-old male anaesthetized Sprague–Dawley rats subjected to pretreatment with either the nitric oxide synthase (NOS) inhibitor, N v -nitro-L-arginine methyl ester (L-NAME; 0.7 mg/ ml by mouth for 5 days), or vehicle (control). Results Ghrelin produced a significant decrease in MAP at 20 min (P < 0.05) after infusion in the control group, without any change in heart rate. The MAP recovered partially over 1 h. The ghrelin-evoked decrease in MAP was much greater (P < 0.01) and was sustained for 1 h in rats subjected to NOS inhibition. Pretreatment with the cyclo-oxygenase inhibitor, indomethacin, failed to affect the responses in either group. Intravenous infusion of 50 mg/kg each of apamin and charybdotoxin (ChTX), a combination that is known to block Ca 2R -activated K R channels or the endothelium-derived hyperpolarization process, attenuated the decrease in MAP evoked by ghrelin in both control and NOS-inhibited rats. A sodium nitroprusside-induced decrease in MAP was unaffected in the presence of apamin–ChTX, but acetylcholine-evoked hypotension was significantly reduced in both groups. Conclusion These data suggest that the Ca 2R -activated, K R -channel-mediated, ghrelin-evoked decrease in MAP may be significant in states of endothelial dysfunction associated with reduced nitric oxide availability. J Hypertens 23:779–784 Q 2005 Lippincott Williams & Wilkins. Journal of Hypertension 2005, 23:779–784 Keywords: ghrelin, mean arterial pressure, nitric oxide, K R channels, heart rate Department of Pharmacology, College of Medicine, University of Saskatchewan, Saskatoon, SK, S7N 5E5 Canada. Sponsorship: This work was supported by grants-in-aid from the Canadian Institutes of Health Research (MOP-53293) and the Heart and Stroke Foundation of Saskatchewan. Correspondence and requests for reprints to: Venkat Gopalakrishnan PhD, Department of Pharmacology, College of Medicine, University of Saskatchewan, Saskatoon, SK, S7N 5E5 Canada. Tel: +1 306 966 6293; fax: +1 306 966 6220; e-mail: gopal@usask.ca Received 27 August 2004 Revised 17 November 2004 Accepted 6 December 2004 See editorial commentary page 713 Introduction Ghrelin, a 28-amino acid acylated peptide hormone released from the stomach, is now established as the cognate endogenous ligand that activates the growth hormone secretagogue (GHS) receptor [1]. Ghrelin pro- motes beneficial cardiovascular responses that are inde- pendent of its growth hormone (GH)-releasing effect [2 – 5]. A single bolus dose of ghrelin led to a long-lasting decrease in mean arterial pressure (MAP) without sig- nificant changes in heart rate, total plasma protein and plasma osmolality, in both humans and rats [2,6]. Intra- arterial infusion of ghrelin in the human forearm caused vasodilatation that was not mediated by GH, insulin-like growth factor (IGF)-I or nitric oxide (NO)/cGMP release [5]. The vasoconstrictor responses to endothelin (ET)-1 in human internal mammary artery rings were reduced by ghrelin, confirming its direct vasodilator effect [7]. In support of these in-vitro and in-vivo findings, recent studies have also reaffirmed that both systemic [intrave- nous (i.v.)] and central (intracerebroventricular) admin- istration of ghrelin decrease MAP and suppress sympathetic activity in conscious rabbits [4,8]. A negative correlation between plasma concentration of ghrelin and systemic blood pressure in pregnant women has been reported [9]. In addition, ghrelin administration is known to attenuate monocrotaline-induced pulmonary hyper- tension, pulmonary vascular remodelling and right ven- tricular hypertrophy in adult Wistar rats [10]. Specific, high-affinity binding sites for iodine-125-labelled ghrelin have been characterized in both rat and human vascular tissues; in fact, ghrelin receptor density is four-fold greater in atherosclerotic coronary arteries than in normal vessels [11]. Despite these observations, there are no reports on the mechanism of the in-vivo vasodilator action of ghrelin. The present study attempted to address this issue by examining the effect of a single i.v. bolus dose of ghrelin on MAP and heart rate in anaesthetized normotensive Sprague–Dawley rats. Methods The experimental procedures adhered to the current laws governing animal experimentation in Europe, and were approved by the Animal Care Committee of the Uni- versity of Saskatchewan. Male Sprague–Dawley rats, Original article 779 0263-6352 ß 2005 Lippincott Williams & Wilkins