IJCS | Volume 33, Nº4, July and August 2020

322 deflections (waves) that reflect the alternate contraction of the atria and the ventricles. 14 The first wave, P, is due to atrial contraction and its prolongation has been associated with hypertension caused by endothelial dysfunction and interatrial conduction delay. 12,15 The Q, R, S complex indicates ventricular depolarization. 12 The duration of the QRS interval and the amplitude of the waves separately are related to mortality in hypertension. 16,17 The final wave, named T, represents the repolarization of the ventricles and its correlation with the QRS complex (QT-interval). It is an important marker of ventricular activity and has been shown to have clinical utility. 12,18 In the present study, we aimed to conduct a comparative ECG analysis between two hypertension models (L-NAME and SHR) and their controls (Wistar and Wistar-Kyoto). Methods Animals Studies were conducted with adult, male, six and 15-week-old Wistar (HanUnib:WH), Wistar-Kyoto (NTacUnib:WKY) and SHR (SHR/NTacUnib) rats ( Rattus norvegicus ). All animals were provided by the Multidisciplinary Center for Biological Research (CEMIB - UNICAMP). The rats were housed in collective cages (3 rats per cage) at 22 o C on a 12 h light-dark cycle (lights on at 06:30 a.m.) with ad libitum access to standard chow (Labina Purina ® ) and filtered water. All animal housing, animal care and experimental procedures, and sample size were approved by the Ethics Committee on Animal Experimentation (CEUA) of the Institute of Biology of Unicamp in Campinas, Brazil (no. 2615-1), in accordance with the NIH guidelines. The animals were divided into four groups: control Wistar (WIS, n = 6), induced hypertension (L-NAME, n = 6), control Wistar Kyoto (WKY, n = 6), and genetic hypertension (SHR, n = 6). For the L-NAME group, we inhibited nitric oxide synthesis by administration of L-NAME (Enzo Life Sciences International, Inc.5120 Butler Pike, Plymouth Meeting, PA 19462) 40 mg/kg/day, for 5 weeks in the drinking water, started at the 10 th week of life of WIS. 19 Water was changed three times a week, with correction in dose/weight. The rats were anesthetized with tiletamine 29 mg kg -1 and zolazepam 29 mg kg -1 , i.p. (Zoletil 50 ® - Virbac Laboratories, Carros, France); and xylazine 12.88 mg kg -1 , i.p. (Anasedan ® - Sespo Ind. e Com. Ltda, Paulínia/SP, Brazil). Blood Pressure Blood pressuremonitoringwas performed at the end of the 15 th week under anaesthesia. Blood catheterizationwas performed by insertion of a cannula (PE 50) into the right carotid artery of anesthetized animals, attached to a strain- gauge pressure transducer thatwas connected to aMLS370 amplifier/7 blood pressure Module (ADInstruments - Australia), and to the data acquisition system PowerLab 8/30. For analysis of the results, we used the Software LabChart Pro (ADInstruments - Australia). 20 Electrocardiography Electrocardiography was performed in anesthetized rats in the supine position during spontaneous breathing. Recordings were performed at 6 and 15 weeks of age, using hypodermic needle electrodes, with computerized electrocardiography (MLS360/7 ECG Analysis Module, ADInstruments, Australia), for five minutes. 21 Statistical Methods Data are presented as mean ± SD. Normality of data distribution was confirmed by Kolmogorov-Smirnov test and then we performed unpaired Student’s t-test for parametric and theMann-Whitney test for nonparametric data. All statistical analysis was performed with Graph Pad Prism version 7.00 for Windows (Graph Pad Software, San Diego, California, USA). The accepted level of significance was p < 0.05. Results Blood pressure L-NAME and SHR 15-week-old rats exhibited increased systolic blood pressure, diastolic blood pressure, mean diastolic pressure and mean pressure when compared to their respective controls, WIS and WKY rats. There were no differences in systolic blood pressure, diastolic blood pressure, mean diastolic pressure and mean pressure between hypertensive rats (L-NAME vs. SHR) and control groups (WIS vs. WKY). Peak time was higher in SHRwhen comparedwithWKY but was not different when comparedwith L-NAME. No difference was observed in this parameter in control rats. There were no differences in pulse pressure, ejection and non-ejection time and cycle duration between the study groups (Table 1). Grassi-Kassisse et al. ECG in hypertension models Int J Cardiovasc Sci. 2020; 33(4):321-328 Original Article