| Arterial baroreflex(ABR) buffers the increase and decrease in blood pressure and is one of the most important mechanisms in the regulation of cardiovascular activities. Baroreflex sensitivity(BRS) is a recognized autonomic marker. Large multicenter prospective studies of subjects with or without cardiovascular history show that BRS is a strong predictor of mortality in hypertension, arrhythmias, stroke, heart failure, myocardial infarction, etc.We hypothesized that artificial selection of rats based on deficient and normal intrinsic BRS would yield models that also contrast for disease risk. Based on these animal models, this study was also designed to investigate the mechanism of baroreflex impairment.The founder population was 55 male and 61 female Sprague-Dawley(SD) rats. Their blood pressure and BRS were measured in conscious state. The average BRS and systolic blood pressure(SBP) of the founder population were 0.77 ms/mmHg and 130 mmHg, respectively. There was no difference between male and female rats. According to the distribution of BRS, we defined rats with BRS<0.6 ms/mmHg as arterial baroreflex deficient rats(ABR-DRs), and rats with BRS>0.8 ms/mmHg as arterial baroreflex normal rats(ABR-NRs). We only bred rats with normal blood pressure to avoid the influence of hypertension on BRS. Using above criteria, we selected 20 rats(♂:♀=1:1) as the founder of ABR-DR and 20 rats(♂:♀=1:1) as the founder of ABR-NR. For each line, they were paired randomly for mating. For the subsequent generations, rats underwent strict brother-sister mating. We only screened and identified male rats to lighten the burden of selective breeding. After the offspring were weaned, only their fathers underwent blood pressure continuous recording and BRS measurement. If both blood pressure and BRS of their fathers met above criteria of the original line, rats had the chance to grow up. Otherwise, they were sacrificed.Twenty generations of selective breeding results in two divergent strains. At Generation 20, the BRS of ABR-DRs was significantly lower than that of ABR-NRs(0.46 ± 0.12 ms/mmHg vs 1.20 ± 0.26 ms/mmHg, P<0.01). In our study, although all hypertensive rats of ABR-DRs or ABR-NRs were discarded during selective breeding, the ABR-DRs had higher blood pressure than ABR-NRs by about 10 mmHg from Generation 9. Their heart rate had no significant difference.We measured BRS in male ABR-NRs and ABR-DRs at different ages(1, 2, 4, 6 and 8 months). In 1-month-old animals, the BRS in ABR-DRs was significantly lower than that in ABR-NRs(0.25 ± 0.08 ms/mmHg vs 0.72 ± 0.25 ms/mmHg, P<0.01). In 8-month-old rats, the BRS of ABR-NRs increased to 1.19 ms/mmHg, and that of ABR-DRs only increased to 0.45 ms/mmHg(P<0.01).We also measured BRS and hemodynamic parameters in 6-month-old female rats both at generation 19 and 20 and got similar results. Accordingly, for the rest of this study, we only used male rats from generation 19 and 20, to test our hypothesis that risk factors for cardiovascular diseases segregate with variation in intrinsic BRS.At generation 20, we measured the blood pressure of ABR-NRs and ABR-DRs at 5 different ages(1, 2, 4, 6 and 8-month-old) by catherization. Both SBP and diastolic blood pressure(DBP) of ABR-DRs were significantly higher than that of ABR-NRs by about 13 mmHg in 5 different age groups. The heart rate(HR) of ABR-DRs was higher than that of ABR-NRs at the first month. Then there was no difference in heart rate for subsequent months. The SBP, DBP and HR by noninvasive tail-cuff method were similar to above results. More importantly, the hypertension rate of ABR-DRs was much higher than that of ABR-NRs. These results showed that ABR-DRs had higher risk to hypertension.The serum biochemical factors related to glucose and lipids metabolism, liver function and kidney function were also analyzed at the age of two months. ABR-DRs had higher level of random glucose, fasting glucose, triglyceride, cholestrol, low-density lipoprotein, high-density lipoprotein and leptin. But their fasting insulin liver function and kidney function were not different from those of ABR-NRs. At the age of 8 months, ABR-DRs had impaired glucose tolerance and insulin tolerance. The insulin levels of ABR-DRs during glucose tolerance test revealed that their insulin secretion was preserved. These results indicated that ABR-DRs showed hyperglycemia, impaired glucose tolerance, insulin resistance and hyperlipidemia.We also monitored body weight and the amount of food intake at different ages. From 7-month old, ABR-DRs were heavier than ABR-NRs. At the first two months, ABR-DRs consumed more food than ABR-NRs. ABR-DRs consumed the same amount of food from the third month to the eighth month(except the sixth month) but ate less from 9-month old. These trends lasted to the age of 24 months. At the age of ten months, ABR-DRs exhibited higher ratio of total fat, subcutaneous fat and visceral fat. Thus overweight existed in ABR-DRs.At generation 20, we also recorded the cardiac function by echocardiographic method in both ABR-DRs and ABR-NRs at 8-month-old. Although ABR-DRs did not exhibit impaired cardiac function, they had an increased thickness of left ventricle wall. By using the morphological examination, we also found that ABR-DRs had heavier heart and left venricle, which also demonstrated that cardiac hypertrophy existed in ABR-DRs. The weight of aorta(mg/cm) in ABR-DRs was also bigger than that in ABR-NRs. There was no difference in the kidney weight between two strains.In an acute model of myocardial infarction, ABR-DRs significantly aggravated the infarct volume induced by the occlusion of left anterior descending coronary artery. In middle cerebral artery occlusion model of brain ischemia, ABR-DRs also exhibited an increase in the infarct size and neurological deficit score.Compared with ABR-NRs, the swimming time of ABR-DRs was much shorter in weight-loaded forced swimming test at different ages; ABR-DRs also had shorter treadmill running time to exhaustion. Thus ABR-DRs had lower aerobic exercise capacity.The survival time of ABR-DRs was significantly shorter than that of ABR-NRs.Based on these animal models, the mechanism of baroreflex impairment was investigated. Compared with ABR-NRs, ABR-DRs had a similar baseline of RSNA. We then activated baroreflex by increasing the blood pressure(about 50 mmHg) to observe the control of BRS on the sympathetic activation. The change of RSNA had no difference between ABR-DRs and ABR-NRs. But the heart rate of ABR-DRs decreased less than ABR-NRs. These data indicated that ABR-DRs had normal sympathetic activity, but their parasympathetic function was impaired.ABR-DRs had lower spontaneous firing frequency of cardiac vagal preganglionic neurons(CVPNs) in nucleus ambiguus and decreased vagal outflow. Exogenous glutamate could excite CVPNs and increase their firing frequency. But the increase rate of ABR-DRs was much lower than that of ABR-NRs. Inhibiting GABAA receptors or glutamate NMDA receptors could not abolish this difference in firing frequency. But the inhibition of AMPA receptors could abolish this difference and exogenous glutamate could not increase the firing frequency in this state. Thus, AMPA receptors of CVPNs in nucleus ambiguus were involved in the underlying mechanism.Conclusions: ABR-DRs scored higher on cardiovascular risk factors, including hypertension, hyperglycemia, hyperlipemia, overweight, low aerobic capacity, etc. ABR-DRs also aggravated the heart and brain ischemic injury. Impaird BRS could shorten survival time. Our data showed that impaired BRS or the dysfunction of parasympathetic outflow may be a common risk factor linking to cardiovascular diseases. AMPA receptors of CVPNs in nucleus ambiguus were involved in the underlying mechanism. |
