Ding-Feng Su

Arterial baroreflex deficit induced organ damage in sinoaortic denervated rats.

&NA; To verify the independent role of the arterial baroreceptor dysfunction involved in target‐organ damage in hypertension, sinoaortic denervated (SAD) rats were used as a model of arterial baroreflex (ABR) deficit. SAD, isolated aortic‐denervated (AD), and isolated sinus‐denervated (SD) rats were instrumented to record blood pressure (BP), heart rate (HR), BP variability (BPV), HR variability (HRV), ABR function control of heart period (ABR‐HP), and BP (ABR‐BP). Vascular maximum contractile/relaxant function was determined and organ damage was estimated by observation of morphologic changes. Short‐term (postoperative 1 week) SAD caused hypertension and tachycardia in rats. Eighteen weeks after operation, BP and HR values in SAD and SD rats were not different from those in sham‐operated rats, but AD rats were hypertensive compared with control group. Although 24‐h mean BP values of long‐term SAD rats were not different from those of sham‐operated rats, 24‐h BPV of SAD rats was significantly higher than that of sham‐operated rats. Arterial baroreflex function in short‐term SAD rats was significantly less than in sham‐operated rats, whereas in long‐term SAD rats, ABR‐HP and ABR‐BP were higher than those in short‐term SAD rats, but were still significantly lower than those in control groups. At postoperative 18 weeks, baroreflex function in SAD and AD rats was significantly less than function in SD and control groups. SBPmax after phenylephrine and DBPmin after nitroprusside were significantly higher in SAD, AD, and SD rats than in control rats. Baroreflex function was negatively correlated to DBPmin and SBPmax in all denervated rats (n = 44). Some morphologic changes were found 18 weeks after denervation in heart, kidney, and small artery in SAD, AD, and SD rats. Baroreflex function in all denervated rats was negatively related to 24‐h BPV values. In contrast, 24‐h BPV values in SAD, AD, and SD rats were positively related to organ‐damage score. A negative correlation between ABR function and end‐organ damage score was found. Arterial baroreflex deficit played an independent and important role in organ‐damage in SAD rats with significantly elevated 24‐h BPV.

Relationship between baroreceptor reflex function and end-organ damage in spontaneously hypertensive rats

The purpose of this study was to further illustrate the relationship between baroreceptor reflex sensitivity (BRS) and hypertensive end-organ damage (EOD) and to test the hypothesis that impairment of BRS aggravates EOD in hypertension. We studied baroreflex-mediated changes in heart rate [expressed as baroreceptor sensitivity to heart rate control (BRS(HR))] and blood pressure [expressed as baroreceptor sensitivity to blood pressure control (BRS(BP))] in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) that were used as controls, both at the age of 50-52 wk. Rats were also instrumented to record BP, HR, and BP variability (BPV) in the conscious, unrestrained state. In SHR compared with WKY, BP and BPV were significantly increased, whereas BRS(HR) and BRS(BP) were significantly decreased. SHR had remarkable EOD when compared with WKY (EOD score: 6.3 +/- 2.5 vs. 2.9 +/- 0.8, P < 0.01). Univariate regressive analysis demonstrated that EOD score was increased with BP and BPV and decreased with BRS. In multivariate analysis, EOD score was predicted by greater systolic BP and lower BRS and HR variability. These results indicate that BRS is negatively related to BPV and EOD score, and impaired BRS might be one of the major causes for hypertensive EOD.The purpose of this study was to further illustrate the relationship between baroreceptor reflex sensitivity (BRS) and hypertensive end-organ damage (EOD) and to test the hypothesis that impairment of BRS aggravates EOD in hypertension. We studied baroreflex-mediated changes in heart rate [expressed as baroreceptor sensitivity to heart rate control (BRSHR)] and blood pressure [expressed as baroreceptor sensitivity to blood pressure control (BRSBP)] in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) that were used as controls, both at the age of 50-52 wk. Rats were also instrumented to record BP, HR, and BP variability (BPV) in the conscious, unrestrained state. In SHR compared with WKY, BP and BPV were significantly increased, whereas BRSHR and BRSBP were significantly decreased. SHR had remarkable EOD when compared with WKY (EOD score: 6.3 ± 2.5 vs. 2.9 ± 0.8, P < 0.01). Univariate regressive analysis demonstrated that EOD score was increased with BP and BPV and decreased with BRS. In multivariate analysis, EOD score was predicted by greater systolic BP and lower BRS and HR variability. These results indicate that BRS is negatively related to BPV and EOD score, and impaired BRS might be one of the major causes for hypertensive EOD.

Blockade of N-methyl-D-aspartate receptors within the rostral ventrolateral medulla antagonizes clonidine-induced cardiovascular effects

There is wide agreement that the rostral ventrolateral medulla (RVLM) plays a crucial role in the regulation of blood pressure (BP), and that there may be a close correlation between the actions of centrally acting antihypertensive agents and N-methyl-D-aspartate (NMDA) receptor functional states. The present study was done to test the hypothesis that NMDA receptors within the RVLM were involved in the cardiovascular effects of centrally acting antihypertensive drug clonidine in anesthetized and paralyzed rats. Prior unilateral microinjection of NMDA receptor antagonist dizocilpine (MK801, 500 pmol) into the RVLM significantly attenuated (p<0.01, n=9) the reductions of BP (-24+/-6 to -8+/-4 mm Hg) and heart rate (-49+/-9 to -14+/-7 bpm) induced by unilaterally injected clonidine (5 nmol) into the RVLM. Prior bilateral microinjection of MK801 (500 pmol for each side) into the RVLM effectively (p<0.01, n=7) antagonized the hypotension (-25+/-5 to -8+/-2 mm Hg) and bradycardia (-43+/-7 to -11+/-4 bpm) of intravenously administered clonidine (10 microg kg(-1)). Importantly, iontophoretic application of MK801 (60 nA) significantly (p<0.01, n=9) prevented the inhibitory effect of intravenously (10 microg kg(-1)) injected clonidine on the discharge of presympathetic neurons in the RVLM (neuronal inhibition: -39+/-6 to -10+/-2%). In conclusion, the present study shows that the RVLM administrated MK801 effectively antagonizes clonidine-induced cardiovascular effects, and suggests that NMDA receptors within the RVLM contribute to clonidine actions.

Overexpression of Angiotensin-Converting Enzyme 2 Attenuates Tonically Active Glutamatergic Input to the Rostral Ventrolateral Medulla in Hypertensive Rats

The rostral ventrolateral medulla (RVLM) plays a key role in cardiovascular regulation. It has been reported that tonically active glutamatergic input to the RVLM is increased in hypertensive rats, whereas angiotensin-converting enzyme 2 (ACE2) in the brain has been suggested to be beneficial to hypertension. This study was designed to determine the effect of ACE2 gene transfer into the RVLM on tonically active glutamatergic input in spontaneously hypertensive rats (SHRs). Lentiviral particles containing enhanced green fluorescent protein (lenti-GFP) or ACE2 (lenti-ACE2) were injected bilaterally into the RVLM. Both protein expression and activity of ACE2 in the RVLM were increased in SHRs after overexpression of ACE2. A significant reduction in blood pressure and heart rate in SHRs was observed 6 wk after lenti-ACE2 injected into the RVLM. The concentration of glutamate in microdialysis fluid from the RVLM was significantly reduced by an average of 61% in SHRs with lenti-ACE2 compared with lenti-GFP. ACE2 overexpression significantly attenuated the decrease in blood pressure and renal sympathetic nerve activity evoked by bilateral injection of the glutamate receptor antagonist kynurenic acid (2.7 nmol in 100 nl) into the RVLM in SHRs. Therefore, we suggest that ACE2 overexpression in the RVLM attenuates the enhanced tonically active glutamatergic input in SHRs, which may be an important mechanism underlying the beneficial effect of central ACE2 to hypertension.

Changes of central norepinephrine, beta-endorphin, LEU-enkephalin, peripheral arginine-vasopressin, and angiotensin II levels in acute and chronic phases of sino-aortic denervation in rats.

We and others have demonstrated that impaired arterial baroreceptor reflex (ABR) function is one of the major causes of hypertension-associated end organ damage. The goal of this study was to clarify the potential neuro-humoral mechanisms responsible for impaired ABR-induced end organ damage. The sino-aortic denervated (SAD) rat was used as an animal model of ABR dysfunction. One-week SAD rats were characterized by hypertension, tachycardia, increased norepinephrine content, and decreased &bgr;-endorphin and leu-enkephalin content in hypothalamus and medulla oblongata, and increased plasma levels of arginine-vasopressin. In 18-week SAD rats, the 24-hour average arterial pressure, heart rate, &bgr;-endorphin, and leu-enkephalin content in hypothalamus and medulla oblongata and plasma levels of arginine-vasopressin and angiotensin II were not different from those measured in ABR-intact rats. However, blood pressure variability and angiotensin II content in kidney and left ventricle increased. When exposed to chronic stress, exaggerated changes in arterial pressure, blood pressure variability, the levels of central norepinephrine, &bgr;-endorphin and leu-enkephalin, plasma arginine-vasopressin and angiotensin II, and tissue angiotensin II were found in 18-week SAD rats. These data indicate that a long-term impairment of ABR leads to chronic activation of central noradrenergic neurons and tissue renin-angiotensin system, and that stress induces exaggerated responses of neuro-humoral factors and hemodynamics in SAD rats. Thus, if the present results hold true for humans, one can expect abnormal neurotransmitter/neuromodulator responses to environmental insults in patients with impaired ABR function.

Interaction between clonidine and N -methyl-D-aspartate receptors in the caudal ventrolateral medulla of rats

It is known that the caudal ventrolateral medulla (CVLM) plays an important role in controlling blood pressure and mediating the cardiovascular effects of centrally acting antihypertensive drugs such as clonidine. Recently, the effect of clonidine was believed to be related to the functional states of N-methyl-D-aspartate (NMDA) receptors. The present work was designed to observe the interactions between clonidine and NMDA receptor in the CVLM. Unilaterally injected clonidine (6xa0nmol) into the CVLM not only produced a pressor action, but also effectively (P<0.01, n=8) antagonized the decreases in both mean arterial pressure (MAP) (−22.3±5.0 to −7.9±2.3xa0mmHg) and heart rate (HR) (−31.9±5.9 to −10.3±2.7xa0beats/min) evoked by L-glutamate in the CVLM. Unilaterally injected NMDA receptor antagonist MK801 (200xa0pmol) into the CVLM significantly increased MAP by 26.5±3.7xa0mmHg and HR by 37.1±7.6xa0beats/min, and completely (P<0.01, n=10) abolished the pressor effect (16.1±6.6 to 1.5±2.8xa0mmHg) of clonidine in the CVLM. In conclusion, these findings show that NMDA receptors within the CVLM contribute to clonidine-induced action, and suggest that the CVLM plays an important role in the interaction between clonidine and NMDA receptors.

Role of I1-imidazoline receptors within the caudal ventrolateral medulla in cardiovascular responses to clonidine in rats.

&NA; Although it is recognized that imidazoline receptors play an important role in the central regulation of cardiovascular activities, little is known about their role in the caudal ventrolateral medulla. In male Sprague‐Dawley rats anesthetized with urethane, we used antagonists of I1‐imidazoline receptor or &agr;2‐adrenoceptor to assess the function of these receptors in the caudal ventrolateral medulla in controlling the cardiovascular effects of clonidine. Unilateral microinjection of clonidine (6 nmol/50 nl) into the caudal ventrolateral medulla significantly (P < 0.01) increased blood pressure and the discharge of the rostral ventrolateral medulla presympathetic neurons, while heart rate remained unchanged. Microinjection of yohimbine (a selective &agr;2‐adrenoceptor antagonist, 500 pmol/50 nl) into the caudal ventrolateral medulla did not modify blood pressure, heart rate, or the discharge of the rostral ventrolateral medulla presympathetic neurons, and failed to attenuate the local caudal ventrolateral medulla clonidine‐induced blood pressure elevation. However, unilateral microinjection of idazoxan (a mixed antagonist of imidazoline receptor and &agr;2‐adrenoceptor, 2 nmol/50 nl) into the caudal ventrolateral medulla significantly (P < 0.01) decreased mean arterial pressure, heart rate, and the discharge of the rostral ventrolateral medulla presympathetic neurons, and completely abolished the pressor effect of clonidine. In addition, bilateral microinjection of idazoxan (4 nmol in 100 nl for each side) into the caudal ventrolateral medulla effectively (P < 0.01) blocked the depressor effects of clonidine administered intravenously (5 and 50 &mgr;g/kg). These results confirm that I1‐imidazoline receptors within the caudal ventrolateral medulla are involved in maintaining the tonic cardiovascular activities and in the pressor effect of clonidine in the caudal ventrolateral medulla. In addition, it seems that the caudal ventrolateral medulla plays an important role in the antihypertensive effects of systemically administered clonidine in rats.

Hemodynamic responses to endothelin-1 and endothelin antagonists microinjected into the nucleus tractus solitarius in rats.

The role of endothelin-1 (ET-1) within the nucleus tractus solitarius (NTS) in central cardiovascular control was investigated by local microinjections of ET-1 and ET-receptor antagonists. In urethane-anesthetized Sprague-Dawley rats, a unilateral microinjection of ET-1 (1.0, 3.3, and 10.0 pmol) into the NTS significantly increased arterial pressure, left ventricular systolic pressure, and dP/dt(max) in a dose-dependent manner, and slightly decreased heart rate in a dose-independent manner. The pressor effect lasted >90 min. In normotensive rats, neither PD147953, a selective ETA-receptor antagonist, nor PD142893, a mixed ETA- and ETB-receptor antagonist, microinjected into the NTS elicited any changes in arterial pressure or heart rate. The pressor and bradycardic effects evoked by microinjection of ET-1 into the NTS could be blocked by local pretreatment with PD147953 and completely eliminated by intravenous pretreatment with the ganglionic blocker hexamethonium. The arterial baroreflex sensitivity was almost totally suppressed by microinjection of ET-1 (3.3 pmol) in alpha-chloralose-anesthetized Sprague-Dawley rats. A similar pattern of changes in the hemodynamic variables was elicited by microinjection of ET-1 (3.3 pmol) into the NTS in spontaneously hypertensive rats (SHRs) compared with Wistar-Kyoto (WKY) rats. In SHRs, microinjection of PD142893 did not elicit any changes in arterial pressure or heart rate. These results suggest that ET-1 modulates reflex control of hemodynamics by activation of autonomic nerve via ETA receptors in the NTS, and that the responsiveness of SHRs to ET-1 or PD142893 is similar to that of WKY rats.

Comparative study of NMDA and AMPA/kainate receptors involved in cardiovascular inhibition produced by imidazoline-like drugs in anaesthetized rats

The depressor mechanism of imidazoline‐like drugs is believed to result from activation of I1‐imidazoline receptors (I1R) and/or α2‐adrenoceptors within the central nervous system, which are associated with the glutamatergic system. The rostral ventrolateral medulla (RVLM) has been recognized as a specific target area that mediates the depressor action of imidazoline‐like drugs. The objective of this study was to determine the comparative effects of blockade of the central glutamate receptor subtypes N‐methyl‐d‐aspartate (NMDA) or α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA)/kainate on the cardiovascular actions of imidazoline‐like drugs (clonidine and moxonidine) in anaesthetized rats. Intracerebroventricular (i.c.v.) injection of the NMDA receptor antagonist MK801 or the AMPA/kainate receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) produced similar reductions in blood pressure (BP) and heart rate (HR) to those induced by i.c.v. injection of clonidine. Intracerebroventricular injection of the glutamate receptor antagonist kynurenic acid not only abolished clonidine‐induced hypotension and bradycardia but converted the responses to a pressor action and tachycardia. Unilateral injection of MK801 or CNQX into RVLM significantly attenuated intra‐RVLM clonidine‐induced decreases in BP and HR. We also found that unilateral injection of a selective I1R agonist, moxonidine, significantly decreased BP and HR, which were also attenuated to a similar extent by prior injection of MK801 or CNQX. In conclusion, these data show that blockade of central (RVLM) NMDA and AMPA/kainate receptors produces similar attenuation of the decrease in BP and HR induced by clonidine or moxonidine. It is suggested that both NMDA and AMPA/kainate receptors are involved in the cardiovascular inhibition produced by imidazoline‐like drugs, which is probably at least partly dependent on an I1R mechanism in the RVLM.

Electrophysiological evidences for the contribution of NMDA receptors to the inhibition of clonidine on the RVLM presympathetic neurons.

The main objective of this study is to test the hypothesis that N-methyl-D-aspartate (NMDA) receptors within the rostral ventrolateral medulla (RVLM) are involved in the inhibition of clonidine on the RVLM presympathetic neurons. Totally, 22 presympathetic neurons were recorded in anesthetized and paralyzed rats. The majority of these neurons (n=16 of 22) were significantly inhibited by iontophoretic (30 nA) clonidine, the other 6 neurons were insensitive to clonidine. In seven clonidine-sensitive neurons, iontophoretic clonidine (30 nA) antagonized the neuronal excitation of iontophoretic NMDA receptor agonist NMDA (20 nA). In remaining nine clonidine-sensitive neurons, iontophoretic NMDA receptor antagonist MK801 (60 nA) significantly attenuated the neuronal inhibition of iontophoretic (30 nA) clonidine. In conclusion, these results suggest that NMDA receptors contribute to the inhibition of clonidine on the RVLM presympathetic neurons.