Glomerular and tubular responses to N(G)-nitro-L-arginine methyl ester are age dependent in conscious lambs

The present experiments were carried out to investigate the role of endogenously produced NO in modulating renal function during postnatal maturation under physiological conditions. In conscious, chronically instrumented lambs aged approximately 1 (n = 8) and approximately 6 wk (n = 8) of postnatal life, various parameters of glomerular and tubular function were measured for 1 h before and 1 h after intravenous injection of 20 mg/kg of N(G)-nitro-L-arginine methyl ester (L-NAME; experiment 1) or its inactive isomer D-NAME (experiment 2). After administration of L-NAME to 1-wk-old lambs, glomerular filtration rate (GFR) and filtration factor (FF) decreased by approximately 50% at 20 min, remaining decreased at 60 min. In 6-wk-old lambs, GFR and FF remained constant after L-NAME. Proximal fractional Na(+) reabsorption decreased after L-NAME administration to lambs aged 6 wk, resulting in a prompt natriuresis; this was sustained for 60 min. There were no effects of L-NAME on proximal fractional Na(+) reabsorption in 1-wk-old lambs. In 6-wk-old lambs, urinary flow rate increased by approximately 500%, free water clearance increased by approximately 50%, and urinary osmolality decreased by approximately 60% after L-NAME administration; no effects on these variables were measured in 1-wk-old lambs. The diuresis after L-NAME administration to 6-wk-old lambs was unaccompanied by any changes in plasma levels of arginine vasopressin. There were no effects of D-NAME on any of the measured variables. We conclude that endogenously produced nitric oxide modulates glomerular and tubular function in an age-dependent manner.     

Baroreflex regulation of renal sympathetic nerve activity and heart rate in renal wrap hypertensive rats

Despite its usefulness as a nongenetic model of hypertension, little information is available regarding baroreflex function in the Grollman, renal wrap model of hypertension in the rat. Baroreflex regulation of renal sympathetic nerve activity (RSNA) and heart rate (HR) were studied in male, Sprague-Dawley rats hypertensive (HT) for 1 or 4-6 wk after unilateral nephrectomy and figure-8 ligature around the remaining kidney or normotensive (NT) after sham surgery. Rats were anesthetized with Inactin and RSNA, and HR was recorded during intravenous infusions of sodium nitroprusside or phenylephrine to lower or raise mean arterial pressure (MAP). Response curves were analyzed using a logistic sigmoid function. In 1- and 4-wk HT rats the midpoints of RSNA and HR reflex curves were shifted to the right (P < 0.05). Comparing NT to 1- or 4-wk HT rats, the gain of RSNA-MAP curves was no different; however, gain was reduced in the HR-MAP curves at both 1 and 4 wk in HT rats (P < 0.05). In anesthetized rats the HR range was small; therefore, MAP and HR were measured in conscious rats during intravenous injections of three doses of phenylephrine and three doses of sodium nitroprusside. Linear regressions revealed a reduced slope in both 1- and 4-wk HT rats compared with NT rats (P < 0.05). The results indicate that baroreflex curves are shifted to the right, to higher pressures, in hypertension. After 1-4 wk of hypertension the gain of baroreflex regulation of RSNA is not altered; however, the gain of HR regulation is reduced.     

Nitric oxide at the CVLM is involved in the attenuation of the reflex bradycardia in renovascular hypertensive rats

Hypertension is associated to an increase in central oxidative stress and an attenuation of the baroreflex control of arterial pressure. The present study evaluated the effect of alterations in the levels of nitric oxide (NO) and superoxide anion in the caudal ventrolateral medulla (CVLM), a key area of the brainstem for the baroreflex control of arterial pressure, in renovascular hypertensive rats (2K1C). Baseline mean arterial pressure (MAP), heart rate (HR), and reflex bradycardia were evaluated 30 days after renal artery occlusion in anesthetized (urethane, 1.2 g/kg, i.p.) 2K1C or normotensive (SHAM) rats. The MAP, HR, and baroreflex control of HR were evaluated before and after CVLM microinjections of the non-selective NOS inhibitor L-NAME (10 nmol), the NO precursor L-ARG (50 nmol), or the antioxidant ascorbic acid, Vit C (10 nmol). In both 2K1C and SHAM animals, CVLM microinjection of L-NAME produced a decrease in MAP, whereas L-ARG induced a significant increase in MAP. However, microinjection of Vit C into the CVLM produced a decrease in MAP and HR only in 2K1C and not in SHAM rats. Cardiovascular effects produced by microinjection of l-ARG into the CVLM were abolished by prior microinjection of L-NAME in the CVLM of 2K1C and SHAM rats. Microinjection of L-NAME into the CVLM increased the sensitivity of reflex bradycardia in 2K1C animals. In contrast, the CVLM microinjection of L-ARG reduced reflex bradycardia only in SHAM rats. Vit C in the CVLM did not change reflex bradycardia in either 2K1C or in SHAM rats. These results suggest that increased oxidative stress in the CVLM during hypertension contributes to the reduced baroreflex sensitivity and to maintain hypertension in the 2K1C model.     

Effect of blockade of endogenous angiotensin II on baroreflex function in conscious diabetic rats

Little is known about baroreflex control of renal nerve sympathetic activity (RSNA) or the effect of angiotensin II (ANG II) on the baroreflex in diabetes. We examined baroreflex control of RSNA and heart rate (HR) in conscious, chronically instrumented rats 2 wk after citrate vehicle (normal) or 55 mg/kg iv streptozotocin (diabetic) before and after losartan (5 mg/kg iv) or enalapril (2.5 mg/kg iv). Resting HR and RSNA were lower in diabetic versus normal rats. The range of baroreflex control of HR and the gain of baroreflex-mediated bradycardia were impaired in diabetic rats. Maximum gain was unchanged. The baroreflex control of RSNA was reset to lower pressures in the diabetic rats but remained otherwise unchanged. Losartan decreased mean arterial pressure (MAP) and increased HR and RSNA in both groups but had no influence on the baroreflex. Enalapril decreased MAP only in normal rats, yet the increase in HR and RSNA was similar in both groups. Thus in diabetic rats enalapril produced a pressure-independent increase in HR and RSNA. Enalapril exerted no effect on the baroreflex control of HR or RSNA in either group. These data indicate that in conscious rats resting RSNA is lower but baroreflex control of RSNA is preserved after 2 wk of diabetes. At this time, the baroreflex control of HR is already impaired and blockade of endogenous ANG II does not improve this dysfunction.     

The influence of nitric oxide synthase 1 on blood flow and interstitial nitric oxide in the kidney

The role of nitric oxide (NO) produced by NO synthase 1 (NOS1) in the renal vasculature remains undetermined. In the present study, we investigated the influence of systemic inhibition of NOS1 by intravenous administration of N(omega)-propyl-L-arginine (L-NPA; 1 mg. kg(-1). h(-1)) and N(5)-(1-imino-3-butenyl)-L-ornithine (v-NIO; 1 mg. kg(-1). h(-1)), highly selective NOS1 inhibitors, on renal cortical and medullary blood flow and interstitial NO concentration in Sprague-Dawley rats. Arterial blood pressure was significantly decreased by administration of both NOS1-selective inhibitors (-11 +/- 1 mmHg with L-NPA and -7 +/- 1 mmHg with v-NIO; n = 9/group). Laser-Doppler flowmetry experiments demonstrated that blood flow in the renal cortex and medulla was not significantly altered following administration of either NOS1-selective inhibitor. In contrast, the renal interstitial level of NO assessed by an in vivo microdialysis oxyhemoglobin-trapping technique was significantly decreased in both the renal cortex (by 36-42%) and medulla (by 32-40%) following administration of L-NPA (n = 8) or v-NIO (n = 8). Subsequent infusion of the nonspecific NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 50 mg. kg(-1). h(-1)) to rats pretreated with either of the NOS1-selective inhibitors significantly increased mean arterial pressure by 38-45 mmHg and significantly decreased cortical (25-29%) and medullary (37-43%) blood flow. In addition, L-NAME further decreased NO in the renal cortex (73-77%) and medulla (62-71%). To determine if a 40% decrease in NO could alter renal blood flow, a lower dose of L-NAME (5 mg. kg(-1). h(-1); n = 8) was administered to a separate group of rats. The low dose of L-NAME reduced interstitial NO (cortex 39%, medulla 38%) and significantly decreased blood flow (cortex 23-24%, medulla 31-33%). These results suggest that NOS1 does not regulate basal blood flow in the renal cortex or medulla, despite the observation that a considerable portion of NO in the renal interstitial space appears to be produced by NOS1.     

Independent modification of baroreceptor and exercise pressor reflex function by nitric oxide in nucleus tractus solitarius

It has been suggested that nitric oxide (NO) is a key modulator of both baroreceptor and exercise pressor reflex afferent signals processed within the nucleus tractus solitarius (NTS). However, studies investigating the independent effects of NO within the NTS on the function of each reflex have produced inconsistent results. To address these concerns, the effects of microdialyzing 10 mM L-arginine, an NO precursor, and 20 mM N(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, into the NTS on baroreceptor and exercise pressor reflex function were examined in 17 anesthetized cats. Arterial baroreflex regulation of heart rate was quantified using vasoactive drugs to induce acute changes in mean arterial pressure (MAP). To activate the exercise pressor reflex, static hindlimb contractions were induced by electrical stimulation of spinal ventral roots. To isolate the exercise pressor reflex, contractions were repeated after barodenervation. The gain coefficient of the arterial cardiac baroreflex was significantly different from control (-0.24 +/- 0.04 beats.min(-1).mmHg(-1)) after the dialysis of L-arginine (-0.18 +/- 0.02 beats.min(-1).mmHg(-1)) and L-NAME (-0.29 +/- 0.02 beats.min(-1).mmHg(-1)). In barodenervated animals, the peak MAP response to activation of the exercise pressor reflex (change in MAP from baseline, 39 +/- 7 mmHg) was significantly attenuated by the dialysis of L-arginine (change in MAP from baseline, 29 +/- 6 mmHg). The results demonstrate that NO within the NTS can independently modulate both the arterial cardiac baroreflex and the exercise pressor reflex. Collectively, these findings provide a neuroanatomical and chemical basis for the regulation of baroreflex and exercise pressor reflex function within the central nervous system.     

Arterial baroreflex control of heart rate during exercise in postural tachycardia syndrome.

Patients with postural tachycardia syndrome (POTS) have excessive tachycardia without hypotension during orthostasis as well as exercise. We tested the hypothesis that excessive tachycardia during exercise in POTS is not related to abnormal baroreflex control of heart rate (HR). Patients (n = 13) and healthy controls (n = 10) performed graded cycle exercise at 25, 50, and 75 W in both supine and upright positions while arterial pressure (arterial catheter) and HR (ECG) were measured. Baroreflex sensitivity of HR was assessed by bolus intravenous infusion of phenylephrine at each workload. In both positions, HR was higher in the patients than the controls during exercise. Supine baroreflex sensitivity (HR/systolic pressure) in POTS patients was -1.3 +/- 0.1 beats.min(-1).mmHg(-1) at rest and decreased to -0.6 +/- 0.1 beats.min(-1).mmHg(-1) during 75-W exercise, neither significantly different from the controls (P > 0.6). In the upright position, baroreflex sensitivity in POTS patients at rest (-1.4 +/- 0.1 beats.min(-1).mmHg(-1)) was higher than the controls (-1.0 +/- 0.1 beats.min(-1).mmHg(-1)) (P < 0.05), and it decreased to -0.1 +/- 0.04 beats.min(-1).mmHg(-1) during 75-W exercise, lower than the controls (-0.3 +/- 0.09 beats.min(-1).mmHg(-1)) (P < 0.05). The reduced arterial baroreflex sensitivity of HR during upright exercise was accompanied by greater fluctuations in systolic and pulse pressure in the patients than in the controls with 56 and 90% higher coefficient of variations, respectively (P < 0.01). However, when baroreflex control of HR was corrected for differences in HR, it was similar between the patients and controls during upright exercise. These results suggest that the tachycardia during exercise in POTS was not due to abnormal baroreflex control of HR.     

Influence of physical training on heart rate variability and baroreflex circulatory control

Nineteen males (aged 45-68 yr) were studied before and after either a period of regular endurance exercise [walk/jog 3-4 days/wk for 30 +/- 1 (SE) wk, n = 11] or unchanged physical activity (38 +/- 2 wk, n = 8) (controls) to determine the influence of physical training on cardiac parasympathetic (vagal) tone and baroreflex control of heart rate (HR) and limb vascular resistance (VR) at rest in middle-aged and older men. Training resulted in a marked increase in maximal O2 uptake (31.6 +/- 1.2 vs. 41.0 +/- 1.8 ml.kg-1.min-1, 2.56 +/- 0.16 vs. 3.20 +/- 0.18 l/min, P less than 0.05) and small (P less than 0.05) reductions in body weight (81.2 +/- 3.5 vs. 78.7 +/- 4.0 kg) and body fat (23.8 +/- 1.3 vs. 20.9 +/- 1.3%). HR at rest was slightly, but consistently, lower after training (63 +/- 2 vs. 58 +/- 1 beats/min, P less than 0.05). In general, HR variability (index of cardiac vagal tone) was greater after training. Chronotropic responsiveness to either brief carotid baroreflex stimulation (neck suction) or inhibition (neck pressure), or to non-specific arterial baroreflex inhibition induced by a hypotensive level of lower body suction, was unchanged after training. In contrast, the magnitude of the reflex increase in forearm VR in response to three levels of lower body suction was markedly attenuated after training (38-59%; P less than 0.05 at -10 and -30 mmHg; P = 0.07 at -20 mmHg). None of these variables or responses was altered over time in the controls. These findings indicate that in healthy, previously sedentary, middle-aged and older men, strenuous and prolonged endurance training 1) elicits large increases in maximal exercise capacity and small reductions in HR at rest, 2) may increase cardiac vagal tone at rest, 3) does not alter arterial baroreflex control of HR, and 4) results in a diminished forearm vasoconstrictor response to reductions in baroreflex sympathoinhibition.     

Exercise training enhances baroreflex control of heart rate by a vagal mechanism in rabbits with heart failure.

Moderate exercise training (Ex) enhances work capacity and quality of life in patients with chronic heart failure (CHF). We investigated the autonomic components of resting heart rate (HR) and the baroreflex control of HR in conscious, instrumented rabbits with pacing-induced CHF after Ex. Sham and CHF rabbits were exercise trained for 4 wk at 15-18 m/min, 6 days/wk. Arterial pressure and HR were recorded before and after metoprolol (1 mg/kg iv) or after atropine (0.2 mg/kg iv). Mean arterial pressure was altered by infusions of sodium nitroprusside and phenylephrine. The data were fit to a sigmoid (logistic) function. Baseline HRs were 266.5 +/- 8.4 and 232.1 +/- 1.6 beats/min in CHF and CHF Ex rabbits, respectively (P < 0.05). In the unblocked state, CHF rabbits had a significantly depressed peak baroreflex slope (1.7 +/- 0.3 vs. 5.6 +/- 0.7 beats. min(-1). mmHg(-1); P < 0.001) and HR range (128.6 +/- 34.5 vs. 253.2 +/- 20.3 beats/min; P < 0.05) compared with normal subjects. Ex increased baroreflex slope to 4.9 +/- 0.3 from 1.7 +/- 0.3 beats. min(-1). mmHg(-1) in unblocked rabbits (P < 0.001 compared with CHF non-Ex). Ex did not alter baroreflex function in sham animals. After metoprolol, baroreflex slope was significantly increased in CHF Ex rabbits (1.5 +/- 0.2 vs. 3.0 +/- 0.2 beats. min(-1). mmHg(-1); P < 0.05). After atropine, there was no significant change in baroreflex slope or HR range between CHF Ex and CHF rabbits. These data support the view that enhancement of baroreflex control of HR after Ex is due to an augmentation of vagal tone.     

Respiratory muscle training improves hemodynamics, autonomic function, baroreceptor sensitivity, and respiratory mechanics in rats with heart failure

Respiratory muscle training (RMT) improves functional capacity in chronic heart-failure (HF) patients, but the basis for this improvement remains unclear. We evaluate the effects of RMT on the hemodynamic and autonomic function, arterial baroreflex sensitivity (BRS), and respiratory mechanics in rats with HF. Rats were assigned to one of four groups: sedentary sham (n = 8), trained sham (n = 8), sedentary HF (n = 8), or trained HF (n = 8). Trained animals underwent a RMT protocol (30 min/day, 5 day/wk, 6 wk of breathing through a resistor), whereas sedentary animals did not. In HF rats, RMT had significant effects on several parameters. It reduced left ventricular (LV) end-diastolic pressure (P < 0.01), increased LV systolic pressure (P < 0.01), and reduced right ventricular hypertrophy (P < 0.01) and pulmonary (P < 0.001) and hepatic (P < 0.001) congestion. It also decreased resting heart rate (HR; P < 0.05), indicating a decrease in the sympathetic and an increase in the vagal modulation of HR. There was also an increase in baroreflex gain (P < 0.05). The respiratory system resistance was reduced (P < 0.001), which was associated with the reduction in tissue resistance after RMT (P < 0.01). The respiratory system and tissue elastance (Est) were also reduced by RMT (P < 0.01 and P < 0.05, respectively). Additionally, the quasistatic Est was reduced after RMT (P < 0.01). These findings show that a 6-wk RMT protocol in HF rats promotes an improvement in hemodynamic function, sympathetic and vagal heart modulation, arterial BRS, and respiratory mechanics, all of which are benefits associated with improvements in cardiopulmonary interaction.     

Kappa opioids modulate the arterial baroreflex control of heart rate in conscious young sheep

The present study tested the hypothesis that kappa-opioids modulate the arterial baroreflex control of heart rate in conscious young sheep. Various parameters governing the arterial baroreflex control of heart rate were assessed before and after activation of kappa-opiate receptors (KOR) by i.v. administration of the specific KOR agonist U-50488H (experiment 1) or vehicle (experiment 2) to conscious, chronically instrumented lambs aged 42 +/- 2 days (n = 6). The 2 experiments were administered in random order at minimum intervals of 48 h. Thirty min after U-50488H treatment, there was an increase in diastolic and mean arterial pressure and in heart rate, returning to control levels by 90 min. A significant increase in the arterial pressure at the midpoint of the baroreflex range and in the minimum heart rate as well as a significant decrease in the heart rate range over which the arterial baroreflex operates were also seen at 30 min after U-50488H, gradually returning to control levels over 120 min. Vehicle had no effect on any of the parameters governing the arterial baroreflex control of heart rate. These data provide the first direct evidence that under physiological conditions in young lambs, the arterial baroreflex control of heart rate is altered after administration of the specific KOR agonist U-50488H, revealing a previously unidentified role for this opioid receptor.     

Systemic arterial pressure response to two weeks of Tempol therapy in SHR: involvement of NO, the RAS, and oxidative stress

The roles of nitric oxide (NO) and plasma renin activity (PRA) in the depressor response to chronic administration of Tempol in spontaneously hypertensive rats (SHR) are not clear. The present study was done to determine the effect of 2 wk of Tempol treatment on blood pressure [mean arterial pressure (MAP)], oxidative stress, and PRA in the presence or absence of chronic NO synthase inhibition. SHR were divided into four groups: control, Tempol (1 mmol/l) alone, nitro-L-arginine methyl ester (L-NAME, 4.5 mg x g(-1).day(-1)) alone, and Tempol + L-NAME or 2 wk. With Tempol, MAP decreased by 22%: 191 +/- 3 and 162 +/- 21 mmHg for control and Tempol, respectively (P < 0.05). L-NAME increased MAP by 16% (222 +/- 2 mmHg, P < 0.01), and L-NAME + Tempol abolished the depressor response to Tempol (215 +/- 3 mmHg, P < 0.01). PRA was not affected by Tempol but was increased slightly with L-NAME alone and 4.4-fold with L-NAME + Tempol. Urinary nitrate/nitrite increased with Tempol and decreased with L-NAME and L-NAME + Tempol. Tempol significantly reduced oxidative stress in the presence and absence of L-NAME. In conclusion, in SHR, Tempol administration for 2 wk reduces oxidative stress in the presence or absence of NO, but in the absence of NO, Tempol is unable to reduce MAP. Therefore, NO, but not changes in PRA, plays a major role in the blood pressure-lowering effects of Tempol. These data suggest that, in hypertensive individuals with endothelial damage and chronic NO deficiency, antioxidants may be able to reduce oxidative stress but not blood pressure.     

L-NAME- and ADMA-induced sympathetic neural activation in conscious rats

Although studies in anesthetized, sino-aortic denervated animals indicate that inhibition of central nitric oxide (NO) causes an excitatory influence on efferent sympathetic nerve activity (SNA) that is normally offset by baroreflex activation, studies in conscious animals have not provided clear-cut evidence for a sympathoexcitatory effect of N(omega)-nitro-l-arginine methyl ester (L-NAME) or the endogenous circulating NO synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Thus our goals were to 1) use surgical sino-aortic denervation to test for a sympathoexcititatory effect of intravenous l-NAME in conscious rats, and 2) to determine whether SNA responses to intravenous L-NAME can be extrapolated directly to intravenous ADMA. We recorded mean arterial blood pressure and renal SNA in both intact and sino-aortic-denervated conscious rats during 3 h of continuous intravenous infusion with either L-NAME or ADMA. When we eliminated the confounding influence of the sino-aortic baroreceptors, L-NAME produced a progressive increase in SNA with the peak response exceeding the baseline level of nerve firing by 150%. The same type of frank sympathetic activation was observed with intravenous ADMA. Taken together, these data offer straightforward evidence for l-NAME, as well as ADMA-induced sympathetic activation with direct recordings of SNA in conscious animals. These data confirm and extend the concept that circulating endogenous NOS inhibitors can constitute an excitatory signal to SNA.     

alpha(2)-Adrenergic receptors in NTS facilitate baroreflex function in adult spontaneously hypertensive rats

We examined the effect of alpha(2)-adrenoreceptor blockade in the nucleus of the solitary tract (NTS) on baroreflex responses elicited by electrical stimulation of the left aortic depressor nerve (ADN) in urethane-anesthetized spontaneously hypertensive rats (SHR, n = 11) and normotensive Wistar-Kyoto rats (WKY, n = 11). ADN stimulation produced a frequency-dependent decrease in mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and heart rate (HR). In SHR, unilateral microinjection of idazoxan into the NTS markedly reduced baroreflex control of MAP, RSNA, and HR and had a disproportionately greater influence on baroreflex control of MAP than of RSNA. In WKY, idazoxan microinjections did not significantly alter baroreflex function relative to control vehicle injections. These results suggest that baroreflex regulation of arterial pressure in SHR is highly dependent on NTS adrenergic mechanisms. The reflex regulation of sympathetic outflow to the kidney is less influenced by the altered alpha(2)-adrenoreceptor mechanisms in SHR.     

Long-term inhibition of nitric oxide synthesis increases arterial thrombogenecity in rat carotid artery

Reduced activity of endothelial nitric oxide (NO) may be involved in thrombus formation on atherosclerotic plaques, a major cause of acute coronary syndrome. However, mechanisms of such increase in arterial thrombogenecity have not been fully understood. We previously reported that long-term inhibition of NO synthesis by administration of N(G)-nitro-L-arginine methyl ester (L-NAME) causes hypertension and activates vascular tissue angiotensin-converting enzyme (ACE) activity. We used this model to investigate the mechanism by which long-term impairment of NO activity increases arterial thrombogenecity. We observed cyclic flow variations (CFVs), a reliable marker of platelet thrombi, after the production of stenosis of the carotid artery in rats treated with L-NAME for 4 wk. The thrombin antagonist argatroban suppressed the CFVs. The CFVs were detected in rats receiving L-NAME plus hydralazine but not in rats receiving L-NAME plus an ACE inhibitor (imidapril). Treatment with the ACE inhibitor imidapril, but not with hydralazine, prevented L-NAME-induced increases in carotid arterial ACE activity and attenuated tissue factor expression. These results suggest that long-term inhibition of endothelial NO synthesis may increase arterial thrombogenecity at least in part through angiotensin II-induced induction of tissue factor and the resultant thrombin generation. These data provide a new insight as to how endothelial NO exhibits antithrombogenic properties of the endothelium.     

Baroreflex modulation by angiotensins at the rat rostral and caudal ventrolateral medulla

We determined the effect of microinjection of ANG-(1-7) and ANG II into two key regions of the medulla that control the circulation [rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively)] on baroreflex control of heart rate (HR) in anesthetized rats. Reflex bradycardia and tachycardia were induced by increases and decreases in mean arterial pressure produced by intravenous phenylephrine and sodium nitroprusside, respectively. The pressor effects of ANG-(1-7) and ANG II (25 pmol) after RVLM microinjection (11 +/- 0.8 and 10 +/- 2 mmHg, respectively) were not accompanied by consistent changes in HR. In addition, RVLM microinjection of these angiotensin peptides did not alter the bradycardic or tachycardic component of the baroreflex. CVLM microinjections of ANG-(1-7) and ANG II produced hypotension (-11 +/- 1.5 and -11 +/- 1.9 mmHg, respectively) that was similarly not accompanied by significant changes in HR. However, CVLM microinjections of angiotensins induced differential changes in the baroreflex control of HR. ANG-(1-7) attenuated the baroreflex bradycardia (0.26 +/- 0.06 ms/mmHg vs. 0.42 +/- 0.08 ms/mmHg before treatment) and facilitated the baroreflex tachycardia (0.86 +/- 0.19 ms/mmHg vs. 0.42 +/- 0.10 ms/mmHg before treatment); ANG II produced the opposite effect, attenuating baroreflex tachycardia (0.09 +/- 0.06 ms/mmHg vs. 0.31 +/- 0.07 ms/mmHg before treatment) and facilitating the baroreflex bradycardia (0.67 +/- 0.16 ms/mmHg vs. 0.41 +/- 0.05 ms/mmHg before treatment). The modulatory effect of ANG II and ANG-(1-7) on baroreflex sensitivity was completely abolished by peripheral administration of methylatropine. These results suggest that ANG II and ANG-(1-7) at the CVLM produce a differential modulation of the baroreflex control of HR, probably through distinct effects on the parasympathetic drive to the heart.     

Effects of whole body heating on dynamic baroreflex regulation of heart rate in humans

The purpose of this project was to identify whether dynamic baroreflex regulation of heart rate (HR) is altered during whole body heating. In 14 subjects, dynamic baroreflex regulation of HR was assessed using transfer function analysis. In normothermic and heat-stressed conditions, each subject breathed at a fixed rate (0. 25 Hz) while beat-by-beat HR and systolic blood pressure (SBP) were obtained. Whole body heating significantly increased sublingual temperature, HR, and forearm skin blood flow. Spectral analysis of HR and SBP revealed that the heat stress significantly reduced HR and SBP variability within the high-frequency range (0.2-0.3 Hz), reduced SBP variability within the low-frequency range (0.03-0.15 Hz), and increased the ratio of low- to high-frequency HR variability (all P < 0.01). Transfer function gain analysis showed that the heat stress reduced dynamic baroreflex regulation of HR within the high-frequency range (from 1.04 +/- 0.06 to 0.54 +/- 0.6 beats. min(-1). mmHg(-1); P < 0.001) without significantly affecting the gain in the low-frequency range (P = 0.63). These data suggest that whole body heating reduced high-frequency dynamic baroreflex regulation of HR associated with spontaneous changes in blood pressure. Reduced vagal baroreflex regulation of HR may contribute to reduced orthostatic tolerance known to occur in humans during heat stress.     

Rho kinase and Ca2+ entry mediate increased pulmonary and systemic vascular resistance in L-NAME-treated rats

The small GTP-binding protein and its downstream effector Rho kinase play an important role in the regulation of vasoconstrictor tone. Rho kinase activation maintains increased pulmonary vascular tone and mediates the vasoconstrictor response to nitric oxide (NO) synthesis inhibition in chronically hypoxic rats and in the ovine fetal lung. However, the role of Rho kinase in mediating pulmonary vasoconstriction after NO synthesis inhibition has not been examined in the intact rat. To address this question, cardiovascular responses to the Rho kinase inhibitor fasudil were studied at baseline and after administration of an NO synthesis inhibitor. In the intact rat, intravenous injections of fasudil cause dose-dependent decreases in systemic arterial pressure, small decreases in pulmonary arterial pressure, and increases in cardiac output. L-NAME caused a significant increase in pulmonary and systemic arterial pressures and a decrease in cardiac output. The intravenous injections of fasudil after L-NAME caused dose-dependent decreases in pulmonary and systemic arterial pressure and increases in cardiac output, and the percent decreases in pulmonary arterial pressure in response to the lower doses of fasudil were greater than decreases in systemic arterial pressure. The Ca(++) entry blocker isradipine also decreased pulmonary and systemic arterial pressure in L-NAME-treated rats. Infusion of sodium nitroprusside restored pulmonary arterial pressure to baseline values after administration of L-NAME. These data provide evidence in support of the hypothesis that increases in pulmonary and systemic vascular resistance following L-NAME treatment are mediated by Rho kinase and Ca(++) entry through L-type channels, and that responses to L-NAME can be reversed by an NO donor.     

Hypotensive effect of ANG II and ANG-(1-7) at the caudal ventrolateral medulla involves different mechanisms

The objective of the present study was to determine the contribution of the autonomic nervous system and nitric oxide to the depressor effect produced by unilateral microinjection of ANG-(1-7) and ANG II into the caudal ventrolateral medulla (CVLM). Unilateral microinjection of ANG-(1-7), ANG II (40 pmol), or saline (100 nl) was made into the CVLM of male Wistar rats anesthetized with urethane before and after intravenous injection of 1) methyl-atropine, 2.5 mg/kg; 2) prazosin, 25 microg/kg; 3) the nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), 5 mg/kg; or 4) the specific inhibitor of neuronal NOS, 7-nitroindazole (7-NI), 45 mg/kg. Arterial pressure and heart rate (HR) were continuously monitored. Microinjection of ANG-(1-7) or ANG II into the CVLM produced a significant decrease in mean arterial pressure (MAP; -11 +/- 1 mmHg, n = 12 and -10 +/- 1 mmHg, n = 10, respectively) that was not accompanied by consistent changes in HR or in cardiac output. The effect of ANG-(1-7) was abolished after treatment with methyl-atropine (-3 +/- 0.6 mmHg, n = 9) or L-NAME (-2.3 +/- 0.5 mmHg, n = 8) or 7-NI (-2.8 +/- 0.6 mmHg, n = 5). In contrast, these treatments did not significantly interfere with the ANG II effect (-10 +/- 2.6 mmHg, n = 8; -8 +/- 1.5 mmHg, n = 8; and -12 +/- 3.6 mmHg, n = 6; respectively). Peripheral treatment with prazosin abolished the hypotensive effect of ANG-(1-7) and ANG II. Microinjection of saline did not produce any significant change in MAP or in HR. These results suggest that the hypotensive effect produced by ANG II at the CVLM depends on changes in adrenergic vascular tonus and, more importantly, the hypotensive effect produced by ANG-(1-7) also involves a nitric oxide-related mechanism.     

Transfer function analysis between arterial pressure and renal sympathetic nerve activity at cardiac pacing frequencies in the rat.

This study examined the possible influence of changes in heart rate (HR) on the gain of the transfer function relating renal sympathetic nerve activity (RSNA) to arterial pressure (AP) at HR frequency in rats. In seven urethane-anesthetized rats, AP and RSNA were recorded under baseline conditions (spontaneous HR = 338 +/- 6 beats/min, i.e., 5.6 +/- 0.1 Hz) and during 70-s periods of cardiac pacing at 6-9 Hz applied in random order. Cardiac pacing slightly increased mean AP (0.8 +/- 0.2 mmHg/Hz) and decreased pulse pressure (-3.6 +/- 0.3 mmHg/Hz) while leaving the mean level of RSNA essentially unaltered (P = 0.680, repeated-measures ANOVA). The gain of the transfer function from AP to RSNA measured at HR frequency was always associated with a strong, significant coherence and was stable between 6 and 9 Hz (P = 0.185). The transfer function gain measured under baseline conditions [2.44 +/- 0.28 normalized units (NU)/mmHg] did not differ from that measured during cardiac pacing (2.46 +/- 0.27 NU/mmHg). On the contrary, phase decreased linearly as a function of HR, which indicated the presence of a fixed time delay (97 +/- 6 ms) between AP and RSNA. In conclusion, the dynamic properties of arterial baroreflex pathways do not affect the gain of the transfer function between AP and RSNA measured at HR frequency in the upper part of the physiological range of HR variations in the rat.