Orexin neuron-mediated skeletal muscle vasodilation and shift of baroreflex during defense response in mice

We have previously shown that some features of the defense response, such as increases in arterial blood pressure (AP), heart rate (HR), and ventilation were attenuated in prepro-orexin knockout (ORX-KO) mice. Here, we examined whether the same was true in orexin neuron-ablated [orexin/ataxin-3 transgenic mice (ORX/ATX-Tg)] mice. In addition, we examined other features of the defense response: skeletal muscular vasodilation and shift of baroreceptor reflex. In both anesthetized and conscious conditions, basal AP in ORX/ATX-Tg mice was significantly lower by approximately 20 mmHg than in wild-type (WT) controls, as was the case in ORX-KO mice. The difference in AP disappeared after treatment with an alpha-blocker but not with a beta-blocker, indicating lower sympathetic vasoconstrictor outflow. Stimulation of the perifornical area (PFA) in urethane-anesthetized ORX/ATX-Tg mice elicited smaller and shorter-lasting increases in AP, HR, and ventilation, and skeletal muscle vasodilation than in WT controls. In addition, air jet stress-induced elevations of AP and HR were attenuated in conscious ORX/ATX-Tg mice. After pretreatment with a beta-blocker, atenolol, stimulation of PFA suppressed phenylephrine (50 microg/kg iv)-induced bradycardia (DeltaHR=-360+/-29 beats/min without PFA stimulation vs. -166+/-26 during stimulation) in WT. This demonstrated the resetting of the baroreflex. In ORX/ATX-Tg mice, however, no significant suppression was observed (-355+/-16 without stimulation vs. -300+/-30 during stimulation). The present study provided further support for our hypothesis that orexin-containing neurons in PFA play a role as a master switch to activate multiple efferent pathways of the defense response and also operate as a regulator of basal AP.     

Cardiovascular autonomic control in mice lacking angiotensin AT1a receptors

Studies examined the role of angiotensin (ANG) AT1a receptors in cardiovascular autonomic control by measuring arterial pressure (AP) and heart rate (HR) variability and the effect of autonomic blockade in mice lacking AT1a receptors (AT1a -/-). Using radiotelemetry in conscious AT1a +/+ and AT1a -/- mice, we determined 1) AP and pulse interval (PI) variability in time and frequency (spectral analysis) domains, 2) AP response to alpha(1)-adrenergic and ganglionic blockade, and 3) intrinsic HR after ganglionic blockade. Pulsatile AP was recorded (5 kHz) for measurement of AP and PI and respective variability. Steady-state AP responses to prazosin (1 microg/g ip) and hexamethonium (30 microg/g ip) were also measured. AP was lower in AT1a -/- vs. AT1a +/+, whereas HR was not changed. Prazosin and hexamethonium produced greater decreases in mean AP in AT1a -/- than in AT1a +/+. The blood pressure difference was marked after ganglionic blockade (change in mean AP of -44 +/- 10 vs. -18 +/- 2 mmHg, AT1a -/- vs. AT1a +/+ mice). Intrinsic HR was also lower in AT1a -/- mice (431 +/- 32 vs. 524 +/- 22 beats/min, AT1a -/- vs. AT1a +/+). Beat-by-beat series of systolic AP and PI were submitted to autoregressive spectral estimation with variability quantified in low-frequency (LF: 0.1-1 Hz) and high-frequency (HF: 1-5 Hz) ranges. AT1a -/- mice showed a reduction in systolic AP LF variability (4.3 +/- 0.8 vs. 9.8 +/- 1.3 mmHg(2)), with no change in HF (2.7 +/- 0.3 vs. 3.3 +/- 0.6 mmHg(2)). There was a reduction in PI variability of AT1a -/- in both LF (18.7 +/- 3.7 vs. 32.1 +/- 4.2 ms(2)) and HF (17.7 +/- 1.9 vs. 40.3 +/- 7.3 ms(2)) ranges. The association of lower AP and PI variability in AT1a -/- mice with enhanced AP response to alpha(1)-adrenergic and ganglionic blockade suggests that removal of the ANG AT1a receptor produces autonomic imbalance. This is seen as enhanced sympathetic drive to compensate for the lack of ANG signaling.     

Oxytocinergic regulation of cardiovascular function: studies in oxytocin-deficient mice

Oxytocin (OT) has been implicated in the cardiovascular responses to exercise, stress, and baroreflex adjustments. Studies were conducted to determine the effect of genetic manipulation of the OT gene on blood pressure (BP), heart rate (HR), and autonomic/baroreflex function. OT knockout (OTKO -/-) and control +/+ mice were prepared with chronic arterial catheters. OTKO -/- mice exhibited a mild hypotension (102 +/- 3 vs. 110 +/- 3 mmHg). Sympathetic and vagal tone were tested using beta(1)-adrenergic and cholinergic blockade (atenolol and atropine). Magnitude of sympathetic and vagal tone to the heart and periphery was not significantly different between groups. However, there was an upward shift of sympathetic tone to higher HR values in OTKO -/- mice. This displacement combined with unchanged basal HR led to larger responses to cholinergic blockade (+77 +/- 25 vs. +5 +/- 15 beats/min, OTKO -/- vs. control +/+ group). There was also an increase in baroreflex gain (-13.1 +/- 2.5 vs. -4.1 +/- 1.2 beats x min(-1) x mmHg(-1), OTKO -/- vs. control +/+ group) over a smaller BP range. Results show that OTKO -/- mice are characterized by 1) hypotension, suggesting that OT is involved in tonic BP maintenance; 2) enhanced baroreflex gain over a small BP range, suggesting that OT extends the functional range of arterial baroreceptor reflex; and 3) shift in autonomic balance, indicating that OT reduces the sympathetic reserve.     

Effects of neuronal norepinephrine uptake blockade on baroreflex neural and peripheral arc transfer characteristics

Neuronal uptake is the most important mechanism by which norepinephrine (NE) is removed from the synaptic clefts at sympathetic nerve terminals. We examined the effects of neuronal NE uptake blockade on the dynamic sympathetic regulation of the arterial baroreflex because dynamic characteristics are important for understanding the system behavior in response to exogenous disturbance. We perturbed intracarotid sinus pressure (CSP) according to a binary white noise sequence in anesthetized rabbits, while recording cardiac sympathetic nerve activity (SNA), arterial pressure (AP), and heart rate (HR). Intravenous administration of desipramine (1 mg/kg) decreased the normalized gain of the neural arc transfer function from CSP to SNA relative to untreated control (1.03 +/- 0.09 vs. 0.60 +/- 0.08 AU/mmHg, mean +/- SE, P < 0.01) but did not affect that of the peripheral arc transfer function from SNA to AP (1.10 +/- 0.05 vs. 1.08 +/- 0.10 mmHg/AU). The normalized gain of the transfer function from SNA to HR was unaffected (1.01 +/- 0.04 vs. 1.09 +/- 0.12 beats.min(-1).AU(-1)). Desipramine decreased the natural frequency of the transfer function from SNA to AP by 28.7 +/- 7.0% (0.046 +/- 0.007 vs. 0.031 +/- 0.002 Hz, P < 0.05) and that of the transfer function from SNA to HR by 64.4 +/- 2.2% (0.071 +/- 0.003 vs. 0.025 +/- 0.002 Hz, P < 0.01). In conclusion, neuronal NE uptake blockade by intravenous desipramine administration reduced the total buffering capacity of the arterial baroreflex mainly through its action on the neural arc. The differential effects of neuronal NE uptake blockade on the dynamic AP and HR responses to SNA may provide clues for understanding the complex pathophysiology of cardiovascular diseases associated with neuronal NE uptake deficiency.     

Diminished baroreflex control of heart rate responses in otoconia-deficient C57BL/6JEi head tilt mice

The maintenance of stable blood pressure during postural changes is known to involve integration of vestibular and cardiovascular central regulatory mechanisms. Sensory activity in the vestibular system plays an important role in cardiovascular regulation. The purpose of this study was to determine the role of vestibular gravity receptors in normal baroreflex function. Baroreflex heart rate (HR) responses to changes in blood pressure (BP) in otoconia-deficient head tilt (het) mice (n = 8) were compared with their wild-type littermates (n = 12). The study was carried out in conscious male mice chronically implanted with arterial and venous catheters for recording BP and HR and for the infusion of vasoactive drugs. Resting HR was higher in the het mice (661 +/- 13 beats/min) than in the wild-type mice (579 +/- 20 beats/min). BP was comparable in the het (113 +/- 4 mmHg) and wild-type mice (104 +/- 4 mmHg). The slopes of reflex decreases in HR in response to phenylephrine (PE) were blunted in the het mice (-5.5 +/- 1.5 beats x min(-1) x mmHg(-1)) compared with the wild-type mice (-8.5 +/- 0.9 beats x min(-1) x mmHg(-1)). Likewise, reflex tachycardic responses to decreases in BP with sodium nitroprusside (SNP) were significantly blunted in the het mice (-0.8 +/- 0.3 beats x min(-1) x mmHg(-1)) versus the wild-type mice (-2.2 +/- 0.6 beats x min(-1) x mmHg(-1)). Frequency-domain analysis of the HR variability suggests that under resting conditions, parasympathetic contribution was lower in the het versus wild-type mice. Mapping of the expression of immediate-early gene product, c-Fos, in forebrain and brain stem nuclei in response to a BP challenge showed no differences between the wild-type and het mice. These results suggest that tonic activity of gravity receptors modulates and is required for normal function of the cardiac baroreflexes.     

Effect of renal denervation on plasma renin activity after aortic baroreceptor deafferentation.

Renal nerves are thought to play an important role in cardiovascular regulation under both normotensive and hypertensive conditions. In the present study the effect of renal denervation on the changes in plasma renin activity (PRA) after aortic baroreceptor deafferentation (tADN) were investigated in the rat. Bilateral renal denervation did not alter arterial pressure (AP, 100 +/- 4 mmHg; 1 mmHg = 133.32 Pa), heart rate (HR, 363 +/- 12 bpm), or PRA (2.9 +/- 0.6 ng.mL-1.h-1) compared with the respective sham renal denervation values of 106 +/- 3 mmHg (AP), 385 +/- 13 bpm (HR), and 3.3 +/- 0.7 ng.mL-1.h-1 (PRA). On the other hand, bilateral tADN resulted in significant increases in AP, HR, and PRA. One and 3 days after tADN, AP was 130 +/- 4 and 127 +/- 6 mmHg, HR was 461 +/- 15 and 463 +/- 20 bpm, and PRA was 9.1 +/- 3.0 and 11.9 +/- 4.5 ng.mL-1.h-1, respectively. Renal denervation before tADN prevented the increases in AP and PRA, but it did not affect the increase in HR. These data indicate that renal denervation does not alter basal PRA in normotensive animals but prevents the increased renin release observed in neurogenic hypertension. These data suggest that the increased PRA may be one of several factors that contributes to the elevated AP after tADN.     

Deficiency in angiotensin AT1a receptors prevents diabetes-induced hypertension

The renin-angiotensin system has been implicated in the etiology of the cardiovascular complications of diabetes. Our studies extend these findings to show a specific role for angiotensin AT1a receptors in mediating diabetes-induced hypertension. Male angiotensin AT1a knockout (AT1aKO) and wild-type (AT1aWT) mice with arterial telemetric catheters were injected with streptozotocin (STZ; 150 mg/kg ip). The STZ dose was selected on the basis of a dose-response experiment in C57/BL mice. Blood glucose, water intake, body weight, blood pressure (BP), and heart rate (HR) were measured over a 2-wk period. Estimates of BP and HR variance (BPV and HRV) and their low- and high-frequency domains were also determined. STZ induced similar levels of hyperglycemia and polydypsia in the groups. Mean arterial pressure (MAP) was increased from 100 +/- 6 to 124 +/- 6 mmHg in diabetic AT1aWT. MAP was unchanged in AT1aKO (80 +/- 4 vs. 85 +/- 5 mmHg, basal vs. STZ). Treatment with an ACE inhibitor, captopril, produced a greater reduction in MAP (-18%) in diabetic AT1aWT than in AT1aKO (-3.4%). BPV was lower in AT1aKO (19 +/- 0.5 vs. 9 +/- 2 mmHg(2), AT1aWT vs. AT1aKO). Diabetes reduced BPV but only in AT1aWT (19 +/- 0.5 vs. 8 +/- 1 mmHg(2), basal vs. STZ). There were no changes in HR in either group. In AT1aKO, STZ increased HRV and its high-frequency domain with no changes seen in AT1aWT. Results document that ANG AT1a receptors are critical in diabetes-induced hypertension and in cardiac autonomic responses.     

Heart rate dynamics in iNOS knockout mice

Nitric oxide has both an inhibitory and excitatory role in the regulation of pre-ganglionic sympathetic neurons, involving the iNOS and nNOS systems respectively. The aim of the present study was to examine cardiovascular autonomic activity in iNOS knockout mice using spectral analysis of heart rate variability (HRV), and to determine the role of iNOS in altered HRV in endotoxaemia. Electrocardiograms were recorded in anaesthetised mice, and the R-R intervals digitized for spectral analysis of HRV and cardiac rhythm regularity using sample entropy analysis. The basal heart rate was higher in iNOS knockout mice compared with controls (465+/-8 vs 415+/-13 beat/min P<0.05), with a significant increase in the low frequency power of HRV spectra in iNOS knockout mice compared with controls (49.4+/-4.3 vs 33.8+/-5.6 normalized units, P<0.05), consistent with increased cardiac sympathetic activity. Endotoxaemia is known to decrease HRV, but the role of iNOS is unknown. LPS (20 mg/kg i.p) increased basal heart rate in both wild type and iNOS knockout mice, but caused a depression of HRV and sample entropy in both groups. Studies in isolated beating atria showed that the changes of HRV under basal or post-LPS conditions disappeared in vitro, suggesting that the autonomic system is responsible for altered HRV. We conclude that disruption of iNOS gene leads to an increase in the low frequency power of HRV consistent with increased cardiac sympathetic activity. These data also demonstrate that LPS-induced decrease of HRV is independent of iNOS.     

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.     

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.     

Cardiac response to submaximal exercise in dogs susceptible to sudden cardiac death

The hemodynamic response to submaximal exercise was investigated in 38 mongrel dogs with healed anterior wall myocardial infarctions. The dogs were chronically instrumented to measure heart rate (HR), left ventricular pressure (LVP), LVP rate of change, and coronary blood flow. A 2 min coronary occlusion was initiated during the last minute of an exercise stress test and continued for 1 min after cessation of exercise. Nineteen dogs had ventricular fibrillation (susceptible) while 19 animals did not (resistant) during this test. The cardiac response to submaximal exercise was markedly different between the two groups. The susceptible dogs exhibited a significantly higher HR and left ventricular end-diastolic pressure (LVEDP) but a significantly lower left ventricular systolic pressure (LVSP) in response to exercise than did the resistant animals. (For example, response to 6.4 kph at 8% grade; HR, susceptible 201.4 +/- 5.1 beats/min vs. resistant 176.2 +/- 5.6 beats/min; LVEDP, susceptible 19.4 +/- 1.1 mmHg vs. resistant 12.3 +/- 1.7 mmHg; LVSP, susceptible 136.9 +/- 7.9 mmHg vs. resistant 154.6 +/- 9.8 mmHg.) beta-Adrenergic receptor blockade with propranolol reduced the difference noted in the HR response but exacerbated the LVP differences (response to 6.4 kph at 8% grade; HR, susceptible 163.4 +/- 4.7 mmHg vs. resistant 150.3 +/- 6.4 mmHg; LVEDP susceptible 28.4 +/- 2.1 mmHg vs. resistant 19.6 +/- 3.0 mmHg; LVSP, susceptible 122.2 +/- 8.1 mmHg vs. resistant 142.8 +/- 10.7 mmHg). These data indicate that the animals particularly vulnerable to ventricular fibrillation also exhibit a greater degree of left ventricular dysfunction and an increased sympathetic efferent activity.     

Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (T(a)) and that the housing T(a) is typically 21-23 degrees C, well below the thermoneutral zone ( approximately 30 degrees C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in T(a) to demonstrate the sensitivity of the mouse cardiovascular system to T(a). We then determined HR in 1) mice deficient in cardiac sympathetic tone ("beta-less" mice), 2) mice deficient in cardiac vagal tone [muscarinic M(2) receptor (M(2)R(-/-)) mice], and 3) littermate controls. At a T(a) of 30 degrees C, the HR of beta-less mice was identical to that of wild-type mice (351 +/- 11 and 363 +/- 10 beats/min, respectively). However, the HR of M(2)R(-/-) mice was significantly greater (416 +/- 7 beats/min), demonstrating that vagal tone predominates over HR control at this T(a). When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, beta-less, and M(2)R(-/-) mice (DeltaHR = 73 +/- 9, 76 +/- 3, and 73 +/- 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the T(a) was relatively cool, at 23 degrees C, did beta-less mice exhibit a HR (442 +/- 14 beats/min) that was different from that of littermate controls (604 +/- 10 beats/min) and M(2)R(-/-) mice (602 +/- 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.     

Orthostatic hypotension in aging humans

We tested the hypothesis that hypotension occurred in older adults at the onset of orthostatic challenge as a result of vagal dysfunction. Responses of heart rate (HR) and mean arterial pressure (MAP) were compared between 10 healthy older and younger adults during onset and sustained lower body negative pressure (LBNP). A younger group was also assessed after blockade of the parasympathetic nervous system with the use of atropine or glycopyrrolate and after blockade of the beta(1)-adrenoceptor by use of metoprolol. Baseline HR (older vs. younger: 59 +/- 4 vs. 54 +/- 1 beats/min) and MAP (83 +/- 2 vs. 89 +/- 3 mmHg) were not significantly different between the groups. During -40 Torr, significant tachycardia occurred at the first HR response in the younger subjects without hypotension, whereas significant hypotension [change in MAP (DeltaMAP) -7 +/- 2 mmHg] was observed in the elderly without tachycardia. After the parasympathetic blockade, tachycardiac responses of younger subjects were diminished and associated with a significant hypotension at the onset of LBNP. However, MAP was not affected after the cardiac sympathetic blockade. We concluded that the elderly experienced orthostatic hypotension at the onset of orthostatic challenge because of a diminished HR response. However, an augmented vasoconstriction helped with the maintenance of their blood pressure during sustained LBNP.     

Impairment of sympathetic baroreceptor reflexes in obese Zucker rats

Adult obese Zucker rats (OZRs) have elevated sympathetic vasomotor tone and arterial pressure (AP) with blunted baroreflex-mediated changes in heart rate (HR) compared with adult lean Zucker rats (LZRs). The present study examined whether compromised cardiac baroreflexes are indicative of attenuated sympathetic responses. In addition, because juvenile OZRs have a normal mean AP, we determined whether baroreflexes are fully functional prior to hypertension. At 13 wk, adult OZRs had an elevated baseline mean AP compared with LZRs (137 +/- 3 vs. 123 +/- 5 mmHg, P < 0.05) under urethane anesthesia. Phenylephrine-induced increases in AP evoked smaller inhibitions of splanchnic sympathetic nerve activity (SNA) and HR in OZRs compared with LZRs. In addition, sympathoexcitatory responses to nitroprusside-induced hypotension were also blunted in OZRs. Sigmoid analysis revealed a decreased gain, a higher mean AP at the midpoint of the curve (AP(50)), and a reduced range of changes in SNA in OZRs. In contrast, at 7 wk of age, although juvenile OZRs weighed more than LZRs (313 +/- 13 vs. 204 +/- 4 g, P < 0.05), mean AP was comparable in both groups (122 +/- 5 vs. 121 +/- 4 mmHg, not significant). In these rats, rapid changes in AP evoked comparable changes in SNA and HR in OZRs and LZRs. Sigmoid analysis revealed that, although the gain of the reflex was blunted in OZRs (P < 0.05), the mean AP(50) and range of changes in SNA were comparable in OZRs and LZRs. Together, these data indicate that in adult OZRs, sympathetic responses to acute changes in AP are smaller than those observed in adult LZRs and that impairment of baroreceptor reflexes in OZR is not limited to the regulation of HR but extends to sympathetic vasomotor control. In addition, most of these deficits in baroreflex control of SNA develop in adulthood long after the onset of obesity and when other deficits in cardiovascular regulation are present.     

Knockout of the Na,K-ATPase α₂-isoform in the cardiovascular system does not alter basal blood pressure but prevents ACTH-induced hypertension

The α(2)-isoform of Na,K-ATPase (α(2)) is thought to play a role in blood pressure regulation, but the specific cell type(s) involved have not been identified. Therefore, it is important to study the role of the α(2) in individual cell types in the cardiovascular system. The present study demonstrates the role of vascular smooth muscle α(2) in the regulation of cardiovascular hemodynamics. To accomplish this, we developed a mouse model utilizing the Cre/LoxP system to generate a cell type-specific knockout of the α(2) in vascular smooth muscle cells using the SM22α Cre. We achieved a 90% reduction in the α(2)-expression in heart and vascular smooth muscle in the knockout mice. Interestingly, tail-cuff blood pressure analysis reveals that basal systolic blood pressure is unaffected by the knockout of α(2) in the knockout mice. However, knockout mice do fail to develop ACTH-induced hypertension, as seen in wild-type mice, following 5 days of treatment with ACTH (Cortrosyn; wild type = 119.0 ± 6.8 mmHg; knockout = 103.0 ± 2.0 mmHg). These results demonstrate that α(2)-expression in heart and vascular smooth muscle is not essential for regulation of basal systolic blood pressure, but α(2) is critical for blood pressure regulation under chronic stress such as ACTH-induced hypertension.     

Sex differences in the development of angiotensin II-induced hypertension in conscious mice

Sex has an important influence on blood pressure (BP) regulation. There is increasing evidence that sex hormones interfere with the renin-angiotensin system. Thus the purpose of this study was to determine whether there are sex differences in the development of ANG II-induced hypertension in conscious male and female mice. We used telemetry implants to measure aortic BP and heart rate (HR) in conscious, freely moving animals. ANG II (800 ng.kg(-1).min(-1)) was delivered via an osmotic pump implanted subcutaneously. Our results showed baseline BP in male and female mice to be similar. Chronic systemic infusion of ANG II induced a greater increase in BP in male (35.1 +/- 5.7 mmHg) than in female mice (7.2 +/- 2.0 mmHg). Gonadectomy attenuated ANG II-induced hypertension in male mice (15.2 +/- 2.4 mmHg) and augmented it in female mice (23.1 +/- 1.0 mmHg). Baseline HR was significantly higher in females relative to males (630.1 +/- 7.9 vs. 544.8 +/- 16.2 beats/min). In females, ANG II infusion significantly decreased HR. However, the increase in BP with ANG II did not result in the expected decrease in HR in either intact male or gonadectomized mice. Moreover, the slope of the baroreflex bradycardia to phenylephrine was blunted in males (-5.6 +/- 0.3 to -2.9 +/- 0.5) but not in females (-6.5 +/- 0.5 to -5.6 +/- 0.3) during infusion of ANG II, suggesting that, in male mice, infusion of ANG II results in a resetting of the baroreflex control of HR. Ganglionic blockade resulted in greater reduction in BP on day 7 after ANG II infusion in males compared with females (-61.0 +/- 8.9 vs. -36.6 +/- 6.6 mmHg), suggesting an increased contribution of sympathetic nerve activity in arterial BP maintenance in male mice. Together, these data indicate that there are sex differences in the development of chronic ANG II-induced hypertension in conscious mice and that females may be protected from the increases in BP induced by ANG II.     

Mice lacking melanin-concentrating hormone receptor 1 demonstrate increased heart rate associated with altered autonomic activity

Melanin-concentrating hormone (MCH) plays an important role in energy balance. The current studies were carried out on a new line of mice lacking the rodent MCH receptor (MCHR1(-/-) mice). These mice confirmed the previously reported lean phenotype characterized by increased energy expenditure and modestly increased caloric intake. Because MCH is expressed in the lateral hypothalamic area, which also has an important role in the regulation of the autonomic nervous system, heart rate and blood pressure were measured by a telemetric method to investigate whether the increased energy expenditure in these mice might be due to altered autonomic nervous system activity. Male MCHR1(-/-) mice demonstrated a significantly increased heart rate [24-h period: wild type 495 +/- 4 vs. MCHR1(-/-) 561 +/- 8 beats/min (P < 0.001); dark phase: wild type 506 +/- 8 vs. MCHR1(-/-) 582 +/- 9 beats/min (P < 0.001); light phase: wild type 484 +/- 13 vs. MCHR1(-/-) 539 +/- 9 beats/min (P < 0.005)] with no significant difference in mean arterial pressure [wild type 110 +/- 0.3 vs. MCHR1(-/-) 113 +/- 0.4 mmHg (P > 0.05)]. Locomotor activity and core body temperature were higher in the MCHR1(-/-) mice during the dark phase only and thus temporally dissociated from heart rate differences. On fasting, wild-type animals rapidly downregulated body temperature and heart rate. MCHR1(-/-) mice displayed a distinct delay in the onset of this downregulation. To investigate the mechanism underlying these differences, autonomic blockade experiments were carried out. Administration of the adrenergic antagonist metoprolol completely reversed the tachycardia seen in MCHR1(-/-) mice, suggesting an increased sympathetic tone.     

Baroreflex depression persists in the early phase after hypertension reversal

The baroreflex control of heart rate (HR) was evaluated in conscious chronic renal hypertensive rats (RHR; 1K-1C, 2 mo) under control conditions and after reversal of hypertension by unclipping the renal artery or sodium nitroprusside infusion. Unclipping and nitroprusside infusion were both followed by significant decreases in the mean arterial pressure (unclipping: from 199 +/- 4 to 153 +/- 8 mmHg; nitroprusside infusion: from 197 +/- 9 to 166 +/- 6 mmHg) as well as slight and significant increases, respectively, in the baroreflex bradycardic response index (unclipping: from 0.2 +/- 0.04 to 0.6 +/- 0.1 beats x min(-1) x mmHg(-1); nitroprusside infusion: from 0.1 +/- 0.04 to 0.5 +/- 0.1 beats x min(-1) x mmHg(-1)). However, this index was still depressed compared with that for normotensive control rats (2.1 +/- 0.2 beats x min(-1) x mmHg(-1)). The index for the baroreflex tachycardic response was also depressed under control conditions and remained unchanged after hypertension reversal. RHR possessed markedly attenuated vagal tone as demonstrated by pharmacological blockade of parasympathetic and sympathetic control of HR with methylatropine and propranolol, respectively. A reduced bradycardic response was also observed in anesthetized RHR during electrical stimulation of the vagus nerve or methacholine chloride injection, indicating impairment of efferent vagal influence over the HR. Together, these data indicate that 2 h after hypertension reversal in RHR, the previously described normalization of baroreceptor gain occurs independent of the minimal or lack of recovery of baroreflex control over HR.     

Intrarenal infusion of bradykinin elicits a pressor response in conscious rats via a B2-receptor mechanism.

Bradykinin (BK) is a peptide known to activate afferent nerve fibers from the kidney and elicit reflex changes in the cardiovascular system. The present study was specifically designed to test the hypothesis that bradykinin B2 receptors mediated the pressor responses elicited during intrarenal bradykinin administration. Pulsed Doppler flow probes were positioned around the left renal artery to measure renal blood flow (RBF). A catheter, to permit selective intrarenal administration of BK, was advanced into the proximal left renal artery. The femoral artery was cannulated to measure mean arterial pressure (MAP). MAP, heart rate (HR), and RBF were recorded from conscious unrestrained rats while five-point cumulative dose-response curves during an intrarenal infusion of BK (5-80 microg x kg(-1) x min(-1)) were constructed. Intrarenal infusion of BK elicited dose-dependent increases in MAP (maximum pressor response, 26+/-3 mmHg), accompanied by a significant tachycardia (130+/-18 beats/min) and a 28% increase in RBF. Ganglionic blockade abolished the BK-induced increases in MAP (maximum response, -6+/-5 mmHg), HR (maximum response 31+/-14 beats/min), and RBF (maximum response, 7+/-2%). Selective intrarenal B2-receptor blockade with HOE-140 (50 microg/kg intrarenal bolus) abolished the increases in MAP and HR observed during intrarenal infusion of BK (maximum MAP response, -2+/-3 mmHg; maximum HR response, 15+/-11 beats/min). Similarly, the increases in RBF were prevented after HOE-140 treatment. In fact, after HOE-140, intrarenal BK produced a significant decrease in RBF (22%) at the highest dose of BK. Results from this study show that the cardiovascular responses elicited by intrarenal BK are mediated predominantly via a B2-receptor mechanism.     

Quantitative evaluation of ontogenetic change in heart rate and its autonomic regulation in newborn mice with the use of a noninvasive piezoelectric sensor

A reliable basal heart rate (HR) measurement in freely moving newborn mice was accomplished for the first time by using a novel noninvasive piezoelectric transducer (PZT) sensor. The basal HR was approximately 320 beats/min at postnatal day (P)0 and increased with age to approximately 690 beats/min at P14. Contribution of autonomic control to HR was then assessed. Sympathetic blockade with metoprolol significantly reduced basal HR at both P6 (-236 +/- 23 beats/min; mean +/- SE) and P12 (-105 +/- 8 beats/min), but atropine was without effect, indicating the predominant tonic adrenergic stimulation and absence of vagal control for basal HR in newborn mice. In contrast to stable basal HR during 5-min recording, HR measured by ECG (ECG-HR) was markedly decreased because of the restraint stress of attaching ECG electrodes, with accompanying freezing behavior. ECG-HR lowered and further decreased gradually during 5 min (slow cardiodeceleration) at P0-P3 and rapidly decreased and gradually recovered within 5 min (transient bradycardia) at P9-P14. The response was not uniform in P4-P8 mice: they showed either of these two patterns or sustained bradycardia (9-29%), and the number of mice that showed transient bradycardia increased with age (30-100%) during the period. Studies with autonomic blockade suggest that the slow cardiodeceleration and transient bradycardia are mediated mainly by withdrawal of adrenergic stimulation and phasic vagal activation, respectively, and the autonomic control of HR response to restraint stress is likely to change from the withdrawal of adrenergic stimulation to the phasic vagal activation at different stages during P4-P8 in individual mice. The PZT sensor may offer excellent opportunities to monitor basal HR of small animals noninvasively.