Pressor response to chemoreflex activation in awake rats: role of vasopressin and adrenal medulla

Vasopressin: a possible link between hypoxia and hypertension

Cardiovascular and respiratory diseases are frequently associated with transient and prolonged hypoxia, whereas hypoxia exerts pro-hypertensive effects, through stimulation of the sympathetic system and release of pressor endocrine factors. This review is focused on the role of arginine vasopressin (AVP) in dysregulation of the cardiovascular system during hypoxia associated with cardiovascular disorders. AVP is synthesized mainly in the neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON), which send axons to the posterior pituitary and various regions of the central nervous system (CNS). Vasopressinergic neurons are innervated by multiple neuronal projections releasing several neurotransmitters and other regulatory molecules. AVP interacts with V1a, V1b and V2 receptors that are present in the brain and peripheral organs, including the heart, vessels, lungs, and kidneys. Release of vasopressin is intensified during hypernatremia, hypovolemia, inflammation, stress, pain, and hypoxia which frequently occur in cardiovascular patients, and blood AVP concentration is markedly elevated in cardiovascular diseases associated with hypoxemia. There is evidence that hypoxia stimulates AVP release through stimulation of chemoreceptors. It is suggested that acting in the carotid bodies, AVP may fine-tune respiratory and hemodynamic responses to hypoxia and that this effect is intensified in hypertension. There is also evidence that during hypoxia, augmentation of pro-hypertensive effects of vasopressin may result from inappropriate interaction of this hormone with other compounds regulating the cardiovascular system (catecholamines, angiotensins, natriuretic peptides, steroids, nitric oxide). In conclusion, current literature indicates that abnormal mutual interactions between hypoxia and vasopressin may significantly contribute to pathogenesis of hypertension.

Effect of chronic stress on cardiovascular and ventilatory responses activated by both chemoreflex and baroreflex in rats

Chronic stress results in physiological and somatic changes. It has been recognized as a risk factor for several types of cardiovascular dysfunction and changes in autonomic mechanisms, such as baroreflex and chemoreflex activity. However, the effects of different types of chronic stress on these mechanisms are still poorly understood. Therefore, in the present study, we investigated, in adult male rats, the effect of repeated restraint stress (RRS) or chronic variable stress (CVS) on baroreflex, chemoreflex and heart rate variability in a protocol of 14 days of stress sessions. Exposure to RRS and CVS indicated no changes in the basal level of either arterial pressure or heart rate. However, RRS and CVS were able to attenuate sympathovagal modulation and spontaneous baroreflex gain. Additionally, only RRS was able to increase the power of the low-frequency band of the systolic blood pressure spectrum, as well as the slope of linear regression of baroreflex bradycardic and tachycardic responses induced by vasoactive compounds. Additionally, our study is one of the first to show that exposure to RRS and CVS decreases the magnitude of the pressor response and potentiates respiratory responses to chemoreflex activation, which can trigger cardiovascular and respiratory pathologies. Furthermore, the basal respiratory parameters, such as minute ventilation and tidal volume, were significantly decreased by both protocols of chronic stress. However, only CVS increased the basal respiratory frequency. In this way, the findings of the present study demonstrate the impact of chronic stress in terms of not only depressive-like behavior but also alterations of the autonomic baroreflex responses and cardiocirculatory variability (systolic blood pressure and pulse interval).Our results provide evidence that chronic stress promotes autonomic dysregulation, and impairment of baroreflex, chemoreflex and heart rate variability.

α2 Receptors in the lateral parabrachial nucleus generates the pressor response of the cardiovascular chemoreflex, effects of GABAA receptor

The lateral parabrachial nucleus (LPBN) is a pontine area involved in cardiovascular chemoreflex. This study was performed to find the effects of reversible synaptic blockade of the LPBN on the chemoreflex responses, and to find the roles of GABA_A receptor and α₂-adenoreceptor (α₂-AR) in chemoreflex. It also aimed to seek possible interaction between GABA and noradrenergic systems of the LPBN in urethane-anesthetized male rats. Cardiovascular chemoreflex was activated by intravenous injection of potassium cyanide (KCN, 80 μg/kg). The cardiovascular responses of chemoreflex were evaluated before (control), 5 and 15 min after microinjection of each drug (100 nl) into the LPBN. Microinjections of cobalt chloride (5 mM), a reversible synaptic blocker, into the LPBN greatly attenuated the chemoreflex pressor and bradycardic responses indicating that the LPBN plays a main role in chemoreflex. Local injection of yohimbine (10 nmol), an α₂-AR antagonist, attenuated the pressor response with no effect on bradycardic response, suggesting that α₂-adrenoreceptors are involved in producing the pressor response of the chemoreflex. Microinjection of bicuculline methiodide (BMI, 100 pmol), a GABA_A antagonist, into the LPBN augmented the pressor response and attenuated the bradycardic response, indicating that GABA inhibits the sympathetic output to the heart and vasculature. Sequential injection of yohimbine and BMI had no significant effect on the pressor response but attenuated the bradycardia. In conclusion, the LPBN is essential for the chemoreflex responses. The pressor response of the chemoreflex, at least partly, is produced by α₂- adenoreceptors. GABA in the LPBN inhibits the cardiovascular system. Finally, there is no interaction between GABAergic and adrenergic neurons of the LPBN in producing the cardiovascular chemoreflex.

Ventral hippocampus modulates bradycardic response to peripheral chemoreflex activation in awake rats

NEW FINDINGS

What is the central question of this study? Does reversible synaptic inactivation by CoCl2 in the dorsal (DH) or ventral (VH) portions of the hippocampus have a modulatory effect on cardiovascular and respiratory responses evoked by chemoreflex activation in awake rats? What is the main finding and its importance? Using i.v. infusion of KCN to activate the peripheral chemoreflex before and after microinjection of CoCl2 into VH, we showed that the bradycardic response was increased, but not the pressor and tachypnoeic responses even if the tidal volume had been increased. Thus, VH but not DH may be involved in the modulation of the parasympathoexcitatory component of the peripheral chemoreflex. In rats, peripheral chemoreflex activation evokes pressor and bradycardic responses as well as a tachypnoeic response. Studies have shown that limbic structures, such as the hippocampus, can modulate autonomic reflexes. Evidence suggests that the dorsal (DH) and the ventral (VH) portions of the hippocampus are structurally and functionally distinct; therefore, in the present study we tested the hypothesis that local neurotransmission of the DH and VH are involved in the neural pathways of the cardiovascular and ventilatory responses to chemoreflex activation. Thus, the goal of the present study was to compare the chemoreflex responses elicited by i.v. injection of KCN (40 μg per rat) in awake rats before and after DH and VH synaptic transmission was temporarily inhibited by bilateral microinjections of 500 nl of the unspecific synapse blocker, CoCl2 (1  mm). Bilateral inhibition of VH, but not DH, 10 min before KCN infusion was able to enhance the bradycardic response (P < 0.05), with no changes in the typical pressor and tachypnoeic responses evoked by chemoreflex activation (P > 0.05). Furthermore, the tidal volume was significantly increased (P < 0.05) even though no other respiratory parameter had been significantly changed (P > 0.05), suggesting that VH can exert a tonic modulatory action on tidal volume. Therefore, the present study reports, for the first time, that DH neurotransmission did not exert an influence on chemoreflex responses, whereas VH mediates, at least in part, the parasympathoexcitatory component of the peripheral chemoreflex.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.

Detecting activation of the sympatho-adrenal axis from haemodynamic recordings, in conscious rabbits exposed to acute stress

AIMS

When assessing sympathetic activation in acute stress, the attention is often limited to the sympatho-neural axis, whereas sympatho-adrenal activation, that can only be detected with poor time resolution from the concentration of plasma catecholamines, is often neglected. This study is aimed at re-investigating the role and the relevance of the sympatho-adrenal system in acute stress based on the analysis of haemodynamic responses in conscious rabbits.

METHODS

Experiments were carried out on 19 rabbits implanted with chronic probes for arterial blood pressure and for blood flow in the facial artery. Cardiovascular responses to a randomized sequence of acute stressors (pinprick, air jet, oscillation of the cage, inhalation of formaldehyde vapours and im injection of hypertonic saline) were recorded before and after α-adrenergic blockade (phentolamine) and unilateral section of the cervical sympathetic trunk (decentralization). Plasma catecholamine concentrations were analysed in four animals.

RESULTS

All stressors induced an increase in arterial blood pressure and a reduction of vascular conductance in the facial artery ranging on average from 24% (pinprick) to 55% (box oscillation). Such vasoconstrictor response was abolished by phentolamine. In decentralized arteries, the vasoconstriction was delayed by 10-15 s and decreased in magnitude in a stressor-dependent way, indicating an adrenaline-mediated effect in the late phase of the stress response that was confirmed by changes in plasma adrenaline concentration.

CONCLUSIONS

In conscious rabbits, rapid release of adrenaline makes a prominent contribution to vasoconstriction in response to different stressors including box oscillation, muscle pain and air jet but not the nasopharyngeal stimulation.

Modulation of arterial pressure by P2 purinoceptors in the paraventricular nucleus of the hypothalamus of awake rats

In the present study we evaluated the role of purinergic mechanisms in the PVN on the tonic modulation of the autonomic function to the cardiovascular system as well on the cardiovascular responses to peripheral chemoreflex activation in awake rats. Guide-cannulae were bilaterally implanted in the direction of the PVN of male Wistar rats. Femoral artery and vein were catheterized one day before the experiments. Chemoreflex was activated with KCN (80 μg/0.05 ml, i.v.) before and after microinjections of P2 receptors antagonist into the PVN. Microinjection of PPADS, a non selective P2X antagonist, into the PVN (n=6) produced a significant increase in the baseline MAP (99±2 vs 112±3 mmHg) and HR (332±8 vs 375±8 bpm) but had no effect on the pressor and bradycardic responses to chemoreflex activation. Intravenous injection of vasopressin receptors antagonist after microinjection of PPADS into the PVN produced no effect on the increased baseline MAP. Simultaneous microinjection of PPADS and KYN into the PVN (n=6) had no effect in the baseline MAP, HR or in the pressor and bradycardic responses to chemoreflex activation. We conclude that P2 purinoceptors in the PVN are involved in the modulation of baseline autonomic function to the cardiovascular system but not in the cardiovascular responses to chemoreflex activation in awake rats.

GABA and nitric oxide in the PVN are involved in arterial pressure control but not in the chemoreflex responses in rats

GABAergic, nitrergic and glutamatergic mechanisms in the PVN on the baseline mean arterial pressure (MAP), heart rate (HR) and on the cardiovascular responses to chemoreflex activation in awake rat were evaluated. Chemoreflex was activated with KCN before and after microinjections into the PVN. Bicuculline into the PVN increased baseline MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (49+/-5 vs 47+/-6 mmHg) or bradicardic (-213+/-23 vs -256+/-42 bpm) responses (n=7). Kynurenic acid into the PVN (n=6) produced no significant changes in the MAP (98+/-3 vs 100+/-3 mmHg), HR (330+/-5 vs 339+/-12 mmHg) or in the pressor (50+/-4 vs 42+/-4 mmHg) and bradicardic (-252+/-4 vs -285+/-16 bpm) responses to chemoreflex. L-NAME into the PVN (n=8) produced increase in the MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (52+/-5 vs 47+/-6 mmHg) or bradicardic (-253+/-19 vs -320+/-25 bpm) responses to chemoreflex. We conclude that GABA(A) and nitric oxide in the PVN are involved in the maintenance of the baseline MAP but not in the modulation of the responses to chemoreflex. The results also show that Glutamate receptors in the PVN are not involved in maintenance of the baseline MAP, HR or in the cardiovascular responses to chemoreflex in awake rats.