A chronically instrumented rat model to assess the altered baroreflex due to exercise.

We developed a conscious, chronically instrumented, exercise-trained rat model and examined the time course of the training-induced alteration of baroreflex function in response to hypertensive conditions. Exercise-trained (ET) animals ran at 18 m.min-1, 15% grade, for 60 min.d-1, 5 d.wk-1 for 5 wk. Baroreflex tests were conducted on day 6 each week. Regression line slopes relating the change in mean arterial pressure (delta MAP) to the change in heart rate (delta HR) were used to assess baroreflex sensitivity. Intravenous injections of phenylephrine were used to create hypertensive conditions. Compared with the C group, slopes of ET animals were reduced (from 2.1 to 1.2 bpm.mm Hg-1, P < 0.05) as early as week 3 of training in response to increasing doses of PE, and reached 0.8 bpm.mm Hg-1 by the end of training. The reflex bradycardiac response (delta HR) to PE was reduced (P < 0.05) depending on the dose of PE and the duration of training: in micrograms PE.kg-1 body weight, 5 (71% +/- 6% of control at week 2), 3 (70% +/- 7% of control at week 3, and 1 (61% +/- 10% of control at week 4). The pressor (delta MAP) to PE remained constant throughout training. Thus, using a chronically instrumented rat model that maintains the ability to run, we observed that the ability of the arterial baroreflex to produce bradycardia during pressor events was substantially reduced following as few as 2 wk of training.

Naloxone-induced potentiation of cardiac alpha-2 adrenoceptor-mediated depression of neurogenic tachycardia.

The objective of this investigation was to test the hypothesis that naloxone directly activates alpha-2 adrenoceptors to cause depression of neurogenic tachycardia as suggested in an earlier investigation (Naloxone-Induced Bradycardia in Pithed Rats: Evidence for an Interaction with the Peripheral Sympathetic Nervous System and Alpha-2 Adrenoceptors. J. Pharmacol. Exp. Ther. 296: 916-926, 1992). Bolus doses of naloxone in a range of 10-1000 micrograms/kg i.v., administered in the presence of sustained neurogenic tachycardia (108 +/- 10 beats per min), resulted in a dose-dependent inhibition of neurogenic tachycardia with a maximum inhibitory response of 21% and an ED50 of 55 +/- 2.3 micrograms/kg. The inhibition of the naloxone-induced inhibition of neurogenic tachycardia was antagonized by phentolamine (5 mg/kg i.v.) and rauwolscine (0.5 mg/kg i.v.), but not prazosin (0.1 mg/kg i.v.). In the absence of sympathetic nerve activity, low doses of naloxone (10-300 micrograms/kg i.v.) had no effect on heart rate. These data suggest that naloxone in lower doses (10-1000 micrograms/kg i.v.) is a partial agonist at prejunctional alpha-2 adrenoceptors. In the presence of a steady-state maximum response (21% inhibition of neurogenic tachycardia) caused by naloxone infusion (100 and 1000 micrograms/kg/min i.v.), the ED50 of the preferential alpha-2 adrenoceptor agonist, UK14304-18, was not shifted to the right, but instead shifted to the left. This suggests that naloxone-induced depression of the neurogenic tachycardia does not involve the direct activation of alpha-2 adrenoceptors, but involves the potentiation of alpha-2 adrenoceptor-mediated inhibition of heart rate through an unknown mechanism.

Naloxone-induced bradycardia in pithed rats: evidence for an interaction with the peripheral sympathetic nervous system and alpha-2 adrenoceptors.

Earlier experiments performed in this laboratory have demonstrated that naloxone infusion (1 mg/kg/min i.v.) into conscious rats results in a bradycardia that has a peripheral component, is dependent on a certain level of sympathetic activity and is sensitive to alpha adrenoceptor blockade (5 mg/kg of phentolamine i.v.). The main objective of this investigation was to examine the underlying mechanism(s) responsible for the peripherally mediated naloxone-induced bradycardia, and to test the hypothesis that naloxone interacts with peripheral inhibitory alpha adrenoceptors associated with depression of peripheral sympathetic activity. Naloxone infusion (1 mg/kg/min i.v.) in pithed rats, in the absence of sympathetic nerve activation, resulted in a bradycardia that could not be blocked by 1 mg/kg (i.v.) of atropine, 5 mg/kg (i.v.) of phentolamine, 0.1 mg/kg (i.v.) of prazosin or 0.5 mg/kg (i.v.) of rauwolscine. Isoproterenol or norepinephrine-induced tachycardia was not blocked by naloxone infusion, suggesting that naloxone does not antagonize the postjunctional activation of cardiac adrenoceptors to cause bradycardia. In the presence of sympathetic nerve activity, naloxone depresses neurogenic tachycardia. This effect was blocked completely by 5 mg/kg (i.v.) of phentolamine or 0.5 mg/kg (i.v.) of rauwolscine, but not 0.1 mg/kg (i.v.) of prazosin or 1 mg/kg (i.v.) of atropine. The results of this investigation suggest that the naloxone-induced bradycardia in pithed rats is mediated postjunctionally and prejunctionally, and that this prejunctional effect is dependent on sympathetic nerve activity and inhibitory alpha-2 adrenoceptors. Furthermore, these results confirm results obtained from conscious rats in an earlier investigation.

A method for maintaining and protecting chronic arterial and venous catheters in conscious rats.

The ability to monitor arterial blood pressure and heart rate directly, as well as to sample venous blood, or inject pharmaceutical agents intravenously is important in pharmacological studies of the cardiovascular system. The rat is a frequently used and accepted animal model for cardiovascular investigations, especially those relating to hypertension. Even though the rat is a major model for these studies, the size of the rat has made it difficult to maintain catheters for a long period of time. Although there have been previous methods available, the authors report on an improved method to implant, maintain, and protect arterial and venous catheters in conscious rats for extended periods of time. A Silastic/Tygon catheter is implanted intraarterially and intravenously, exteriorized, and protected with a spring device. Catheters remained patent throughout a 5-week period during which time direct blood pressure recordings were obtained and baroreflexes were evaluated in conscious, unrestrained rats. The described design and methods provide an inexpensive means to maintain chronically implanted venous and arterial catheters in the conscious rat. Furthermore, rats may be gang housed.

Psychosocial stress elevates blood pressure via an opioid dependent mechanism in normotensive rats.

Stress is an important risk factor in cardiovascular diseases, including hypertension. Endogenous opioids are known to be elevated in stress and in various models of hypertension with differing etiologies. Blockade of endogenous opioids with naloxone has been demonstrated to attenuate or reverse the elevation in blood pressure in both renovascular and spontaneous hypertension. In the current study, increased blood pressure was induced using a model of psychosocial stress. During the first week of stress, systolic blood pressure rose rapidly to reach a level that was sustained throughout the remaining period of stress. Chronic infusion of the opiate antagonist, naloxone, both prevented and completely reversed the elevated blood pressure due to psychosocial stress. These data demonstrate that elevated endogenous opioids are important factors in cardiovascular regulation and are likely to influence both the development and maintenance of stress-induced hypertension.

Endogenous opiates and the pathogenesis of hypertension.

Opiates are now known to be important modulators of cardiovascular function in both the normotensive and hypertensive states. There is accumulating evidence that endogenous opiates are elevated in models of hypertension of various etiologies including genetic and renovascular hypertension. Early evidence for elevated opiates in hypertension arose from observations that hypertensive humans and rats with genetic or experimental hypertension exhibited hypoalgesia in various tests of pain sensitivity. Because pain and cardiovascular regulatory systems have in common a number of brain loci, cardiovascular effects of opiates and opiate blockade were studied. These studies have shown that opiate blockade can attenuate the development of hypertension and reduce blood pressure in chronic hypertension possibly via actions on the baroreflexes and/or by modulating the centrally mediated pressor actions of angiotensin II.

Endogenous opiate modulation of baroreflexes in normotensive and hypertensive rats.

It is evident that hypertension is associated with elevated endogenous opiates. This study was designed to examine the role of endogenous opiates in the development and/or maintenance of two-kidney renovascular hypertension and in baroreceptor reflex function in conscious hypertensive rats. Naloxone administration during the onset of hypertension significantly attenuated the rise in blood pressure. After one week, systolic blood pressure in naloxone-treated rats was 27 mmHg lower than in 0.9% NaCl-treated hypertensive rats. Acute naloxone infusions in chronic hypertensive animals also significantly lowered blood pressure (-10%) and heart rate (-26%). Baroreceptor function was significantly enhanced in both normotensive (+135%) and hypertensive (+207%) rats after administration of naloxone. Furthermore, naloxone treatment also caused the baroreflex response to shift from the higher reset state toward that seen in normotensive counterparts. The inability of naloxone methyl bromide to alter baroreflex sensitivity indicates that the site(s) of action of opiates resides in the brain. These data support a role for opiates in the development and/or maintenance of renovascular hypertension, which may be related to alterations in baroreceptor reflex function.

Cryoblockade of the ventromedial frontal cortex reverses hypertension in the rat.

The anteroventral part of the hypothalamus adjacent to the third ventricle (AV3V) has been implicated in electrolytic lesion studies as a site crucial to the development and maintenance of hypertension. Cryoblockade is known to alter synaptic and axonal transmission differently at different temperatures. In this study, cooling of the hypothalamus, including the AV3V area, to the temperature known to block only synaptic function did not alter blood pressure in two different models of experimental hypertension in the rat. Cooling sufficient to block both synaptic and axonal transmission, however, reduced blood pressure elevations to near normotensive levels. Synaptic cryoblockade in the ventromedial portion of the frontal cortex lowered experimental hypertension by 21 +/- 3 mm Hg (p less than 0.05). In normotensive controls, blood pressure was not altered by cryoblockade in either the frontal cortex or hypothalamus. Anatomical evidence provided by others shows that cells in the ventromedial frontal cortex project, in part, through the AV3V region to the brainstem cardioregulatory structures. These results indicate that neural activity arising in frontal cortex is axonally projected through the hypothalamus to maintain elevated blood pressure in experimental hypertension.

Chronic sodium depletion suppresses the area postrema pressor pathway.

Sodium depletion in dogs is known to affect both the renin-angiotensin as well as the sympathetic nervous system. The effect of this dietary regime upon the area postrema pressor pathway, as evaluated by the cardiovascular responses to centrally acting angiotensin II, has not been determined previously. With this in mind, male mongrel dogs were maintained on either a normal or a sodium restricted diet supplemented with furosemide and dose-response curves for intravertebral and intravenous angiotensin II (range: 1-20 ng/kg/min) were obtained. Sodium depletion results in not only a blunted intravenous pressor response to angiotensin II but also the abolition of the centrally mediated pressor responses mediated by the area postrema. Because accumulating evidence indicates that in sodium depleted dogs sympathetic nerve activity is reduced while central noradrenergic inhibitory activity is increased the reduced effects of angiotensin II upon the central sympathetically mediated pressor response may in part be related to decreases in sympathetic nerve activity.

Opioid modulation of baroreceptor reflex sensitivity in dogs.

Opioid peptides can modulate cardiovascular function both directly and by modifying specific responses. The action of opioids on baroreceptors was evaluated in anesthetized dogs. Baroreceptor responsiveness was determined by observing the peak heart rate and blood pressure responses as well as by evaluating the slope of a line relating heart period to blood pressure while pharmacologically altering blood pressure with doses of phenylephrine hydrochloride and sodium nitroprusside. These tests were performed before and after administration of naloxone (40 micrograms/kg). Naloxone significantly blunted baroreflex function (-55%). Treatment with atropine and vagotomy revealed that the action of naloxone appears to be dually mediated by the parasympathetic and sympathetic nervous systems. These data suggest that opioids can influence baroreflex function in the dog.

Suppression of renin release by antagonism of endogenous opiates in the dog.

The influence of blockade of endogenous opioids on the release of renin due to partial renal arterial constriction was determined acutely and chronically in unilaterally nephrectomized dogs. In acute preparations changes in plasma renin activity, arterial blood pressure, and heart rate were determined after 15 min of 60% renal arterial constriction before and after administration of either a saline vehicle, the opiate antagonist naloxone (0.05 mg/kg), or morphine (2 mg/kg). Acute antagonism of endogenous opiates abolished the increase in plasma renin activity and mean arterial pressure associated with renal arterial constriction. Repeated renal arterial constrictions in saline- or morphine-treated animals did not alter the humoral or hemodynamic responses. In chronic preparations long-term naloxone infusion attenuated the development of renovascular hypertension and diminished the increase in plasma renin activity. These data suggest that endogenous opioid peptides are modulators in the control of renin release and may be important participants in the pathogenesis of hypertension.

Cardiovascular effects of chronic naloxone infusion in normal dogs.

It has now been established that there is an interaction between the cardiovascular control systems and opiate substances. To assess the role of endogenous opiates in the long-term control of blood pressure, the opiate antagonist naloxone (0.5 mg/kg/24 h) was infused intravenously for 7 days. Chronic blockade of endogenous opiates resulted in an elevation in arterial blood pressure which averaged 11% and was sustained throughout the infusion period. This was accompanied by a significant decrease in plasma renin activity averaging -59%. These data provide support for a tonic role of opioids in the long-term control of the cardiovascular system which may involve baroreceptor function.

Neurogenic suppression of carotid sinus reflexes by vagal afferents in sodium-depleted dogs.

Reduction of sodium intake affects both the renin-angiotensin and sympathetic nervous systems, but the effects on the latter are less well understood. To clarify this relationship, the responsiveness of the baroreceptor reflex was compared in groups of normal and sodium-depleted dogs prepared with a single innervated carotid sinus and left vagus nerve after anesthesia with morphine-chloralose. Three weeks of sodium depletion did not affect the mean arterial blood pressure, but cardiac output (-50%) and stroke volume (-38%) were reduced, whereas there was a large compensatory increase in total peripheral resistance (115%). The pressor response to carotid occlusion in sodium-depleted dogs was significantly less (35 +/- 4 mmHg) than that obtained in normal dogs (61 +/- 5 mmHg). In the dogs treated with cholinergic blockade with atropine, this difference (-51%) persisted. In contrast, the pressor response to carotid occlusion in sodium-depleted dogs was significantly potentiated (112%) and restored in magnitude to that found in normal dogs after severing the only remaining vagus nerve. These data suggest that the cardiopulmonary and/or aortic vagal afferents are responsible for the blunting of baroreceptor reflexes during sodium depletion.

Central opiate system modulation of the area postrema pressor pathway.

Angiotensin II, when given into the vertebral arteries, acts at the area postrema to augment central sympathetic vasomotor activity. The mechanism of action is unknown but recent evidence implicates an interaction with the opiate system. In dogs anesthetized with chloralose either alone or in combination with morphine, naloxone blunted the pressor response to vertebrally administered angiotensin II by 50%. Addition of morphine to dogs anesthetized with chloralose only doubled the pressor response to identical doses of angiotensin II. On the other hand, the magnitude of the pressor responses to intravenously infused angiotensin II were unaltered by either naloxone or morphine. Likewise, responses to norepinephrine given vertebrally and intravenously were not similarly affected. Therefore, naloxone-induced changes in vascular responsiveness were not responsible for the altered sensitivity of the area postrema to angiotensin II following blockade of endogenous opiates. The data suggest that there exists a previously unrecognized interaction of the endogenous opiate system in the medulla in mediating the pressor effects of angiotensin II at the level of the area postrema.

Effect of chronic sodium depletion on cerebrospinal fluid and plasma catecholamines.

To test the role of central neurogenic factors in sodium-depleted states, cerebrospinal fluid (CSF) norepinephrine, epinephrine, and dopamine were measured in mongrel dogs first on a normal sodium intake (65 mEq sodium/day) and then on a 21-day regime of low sodium diet (4 mEq/day combined with diuretics). Plasma catecholamines were measured in the same group of dogs. Three weeks of sodium depletion supplemented with diuretics caused a 24-fold increase in plasma renin activity, hemoconcentration, and elevated serum protein concentration. Both plasma and CSF sodium decreased significantly. After sodium depletion, plasma norepinephrine rose 76% but epinephrine and dopamine did not change. The same pattern was observed whether samples were obtained in conscious or anesthetized animals. In CSF, norepinephrine rose 44% during sodium depletion, while epinephrine and dopamine remained unchanged. The CSF norepinephrine was related inversely to the CSF sodium concentration and directly to plasma renin activity. These observations support the view that the combined procedure of restricted dietary sodium intake and diuretic therapy causes alterations in CSF norepinephrine in a direction compatible with possible overactivity of central noradrenergic neurons.

The role of endogenous opiates in the area postrema pressor pathway.

1. Intra-vertebral artery-administered angiotensin II acts at the area postrema to facilitate central sympathetic vasomotor activity. Recent evidence suggests a possible role of the opiate system in the mechanism of action of angiotensin II at the level of the brain stem. 2. In these experiments, we show that the morphine antagonist naloxone reduces significantly the magnitude of the pressor response to vertebral artery-infused angiotensin II. 3. Morphine, in contrast, doubled the peak of the vertebral response to identical doses of the peptide. Neither naloxone nor morphine affected the pressor responses to intravenously administered angiotensin II. 4. The data suggest that the endogenous opiate system in the medulla modulates the cardiovascular effects of angiotensin II at the level of the area postrema.