Antonio M. Cabral

Chronic experimental myocardial infarction produces antinatriuresis by a renal nerve-dependent mechanism

The present study focused on the role of sympathetic renal nerve activity, in mediating congestive heart failure-induced sodium retention following experimental chronic myocardial infarction. Groups of male Wistar rats (240-260 g) were studied: sham-operated coronary ligation (CON3W, N = 11), coronary ligation and sham-operated renal denervation (INF3W, N = 19), 3 weeks of coronary ligation and sympathetic renal nerve denervation (INF3WDX, N = 6), sham-operated coronary ligation (N = 7), and 16 weeks of coronary ligation (INF16W, N = 7). An acute experimental protocol was used in which the volume overload (VO; 5% of body weight) was applied for 30 min after the equilibration period of continuous iv infusion of saline. Compared to control levels, VO produced an increase (P < 0.01, ANOVA) in urine flow rate (UFR; 570%) and urinary sodium excretion (USE; 1117%) in CON3W. VO induced a smaller increase (P < 0.01) in USE (684%) in INF3W. A similar response was also observed in INF16W. In INF3WDX, VO produced an immediate and large increase (P < 0.01) in UFR (547%) and USE (1211%). Similarly, in INF3W VO increased (P < 0.01) UFR (394%) and USE (894%). Compared with INF3W, VO induced a higher (P < 0.01) USE in INF3WDX, whose values were similar to those for CON3W. These results suggest that renal sympathetic activity may be involved in sodium retention induced by congestive heart failure. This premise is supported by the observation that in bilaterally renal denervated INF3WDX rats myocardial infarction was unable to reduce volume expansion-induced natriuresis. However, the mechanism involved in urinary volume regulation seems to be insensitive to the factors that alter natriuresis.

Neural reflex regulation of arterial pressure in pathophysiological conditions: interplay among the baroreflex, the cardiopulmonary reflexes and the chemoreflex

The maintenance of arterial pressure at levels adequate to perfuse the tissues is a basic requirement for the constancy of the internal environment and survival. The objective of the present review was to provide information about the basic reflex mechanisms that are responsible for the moment-to-moment regulation of the cardiovascular system. We demonstrate that this control is largely provided by the action of arterial and non-arterial reflexes that detect and correct changes in arterial pressure (baroreflex), blood volume or chemical composition (mechano- and chemosensitive cardiopulmonary reflexes), and changes in blood-gas composition (chemoreceptor reflex). The importance of the integration of these cardiovascular reflexes is well understood and it is clear that processing mainly occurs in the nucleus tractus solitarii, although the mechanism is poorly understood. There are several indications that the interactions of baroreflex, chemoreflex and Bezold-Jarisch reflex inputs, and the central nervous system control the activity of autonomic preganglionic neurons through parallel afferent and efferent pathways to achieve cardiovascular homeostasis. It is surprising that so little appears in the literature about the integration of these neural reflexes in cardiovascular function. Thus, our purpose was to review the interplay between peripheral neural reflex mechanisms of arterial blood pressure and blood volume regulation in physiological and pathophysiological states. Special emphasis is placed on the experimental model of arterial hypertension induced by N-nitro-L-arginine methyl ester (L-NAME) in which the interplay of these three reflexes is demonstrable.

Central α2-receptor mechanisms contribute to enhanced renal responses during ketamine-xylazine anesthesia

We have recently developed an experimental approach to study central opioid control of renal function in anesthetized rats. This model system uses the intravenous infusion of the α2-agonist xylazine to enhance basal levels of urine flow rate and urinary sodium excretion in ketamine-anesthetized rats. This study examined the contribution of central and peripheral α2-adrenergic receptor mechanisms in mediating the enhanced renal excretory responses produced by xylazine. In ketamine-anesthetized rats, the enhanced levels of urine flow rate and urinary sodium excretion produced by the intravenous infusion of xylazine were reversed by the intravenous bolus injection of the α2-adrenoceptor antagonist yohimbine but not by the α1-adrenoceptor antagonist terazosin. In separate animals the intracerebroventricular administration of yohimbine only reduced urine flow rate by ∼50% but did not alter urinary sodium excretion. The decrease in urine flow rate produced by intracerebroventricular yohimbine was reversed by the intravenous injection of a selective V2-vasopressin receptor antagonist. In a separate group of ketamine- and xylazine-anesthetized rats, the bilateral microinjection of yohimbine into the hypothalamic paraventricular nucleus (PVN) also significantly decreased urine flow rate by 54% without altering urinary sodium excretion. The microinjection of the β-adrenoceptor antagonist propranolol into the PVN did not alter either renal excretory parameter. These results suggest that during intravenous infusion, xylazine increases urine flow rate by activating α2-adrenergic receptors in the PVN, which in turn decrease vasopressin release. The ability of α-adrenergic mechanisms in the PVN to selectively influence the renal handling of water, but not sodium, may contribute to the reported dissociation of the natriuretic and diuretic responses of α2-adrenoceptor agonists.

The cardiopulmonary reflexes of spontaneously hypertensive rats are normalized after regression of left ventricular hypertrophy and hypertension

Cardiopulmonary reflexes are activated via changes in cardiac filling pressure (volume-sensitive reflex) and chemical stimulation (chemosensitive reflex). The sensitivity of the cardiopulmonary reflexes to these stimuli is impaired in the spontaneously hypertensive rat (SHR) and other models of hypertension and is thought to be associated with cardiac hypertrophy. The present study investigated whether the sensitivity of the cardiopulmonary reflexes in SHR is restored when cardiac hypertrophy and hypertension are reduced by enalapril treatment. Untreated SHR and WKY rats were fed a normal diet. Another groups of rats were treated with enalapril (10 mg kg-1 day-1, mixed in the diet; SHRE or WKYE) for one month. After treatment, the volume-sensitive reflex was evaluated in each group by determining the decrease in magnitude of the efferent renal sympathetic nerve activity (RSNA) produced by acute isotonic saline volume expansion. Chemoreflex sensitivity was evaluated by examining the bradycardia response elicited by phenyldiguanide administration. Cardiac hypertrophy was determined from the left ventricular/body weight (LV/BW) ratio. Volume expansion produced an attenuated renal sympathoinhibitory response in SHR as compared to WKY rats. As compared to the levels observed in normotensive WKY rats, however, enalapril treatment restored the volume expansion-induced decrease in RSNA in SHRE. SHR with established hypertension had a higher LV/BW ratio (45%) as compared to normotensive WKY rats. With enalapril treatment, the LV/BW ratio was reduced to 19% in SHRE. Finally, the reflex-induced bradycardia response produced by phenyldiguanide was significantly attenuated in SHR compared to WKY rats. Unlike the effects on the volume reflex, the sensitivity of the cardiac chemosensitive reflex to phenyldiguanide was not restored by enalapril treatment in SHRE. Taken together, these results indicate that the impairment of the volume-sensitive, but not the chemosensitive, reflex can be restored by treatment of SHR with enalapril. It is possible that by augmenting the gain of the volume-sensitive reflex control of RSNA, enalapril contributed to the reversal of cardiac hypertrophy and normalization of arterial blood pressure in SHR.

Oral administration of L-arginine decreases blood pressure and increases renal excretion of sodium and water in renovascular hypertensive rats

The two-kidney, one-clip renovascular (2K1C) hypertension model is characterized by a reduction in renal flow on the clipped artery that activates the renin-angiotensin system. Endothelium dysfunction, including diminished nitric oxide production, is also believed to play a role in the pathophysiology of this model. Some studies have shown an effect of L-arginine (L-Arg, a nitric oxide precursor) on hypertension. In the present study we determined the ability of L-Arg (7 days of treatment) to reduce blood pressure and alter renal excretions of water, Na+ and K+ in a model of 2K1C-induced hypertension. Under ether anesthesia, male Wistar rats (150-170 g) had a silver clip (0.20 mm) placed around the left renal artery to produce the 2K1C renovascular hypertension model. In the experimental group, the drinking water was replaced with an L-Arg solution (10 mg/ml; average intake of 300 mg/day) from the 7th to the 14th day after surgery. Sham-operated rats were used as controls. At the end of the treatment period, mean blood pressure was measured in conscious animals. The animals were then killed and the kidneys were removed and weighed. There was a significant reduction of mean blood pressure in the L-Arg-treated group when compared to control (129 7 vs 168 6 mmHg, N = 8-10 per group; P<0.05). Concomitantly, a significant enhancement of water and Na+ excretion was observed in the 2K1C L-Arg-treated group when compared to control (water: 13.0 0.7 vs 9.2 0.5 ml/day, P<0.01; Na+: 1.1 0.05 vs 0.8 0.05 mEq/day, respectively, P<0.01). These results show that orally administered L-Arg acts on the kidney, possibly inducing changes in renal hemodynamics or tubular transport due to an increase in nitric oxide formation.

Chlordiazepoxide and morphine reduce pressor response to brain stimulation in awake rats

The effects of intravenous as well as dorsal midbrain injections of morphine and chlordiazepoxide on the blood pressure rise induced by electrical stimulation of the dorsal periaqueductal gray matter (DPAG) were studied in unanesthetized rats. Chlordiazepoxide applied systemically or locally into the DPAG, as well as locally applied but not systemically injected morphine were found to attenuate the centrally-induced hypertension. These data together with others suggest that benzodiazepines as well as local injections of morphine into the DPAG decrease the aversive effect induced by DPAG stimulation.

Inhibition of Nitric Oxide Synthase Causes Profound Enhancement of the Bezold-Jarisch Reflex

The Bezold-Jarisch reflex function was evaluated in rats made hypertensive by the chronic oral intake of a nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, averaging 35 mg/kg/day), for 3, 6, and 12 days (n = 9/group) and in untreated control rats (CR, n = 9/group). L-NAME-treated rats showed a marked hypertension (MAP: 148 +/- 3, 182 +/- 4, and 179 +/- 4 mm Hg, respectively) compared with CR (110 +/- 2 mm Hg). The 6- and 12-day groups showed tachycardia (447 +/- 20 and 466 +/- 13 beats/min, respectively) when compared with CR (355 +/- 10 beats/min). When compared with CR, left ventricular hypertrophy was observed in rats treated with L-NAME for 6 and 12 days. The Bezold-Jarisch reflex, a decrease in heart rate (HR) accompanied by a decrease in diastolic arterial pressure (DAP), was evoked in a dose dependent manner by the intravenous injection of 5-hydroxytryptamine (5-HT, 5 to 10 microg/kg). Relative to responses observed in CR, 5-HT at 10 microg/kg caused a four- to fivefold greater decrease in HR and a two- to threefold greater decrease in DAP in all the L-NAME treatment groups. Using a Langendorff technique, we observed a significant increase in the responsiveness of the pacemaker to acetylcholine (1.25 to 80 microg/mL). These data suggest that the pharmacological inhibition of the nitric oxide synthase causes profound changes in the mechanisms of cardiovascular regulation as shown by a marked enhancement of the Bezold-Jarisch reflex in L-NAME-treated rats. The enhancement of this reflex seems to be in great part due to the hyperresponsiveness of the cardiac pacemaker to cholinergic stimulation.

Differential cardiovascular and renal responses produced by microinjection of the {kappa}-opioid U-50488H [(trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene-acetamide) methane sulfonate] into subregions of the paraventricular nucleus.

κ-Opioids produce a centrally mediated diuresis, antinatriuresis, and renal sympathoexcitation in vivo; however, the specific brain sites mediating these responses are unknown. This study examined the role of the hypothalamic paraventricular nucleus (PVN) and the renal sympathetic nerves in mediating the cardiovascular and renal responses to central κ-opioid receptor activation. In ketamine/xylazine-anesthetized rats, bilateral microinjection of the selective κ-agonist U-50488H [(trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene-acetamide) methane sulfonate; 100 ng] into the posterior magnocellular division of the PVN significantly increased urine flow rate (control, 47 ± 9 μl/min; 40 min, 108 ± 10 μl/min) without changing urinary sodium excretion or cardiovascular function. In other animals, microinjection of U-50488H into the same site elicited a similar water diuresis without a change in renal sympathetic nerve activity. In contrast, microinjection of U-50488H (100 ng) into the parvocellular PVN produced an immediate pressor response (Δ 16 ± 3 mm Hg) that occurred with a potential baroreflex evoked bradycardia (Δ -26 ± 8 beats per minute), renal sympathoinhibition (Δ -18 ± 4%), natriuresis (Δ 38 ± 1%), and delayed (30-min) antidiuresis (Δ -22 ± 9%). These results were prevented by pretreatment with the κ-receptor antagonist nor-binaltorphimine and were not obtained when U-50488H was injected outside the PVN, or when vehicle was injected into the PVN. Together, these results demonstrate that the posterior magnocellular PVN is a brain site where central κ-opioids act to produce diuresis, presumably by inhibiting the secretion of arginine vasopressin. Alternatively, central κ-opioids evoke antinatriuresis via augmenting renal sympathetic nerve activity and/or other neurohumoral sodium retaining pathways at brain sites other than the hypothalamic PVN.

Contractile performance of papillary muscles of renovascular hypertensive and isoproterenol-pretreated rats.

Previous studies suggested that left ventricle papillary muscles from neurogenic hypertensive rats with 15 days of sino-aortic denervation (SAD) develop intense depression of their inotropic state. Considering that these animals have an increased cardiac sympathetic tonus that could produce the observed depression of the contractile response we did the present study simulating a cardiac sympathetic hyperactivity pretreating rats, during 15 days, with isoproterenol (IPA, 0.1 mg/kg, i.p., divided in 3 daily doses). Because SAD rats also develop hypertension, the effect of blood pressure overload was studied in rats with 15 days of renovascular hypertension (RHR), Goldblatt 1K1C, to compare, separated, the effects of sympathetic hyperactivity and hemodynamic overload. Results showed that RHR had muscles that developed larger force than their controls (C: 7.31 +/- 0.34 g/mm2, RHR: 12.8 +/- 0.40 g/mm2), meanwhile IPA pretreated rats had muscles developing less force than their respective controls (C: 13.8 +/- 0.37 g/mm2, IPA: 8.61 +/- 0.24 g/mm2). These results are according to the proposition that the contractile state depression is related to the sympathetic hyperactivity, as suggested by the IPA pre-treated group and because the animals with isolated pressure overload showed a better contractile performance when compared to their uninephrectomized controls.

Cardiac baroreflex properties in myocardial infarcted rats

Recent studies demonstrated that chronic but not acute myocardial infarction impairs the cardiopulmonary reflex. The aim of the present study was to evaluate the baroreflex in awake rats bearing short-term (1 day) or long-term (30 days) myocardial infarction. Left ventricular infarction was produced by ligation of the anterior descending branch of the left coronary artery. In order to examine the baroreceptor reflex function by means of sigmoidal curvefitting analysis in conscious rats, reflex heart rate responses were elicited by alternate intravenous injections of phenylephrine (change, +5 to +40 mmHg) and sodium nitroprusside (change, -5 to -40 mmHg). Infarcted rats showed either hypotension plus tachycardia (1 day) or bradycardia (30 days) in resting conditions. The baroreceptor reflex gain (sensitivity) was significantly increased in 30 days (5.20 +/- 0.33 bpm/mmHg, p < 0.01) but not in 1 day (3.78 +/- 0.20 bpm/mmHg) infarcted rats when compared to sham rats (3.83 +/- 0.16 bpm/mmHg). Transmural antero-medio-lateral infarcted areas spanned over nearly 37% (1 day group) and 35% (30 days group) of the left ventricular circumference. Myocardial hypertrophy was showed in right ventricle (39%, p < 0.01) as well as in right (35%, P < 0.05) and left atria (127%, p < 0.001) in the 30 days but not in the 1 day infarcted group. The enhancement of baroreflex correlated significantly with the extent of myocardial necrosis in the 30 days infarcted group. We conclude that baroreflex control of heart rate is well preserved in short- but exaggerated in long-term myocardial infarction. The enhancement of the baroreflex gain could reflect a compensatory mechanism to the impairment of the cardiopulmonary reflex following chronic myocardial infarction and thus contributing to sustain the arterial pressure and heart rate in low levels.