Alcohol abolishes the hypotensive effect of clonidine in spontaneously hypertensive rats

Reduced Renal Mass, Salt-Sensitive Hypertension Is Resistant to Renal Denervation

Aim: Activation of the sympathetic nervous system is common in resistant hypertension (RHT) and also in chronic kidney disease (CKD), a prevalent condition among resistant hypertensives. However, renal nerve ablation lowers blood pressure (BP) only in some patients with RHT. The influence of loss of nephrons per se on the antihypertensive response to renal denervation (RDNx) is unclear and was the focus of this study.

Methods: Systemic hemodynamics and sympathetically mediated low frequency oscillations of systolic BP were determined continuously from telemetrically acquired BP recordings in rats before and after surgical excision of ∼80% of renal mass and subsequent RDNx.

Results: After reduction of renal mass, rats fed a high salt (HS) diet showed sustained increases in mean arterial pressure (108 ± 3 mmHg to 128 ± 2 mmHg) and suppression of estimated sympathetic activity (∼15%), responses that did not occur with HS before renal ablation. After denervation of the remnant kidney, arterial pressure fell (to 104 ± 4 mmHg), estimated sympathetic activity and heart rate (HR) increased concomitantly, but these changes gradually returned to pre-denervation levels over 2 weeks of follow up. Subsequently, sympathoinhibition with clonidine did not alter arterial pressure while significantly suppressing estimated sympathetic activity and HR.

Conclusion: These results indicate that RDNx does not chronically lower arterial pressure in this model of salt-sensitive hypertension associated with substantial nephron loss, but without ischemia and increased sympathetic activity, thus providing further insight into conditions likely to impact the antihypertensive response to renal-specific sympathoinhibition in subjects with CKD.

Reduced cardiac contractile force due to sympathovagal dysfunction mediates the additive hypotensive effects of limited-access regimens of ethanol and clonidine in spontaneously hypertensive rats

Our previous attempts to investigate the long-term hemodynamic interaction between ethanol and clonidine in telemetered spontaneously hypertensive rats (SHRs) were hampered by the lack of a sustained hypotensive response to continuous clonidine exposure. This limitation was circumvented when we adopted a limited-access clonidine (8:30 AM-4:30 PM) paradigm in a recent study. The latter paradigm was employed here to evaluate the ethanol-clonidine interaction and possible roles of myocardial function and autonomic control in this interaction. Changes in blood pressure (BP), heart rate, maximum rate of rise in BP wave (+dP/dt(max)), and spectral cardiovascular autonomic profiles were measured by radiotelemetry in pair-fed SHRs receiving clonidine (150 μg/kg/day), ethanol [2.5% (w/v)], or their combination during the day for 12 weeks. Ethanol or clonidine elicited long-term decreases in BP, and their combination caused additive hypotensive response. Significant reductions in +dP/dt(max) were observed upon concurrent treatment with ethanol and clonidine, in contrast to no effect for individual treatment. In addition, the combined treatment increased the high-frequency (HF) spectral band of interbeat interval (IBI-HF(nu), 0.75-3 Hz) and decreased low-frequency (IBI-LF(nu), 0.2-0.75 Hz) bands and IBI(LF/HF) ratios. Clonidine-evoked reductions in plasma and urine norepinephrine and BP-LF spectral power (measure of vasomotor sympathetic tone) were not affected by ethanol. In conclusion, concurrent treatment with ethanol and clonidine shifts the sympathovagal balance toward parasympathetic dominance and elicits exaggerated hypotension as a result of a reduction in cardiac contractile force.

Alpha 2A-adrenergic receptor signaling underlies synergistic enhancement of ethanol-induced behavioral impairment by clonidine

BACKGROUND

We tested the hypothesis that central alpha(2A)-adrenergic receptor (alpha(2A)AR) signaling plays a key role in clonidine-ethanol evoked synergistic behavioral impairment.

METHODS

Male Sprague-Dawley rats, with intracisternal and jugular vein cannulae implanted 6 days earlier, were tested for drug-induced behavioral impairment. The latter was assessed as the duration of loss of righting reflex (LORR) and rotorod performance every 15 minutes until the rat recovered to the baseline walk criterion (180 seconds). In a separate cohort, c-Fos expression in locus coeruleus (LC) and cerebellum was determined as a marker of neuronal activity following drug treatment.

RESULTS

Rats that received clonidine (60 microg/kg, i.v.) followed by ethanol (1 g/kg, i.v.) exhibited synergistic impairment of rotorod performance and LORR. The mixed alpha(2A)AR and I(1)-imidazoline receptor agonist clonidine (30, 60, and 90 microg/kg) synergistically and dose-dependently enhanced behavioral impairment elicited by ethanol (1 g/kg). Possible involvement of I(1)-imidazoline receptors was ruled out because selective I(1)-agonist rilmenidine (300 microg/kg, i.v.) did not cause behavioral impairment alone or enhance ethanol-evoked behavioral impairment. Pharmacological blockade of central alpha(2A)AR (RX821002, 0.3 mg i.c.) abolished the synergy between clonidine and ethanol; the behavioral response caused by the drug combination was similar to that caused by ethanol alone. Conversely, involvement of central alpha(2B)AR in the interaction was ruled out because blockade of central alpha(2B)AR (ARC-239) independently evoked a strong sedative effect. Clonidine (60 microg/kg) or ethanol (1 g/kg) alone increased, but their combination decreased, c-Fos levels in LC, while inconsistent c-Fos responses were observed in cerebellum.

CONCLUSIONS

Central alpha(2A)AR, but not I(1)-imidazoline or alpha(2B)AR, signaling is implicated in the synergistic enhancement of ethanol-evoked behavioral impairment by clonidine. Although the mechanism of c-Fos response remains to be investigated, this neurochemical response highlights the LC as a neuroanatomical target for clonidine-ethanol behavioral interaction.

Role of Myocardial Contractility and Autonomic Control in the Hypotensive Response to a Limited Access Ethanol Paradigm in SHRs

BACKGROUND

Previous experimental studies that evaluated the chronic hemodynamic effect of ethanol employed the continuous exposure protocol of ethanol, which does not mimic the pattern of alcohol consumption in humans. This study dealt with the long-term hemodynamic and cardiovascular autonomic effects of ethanol, in a limited-access regimen in telemetered spontaneously hypertensive rats (SHRs).

METHODS

Changes in blood pressure (BP), heart rate (HR), myocardial contractility (dP/dt(max)), and spectral cardiovascular autonomic profiles during the ethanol exposure period (2.5 or 5% w/v, 8 h/d, 8:30 am till 4:30 pm) were followed for 12 weeks.

RESULTS

Compared with control pair-fed SHRs, body weight and urine output, osmolality, and potassium levels were decreased in SHRs receiving 5% but not 2.5% ethanol. Blood pressure showed progressive falls during ethanol-feeding periods with a maximum effect observed at week 5. The peak hypotensive effect was maintained thereafter in SHRs receiving 5% ethanol in contrast to steady rises in BP in the 2.5% ethanol group to near-control levels by the conclusion of the study. Heart rate was slightly but significantly increased by ethanol 5% whereas dP/dt(max) showed persistent reductions. Power spectral analysis showed that ethanol attenuated the baroreflex gain of HR as suggested by the reductions in index alpha, the spectral index of spontaneous baroreflex sensitivity (BRS).

CONCLUSIONS

It is concluded that limited access ethanol drinking in SHRs elicited hypotension that was concentration dependent and mediated, at least partly, through reductions in myocardial contractility. Baroreflex sensitivity attenuation by ethanol appeared to have limited the tachycardic response to ethanol and perhaps its capacity to offset the evoked hypotension.

Alveolar fluid reabsorption is increased in rats with compensated heart failure

Alveolar fluid reabsorption (AFR) is important in keeping the air spaces free of edema. This process is accomplished via active transport of Na+across the alveolo-capillary barrier mostly by apical Na+channels and basolateral Na+-K+-ATPases. Recently, we have reported that acute elevation of left atrial pressures is associated with decreased AFR in isolated rat lungs. However, the effect of chronic elevation of pulmonary capillary pressure, such as seen in patients with congestive heart failure (CHF), on AFR is unknown. CHF was induced by creating an aorto-caval fistula (ACF) in Sprague-Dawley male rats. Seven days after the placement of the fistula, AFR was studied in the isolated perfused rat lung model. AFR in control rats was 0.49 ± 0.02 ml/h (all values are means ± SE) and increased by ∼40% (0.69 ± 0.03 ml/h) in rats with chronic CHF ( P < 0.001). The albumin flux from the pulmonary circulation into the air spaces did not increase in the experimental groups, indicating that lung permeability for large solutes was not increased. Na+-K+-ATPase activity and protein abundance at the plasma membrane of distal alveolar epithelial tissue were significantly increased in CHF rats compared with controls. These changes were associated with increased plasma norepinephrine levels in CHF rats compared with controls. We provide evidence that in a rat model of chronic compensated CHF, AFR is increased, possibly due to increased endogenous norepinephrine upregulating active sodium transport and protecting against alveolar flooding.

Contrasting effects of chronic ethanol feeding on centrally and peripherally evoked hypotension in telemetered female rats

Our previous studies have shown that ethanol compromises hypotension produced by centrally acting antihypertensive agents in normotensive and hypertensive male rats. The present study investigated whether female rats are as susceptible as male rats to the deleterious effect of ethanol on centrally evoked hypotension. The study was extended to investigate ethanol effects on the time-domain indices of variability in blood pressure [standard deviation of mean arterial pressure (SDMAP)] and heart rate [standard deviation of beat-to-beat intervals (SDRR) and root mean square of successive differences in R-R intervals (rMSSD)]. The hemodynamic effects of a single intraperitoneal dose of clonidine (30 microg/kg) were evaluated in radiotelemetered ethanol-fed (5%, 12 weeks) and pair-fed control Sprague-Dawley rats. In control rats, clonidine caused a significant reduction in MAP that continued for at least 6 h and was associated with reductions in SDMAP and SDRR but not rMSSD, suggesting inhibition of central sympathetic tone. A maximum hypotensive response of -16.4+/-1.7 mm Hg was demonstrated 40 min after clonidine administration. Ethanol feeding significantly attenuated clonidine hypotension whereas it potentiated the associated reduction in SDMAP. To verify the selectivity of ethanol-clonidine interaction, the effects of ethanol on peripherally mediated hemodynamic responses to hydralazine (0.5 mg/kg ip) were investigated. In contrast to its antagonistic effect on clonidine hypotension, ethanol significantly potentiated the hypotensive effect of hydralazine. Together, these findings demonstrate that chronic ethanol feeding exerts opposite effects on centrally (attenuation) and peripherally (potentiation) evoked hypotension in female rats. The interaction of ethanol with antihypertensive agents may not be related to changes in hemodynamic variability.

Chronic ethanol administration attenuates imidazoline I1 receptor- or alpha 2-adrenoceptor-mediated reductions in blood pressure and hemodynamic variability in hypertensive rats

Our previous studies have demonstrated that acute ethanol administration counteracts imidazoline I(1) receptor but not alpha(2)-adrenoceptor-mediated hypotension in spontaneously hypertensive rats (SHR). In the present study, we investigated the effect of chronic ethanol administration on hypotensive responses elicited by acute administration of selective imidazoline I(1) receptor (rilmenidine) or alpha(2)-adrenoceptor (alpha-methyldopa) agonist along with ethanol effects on: (i) locomotor activity and (ii) time-domain indices of variability in blood pressure (standard deviation of mean arterial pressure) and heart rate (standard deviation of beat-to-beat intervals and root mean square of successive differences in R-R intervals). Hemodynamic and locomotor responses elicited by rilmenidine or alpha-methyldopa were assessed in radiotelemetered ethanol-fed (2.5% or 5% w/v, 12 week) and control SHR. In control SHR, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) significantly reduced blood pressure. Rilmenidine had no effect on heart rate whereas alpha-methyldopa elicited a biphasic response (tachycardia followed by bradycardia). Blood pressure and heart rate oscillations were also reduced by both drugs, which may conform to sympathoinhibition. The hypotensive effect of rilmenidine or alpha-methyldopa was significantly attenuated by ethanol feeding (2.5% or 5%) in a concentration-dependent manner. In addition, ethanol attenuated alpha-methyldopa-evoked reduction in heart rate, but not blood pressure, variability in marked contrast to attenuating rilmenidine-evoked reductions in blood pressure, but not heart rate, variability. These findings demonstrate that, unlike its acute effects, chronic ethanol attenuates both imidazoline I(1) receptor and alpha(2)-adrenoceptor-mediated hypotension whereas its effect on hemodynamic variability depended on the nature of the hypotensive stimulus.

Chronic ethanol–clonidine hemodynamic interaction in telemetered spontaneously hypertensive rats

Our previous studies have established that ethanol, administered acutely or chronically, attenuates the hypotensive action of the centrally acting drug clonidine. In this study, we employed the radiotelemetry technique to evaluate the long-term hemodynamic interaction between the two drugs administered simultaneously to spontaneously hypertensive rats (SHRs). Changes in blood pressure (BP), heart rate (HR), and their variability was determined in pair-fed rats receiving ethanol (5%, w/v), clonidine pellets (10 mg/pellet, s.c.), or their combination for 28 days. Ethanol feeding caused significant decreases and increases in BP and HR, respectively. The time-domain variability indices of BP and HR were also reduced by ethanol. Clonidine produced significant reductions in BP and HR that were evident for only 1-2 days and disappeared thereafter. In rats receiving the combined ethanol and clonidine treatment, hemodynamic changes were identical to those produced by ethanol alone. These findings suggest (i) long-term exposure of SHRs to moderate amounts of ethanol reduces HR variability, possibly due to diminished cardiac vagal modulation, and (ii) the lack of a maintained hypotensive response to clonidine, administered via timed-release pellets, made the evaluation of its chronic interaction with ethanol unfeasible.

Effects of chronic ethanol feeding on clonidine-evoked reductions in blood pressure, heart rate, and their variability: time-domain analyses

The effects of chronic ethanol administration on the acute hemodynamic effects of clonidine were investigated in conscious radiotelemetered spontaneously hypertensive rats (SHRs). Changes evoked by clonidine (30 micro g/kg i.p.) in blood pressure, heart rate, and their variability were evaluated in ethanol [2.5 or 5% (w/v), 12 weeks] and pair-fed control rats. The blood pressure variability was determined as the standard deviation of the mean arterial pressure (SDMAP). Two heart rate variability indices were used, the standard deviation of beat-to-beat intervals (SDRR) and the root mean square of successive beat-to-beat differences in R-R interval durations (rMSSD). Compared with control rats, ethanol (2.5 and 5%)-fed rats exhibited concentration-related reductions in mean arterial pressure (MAP) and SDMAP versus no change in heart rate variability. In control rats, clonidine caused a significant reduction in MAP that continued for at least 5 h and was associated with significant reductions in SDMAP, SDRR, and rMSSD, responses that are consistent with the inhibition of central sympathetic tone. The hypotensive effect of clonidine was attenuated by ethanol in a concentration-related manner. The maximum reductions in MAP elicited by clonidine in ethanol (2.5 and 5%)-fed rats amounted to -23.4 +/- 2.8 and -15.1 +/- 1.5 mm Hg, respectively, compared with -35.4 +/- 1.2 mm Hg in control rats. The clonidine-induced reductions in SDMAP, SDRR, and rMSSD were also significantly attenuated by ethanol. These findings suggest that the attenuation of MAP and heart rate variability responses elicited by clonidine in ethanol-fed SHRs reflects alterations in the sympathovagal balance, which may be implicated in the antagonistic hemodynamic interaction between the two drugs.

Effect of long-term ethanol feeding on brainstem alpha(2)-receptor binding in Wistar-Kyoto and spontaneously hypertensive rats

Our previous studies have shown that ethanol attenuates baroreflex function in Wistar-Kyoto (WKY) but not in spontaneously hypertensive rats (SHRs). The present study determined the effects of chronic ethanol administration on alpha(2)-binding sites in brainstem areas that modulate baroreflexes. In vitro autoradiography was utilized to evaluate the effect of a 3-month ethanol feeding on the density (B(max)) and affinity (K(D)) of alpha(2)-adrenoceptors in the middle (mNTS) and rostral (rNTS) portions of the nucleus tractus solitarius of SHRs and WKY rats. Autoradiographic examination of brainstem sections preincubated with [125I]p-iodoclonidine revealed no inter-strain differences in alpha(2)-binding in control rats. Ethanol feeding caused strain-dependent changes in alpha(2)-binding activity, which comprised significant (P<0.05) decreases in the density of alpha(2)-binding sites in both areas of the NTS in SHRs versus no effect in WKY rats. These findings do not favor a role for brainstem alpha(2)-adrenoceptors in ethanol-induced attenuation of baroreflexes. Interestingly, the ethanol-evoked reduction in the NTS alpha(2)-receptor density in SHRs may explain reported findings that ethanol abolishes the hypotensive effect of the alpha(2)-adrenoceptor agonist clonidine in this rat model.

Ovariectomy alters the chronic hemodynamic and sympathetic effects of ethanol in radiotelemetered female rats

This study determined the chronic hemodynamic effects of ethanol in telemetered freely moving female Sprague-Dawley rats. The role of ovarian hormones and sympathetic activity in the modulation of ethanol responses was also investigated. Changes in blood pressure (BP), heart rate (HR), and plasma estrogen and norepinephrine (NE, as index of sympathetic activity) were evaluated in pair-fed sham-operated (SO) and ovariectomized (OVX) rats receiving liquid diet with or without ethanol (5%, w/v) for 12 weeks. OVX caused a significant increase (about 40 g) in body weight, compared with the sham operation, which was apparent after two weeks and remained so for the duration of the study. The body weight showed gradual and similar increases in both ethanol and control groups. Ethanol feeding had no effect on the plasma estrogen level in SO or OVX rats. Daily ethanol intake was significantly (P < 0.05) less in OVX compared with SO rats whereas the blood ethanol concentration were similar in the two groups except for a significantly (P < 0.05) higher level in OVX rats at weeks 8, 10, and 11. Ethanol feeding caused significant (P < 0.05) decreases in BP in SO rats that started at week land reached maximal response (approximately 10 mmHg) at week 6 and remained at that level till the end of week 12. In OVX rats, ethanol had no effect on BPduring the first 5 weeks of the study. A slight but significant reduction in BP (about 5 mmHg) by ethanol in OVX rats started to appear at week 6 and remained for the following 5 weeks. The reduced hypotensive effect of ethanol in OVX rats was associated with an increase in the sympathetic activity as indicated by the significant (P < 0.05) increases in plasma NE levels. This sympathoexcitatory effect of ethanol was not demonstrated in SO rats. The HR was not affected by ethanol in the two groups of rats except for significant (P < 0.05) increases at weeks 1 through 3 in SO rats. The present findings suggest that the ovarian hormones modulate, at least partly, the hemodynamic and neurohumoral effects of chronic ethanol feeding in female rats. Ethanol lowers BP in female rats and this effect was delayed and diminished in OVX rats due possibly to the associated increase in sympathetic activity.

Ethanol selectively counteracts hypotension evoked by central I(1)-imidazoline but not alpha2-adrenergic receptor activation in spontaneously hypertensive rats

Previous studies from our laboratory showed that ethanol counteracts hypotensive responses to clonidine in spontaneously hypertensive rats. This study investigated whether this effect of ethanol involves interaction with central alpha2-adrenoceptors or I(1)-imidazoline receptors or both. The effects of ethanol (0.5 or 1 g/kg, i.v.) or an equal volume of saline on hypotensive and bradycardic responses to clonidine (mixed alpha2-adrenoceptor/I(1)-imidazoline receptor agonist), rilmenidine (selective I(1)-imidazoline receptor agonist), or alpha-methylnorepinephrine (selective alpha2-adrenoceptor agonist) were studied in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (0.5 microg), rilmenidine (25 microg), or alpha-methylnorepinephrine (4 microg) elicited similar decreases in mean arterial pressure (MAP; 25-30 mm Hg) that lasted > or =60 min. Subsequent administration of ethanol (0.5 and 1 g/kg, i.v.) counteracted the hypotensive effect of clonidine in a dose-related manner. Ethanol (1 g/kg) increased the blood pressure to levels similar to baseline (preclonidine) levels, and blood pressure remained significantly (p < 0.05) higher compared with the corresponding values in saline-treated rats. Similarly, ethanol (0.5 and 1 g/kg, i.v.) dose-dependently counteracted the hypotensive effect of rilmenidine. The antagonizing effects of ethanol on hypotension evoked by clonidine and rilmenidine were comparable in terms of both magnitude and duration. In contrast, ethanol (0.5 or 1 g/kg) had no effect on hypotension evoked by alpha-methylnorepinephrine. Except for a brief increase in blood pressure by ethanol (1 g/kg) at 5 min, blood pressure values obtained in alpha-methylnorepinephrine-treated rats receiving any of the two doses of ethanol were similar to postsaline values. Ethanol had no effect on bradycardic responses to any of the three hypotensive agents. Blood ethanol concentrations were similar regardless of the antihypertensive drug used. We concluded that the adverse hemodynamic effect of ethanol on centrally mediated hypotensive responses depends on the types of receptors involved in the elicitation of this response. That ethanol counteracts decreases in blood pressure evoked by clonidine and rilmenidine but not by alpha-methylnorepinephrine suggests an interaction between ethanol and central pathways involved in I(1)-imidazoline receptor-mediated hypotension.

Role of cardiac output in ethanol-evoked attenuation of centrally mediated hypotension in conscious rats

Our previous studies have shown that ethanol selectively counteracts centrally mediated hypotensive responses. This study investigated the role of cardiac output and peripheral resistance in the antagonistic interaction between ethanol and antihypertensive drugs. Changes in blood pressure, heart rate, cardiac index, stroke volume, and peripheral resistance elicited by clonidine and subsequent ethanol or saline administration were evaluated in conscious rats. The aortic barodenervated rat was employed because it exhibits greater hypotensive responses to clonidine compared with the intact rat. Aortic barodenervation elicited acute rises in blood pressure, heart rate, and peripheral resistance, whereas cardiac index and stroke volume were not altered. The blood pressure of conscious aortic barodenervated rats returned to sham-operated levels by 48 hours due to concomitant reductions in cardiac index and stroke volume; the peripheral resistance, however, remained significantly elevated. Clonidine (30 microg/kg, I.V.) elicited greater decreases in blood pressure in aortic barodenervated compared with sham-operated rats. The hypotension was caused by decreases in cardiac index and stroke volume because peripheral resistance did not change. Ethanol (1 g/kg, I.V.) counteracted the hypotensive effect of clonidine and raised blood pressure to levels higher than preclonidine values. Significant (P<.05) increases in cardiac index and stroke volume and decreases in peripheral resistance accompanied the pressor effect of ethanol. Additional control groups were included in the study to determine the selectivity of the interaction. A dose of hydralazine (0.5 mg/kg, I.V.) was used that produced similar hypotension to that evoked by clonidine in aortic barodenervated rats. Hydralazine-evoked hypotension was similar in denervated and control rats and resulted from significant reductions in peripheral resistance. Reflex increases in heart rate and stroke volume and hence cardiac output were observed. Ethanol given after hydralazine produced a short-lived pressor effect (<5 minutes versus 40 minutes in the case of clonidine) and counteracted the sympathetically mediated increases in cardiac output, stroke volume, and heart rate. These findings support our hypothesis that ethanol selectively counteracts hypotensive responses of central origin by reversing the reduction in cardiac output elicited by clonidine.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.