A differential action for ethanol on baroreceptor reflex control of heart rate and sympathetic efferent discharge in rats

Effect of Voluntary Ethanol Consumption Combined with Testosterone Treatment on Cardiovascular Function in Rats: Influence of Exercise Training

This study evaluated the effects of voluntary ethanol consumption combined with testosterone treatment on cardiovascular function in rats. Moreover, we investigated the influence of exercise training on these effects. To this end, male rats were submitted to low-intensity training on a treadmill or kept sedentary while concurrently being treated with ethanol for 6 weeks. For voluntary ethanol intake, rats were given access to two bottles, one containing ethanol and other containing water, three 24-hour sessions per week. In the last two weeks (weeks 5 and 6), animals underwent testosterone treatment concurrently with exercise training and exposure to ethanol. Ethanol consumption was not affected by either testosterone treatment or exercise training. Also, drug treatments did not influence the treadmill performance improvement evoked by training. However, testosterone alone, but not in combination with ethanol, reduced resting heart rate. Moreover, combined treatment with testosterone and ethanol reduced the pressor response to the selective α₁-adrenoceptor agonist phenylephrine. Treatment with either testosterone or ethanol alone also affected baroreflex activity and enhanced depressor response to acetylcholine, but these effects were inhibited when drugs were coadministrated. Exercise training restored most cardiovascular effects evoked by drug treatments. Furthermore, both drugs administrated alone increased pressor response to phenylephrine in trained animals. Also, drug treatments inhibited the beneficial effects of training on baroreflex function. In conclusion, the present results suggest a potential interaction between toxic effects of testosterone and ethanol on cardiovascular function. Data also indicate that exercise training is an important factor influencing the effects of these substances.

Facilitation of central imidazoline I(1)-site/extracellular signal-regulated kinase/p38 mitogen-activated protein kinase signalling mediates the hypotensive effect of ethanol in rats with acute renal failure

BACKGROUND AND PURPOSE

This study investigated the role of central sympathetic activity and related mitogen-activated protein kinase (MAPK) signalling in the cardiovascular effects of ethanol in a model of acute renal failure (ARF).

EXPERIMENTAL APPROACH

The effects of pharmacological interventions that inhibit peripheral or central sympathetic activity or MAPK on the cardiovascular actions of ethanol in rats with ARF induced by glycerol were evaluated.

KEY RESULTS

Glycerol (50%, 10 mL.kg(-1), i.m.) caused progressive increases and decreases in blood pressure (BP) and heart rate (HR) respectively. Subsequent i.v. ethanol (0.25 or 1 g.kg(-1)) elicited dose-related changes in BP (decreases) and HR (increases). These effects were replicated after intracisternal (i.c.) administration of ethanol. Blockade of nicotinic cholinoceptors (nAChR, hexamethonium, 20 mg.kg(-1)) or alpha(1)-adrenoceptors (prazosin, 1 mg.kg(-1)) attenuated cardiovascular effects of ethanol. Ethanol hypotension was also attenuated after the centrally acting sympatholytic drug moxonidine (selective I(1)-site agonist, 100 microg.kg(-1) i.v.), but not guanabenz (selective alpha(2)-receptor agonist, 30 microg.kg(-1), i.v.), suggesting involvement of central circuits of I(1) sites in ethanol-evoked hypotension. Selective blockade I(1) sites (efaroxan) but not alpha(2) (yohimbine) adrenoceptors abolished the hypotensive response to ethanol. Intracisternal administration of PD98059 or SB203580, inhibitors of extracellular signal-regulated kinase (ERK 1/2) and p38 MAPK, respectively, reduced the hypotensive action of moxonidine or ethanol. When used simultaneously, the two MAPK inhibitors produced additive attenuation of ethanol hypotension.

CONCLUSIONS AND IMPLICATIONS

Sympathoinhibitory pathways of central I(1)-sites and downstream ERK/p38 MAPK signalling were involved in the hypotensive action of ethanol in ARF.

Chronic ethanol feeding potentiates alpha1-adrenoceptor responsiveness in SHR aortas

Previous studies including ours demonstrated a hypotensive response to ethanol in spontaneously hypertensive rats (SHRs). In this study, we investigated whether this hypotensive effect of ethanol involves alterations in vascular alpha1-adrenergic receptor responsiveness. The contractile responses to the alpha1-receptor agonist phenylephrine were evaluated in aortic rings obtained from pair-fed SHRs receiving liquid diet with or without ethanol (2.5% or 5%, w/v) for 3 months. The responses were measured in aortas with and without endothelium to determine the role of the endothelium in the observed responses. The liquid diet intake was similar in the control and ethanol groups throughout the study whereas the body weight was significantly reduced by ethanol. Cumulative addition of phenylephrine (1 x 10(-9)-1 x 10(-4) M) caused concentration-related contractile responses. These responses were significantly reduced after endothelium denudation suggesting a role for the endothelium in the modulation of alpha1-receptor responsiveness. Ethanol (2.5% and 5%) caused significant and concentration-related increases in the contractile responses elicited by phenylephrine but not KCl. The maximum contraction (Emax) caused by phenylephrine in rings obtained from SHRs treated with 2.5% and 5% ethanol amounted to 413.6 +/- 26.3 and 513.0 +/- 46.7 mg tension/mg tissue, respectively, compared with 383.6 +/- 35.2 mg tension/mg tissue in control rings. The enhancement of alpha1 contractions by ethanol was virtually abolished in rings pretreated with the alpha1-receptor antagonist prazosin, suggesting upregulation of alpha1-receptors in aortas of ethanol-fed rats. Endothelium denudation also abolished ethanol-evoked increases in phenylephrine contractions. These findings suggest that chronic ethanol feeding upregulates aortic alpha1-receptors, which may be a consequence of chronic alpha1-receptor blockade by ethanol. The latter may account, at least in part, for the hypotensive response elicited by ethanol in SHRs.

Catecholamine depletion by reserpine blocks the anxiolytic actions of ethanol in the rat

Neurochemical investigations of the anti-anxiety action of ethanol demonstrate that increased dopaminergic, noradrenergic, and serotonergic activity mediates the anxiolytic actions of ethanol. Results of studies with animals and human beings also confirm an involvement of the sympathetic nervous system in the behavioral actions of ethanol. Because enhanced sympathetic activity increases the release of norepinephrine from sympathetic terminals, the interruption of normal sympathetic activity might disrupt the anxiolytic action of ethanol. The present study examined the effect of chemical sympathectomy by reserpine pretreatment on the subsequent anxiolytic action of ethanol. Seventy-one, female, Sprague-Dawley rats were administered reserpine (0 or 5 mg/kg), followed 17-19 h later by ethanol (0, 0.5, or 1.0 g/kg). Animals were then tested in the elevated plus maze. Compared with saline pretreatment, which did not attenuate the anxiolytic actions of a 1.0-g/kg dose of ethanol, reserpine pretreatment completely blocked the anxiolytic action of a 1.0-g/kg dose of ethanol. Reserpine, by itself, did not possess anxiolytic or anxiogenic actions. Because the anxiolytic action of ethanol involves increased catecholamine activity and the depletion of norepinephrine and dopamine from sympathetic nerve terminals by reserpine blocked the anxiolytic action of ethanol, it is concluded that the anxiolytic action of ethanol requires the presence of normal catecholamine activity. We suggest that prostaglandin activity normally evoked by enhanced sympathetic nervous system activity might be involved in the anxiolytic action of ethanol.

Ethanol abolishes clonidine-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies showed that the ability of ethanol or clonidine to alter the baroreflex control of heart rate (baroreflex sensitivity, BRS) depends on the functional activity of aortic baroreflexes. In this study, we investigated the interaction between the two drugs on BRS in conscious rats with intact baroreflexes (shamoperated, SO) and after aortic baroreceptor denervation (ABD). The slope of the curve relating increments in mean arterial pressure induced by phenylephrine to corresponding reflex bradycardic responses was taken as an index of BRS. Ethanol (1 g/kg i.v.) significantly (p < 0.05) attenuated BRS in SO rats (-1.7 +/- 0.13 versus -1.04 +/- 0.15 beats/min/mm Hg) but not in ABD rats. Clonidine (30 microg/kg, i.v.) elicited significantly (p < 0.05) greater hypotensive responses in conscious ABD compared with SO rats. The BRS was not affected by clonidine administration in SO rats but showed significant (p < 0.05) reductions in ABD rats. Ethanol (1 g/kg, i.v.) had no effect on the hypotensive response to subsequently administered clonidine in ABD and SO rats; however, the effect of the two drugs on BRS was variable. In ABD rats, the BRS values before and after administration of ethanol and clonidine were similar, suggesting that pretreatment with ethanol counteracted clonidine-evoked attenuation of BRS in this rat preparation. In SO rats, the ethanol-clonidine combination produced a significant (p < 0.05) decrease in BRS, similar to the effect of ethanol when administered alone. These data confirm earlier findings that the aortic baroreflex arc modulates the interaction of ethanol and clonidine with baroreflex function. Further, the ability of ethanol to abolish clonidine-induced attenuation of BRS in ABD rats may relate to the compound effects of the two drugs on neuronal pathways participating in the central processing of baroreflexes in these rats.

Ovariectomy abolishes ethanol-induced impairment of baroreflex control of heart rate in conscious rats

Our previous studies have shown that ethanol attenuates baroreflex control of heart rate in male rats. The present study investigated whether this effect of ethanol is gender-related, and whether it involves hormonal factors. The effect of intragastric administration of ethanol or equal volume of water on baroreflex-mediated decreases in heart rate in response to increments in blood pressure evoked by phenylephrine were evaluated in conscious age-matched male and female Sprague-Dawley rats as well as in ovariectomized rats. Baroreflex curves relating changes in blood pressure and associated heart rate responses were constructed, and the slopes of the regression lines were taken as a measure of baroreflex sensitivity. Phenylephrine (1-16 microg kg(-1), i.v.) elicited dose-dependent pressor responses that were similar in all groups of rats. However, the associated reflex bradycardic responses depended on the rat preparation and the dose of ethanol employed. In water-treated (control) animals, significantly (P < 0.05) lesser reflex bradycardic responses were observed in female compared with male rats (baroreflex sensitivity, -1.21 +/- 0.12 vs. -1.67 +/- 0.12 beats min(-1) mmHg(-1)). Ovariectomy resulted in a further reduction in baroreflex sensitivity (-0.82 +/- 0.06 beats min(-1) mmHg(-1)), suggesting a favorable role for ovarian hormones in baroreflex modulation. In male rats, ethanol (0.25, 0.5, or 1 g kg(-1), intragastric) elicited dose-related decreases in reflex bradycardic responses. The reduction in the regression coefficient obtained by the two higher doses (0.5 and 1 g kg(-1)) of ethanol was statistically significant compared with control values. The ability of ethanol to reduce baroreflex sensitivity appears to be gender-independent as it was similarly demonstrated in intact female rats. In contrast, ethanol had no effect on reflex bradycardic responses in ovariectomized rats at any of the doses tested. The data suggest that ethanol reduces baroreflex control of heart rate irrespective of the rat gender. Further, the lack of an effect of ethanol on baroreflex sensitivity in ovariectomized rats may suggest a role for ovarian hormones in ethanol-evoked baroreflex attenuation.

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.

Suppression of alcohol-induced hypertension by dexamethasone

BACKGROUND

Alcohol consumption is associated with an increased incidence of hypertension and stroke, but the triggering mechanisms are unclear. In animals, alcohol causes activation of the sympathetic nervous system and also stimulates the release of corticotropin-releasing hormone (CRH), which has sympatho-excitatory effects when administered centrally.

METHODS

To determine whether alcohol evokes sympathetic activation and whether such activation is attenuated by the inhibition of CRH release, we measured blood pressure, heart rate, and sympathetic-nerve action potentials (using intraneural microelectrodes) in nine normal subjects before and during an intravenous infusion of alcohol (0.5 g per kilogram of body weight over a period of 45 minutes) and for 75 minutes after the infusion. Each subject received two infusions, one after the administration of dexamethasone (2 mg per day) and one after the administration of a placebo for 48 hours.

RESULTS

The infusion of alcohol alone evoked a marked (P < 0.001) and progressive increase in the mean (+/- SD) rate of sympathetic discharge, from 16 +/- 3 bursts per minute at base line to 30 +/- 8 bursts per minute at the end of the two-hour period. This sympathetic activation was accompanied during the second hour by an increase in mean arterial pressure of 10 +/- 5 mm Hg (P < 0.001). After the administration of dexamethasone, the alcohol infusion had no detectable sympathetic effect. The dexamethasone-induced suppression of sympathetic activation was associated with a decrease in mean arterial pressure of 7 +/- 6 mm Hg (P < 0.001) during the alcohol infusion and with suppression of the pressor effect during the second hour.

CONCLUSIONS

Alcohol induces pressor effects by sympathetic activation that appear to be centrally mediated. It is possible that these alcohol-induced hemodynamic and sympathetic actions could participate in triggering cardiovascular events.

Alcohol abolishes the hypotensive effect of clonidine in spontaneously hypertensive rats

This study tested the hypothesis that concurrent ethanol administration attenuates the hypotensive effect of clonidine. Four groups of spontaneously hypertensive rats matched for baseline systolic pressure and body weight were randomly assigned the following treatments: (1) water (control), (2) ethanol, (3) clonidine, and (4) ethanol plus clonidine for 13 weeks. Ethanol was provided in the drinking water as 5% for 1 week, 10% for the next 2 weeks, and 20% from week 4 to 13. Starting from similar baseline systolic blood pressures, the blood pressure of the control group increased 10 to 15 mm Hg over the 13-week treatment period. A similar rise in systolic blood pressure occurred in ethanol-treated rats despite a drastic (40% to 50%, P < .05) reduction in fluid intake. Clonidine (300 micrograms/kg per day) caused a smaller and shorter reduction in fluid intake. The fluid intake of the combined treatment group was similar to that of the ethanol group. Either treatment caused a significant and additive reduction in body weight gain. Treatment-related mortality (20%) occurred only in the combined treatment group by the 12th week. Clonidine elicited a slowly developing hypotensive response (P < .05) that started 2 to 3 weeks after treatment was initiated and lasted throughout the treatment period. Ethanol abolished the hypotensive effect of clonidine and resulted in blood pressure values that were not significantly different from those of the control or the ethanol group. Blood ethanol concentration was similar in the presence or absence of clonidine (5.5 +/- 1.9 versus 6.5 +/- 3 mmol/L).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of ethanol on baroreceptor heart rate responses of conscious spontaneously hypertensive and normotensive rats

This study investigated the acute effects of ethanol (0.25, 0.5, or 1 g/kg, iv) on mean arterial pressure, heart rate, and gain of the baroreceptor reflex control of heart rate (BRS) in conscious spontaneously hypertensive rats (SHRs) and age-matched Wistar rats. BRS was substantially lower in the SHRs as compared with the Wistar rats when measured by phenylephrine (-0.95 +/- 0.16 vs. -2.02 +/- 0.22 beats/min/mm Hg) or nitroprusside (-1.90 +/- 0.2 vs -3.02 +/- 0.32 beats/min/mm Hg). None of the doses of ethanol influenced BRS in the SHR. In contrast, ethanol attenuated BRS in Wistar rats, but this effect was dependent on the dose used and the type of response. The lower doses attenuated the reflex tachycardic response, but had no effect on the reflex bradycardic response. On the other hand, the 1 g/kg dose of ethanol attenuated the reflex bradycardic but not the tachycardic response. Ethanol produced a pressor effect (15-25 mm Hg; 5-min duration), which was neither dose- nor strain-dependent. However, only in the SHR mean arterial pressure remained elevated (10-15 mm Hg above control) for 20 min in response to the 0.5 g/kg dose of ethanol, and its recovery coincided with the occurrence of a slowly developing negative chronotropic response. Ethanol produced a dose-related negative chronotropic effect in both strains of rats that was of longer duration in the SHR, particularly with the 1 g/kg dose. It is possible that the bradycardic effect of ethanol influenced negatively its pressor effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute alcohol administration stimulates baroreceptor discharge in the dog

It has been shown that alcohol administration causes baroreceptor reflex inhibition. The site of action of alcohol could reside anywhere within the baroreceptor reflex arc. Therefore, the goal of this study was to determine the effects of acute administration of alcohol on carotid sinus baroreceptor discharge characteristics. In pentobarbital-anesthetized dogs, the carotid sinus was isolated and perfused. Single unit baroreceptor discharge was recorded from the carotid sinus nerve along with carotid sinus diameter using sonomicrometry. Carotid sinus pressure-baroreceptor discharge and carotid sinus pressure-diameter curves were constructed. Perfusion of the carotid sinus with alcohol (100 mmol/L) significantly decreased the pressure threshold from 91.1 +/- 2.8 to 86.4 +/- 2.9 mm Hg (p < 0.05) and increased the peak discharge rate from 45.8 +/- 3.4 to 52.8 +/- 3.6 spikes per second (p < 0.01). The same phenomenon was seen during perfusion of the carotid sinus with acetaldehyde (2.5 mmol/L) but was not seen during perfusion with acetate (2.5 mmol/L). During perfusion of the carotid sinus with alcohol, the carotid sinus pressure-carotid sinus diameter relation did not change. The baroreceptor sensitization induced by alcohol is not an endothelium-dependent mechanism, because endothelial denudation did not block this alcohol-induced effect. Measurement of the duration of postexcitatory depression of carotid sinus baroreceptors, which is related to Na+,K(+)-ATPase activity, showed that perfusion of the carotid sinus with alcohol or acetaldehyde significantly reduced the duration of postexcitatory depression, indicating that the alcohol- and acetaldehyde-induced effect on baroreceptor discharge is most likely mediated by an inhibition of Na+,K(+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal by ethanol of the hypotensive effect of clonidine in conscious spontaneously hypertensive rats

We studied the acute effect of ethanol on the hypotensive response to clonidine in conscious spontaneously hypertensive rats. When administered during the hypotensive response to clonidine, ethanol not only reversed the response but also caused a slight but significant short-lived pressor effect. The maximal hypotensive effect of graded doses of clonidine was significantly (p less than 0.05) attenuated by a dose of 1 g/kg ethanol, which resulted in a peak blood ethanol concentration of 54.2 +/- 6.3 mg/dl. The data strongly suggest the adverse effect of ethanol on the hypotensive response to clonidine is ethanol mediated and that their antagonistic interaction is both reversible and reproducible because: 1) an equal volume of saline had no effect on the hemodynamic responses to clonidine and 2) crossing over ethanol and saline treatments on days 2 and 3, which allowed longitudinal comparisons, showed that the effect of ethanol was similar both in naive rats (day 1) and in rats that were pre-exposed to ethanol (day 3). Whether this negative effect of ethanol also involves other antihypertensive agents that do not act primarily by a central nervous system mechanism was investigated. The same dose of ethanol had little or no effect on the hypotensive response to hydralazine, suggesting the negative effect of ethanol is selective to centrally acting antihypertensive agents. Although the mechanism by which ethanol reverses the hypotensive effect of clonidine is not known, it is possible that it involves an ethanol-evoked increase in plasma catecholamine levels, which are known to be decreased by clonidine. That ethanol did not reverse the hypotensive effect of hydralazine, which is also known to be associated with increased plasma catecholamine levels, supports this notion. The findings of the present study may explain, at least in part, why regular use of alcohol is associated with an inadequate control of blood pressure in treated hypertensive patients who are regular consumers of alcohol.

Impairment of baroreceptor reflex control of heart rate but not sympathetic efferent discharge by central neuroadministration of ethanol

We investigated the acute hemodynamic effects of ethanol microinjection into brain areas known to influence cardiovascular function and reflexes. In chloralose-anesthetized rats, ethanol had no effect on baseline mean arterial pressure, heart rate, or sympathetic efferent discharge when microinjected into the nucleus tractus solitarius, the dorsal motor nucleus of the vagus, the rostral ventrolateral medulla, or the posterior hypothalamus. On the other hand, ethanol microinjection into the anterior hypothalamus caused a site-dependent pressor effect and an increase in sympathetic efferent discharge. Baroreceptor heart rate response but not sympathetic efferent discharge response was impaired by ethanol microinjection into the nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the rostral ventrolateral medulla, suggesting that ethanol involves one or more of these areas in its inhibitory effect on baroreceptor heart rate response and that ethanol has a selective action on baroreceptor reflex control of heart rate. The findings that 1) the effect was dose dependent and 2) injection of ethanol outside of, or an equal volume of cerebrospinal fluid into, the nucleus tractus solitarius had no effect on the response strongly suggest that the observed effect on baroreceptor heart rate response was ethanol mediated. Ethanol microinjection into the dorsal motor nucleus of the vagus impaired the heart rate response, thus raising the possibility that leakage of ethanol to that area from the nucleus tractus solitarius might have contributed to its effect. These findings show that ethanol has a pressor and sympathoexcitatory site of action within the anterior hypothalamus and that it selectively impairs baroreceptor heart rate response via a central site of action; the mechanisms by which ethanol produces these effects remain to be elucidated.