Kurt J. Varner

Cardiovascular and sympathetic responses and reflex changes elicited by MDMA

The recreational use of 3,4-methylenedioxymethamphetamine (MDMA) has increased as have the number of clinical reports linking MDMA use with cardiovascular toxicity. Nonetheless, the cardiovascular and sympathetic nerve responses elicited by MDMA have not been well characterized. The purpose of this study was to characterize the mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve responses elicited by the acute administration of MDMA and to determine whether neurotoxic doses of MDMA change cardiovascular and/or cardiovascular reflex function. In conscious rats, MDMA or d-amphetamine elicited similar dose-dependent increases in MAP. MDMA elicited significant bradycardia at doses above 1.0 mg/kg. Pretreatment with phentolamine significantly reduced the duration but not the magnitude of the pressor response elicited by MDMA. In pentobarbital-anesthetized rats, MDMA (0.1 mg/kg) increased renal sympathetic nerve activity (RSNA; 33 +/- 10%), while larger doses significantly decreased RSNA (-91 +/- 3%, max). Neurotoxic doses of MDMA (20 mg/kg, s.c., b.i.d. for 4 days) significantly enhanced the bradycardic component of the Bezold-Jarisch reflex elicited by i.v. serotonin when tested either 2 days or 2 weeks after the last neurotoxic treatment. However, neurotoxic treatment did not significantly affect baroreceptor reflex function. These results indicate that the acute administration of MDMA and d-amphetamine produce similar cardiovascular and sympathetic responses. Neurotoxic doses of MDMA can also significantly alter cardiovascular reflex function. These findings raise the possibility that MDMA may have the potential to produce cardiovascular and/or cardiac toxicity similar to that elicited by other amphetamine analogs.

Oxidative stress contributes to methamphetamine-induced left ventricular dysfunction

AIMS Our aim was to test the hypothesis that the repeated, binge administration of methamphetamine would produce oxidative stress in the myocardium leading to structural remodeling and impaired left ventricular function. METHODS AND RESULTS Echocardiography and Millar pressure-volume catheters were used to monitor left ventricular structure and function in rats subjected to four methamphetamine binges (3 mg/kg, iv for 4 days, separated by a 10-day drug-free period). Hearts from treated and control rats were used for histological or proteomic analysis. When compared with saline treatment, four methamphetamine binges produced eccentric left ventricular hypertrophy. The drug also significantly impaired systolic function (decreased fractional shortening, ejection fraction, and adjusted maximal power) and produced significant diastolic dysfunction (increased -dP/dt and tau). Dihydroethedium staining showed that methamphetamine significantly increased (285%) the levels of reactive oxygen species in the left ventricle. Treatment with methamphetamine also resulted in the tyrosine nitration of myofilament (desmin, myosin light chain) and mitochondrial (ATP synthase, NADH dehydrogenase, cytochrome c oxidase, prohibitin) proteins. Treatment with the superoxide dismutase mimetic, tempol in the drinking water prevented methamphetamine-induced left ventricular dilation and systolic dysfunction; however, tempol (2.5 mM) did not prevent the diastolic dysfunction. Tempol significantly reduced, but did not eliminate dihydroethedium staining in the left ventricle, nor did it prevent the tyrosine nitration of mitochondrial and contractile proteins. CONCLUSION This study shows that oxidative stress plays a significant role in mediating methamphetamine-induced eccentric left ventricular dilation and systolic dysfunction.

Effects of mercury on the arterial blood pressure of anesthetized rats

The available data suggests that hypotension caused by Hg2+ administration may be produced by a reduction of cardiac contractility or by cholinergic mechanisms. The hemodynamic effects of an intravenous injection of HgCl2 (5 mg/kg) were studied in anesthetized rats (N = 12) by monitoring left and right ventricular (LV and RV) systolic and diastolic pressures for 120 min. After HgCl2 administration the LV systolic pressure decreased only after 40 min (99 +/- 3.3 to 85 +/- 8.8 mmHg at 80 min). However, RV systolic pressure increased, initially slowly but faster after 30 min (25 +/- 1.8 to 42 +/- 1.6 mmHg at 80 min). Both right and left diastolic pressures increased after HgCl2 treatment, suggesting the development of diastolic ventricular dysfunction. Since HgCl2 could be increasing pulmonary vascular resistance, isolated lungs (N = 10) were perfused for 80 min with Krebs solution (continuous flow of 10 ml/min) containing or not 5 microM HgCl2. A continuous increase in pulmonary vascular resistance was observed, suggesting the direct effect of Hg2+ on the pulmonary vessels (12 +/- 0.4 to 29 +/- 3.2 mmHg at 30 min). To examine the interactions of Hg2+ and changes in cholinergic activity we analyzed the effects of acetylcholine (Ach) on mean arterial blood pressure (ABP) in anesthetized rats (N = 9) before and after Hg2+ treatment (5 mg/kg). Using the same amount and route used to study the hemodynamic effects we also examined the effects of Hg2+ administration on heart and plasma cholinesterase activity (N = 10). The in vivo hypotensive response to Ach (0.035 to 10.5 microg) was reduced after Hg2+ treatment. Cholinesterase activity (microM h-1 mg protein-1) increased in heart and plasma (32 and 65%, respectively) after Hg2+ treatment. In conclusion, the reduction in ABP produced by Hg2+ is not dependent on a putative increase in cholinergic activity. HgCl2 mainly affects cardiac function. The increased pulmonary vascular resistance and cardiac failure due to diastolic dysfunction of both ventricles are factors that might contribute to the reduction of cardiac output and the fall in arterial pressure.

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.

Role of Renal Nerves in Experimental Hypertension: Evaluation of Neurogenic Mechanisms

To evaluate neurogenic mechanisms underlying variations in arterial pressure associated with removal of baroreflexes, renal sympathetic nerve activity (RSNA) was recorded in conscious unrestrained rats 1 day and 14 days following sinoaortic deafferentation (SAD) or sham operation. Fluctuations in RSNA and heart rate (HR) were correlated stastistically with moment to moment changes in pressure. One day and 14 days after SAD, the lability of mean arterial pressure (MAP) was increased, whereas the lability of RSNA and HR were reduced at 1 day and unchanged at 14 days. Arterial pressure and RSNA were negatively correlated in sham rats, however in rats with SAD negative correlations were virtually absent and positive correlations appeared only infrequently. These results indicate that SAD reduces variability of both RSNA and HR and that lability of arterial pressure appears to not be driven by variations in sympathetic discharge. To examine the central origins of RSNA in anesthetized rats we blocked neuronal transmission in two vasomotor regions of rostral medulla, rostral ventrolateral medulla (RVLM) and rostral ventromedial medulla (RVMM) using bilateral microinjections of lidocaine. Blockade of either or both RVLM and RVMM produced an equivalent marked reduction in arterial pressure but reduced RSNA to only 40% of control. Ganglionic blockade had little additional effect on arterial pressure but abolished the residual RSNA. These findings suggest that a substantial fraction of RSNA may be non-vasomotor in function and that this activity may originate from spinal sites or from supraspinal sites other than RVLM or RVMM.

Ecstasy produces left ventricular dysfunction and oxidative stress in rats

AIMS Our aim was to determine whether the repeated, binge administration of 3,4-methylenedioxymethamphetamine (ecstasy; MDMA) produces structural and/or functional changes in the myocardium that are associated with oxidative stress. METHODS AND RESULTS Echocardiography and pressure-volume conductance catheters were used to assess left ventricular (LV) structure and function in rats subjected to four ecstasy binges (9 mg/kg i.v. for 4 days, separated by a 10 day drug-free period). Hearts from treated and control rats were used for either biochemical and proteomic analysis or the isolation of adult LV myocytes. After the fourth binge, treated hearts showed eccentric LV dilation and diastolic dysfunction. Systolic function was not altered in vivo; however, the magnitude of the contractile responses to electrical stimulation was significantly smaller in myocytes from rats treated in vivo with ecstasy compared with myocytes from control rats. The magnitude of the peak increase in intracellular calcium (measured by Fura-2) was also significantly smaller in myocytes from ecstasy-treated vs. control rats. The relaxation kinetics of the intracellular calcium transients were significantly longer in myocytes from ecstasy-treated rats. Ecstasy significantly increased nitrotyrosine content in the left ventricle. Proteomic analysis revealed increased nitration of contractile proteins (troponin-T, tropomyosin alpha-1 chain, myosin light polypeptide, and myosin regulatory light chain), mitochondrial proteins (Ub-cytochrome-c reductase and ATP synthase), and sarcoplasmic reticulum calcium ATPase. CONCLUSION The repeated binge administration of ecstasy produces eccentric LV dilation and dysfunction that is accompanied by oxidative stress. These functional responses may result from the redox modification of proteins involved in excitation-contraction coupling and/or mitochondrial energy production. Together, these results indicate that ecstasy has the potential to produce serious cardiac toxicity and ventricular dysfunction.

The Cardiovascular and Cardiac Actions of Ecstasy and its Metabolites

The recreational use of 3, 4 methylenedioxymethamphetamine (ecstasy or MDMA) has increased dramatically over the past thirty years due to its ability to increase stamina and produce feelings of emotional closeness and wellbeing. In spite of the popular perception that MDMA is a safe drug, there is a large literature documenting that the drug can produce significant neurotoxicity, especially in serotonergic and catecholaminergic systems. There are also experimental and clinical data which document that MDMA can alter cardiovascular function and produce cardiac toxicity, including rhythm disturbances, infarction and sudden death. This manuscript will review the literature documenting the cardiovascular responses elicited by MDMA in humans and experimental animals and will examine the underlying mechanisms mediating these responses. We will also review the available clinical, autopsy and experimental data linking MDMA with cardiac toxicity. Most available data indicate that oxidative stress plays an important role in the cardiotoxic actions of MDMA. Moreover, new data indicates that redox active metabolites of MDMA may play especially important roles in MDMA induced toxicity.

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.

Mechanisms underlying the sympathomimetic cardiovascular responses elicited by γ-hydroxybutyrate

γ-Hydroxybutyrate (GHB) is generally thought to be a central nervous system depressant; however, GHB also has sympathomimetic cardiovascular actions. Radio telemetry was used to record the cardiovascular responses elicited by GHB (180-1000 mg/kg IV) in conscious rats. GHB elicited increases in mean arterial pressure (MAP) (24 ± 3 to 60 ± 5 mm Hg) lasting from 28 ± 8 to 227 ± 37 minutes. GHB (560 and 1000 mg/kg IV) also elicited a prolonged tachycardic response (85 ± 23 and 95 ± 22 bpm). The hypertension and tachycardia elicited by GHB (560 mg/kg) were reversed by the intravenous and intracerebroventricular administration of the GABAb receptor antagonist CGP 35348. CGP 35348 also reversed GHB-mediated increases in renal sympathetic nerve activity (RSNA). Administration of the purported GHB receptor antagonist NCS-382 reversed the increase in heart rate but not the pressor response elicited by GHB in telemetered rats. These data indicate that the intravenous administration of GHB markedly increases MAP, heart rate, and RSNA in conscious rats via activation of central GABAb receptors. In addition, GHB receptors appear to selectively mediate the increase in heart rate elicited by large doses of GHB.

Lesions in rostral ventromedial or rostral ventrolateral medulla block neurogenic hypertension.

Neurogenic hypertension results from the removal of inhibitory baroreceptor afferent input to vasomotor systems in the central nervous system. We sought to determine whether the bilateral destruction of neurons in the rostral ventrolateral or rostral ventromedial medulla, made using microinjections of N-methyl-D-aspartic acid (30 nmol in 200 nL), would block the acute increase in arterial pressure after sinoaortic deafferentation in pentobarbital-anesthetized rats. Bilateral lesions of the rostral ventrolateral or rostral ventromedial medulla decreased mean arterial pressure (107 +/- 4 to 78 +/- 5 and 115 +/- 3 to 94 +/- 3 mm Hg, respectively). In rostral ventrolateral or rostral ventromedial medulla lesioned rats, sinoaortic deafferentation failed to increase arterial pressure. Sham lesions or lesions placed rostral to the rostral ventrolateral or rostral ventromedial medulla did not significantly lower arterial pressure. Subsequent sinoaortic deafferentation significantly increased mean arterial pressure (109 +/- 3 to 145 +/- 4 and 109 +/- 5 to 141 +/- 3 mm Hg, respectively). In eight rats we used an infusion of angiotensin II to return arterial pressure to control levels after lesion of the rostral ventrolateral (n = 4) or rostral ventromedial (n = 4) medulla. In these animals, sinoaortic deafferentation failed to increase arterial pressure. We conclude that neurons in the rostral ventrolateral and rostral ventromedial medulla are involved in the normal maintenance of arterial pressure and the development of hypertension after sinoaortic deafferentation in pentobarbital-anesthetized rats.