Expression of Group I metabotropic glutamate receptors on phenotypically different cells within the nucleus of the solitary tract in the rat.

Group I metabotropic glutamate receptors (mGluRs) are G-coupled receptors that modulate synaptic activity. Previous studies have shown that Group I mGluRs are present in the nucleus of the solitary tract (NTS), in which many visceral afferents terminate. Microinjection of selective Group I mGluR agonists into the NTS results in a depressor response and decrease in sympathetic nerve activity. There is, however, little evidence detailing which phenotypes of neurons within the NTS express Group I mGluRs. In brainstem slices, we performed immunohistochemical localization of Group I mGluRs and either glutamic acid decarboxylase 67 kDa isoform (GAD67), neuronal nitric oxide synthase (nNOS) or tyrosine hydroxylase (TH). Fluoro-Gold (FG, 2%; 15 nl) was microinjected in the caudal ventrolateral medulla (CVLM) of the rat to retrogradely label NTS neurons that project to CVLM. Group I mGluRs were distributed throughout the rostral-caudal extent of the NTS and were found within most NTS subregions. The relative percentages of Group I mGluR expressing neurons colabeled with the different markers were FG (6.9+/-0.7) nNOS (5.6+/-0.9), TH (3.9+/-1.0), and GAD67 (3.1+/-1.4). The percentage of FG containing cells colabeled with Group I mGluR (13.6+/-2.0) was greater than the percent colabeled with GAD67 (3.1+/-0.5), nNOS (4.7+/-0.5), and TH (0.1+/-0.08). Cells triple labeled for FG, nNOS, and Group I mGluRs were identified in the NTS. Thus, these data provide an anatomical substrate by which Group I mGluRs could modulate activity of CVLM projecting neurons in the NTS.

Proposed role of the paraventricular nucleus in cardiovascular deconditioning.

AIM: Cardiovascular deconditioning occurs in individuals exposed to prolonged spaceflight or bedrest and is associated with the development of orthostatic intolerance. Although the precise mechanisms remain to be fully elucidated, astronauts returning from space or bedrest patients returning to normal upright posture present with decreases in plasma volume and alterations in autonomic function. The hindlimb unloaded (HU) rat has been a useful model to study the effects of cardiovascular deconditioning as it mimics many of the changes that occur after spaceflight and bedrest. RESULTS: Experiments performed in HU rats suggest that cardiovascular deconditioning attenuates baroreflex mediated sympathoexcitation and enhances cardiopulmonary receptor mediated sympathoinhibition. These alterations appear to be due to changes in the central nervous system and may contribute to the pre disposition towards orthostatic intolerance associated with cardiovascular deconditioning. The paraventricular nucleus (PVN) of the hypothalamus is important in basal and reflex control of sympathetic outflow. Recent evidence suggests that nitric oxide (NO) is an important inhibitory neurotransmitter in the PVN and that alterations in nitroxidergic transmission in the PVN may be involved in elevated sympathetic tone in certain disease states. CONCLUSION: Based on evidence from other laboratories and published and preliminary data from our own laboratories, this review proposes a role for the PVN in cardiovascular deconditioning. In particular, we discuss the hypothesis that increased NO in the PVN contributes to the altered cardiovascular reflexes observed following deconditioning and how these reflexes may be related to the orthostatic intolerance observed after prolonged spaceflight or bedrest.

Staphylococcus aureus MDR efflux pump inhibitors from a Berberis and a Mahonia (sensu strictu) species.

Bioactive fractionation, based on multi-drug resistance (MDR) pump inhibition in Staphylococcus aureus, resulted in the isolation of the active inhibitors 5'-methoxyhydnocarpin-D from leaves of Berberis (formerly Mahonia) trifoliolata and pheophorbide a from Berberis fendleri. The hydnocarpin derivative was not found in the latter species. Pheophytin a (the phytol derivative of pheophorbide a) was identified from both species, but it proved to have no MDR pump inhibitory activity. The somewhat uncommon, and inactive, flavonoid tricin was identified from B. trifoliolata. The occurrence of a flavonolignan in Mahonia-tpe species and its absence in Berberis sensu strictu may provide a chemical differentiation between the two groups which are now recombined on the basis of DNA studies. The strong bacterial efflux pump inhibition of pheophorbide a could be of importance as a plant defense against natural pathogens.

Functional anatomy of the vagal innervation of the cervical trachea of the dog.

The canine cervical trachea has been used for numerous studies regarding the neural control of tracheal smooth muscle. The purpose of the present study was to determine whether there is lateral dominance by either the left or right vagal innervation of the canine cervical trachea. In anesthetized dogs, pressure in the cuff of the endotracheal tube was used as an index of smooth muscle tone in the trachea. After establishment of tracheal tone, as indicated by increased cuff pressure, either the right or left vagus nerve was sectioned followed by section of the contralateral vagus. Sectioning the right vagus first resulted in total loss of tone in the cervical trachea, whereas sectioning the left vagus first produced either a partial or no decrease in tracheal tone. After bilateral section of the vagi, cuff pressure was recorded during electrical stimulation of the rostral end of the right or left vagus. At the maximum current strength used, stimulation of the left vagus produced tracheal constriction that averaged 28.5% of the response to stimulation of the right vagus (9.0 +/- 1.8 and 31.6 +/- 2.5 mmHg, respectively). In conclusion, the musculature of cervical trachea in the dog appears to be predominantly controlled by vagal efferents in the right vagus nerve.

Review of evidence for a novel model of cocaine-induced cardiovascular toxicity.

Cocaine is known to produce life-threatening cardiovascular complications in some but not all individuals. This review considers the premise that an appropriate animal model for cocaine-induced cardiotoxicity should be characterized by varying sensitivity in the population to the deleterious effects of cocaine. We have studied such a model in which physiological, biochemical, and pathological sensitivity to cocaine varies in rats. Our studies have identified a subset of rats that respond to cocaine with a decrease in cardiac output and a substantial increase in systemic vascular resistance (named vascular responders). In contrast, another group, designated mixed responders, is characterized by a smaller increase in systemic vascular resistance and a small increase in cardiac output. We reported that vascular responders are more likely to develop hypertension and cardiomyopathies with repeated cocaine administration. Under chloralose anesthesia, vascular responders have more profound pressor responses to cocaine and an initial brief spike in renal sympathetic nerve activity not usually noted in mixed responders. Vascular responders have higher resting and cocaine-induced dopamine turnover in the striatum. In addition, vascular responders have higher alpha-adrenergic vasoconstrictor tone, whereas mixed responders have higher adrenergic cardiac tone. The difference in cardiac output and systemic vascular resistance responses to cocaine in these two subsets of the population can be prevented by L-type calcium channel, muscarinic, or alpha-adrenergic blockade. Similar hemodynamic response variability is noted with other psychoactive agents and with acute stress, suggesting that the response patterns are not unique to cocaine. We propose that individual hemodynamic response variability is dependent on differences in CNS responsiveness and correlated with the incidence of cardiovascular disease.

Cardiovascular response to group I metabotropic glutamate receptor activation in NTS.

Glutamate is the proposed neurotransmitter of baroreceptor afferents at the level of the nucleus tractus solitarius (NTS). Exogenous glutamate in the NTS activates neurons through ionotropic and metabotropic glutamate receptors (mGluRs). This study tested the hypothesis that group I mGluRs in the NTS produce depressor, bradycardic, and sympathoinhibitory responses. In urethan-anesthetized rats, unilateral 30-nl microinjections of the group I-selective mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) into the NTS decreased mean arterial pressure, heart rate, and lumbar sympathetic nerve activity. The dose of drug that produced 50% of the maximal response (ED50) was 50-100 microM. The response to microinjection of equal concentrations of DHPG or the general mGluR agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD) produced similar cardiovascular effects. The cardiovascular response to injection of DHPG or ACPD was abolished by NTS blockade of mGluRs with alpha-methyl-4-carboxyphenylglycine (MCPG). Blockade of ionotropic glutamate receptors with kynurenic acid did not attenuate the response to DHPG or ACPD injection. These data suggest that DHPG and ACPD activate mGluRs in the NTS and do not require ionotropic glutamate receptors to produce their cardiovascular response. In the NTS the group I mGluRs produce responses that are consistent with excitation of neurons involved in reducing sympathetic outflow, heart rate, and arterial pressure.

alpha1-adrenergic-receptor responsiveness in skeletal muscle during dynamic exercise.

Attenuation of sympathetic vasoconstriction (sympatholysis) in working muscles during dynamic exercise is controversial. One potential mechanism is a reduction in alpha1-adrenergic-receptor responsiveness. The purpose of this study was to examine alpha1-adrenergic-receptor-mediated vasoconstriction in resting and working skeletal muscles by using intra-arterial infusions of a selective agonist. Seven mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. A selective alpha1-adrenergic-receptor agonist (phenylephrine) was infused as a bolus into the femoral artery catheter at rest and during exercise. All dogs ran on a motorized treadmill at two exercise intensities (3 and 6 miles/h). Intra-arterial infusions of the same effective concentration of phenylephrine elicited reductions in vascular conductance of 76 +/- 4, 76 +/- 6, and 67 +/- 5% (P > 0.05) at rest, 3 miles/h, and 6 miles/h, respectively. Systemic blood pressure and blood flow in the contralateral iliac artery were unaffected by phenylephrine. These results do not demonstrate an attenuation of vasoconstriction to a selective alpha1-agonist during exercise and do not support the concept of sympatholysis.

Renal hemodynamic responses to dynamic exercise in rabbits.

Cardiovascular hemodynamics, including renal blood flow, were measured in rabbits with one intact and one denervated kidney during various intensities of treadmill exercise. Within the first 10 s of exercise, there was rapid vasoconstriction in the innervated kidney associated with decreases in renal blood flow (range -10 to -17%). The vasoconstriction in the innervated kidney was evident at all workloads and was intensity dependent. There was no significant vasoconstriction or change in renal blood flow (range 0.5 to -3.1%) in the denervated kidney at the onset of exercise. However, a slowly developing vasoconstriction occurred in the denervated kidney as exercise progressed to 2 min at all workloads. Examination of responses to exercise performed under alpha-adrenergic blockade with phentolamine (5 mg/kg iv) revealed that the vasoconstriction in the innervated kidney at the onset of exercise and the delayed vasoconstriction in the denervated kidney were due primarily to activation of alpha-adrenergic receptors. In addition, a residual vasoconstriction was also present in the innervated kidney after alpha-adrenergic blockade, suggesting that, during exercise, activation of other renal vasoconstrictor mechanisms occurs which is dependent on the presence of renal nerves.

Skeletal muscle vasodilation at the onset of exercise.

The purpose of this study was to determine whether beta-adrenergic or muscarinic receptors are involved in skeletal muscle vasodilation at the onset of exercise. Mongrel dogs (n = 7) were instrumented with flow probes on both external iliac arteries and a catheter in one femoral artery. Propranolol (1 mg), atropine (500 microgram), both drugs, or saline was infused intra-arterially immediately before treadmill exercise at 3 miles/h, 0% grade. Immediate and rapid increases in iliac blood flow occurred with initiation of exercise under all conditions. Peak blood flows were not significantly different among conditions (682 +/- 35, 646 +/- 49, 637 +/- 68, and 705 +/- 50 ml/min, respectively). Although the doses of antagonists employed had no effect on heart rate or systemic blood pressure, they were adequate to abolish agonist-induced increases in iliac blood flow. Because neither propranolol nor atropine affected iliac blood flow, we conclude that activation of beta-adrenergic and muscarinic receptors is not essential for the rapid vasodilation in active skeletal muscle at the onset of exercise in dogs.

Mechanisms of hemodynamic responses to cocaine in conscious rats.

Several agents have been used to treat cocaine-related cardiovascular complications and toxicity occurring in sensitive individuals, yet the causes of hemodynamic responsiveness and differential sensitivity to cocaine are unknown. In this study, we sought to examine the role of different mediators in a model of variable cardiovascular responses to cocaine. As noted previously in conscious rats, cardiac output (CO) and systemic vascular resistance (SVR) responses to cocaine (5 mg/kg, i.v.) varied widely. Twenty of 34 rats exhibited cocaine-induced decreases in CO of > or =8% and large increases in SVR (designated vascular responders). The remaining rats with little change or an increase in CO and smaller increases in SVR were named mixed responders. Pretreatment with propranolol (1 mg/kg) or metoprolol (1 mg/kg) reduced heart rate. In mixed responders, propranolol or metoprolol reversed the cocaine-induced increase in CO and stroke volume and enhanced the increase in SVR, making these rats respond like vascular responders. Nicardipine (25 microg/kg) reduced the pressor response and selectively reversed the CO responses in vascular responders. N omega-nitro-L-arginine methyl ester (L-NAME; 2.7 mg/kg) increased arterial pressure by increasing SVR. Cocaine induced greater pressor and SVR responses apparently because of a shift in baseline values elicited by L-NAME alone. Therefore, differences in hemodynamic responses patterns may be the result of differences in beta-adrenergic activation or subsequent calcium channel activation or both. We predict that calcium channel antagonists may be useful to treat cocaine-induced cardiovascular complications, whereas beta-adrenergic antagonists are not likely to be beneficial.

Ethanol alters hemodynamic responses to cocaine in rats.

Cocaine is often used while consuming ethanol despite evidence that this combination may enhance the toxicity of cocaine. In the present study, we examined the cardiovascular effects of ethanol (475 or 950 mg/kg, i.v.) alone and in combination with cocaine (5 mg/kg, i.v.) in conscious rats. Ethanol or cocaine administration produced a consistent pressor response but highly variable cardiac output and systemic vascular resistance responses. The hemodynamic response patterns in individual rats to either drug were similar and related within rats. After ethanol pretreatment, cocaine produced greater decreases in cardiac output. We have proposed that this pattern of responses may reflect a predisposition in individual rats to cocaine-induced cardiomyopathies and hypertension. Furthermore, these data suggest that ethanol administration elicits a similar pattern of hemodynamic responses as previously reported for cocaine or amphetamine administration or acute behavioral stress.

Autonomic control of skeletal muscle vasodilation during exercise.

Despite extensive investigation, the control of blood flow during dynamic exercise is not fully understood. The purpose of this study was to determine whether beta-adrenergic or muscarinic receptors are involved in the vasodilation in exercising skeletal muscle. Six mongrel dogs were instrumented with ultrasonic flow probes on both external iliac arteries and with a catheter in a branch of one femoral artery. The dogs exercised on a treadmill at 6 miles/h while drugs were injected intra-arterially into one hindlimb. Isoproterenol (0.2 microg) or acetylcholine (1 microg) elicited increases in iliac blood flow of 89.8 +/- 14.4 and 95.6 +/- 17.4%, respectively, without affecting systemic blood pressure or blood flow in the contralateral iliac artery. Intra-arterial propranolol (1 mg) or atropine (500 microg) had no effect on iliac blood flow, although they abolished the isoproterenol and acetylcholine-induced increases in iliac blood flow. These data indicate that exogenous activation of beta-adrenergic or muscarinic receptors in the hindlimb vasculature increases blood flow to dynamically exercising muscle. More importantly, because neither propranolol nor atropine affected iliac blood flow, we conclude that beta-adrenergic and muscarinic receptors are not involved in the control of blood flow to skeletal muscle during moderate steady-state dynamic exercise in dogs.

Sympathetic vasoconstriction in active skeletal muscles during dynamic exercise.

Studies utilizing systemic administration of alpha-adrenergic antagonists have failed to demonstrate sympathetic vasoconstriction in working muscles during dynamic exercise. The purpose of this study was to examine the existence of active sympathetic vasoconstriction in working skeletal muscles by using selective intra-arterial blockade. Six mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and with a catheter in one femoral artery. All dogs ran on a motorized treadmill at three intensities on separate days. After 2 min, the selective alpha 1-adrenergic antagonist prazosin (0.1 mg) was infused as a bolus into the femoral artery catheter. At mild, moderate, and heavy workloads, there were immediate increases in iliac conductance of 76 +/- 7, 54 +/- 11, and 22 +/- 6% (mean +/- SE), respectively. Systemic blood pressure and blood flow in the contralateral iliac artery were unaffected. These results demonstrate that there is sympathetic vasoconstriction in active skeletal muscles even at high exercise intensities.

Effect of increasing work rate on metabolic responses of the donkey (Equus asinus).

Oxygen consumption (VO2) and concentration of venous blood metabolites were measured in donkeys trained to run and to pull loads on a treadmill. VO2 in two donkeys running at maximal speed on a 9.8% slope was 110 +/- 2 ml.min-1.kg-1, approximately 22 times preexercise VO2. Average heart rate at maximal VO2 (VO2max) was 223 +/- 2 beats/min, five times the preexercise heart rate. Blood lactate increased 14-fold, and blood glucose did not change (P > 0.05). Animals running up a 4% incline and incremental draft loading of five donkeys walking on the level were also studied. The total energy cost of walking unloaded was 2.86 +/- 0.06 J.m-1.kg live wt-1. During low- to medium-intensity draft work for 25 min, glucose fell below preexercise values (P < 0.05), whereas plasma hematocrit and cortisol increased (P < 0.05). Blood lactate remained unchanged up to approximately 40% VO2 max but increased 170% at approximately 60% VO2max. The responses in donkeys are similar to those of exercising horses except for the rapid decline in blood glucose observed during low-intensity exercise and the lower lactate levels at both the high-intensity exercise and the apparent anaerobic threshold.

Coronary vascular effects of cocaine in rats.

It has been suggested that ischemia secondary to coronary vasoconstriction is responsible for adverse cardiovascular effects of cocaine. However, the reported coronary vascular effects of cocaine vary considerably. We sought to determine the effects of cocaine on the coronary vasculature in anesthetized and conscious rats. Rats anesthetized with chloralose were instrumented for estimation of ascending aortic and coronary blood flows using pulsed Doppler velocitometry. Cocaine administration resulted in bradycardia and a biphasic mean arterial pressure response. Cocaine elicited highly variable increases in coronary vascular resistance and decreases in cardiac output. Decreases in coronary blood flow and rate-pressure product were directly correlated. Prazosin significantly attenuated the cardiac output but not the coronary vascular responses to cocaine. Propranolol, on the other hand, significantly shortened the duration of both responses. Conscious rats, instrumented for coronary blood flow determination, also exhibited cocaine-induced increases in coronary vascular resistance, yet the changes in coronary blood flow were not correlated with the rate-pressure product. These results provide the first evidence that cocaine produces equivalent increases in coronary vascular resistance in conscious and anesthetized rats. However, because the relationship between coronary blood flow and rate-pressure is different between the two preparations, as are other cardiovascular responses, we suggest that anesthesia alters the mechanism(s) by which cocaine affects the rat coronary vasculature.

Stress and cocaine elicit similar cardiac output responses in individual rats.

Cocaine use and behavioral stress elicit variable cardiovascular responses in individuals. In the present study, we examined the effects of cocaine or stress on arterial pressure, heart rate, and cardiac output in conscious rats. Rats were instrumented for determination of ascending aortic blood flow as an index of cardiac output using pulsed Doppler flow-metry. Cocaine administration elicited consistent decreases in cardiac output in some rats, whereas others had increases. In contrast, the pressor and heart rate responses were similar in these two groups of animals. Air jet stress also elicited a decrease in cardiac output only in a subset of conscious rats, yet produced equivalent pressor responses in all rats. Cardiac output responses to cocaine and air jet stress were closely correlated in individual rats, indicating that these stimuli evoke similar hemodynamic responses in individual rats. These observations suggest that the rat may provide a model for understanding differential cardiovascular sensitivity to cocaine and/or stress in humans.