Stephen V. Rendig

Reversal of reflex-induced myocardial ischemia by median nerve stimulation: A feline model of electroacupuncture

BACKGROUND Acupuncture is reported to reduce myocardial ischemia, arrhythmias, and hypertension. To investigate the physiological mechanisms underlying these observations, a model of reflex-induced, reversible myocardial ischemia was developed to test the effects of median nerve stimulation as a surrogate for electroacupuncture. METHODS AND RESULTS Chloralose-anesthetized cats were instrumented to measure arterial blood pressure, left ventricular pressure, left ventricular dP/dt, heart rate, left anterior descending (LAD) coronary blood velocity, and regional wall motion. The LAD artery either was partially occluded or a small diagonal branch was ligated. Subsequently, transient reflex activation of the cardiovascular system was evoked by application of bradykinin (typically 1 microg/mL) to the gallbladder, which significantly increased myocardial oxygen demand (double product), left ventricular dP/dt, and coronary blood velocity and caused ischemia-induced regional dysfunction, evidenced by significant (P<.05) reduction in normalized wall thickening (10.7+/-4.2% versus -23.6+/-2.9%; control versus ischemia; n=7). However, when median nerves were stimulated with low frequency (5 Hz) to mimic electroacupuncture, bradykinin-induced change in normalized wall thickening was significantly improved (-23.6+/-2.9% versus 9.8+/-4.9%; ischemia versus median nerve stimulation, P<.05) and remained augmented > or = 1 hour. Results were similar in partial and complete occlusion groups. Significant improvement in wall thickening was associated with unchanged increment of coronary blood velocity and significantly diminished increments of double product and diastolic blood pressure. CONCLUSIONS These results suggest that stimulation of the median nerve to mimic electroacupuncture diminishes regional myocardial ischemia triggered by a sympathetically mediated increase in cardiac oxygen demand. The mechanism of this effect is related to reduction in cardiac oxygen demand, secondary to a diminished pressor response. These data provide the first documentation of the physiological mechanisms underlying the possible beneficial effect of electroacupuncture in the context of restricted coronary blood flow and augmented myocardial oxygen demand.

Cardiac PET imaging in mice with simultaneous cardiac and respiratory gating

Gating firmware and software were developed for the microPET II small animal scanner. The measured cardiac and respiratory signals were collected and converted to TTL gating signals by a Biopac MP150 data acquisition system and sent to microPET II through two BNC connectors on the front panel. During acquisition, the coincidence monitor takes the average of the last eight gate input cycles and inserts this into the list mode data stream on the falling edge of the gating pulse. This value is then used to determine the current time interval of the next gate cycle when the list mode data are sorted into sinograms. The gating firmware and software were validated by an experiment using a rotating point source. Mouse heart (18F-FDG) and bone (18F(-)) imaging was performed with simultaneous cardiac and respiratory gating. It was clearly demonstrated that the contractile function of the mouse heart can be studied by cardiac-gated imaging with microPET II. The left ventricular volumes at different times of the cardiac cycle were measured and the ejection fraction was calculated. In the bone scan, no detectable movement caused by heart contraction was observed. Respiratory motion was more subtle with virtually no motion for more than 75% of the respiratory cycle. The motion of the mouse heart and bones in the thorax caused by respiration was less than 1 mm. It appears with the current resolution of PET, and the small fraction of the respiratory cycle in which motion occurs, that respiratory gating is probably not necessary for most mouse cardiac studies.

Alcohol-induced cardiac hemodynamic and Ca2+ flux dysfunctions are reversible☆

Abstract Cardiac hemodynamic function and calcium accumulation by cardiac sarcoplasmic reticulum membrane vesicles were studied in control rats, in rats that had consumed alcohol as 39% of their daily calories for 10 months, and in alcoholic rats after a period of withdrawal from alcohol to determine if alcohol-induced alterations were reversible. Cardiac pump function was assessed with an isolated working whole heart preparation. Alcoholic rat hearts exhibited decreased response of left ventricular peak systolic pressure, d P d t max , peak aortic flow, and − d P d t max to challenge with the sympathomimetic agent, dobutamine (0.1 μ m ). After alcohol was withdrawn from the diet for 4 to 6 months, cardiac hemodynamic response to dobutamine was not significantly different from controls. In alcoholic rats, Ca 2+ binding and uptake were significantly lower than in controls. Four to six months following withdrawal of alcohol, Ca 2+ binding and uptake had returned to control values. The data indicate that chronic alcohol consumption results in cardiac impairment characterized by inability of the heart to fully respond to β-adrenergic stimulation. This adrenergic subsensitivity may be caused, at least in part, by defect(s) in relaxation and excitation-contraction coupling mediated by intracellular Ca 2+ fluxes. These defects were reversible since cardiac pump function and calcium accumulation returned to normal after withdrawing alcohol.

Effects of red wine, alcohol, and quercetin on coronary resistance and conductance arteries

Summary: Moderate consumption of alcoholic beverages is associated with a reduced risk of coronary heart disease (CHD). Some evidence suggests that red wine is particularly beneficial in this regard and may account in part for the French paradox, although the mechanism of this effect is unknown. We assessed the effects of red wine, ethanol, and quercetin, a major flavonoid constituent of red wine, in coronary resistance vessels (80‐150 &mgr;m, i.d.) and conductance vessels (300‐525 &mgr;m, i.d.) of the rabbit. Vessel wall tension was measured in isolated segments maintained in a wire‐type myograph (37°C) and preconstricted with 30 mM K+. At an alcohol concentration (14 mM) equivalent to moderate consumption, red wine evoked a small, transient constrictor effect in resistance and conductance vessels (9 ± 4%, n = 5; 8 ± 1%, n = 7, respectively; p < 0.05). Ethanol alone at this concentration was without effect. Quercetin (5.6, 8, and 30 &mgr;M) significantly relaxed resistance (‐32 ± 4%, n = 10; ‐47 ± 2%, n = 7; ‐82 ± 6%, n = 8, respectively) and conductance (‐20 ± 3%, n = 8; ‐32 ± 4%, n = 8; ‐72 ± 7%, n = 8, respectively) coronary arteries. Vasorelaxation by quercetin was endothelium‐independent and was significantly greater in resistance than in conductance vessels. These data suggest that red wine and ethanol do not evoke relaxation in small coronary arteries at concentrations associated with moderate consumption. Quercetin elicits marked coronary vasorelaxation that is endothelium‐independent. However, the concentrations of quercetin necessary to achieve this action are not attained with moderate red wine consumption.

Limitation of myocardial infarct size in pigs with a dual lipoxygenase-cyclooxygenase blocking agent by inhibition of neutrophil activity without reduction of neutrophil migration

OBJECTIVES The purpose of this study was to assess the effect of the dual cyclooxygenase-lipoxygenase blocking agent BW755C on the extent of myocardial infarction in the pig and to identify the mechanisms of any cardioprotective action of this drug. BACKGROUND Activated neutrophils contribute to reperfusion injury after myocardial infarction and inhibition of neutrophil function can limit infarct size. METHODS In 9 control and 10 study pigs pretreated with intravenous BW755C (10 mg/kg body weight) 30 min before coronary occlusion, ischemia was induced by a 50-min occlusion of the mid-left anterior descending coronary artery, followed by 3 h of reperfusion. Heart rate, arterial pressure, left ventricular end-diastolic pressure, the first derivative of left ventricular pressure (dP/dt) and regional myocardial blood flow were measured during control, occlusion and reperfusion periods. Infarct size was determined by histochemical staining; and myeloperoxidase activity, a marker for tissue neutrophil content, was assessed in normal and infarcted myocardium. The effect of BW755C on the function of isolated neutrophils stimulated with zymosan-activated serum was evaluated by measuring neutrophil degranulation, leukotriene B4 production, superoxide generation and chemotaxis. RESULTS Hemodynamic function and regional myocardial blood flow were similar in control and BW755C-treated animals. BW755C significantly reduced myocardial infarct size compared with that in control animals, as measured by infarct/risk areas by histochemical staining (39 +/- 5% vs. 63 +/- 7%, p < 0.05). Myocardial myeloperoxidase activity was similar in normal, salvaged and infarcted areas in the control and treated groups, indicating that neutrophil accumulation in injured myocardium was unaltered by BW755C. However, this agent attenuated function of isolated, stimulated (zymosan-activated serum) neutrophils. At a concentration of 0.03 mg/ml, BW755C inhibited degranulation (-46%), leukotriene B4 production (-48%) and superoxide generation (-74%), but there was minimal inhibition of chemotaxis in vitro. CONCLUSIONS These findings demonstrate that myocardial infarct size can be reduced by selective inhibition of neutrophil cytotoxic activity without affecting neutrophil migration into injured myocardium.

Depression of Myocardial Contractile Function by Propoxyphene and Norpropoxyphene

Summary: Propoxyphene and its principal metabolite, norpropoxyphene. depressed maximum developed isometric tension (T) and its first derivative. dT/dt. in isolated cat right ventricular papillary muscles. T and dT/dt were reduced by 10 3 M concentrations of each drug (p < 0.005). and this effect was markedly intensified at 10 1 M propoxyphene, 49% reduction of T (p < 0.001): norpropoxyphene, 36% reduction of T (p < 0.001). Nonresponsiveness to electrical stimulation developed in the majority of muscles at the 10 3 M concentration of each drug and in all muscles at 10 3 M. The cardiac depressant actions of the two drugs administered simultaneously at a concentration of 10 3 M each were additive, resulting in a 15.6% decline in T. The contractile actions of propoxyphene and norpropoxyphene were not altered by the specific narcotic blocking agent, naloxone, indicating these effects were produced by a nonopiate mechanism. In contrast to propoxyphene and norpropoxyphene, morphine produced no reductions in T or dT/dt at a concentration of 10 5 M and only a slight, albeit significant (p < 0.05), depression of T (−4%) and dT/dt (−6%) at 10 4 M. The cardiac depressant effects of propoxyphene and norpropoxyphene were at least partially reversible, as indicated by substantial recovery of contractile function after drug removal (to 69–75% of control T) and by a positive inotropic response to isoproterenol. These results indicate that at high concentrations propoxyphene and norpropoxyphene have a cardiac depressant action characterized by decreased electrical excitability and reversible contractile depression, effects which are consistent with a local anesthetic action. Depression of cardiac contractile and electrophysiologic functions by propoxyphene and norpropoxyphene may play a role in the clinical toxicity associated with propoxyphene overdose.

Isolated Hearts from Obese Rats Show Impaired Function during Hypoxia

Summary Cardiac hemodynamic function of genetically obese male Zucker rats and lean male littermates was studied using an isolated ejecting whole heart preparation. Minimal differences in function were observed between hearts from 9-week-old lean and obese rats. In contrast, hearts from 19-week-old obese rats exhibited depressed ventricular function curves accom-panied by decreased stroke work, maximum power, and efficiency in hypoxia, and reduced response to dobutamine in hypoxia compared with controls. These findings suggest that chronic severe obesity in the Zucker rat is associated with the development of diminished cardiac resistance to hypoxic stress.

Sodium pentobarbital effects on cardiac function and response to dobutamine

Summary: This study was undertaken to determine if sodium pentobarbital had persistent effects on isolated working rat heart function that altered the response of the heart to subsequently administered dobutamine. The effects of in vitro sodium pentobarbital treatment were studied using isolated working hearts from rats that were killed after pentobarbital anesthesia or by cervical fracture. Hemodynamic and contractile functions of the isolated hearts were significantly depressed in the presence of 0.4 mM sodium pentobarbital in vitro. After the pentobarbital was washed out of the preparation, all indices of cardiac function recovered fully except for −dP/dtmax, which remained depressed by ˜7%. The subsequent inotropic response of the hearts to 5 × 10−8 M dobutamine was minimally affected by the prior exposure to pentobarbital in vitro. The data confirmed that hearts from rats killed by cervical fracture exhibited a smaller inotropic response to dobutamine than hearts from pentobarbital-anesthetized rats. In addition, hearts excised from ventilated animals had significantly higher left ventricular peak systolic pressure, peak aortic flow rate, and peak power than hearts from nonventilated animals, indicating the importance of ventilating the animal during cardiac excision in order to obtain optimum isolated heart function.

Positive Inotropic Action of Amrinone: Effect of Elevated External Ca2+

To investigate the positive inotropic effect of amrinone under conditions of maximal contractile stimulation, we compared cumulative amrinone one-half log dose-response curves (dose range, 3.1 × 10-5-3.1 × 10-3 M amrinone) of isolated cat right ventricular papillary muscle in normal (2.5 m M) and high (7.5 m M) calcium Krebs solution. In 2.5 m M calcium, amrinone induced significant (p < 0.01) dose-dependent increases in peak developed tension (T), maximum dT/dt, and maximum relaxation rate (dT/dt), which were accompanied by small but significant decreases in time-to-peak tension (TTP). At 10-3 M, changes in T, dT/dt, and TTP were 61.9 (2.7 g/mm2), 98.1 (33.2 g/s/mm2), and - 13.1% (-26 ms), respectively. In 7.5 m M calcium, the increases in T, dT/dt, and - dT/dt, and the reduction of TTP, were markedly less at each concentration. At 10-3 M amrinone, T and dT/dt increased significantly by 16.7 (1.3 g/mm2) and 23.5% (17.4 g/s/mm2), respectively, with no change in TTP. The amrinone-induced reductions in one-half relaxation time and twitch duration persisted in high calcium and were similar in magnitude to those in 2.5 m M calcium. The substantial reduction of amrinones positive inotropic effect in high Ca2+ suggests that a major part of the drugs action involves an augmentation of contractile Ca2+. An amrinone-induced effect on rate of Ca2+ delivery to the contractile apparatus or on myofilament Ca2+ sensitivity would be consistent with a persistent inotropic action in high Ca2+. Contractile changes induced by amrinone are discussed in relation to a possible effect on intracellular cyclic AMP levels.

Bradykinin BK2 receptors contribute to reflex cardiovascular responses during brief abdominal ischemia.

Ischemically sensitive visceral sympathetic nerve fibers, which are thought to represent the afferent limb of a strong cardiovascular pressor reflex, can be stimulated by exogenously applied bradykinin (BK). During ischemia, BK also is known to be produced locally and to serve as an endogenous stimulus for activation of ischemically sensitive nerve endings. It is unclear, however, whether ischemically induced BK production is sufficient to elicit a reflex cardiovascular response. Accordingly, femoral arterial and venous catheters were positioned in anesthetized cats, and the superior mesenteric and celiac arteries were isolated for placement of snare occluders. After dual occlusion of these arteries (20 min), one of two chemically dissimilar specific kinin B2(BK2) receptor antagonists, HOE-140 (30-40 μg/kg iv, n = 8) or NPC-17731 (30-40 μg/kg iv, n= 11), was administered and dual occlusion was repeated. The reflex rise of mean arterial blood pressure (BP) of 16 ± 3.7% was significantly ( P < 0.05) reduced by HOE-140 to 8.4 ± 2.0%. NPC-17731 similarly attenuated the reflex BP increment from 13 ± 1.2 to 6.2 ± 1.6% ( P < 0.05). In a separate set of control animals the first and second periods of ischemia induced reflex BP increments that did not differ significantly (16 ± 2.7 and 16 ± 5.7%, respectively). Qualitatively similar decrements of the BP response were produced by the BK2 receptor antagonists in two additional groups in which blood flow to the superior mesenteric and celiac arteries was diverted to a venous reservoir to eliminate the initial transient (mechanically induced) rise in BP associated with artery ligation that is known not to be associated with the reflex response. These results indicate that the stimulation of BK2 receptors on visceral afferent nerves by BK is responsible, at least in part, for the reflex cardiovascular response during visceral ischemia.Ischemically sensitive visceral sympathetic nerve fibers, which are thought to represent the afferent limb of a strong cardiovascular pressor reflex, can be stimulated by exogenously applied bradykinin (BK). During ischemia, BK also is known to be produced locally and to serve as an endogenous stimulus for activation of ischemically sensitive nerve endings. It is unclear, however, whether ischemically induced BK production is sufficient to elicit a reflex cardiovascular response. Accordingly, femoral arterial and venous catheters were positioned in anesthetized cats, and the superior mesenteric and celiac arteries were isolated for placement of snare occluders. After dual occlusion of these arteries (20 min), one of two chemically dissimilar specific kinin B2 (BK2) receptor antagonists, HOE-140 (30-40 micrograms/kg iv, n = 8) or NPC-17731 (30-40 micrograms/kg iv, n = 11), was administered and dual occlusion was repeated. The reflex rise of mean arterial blood pressure (BP) of 16 +/- 3.7% was significantly (P < 0.05) reduced by HOE-140 to 8.4 +/- 2.0%. NPC-17731 similarly attenuated the reflex BP increment from 13 +/- 1.2 to 6.2 +/- 1.6% (P < 0.05). In a separate set of control animals the first and second periods of ischemia induced reflex BP increments that did not differ significantly (16 +/- 2.7 and 16 +/- 5.7%, respectively). Qualitatively similar decrements of the BP response were produced by the BK2 receptor antagonists in two additional groups in which blood flow to the superior mesenteric and celiac arteries was diverted to a venous reservoir to eliminate the initial transient (mechanically induced) rise in BP associated with artery ligation that is known not to be associated with the reflex response. These results indicate that the stimulation of BK2 receptors on visceral afferent nerves by BK is responsible, at least in part, for the reflex cardiovascular response during visceral ischemia.