Angiogenic gene therapy for heart disease: a review of animal studies and clinical trials.

The current published clinical literature on angiogenic gene therapy for the treatment of myocardial ischemia does not include a single randomized, placebo-controlled trial. Based on current clinical literature, it is an unproven therapy. Successful animal studies combined with published reports of good outcomes in patients enrolled in uncontrolled trials has led to the expectation that angiogenic gene therapy will ultimately become a clinical reality. The next important landmark in the field will be the publication of data showing a favorable effect of angiogenic gene transfer in placebo-controlled, blinded clinical trials.

Intracoronary delivery of adenovirus encoding adenylyl cyclase VI increases left ventricular function and cAMP-generating capacity.

BACKGROUND: We tested the hypothesis that intracoronary injection of a recombinant adenovirus encoding adenylyl cyclase type VI (AC(VI)) would increase cardiac function in pigs. METHODS AND RESULTS: Left ventricular (LV) dP/dt and cardiac output in response to isoproterenol and NKH477 stimulation were assessed in normal pigs before and 12 days after intracoronary delivery of histamine followed by intracoronary delivery of an adenovirus encoding lacZ (control) or AC(VI) (1.4x10(12) vp). Animals that had received AC(VI) gene transfer showed increases in peak LV dP/dt (average increase of 1267+/-807 mm Hg/s; P=0.0002) and cardiac output (average increase of 39+/-20 mL. kg(-1). min(-1); P<0.0001); control animals showed no changes. Increased LV dP/dt was evident 6 days after gene transfer and persisted for at least 57 days. Basal heart rate, blood pressure, and LV dP/dt were unchanged, despite changes in cardiac responsiveness to catecholamine stimulation. Twenty-three hour ECG recordings showed no change in mean heart rate or ectopic beats and no arrhythmias. LV homogenates from animals receiving AC(VI) gene transfer showed increased AC(VI) protein content (P=0.0007) and stimulated cAMP production (P=0.0006), confirming transgene expression and function; basal LV AC activity was unchanged. Increased cAMP-generating capacity persisted for at least 18 weeks (P<0.0002). CONCLUSIONS: Intracoronary injection of a recombinant adenovirus encoding AC provides enduring increases in cardiac function.

Pulmonary gas exchange during exercise in pigs.

Increased ventilation-perfusion (VA/Q) inequality is observed in approximately 50% of humans during heavy exercise and contributes to the widening of the alveolar-arterial O2 difference (A-aDO2). Despite extensive investigation, the cause remains unknown. As a first step to more direct examination of this problem, we developed an animal model. Eight Yucatan miniswine were studied at rest and during treadmill exercise at approximately 30, 50, and 85% of maximal O2 consumption (VO2 max). Multiple inert-gas, blood-gas, and metabolic data were obtained. The A-aDO2 increased from 0 +/- 3 (SE) Torr at rest to 14 +/- 2 Torr during the heaviest exercise level, but arterial PO2 (PaO2) remained at resting levels during exercise. There was normal VA/Q inequality [log SD of the perfusion distribution (log) = 0.42 +/- 0.04] at rest, and moderate increases (log = 0.68 +/- 0.04, P < 0.0001) were observed with exercise. This result was reproducible on a separate day. The VA/Q inequality changes are similar to those reported in highly trained humans. However, in swine, unlike in humans, there was no inert gas evidence for pulmonary end-capillary diffusion limitation during heavy exercise; there was no systematic difference in the measured PaO2 and the PaO2 as predicted from the inert gases. These data suggest that the pig animal model is well suited for studying the mechanism of exercise-induced VA/Q inequality.

Angiotensin II blockade followed by growth hormone as adjunctive therapy after experimental myocardial infarction.

BACKGROUND: Recombinant human growth hormone (rhGH) has shown beneficial effects on cardiac function after myocardial infarction (MI) in rats. High-dose angiotensin II (AT1) receptor blockade in normal rats inhibited the hypertrophic effect of growth hormone (GH), therefore we investigated whether GH effects after MI would be enhanced by giving it in sequence after remodeling had been inhibited by prior AT1 blockade (losartan, L). METHODS AND RESULTS: Rats given losartan for 10 weeks after MI followed by rhGH for 2 weeks (2 mg/kg twice a day, GH plus losartan) were compared with rats given losartan for 10 weeks followed by placebo for 2 weeks (placebo plus losartan group) and with untreated controls (n = 17-20/group). Average MI sizes and left ventricular (LV) end diastolic (ED) dimensions (echocardiography) did not differ between groups. In GH and losartan, body weight (BW) was increased but left ventricular weight (LVW)/BW was reduced, and the LV fractional shortening and LV dP/dtmax (catheter tip micromanometer) were increased compared with the control group (20.3 vs 15.4% and 5579 vs 4699 mmHg/s, respectively, P < .05). The cardiac index also was significantly increased. In the placebo plus losartan group, the LVW/BW was also reduced and the cardiac index increased versus controls. Stroke volume was increased in GH plus losartan group compared with both placebo plus losartan and controls, and the systemic vascular resistance was significantly decreased only in the GH plus losartan group. The ED posterior wall thickness (noninfarcted wall) was increased in GH plus losartan compared with both control and placebo plus losartan. Left ventricular end diastolic pressure reduction was not significant in GH plus losartan group versus controls but was reduced in placebo plus losartan group, whereas LV relaxation (tau) was improved in both groups versus control rats. Thus, persistent remodeling effects caused by prior AT1 blockade undoubtedly contributed to some responses, but short-term GH given in sequence after chronic AT1 blockade had favorable actions on the failing heart and peripheral circulation by increasing LV wall thickness with partial reversal of unfavorable remodeling, lowering of vascular resistance, improvement of LV contractility, and enhanced LV systolic function and cardiac index relatively late after experimental MI.

Exercise training in swine promotes growth of arteriolar bed and capillary angiogenesis in heart.

Exercise training induces coronary vascular adaptations. The goal of this study was to contrast the effects of training on capillary and arteriolar growth. Minipigs were trained for 1, 3, 8, and 16 wk and compared with controls. Maximal O2 consumption increased continuously throughout the study. Capillary and arteriolar densities and diameters, and proliferation of vascular cells in these vessels, were determined in perfusion-fixed tissue. The arterioles were subdivided into five groups according to diameter: 10-19.9, 20-30, 31-40, 41-70, and 71-120 microgram. The total vascular bed cross-sectional area increased by 37% at 16 wk, mainly because of an increase in the number of the small arterioles and an increase in the diameter of the larger vessels. Capillary density increased at 3 wk and then returned to control levels by 16 wk; concomitantly, the number of arterioles (20-30 microgram) increased at 16 wk. We speculate that the "extra" capillaries observed at 3 wk were the source of the new arterioles.

Increased gene expression of plasminogen activators and inhibitors in left ventricular hypertrophy.

In the early stages of left ventricular hypertrophy (LVH) acute adaptive changes occur in the coronary vasculature as it remodels. Plasminogen activators (PAs) and inhibitors (PAIs) have the potential effects of proteolytic degradation that is relevant to tissue remodeling and angiogenesis. Our study focused on the possible roles of PAI-1, PAI-2, and uPA in tPA in myocyte hypertrophy and angiogenesis in the early and late stages of pressure overload induced left ventricular hypertrophy (LVH). We divided seventeen adult swine, weighing 24.2 +/- 6.5 kg, into four groups: control, sham-operated, early LVH and late heart failure LVH group. At surgery we placed a fixed constrictor on the ascending aorta immediately above the aortic valve. This increased LV systolic pressure from 133 +/- 15 to 193 +/- 24 mm Hg after the surgery. We subdivided the early group into groups of 3 animals each that we euthanized at 8, 24 and 72 h after operation and obtained heart samples for analysis. In the late heart failure group individual animals were euthanized at 55, 59, 62 and 72 days after the detection of congestive heart failure. We also obtained tissue samples from the control and sham-operated swine. Sections for histologic analysis were fixed in 10% buffered formalin. We isolated RNA, size fractionated it using 1% formaldehyde-agarose gel electrophoresis and then did Northern blots. The mRNAs from both PAI-1 and PAI-2 showed a remarkable increase at 8 and 24 h after acute aortic constriction and returned to control by 72 h. Regional differences showed that most of the increases were in the endocardium. Three animals in the late heart failure LVH group were determined to be in congestive heart failure at about 2 months after the onset of aortic constriction. In these animals PAI-1 and PAI-2 were increased in both the left and right ventricles but remained low in an animal of the same elevation in aortic pressure seen by the LV who did not have congestive failure. These data suggest that PA and PAI gene expressions change before morphologic changes occur in the early stages of developing LVH. Also at the time of onset of congestive heart failure this increased expression reappears. PAs and PA inhibitors mRNA levels vary in the different regions of the heart reflecting changing wall stresses. Thus, the PAs and PA inhibitors may play an important role in angiogenesis that occurs during the early stages of LVH. The increased expression in the late stage of LVH may reflect further changes in wall stresses since these animals also showed overt clinical signs of heart failure.

Regional deficits of myocardial blood flow and function in left ventricular pacing-induced heart failure.

BACKGROUND: Pacing-induced congestive hear, failure has become a preferred model for the study of the pathogenesis of dilated cardiomyopathy. However, little is known regarding regional myocardial blood flow and function during the development of heart failure in this model. METHODS AND RESULTS: To determine whether regional differences in myocardial blood flow are associated with regional dysfunction in ventricular pacing-induced heart failure, regional myocardial blood flow (radioactive microspheres) and regional wall thickening (transthoracic echocardiography) were measured in pigs studied at weekly intervals during the progression of heart failure induced by rapid pacing from the lateral wall of the left ventricle (220 +/- 9 bpm for 26 +/- 4 days). Echocardiography and hemodynamic measurements with the pacemaker off showed progressive, severe global left ventricular dysfunction. During pacing over the 3- to 4-week period, a progressive decrease in systolic wall thickening in the lateral wall occurred compared with the interventricular septum (IVS; P = .001); at 21 to 28 days, the difference was 50% (lateral wall, 14 +/- 6%; IVS, 28 +/- 6%; P = .0001). A difference in subendocardial blood flow per beat between the left ventricular lateral wall (the site of stimulation) and the IVS was found immediately on the initiation of pacing (IVS, 0.009 +/- 0.002 mL.min-1.g-1.beat-1; lateral wall, 0.005 +/- 0.001 mL.min-1.g-1.beat-1; P = .001), a difference that was sustained during pacing throughout the study. Subendocardial blood flow per beat was normal in both regions with the pacemaker off throughout the study. CONCLUSIONS: These data indicate that regional myocardial ischemia is associated with the development of contractile dysfunction of the paced wall during prolonged rapid left ventricular pacing and that regional stunning contributes to persistent global left ventricular dysfunction when pacing is discontinued.

Hypertonic saline/dextran treatment for severe pressure-driven hemorrhage in dehydrated conscious swine.

Although dehydration impairs the response to a fixed volume hemorrhage, both 7.5% hypertonic saline/6% dextran 70 (HSD: 4 mL/kg) and standard Ringer's lactate (33 mL/kg) are effective resuscitation fluids. However, the efficacy of resuscitation during continuing hemorrhage remains in question. Using a conscious swine model of continuous pressure-driven hemorrhage, we evaluated the effects of dehydration and HSD resuscitation on survival time, hemorrhage volume, regional blood flows, and central hemodynamics. Three groups of pigs were compared: euhydrated control (EC), dehydrated control (48 h water deprived) (DC); and dehydrated and resuscitated with HSD (D + HSD). All pigs were subjected to an initial 37% blood volume hemorrhage for 60 min followed by a continuous hemorrhage proportional to the instantaneous mean arterial pressure. The D + HSD pigs were resuscitated at the end of the 37% blood volume hemorrhage. Dehydration reduced body weight (-6.5 +/- .3%) and increased hematocrit (8.9 +/- 1.8%), serum osmolality (11.6 +/- .9%), serum sodium (11.9 +/- .9%), and serum total protein (9.4 +/- 1.8%). Compared with the EC group, DC had a greater increase in heart rate and arterial base deficit in response to the pressure-driven hemorrhage and a reduced pH and survival time (159 vs. 107 min). In contrast to the DC group, D + HSD had increased mean arterial pressure, cardiac output, oxygen delivery, and regional blood flows to the gut (superior mesenteric artery), kidneys, liver (hepatic artery), and adrenals at 5 min after HSD resuscitation. The HSD did not increase blood loss but tended to prolong survival (+26 min; p = .1079). thus, dehydration compromises survival time (-33%) and the hemodynamic and metabolic responses to pressure-driven hemorrhage, while treatment with HSD improves the hemodynamic responses.

Cardiovascular effects of insulin-like growth factor-1 and growth hormone in chronic left ventricular failure in the rat.

BACKGROUND: Insulin-like growth factor-1 (IGF-1) appears to have favorable cardiac effects associated with left ventricular remodeling early after myocardial infarction in the rat. The present study was designed to determine whether IGF-1 combined with growth hormone would be beneficial later as well, when infarct healing and cardiac remodeling have occurred. METHODS AND RESULTS: Four weeks after coronary occlusion, 36 rats were randomized to IGF-1 (3 mg.kg-1.d-1) plus growth hormone (0.1 mg BID) or to placebo for 4 weeks. Treated rats had significant increases in body weight (22%), while the ratio of heart weight to body weight was unchanged. Under anesthesia, cardiac output (fluorescent microspheres) increased 46%, and systemic vascular resistance decreased by 21% (P < .001) in the treated group; a significant (22%) increase of the cardiac index was limited to treated rats with large myocardial infarctions. Small increases in the reduced left ventricular ejection fractions and left ventricular dP/dt(max) values with treatment were not significant. Treated rats showed a borderline (16%) increase in left ventricular end-diastolic volume (angiography), whereas the ratio of left ventricular end-diastolic volume to body weight was reduced in the treated group. CONCLUSIONS: IGF-1 plus growth hormone administered to rats with left ventricular failure starting 1 month after MI was associated with substantial body growth, decreased systemic vascular resistance, and increased cardiac output. The failing heart also underwent treatment-induced increases in left and right ventricular weights in proportion to body growth, but left ventricular remodeling was minor, and a decrease in the ratio of left ventricular end-diastolic volume to body weight reflected relatively less chamber dilation compared with controls. A significant interaction between size of the myocardial infarction and treatment was observed for several variables, and IGF-1 and growth hormone increased the cardia index (P < .035) in rats with a large myocardial infarction.

Intracoronary gene transfer of fibroblast growth factor-5 increases blood flow and contractile function in an ischemic region of the heart.

Increased coronary blood vessel development could potentially benefit patients with ischemic heart disease. In a model of stress-induced myocardial ischemia, intracoronary injection of a recombinant adenovirus expressing human fibroblast growth factor-5 (FGF-5) resulted in messenger RNA and protein expression of the transferred gene. Two weeks after gene transfer, regional abnormalities in stress-induced function and blood flow were improved, effects that persisted for 12 weeks. Improved blood flow and function were associated with evidence of angiogenesis. This report documents, for the first time, successful amelioration of abnormalities in myocardial blood flow and function following in vivo gene transfer.

Nitric oxide synthesis inhibition does not improve the hemodynamic response to hemorrhagic shock in dehydrated conscious swine.

Nitric oxide (NO) may influence the hemodynamic response to hemorrhage. To test this hypothesis, the NO synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) was administered to conscious, dehydrated swine during a 37% blood volume hemorrhage and a 180 min recovery period without fluid resuscitation. L-NAME (.75 mg/kg bolus plus constant infusion of .75 mg/kg/h) was given via a central intravenous catheter during the bleed. The selectivity and specificity of L-NAME as a NO synthesis inhibitor in pigs was validated in pilot studies. The present study shows that inhibition of NO synthesis with L-NAME had no significant effect on the major hemodynamic parameters during and after hemorrhage when compared to dehydrated and euhydrated control groups. Only stroke volume and A-VO2 were significantly different from controls. Mortality was 83% for the L-NAME group and 44% for controls at 180 min of recovery (NS). The results suggest that NO synthesis inhibition provides no hemodynamic benefit during hemorrhage in dehydrated, conscious swine.

Myocardial high-energy phosphate and substrate metabolism in swine with moderate left ventricular hypertrophy.

BACKGROUND: Although left ventricular hypertrophy (LVH) is frequently associated with impaired coronary vasodilator reserve, it is uncertain whether this leads to myocardial ischemia under physiological conditions. The goal of the present study was to determine whether swine with moderate LVH exhibit metabolic evidence of ischemia when myocardial oxygen requirements are increased. METHODS AND RESULTS: Myocardial metabolism was evaluated in an open-chest anesthetized preparation at baseline and during dobutamine infusion in 13 adolescent pigs with moderate LVH induced by supravalvular aortic banding and 12 age-matched control pigs. Transmural myocardial blood flow was quantified with radioactive microspheres; the ratio of phosphocreatine to ATP (PCr/ATP) in the anterior LV free wall was measured by 31P-nuclear magnetic resonance; and anterior wall lactate release was quantified from the arterial-coronary venous difference in 14C- or 13C-labeled lactate. In a subset of 5 animals from each group, the metabolic fate of exogenous glucose was determined from the transmyocardial difference in 6-14C-glucose and its metabolites 14C-lactate and 14CO2. Coronary reserve, as assessed by the ratio of blood flow during adenosine infusion to baseline blood flow, was significantly lower in the LVH pigs compared with controls (3.5 +/- 0.4 versus 5.5 +/- 0.4 mL/g.min, P < .05); however, transmural myocardial blood flow was similar in both groups of pigs, both at baseline and with dobutamine stimulation, probably reflecting the higher coronary perfusion pressure in the LVH pigs. At baseline, PCr/ATP tended to be lower in the LVH pigs (P = .09) but decreased similarly with dobutamine infusion in both groups. Isotopically measured anterior wall lactate release did not differ between the groups at baseline, nor did the increase in lactate release differ during dobutamine stimulation. The uptake of glucose, lactate, and free fatty acids did not differ between the groups in the basal state. However, during dobutamine stimulation, glucose uptake was greater in the LVH group (0.84 +/- 0.09 mumol/g.min versus 0.59 +/- 0.08 mumol/g.min, P < .05). In a subset of animals, 14C-glucose was used to assess glucose oxidation. These data showed that the LVH animals had a greater rate of glucose oxidation (0.6 +/- 0.10 versus 0.28 +/- 0.08 mumol/g.min, P < .05) and a greater rate of glucose conversion to lactate (0.20 +/- 0.04 versus 0.09 +/- 0.02 mumol/g.min, P < .05) compared with the control pigs. CONCLUSIONS: These results suggest that despite their reduced coronary vasodilator reserve and the absence of a greater rise in myocardial blood flow to compensate for a substantially higher LV double product, pigs with this model of moderate LVH do not exhibit a greater susceptibility to myocardial ischemia during dobutamine stress. However, LVH pigs exhibit significantly greater use of exogenous glucose during dobutamine stress, as evidenced by increases in both glucose oxidation and anaerobic glycolysis.

Hypertonic saline/dextran versus lactated Ringer's treatment for hemorrhage in dehydrated swine.

To determine the efficacy of low-volume resuscitation in dehydrated subjects, 7.5% hypertonic saline/6% dextran 70 (HSD) and lactated Ringer's (LR) treatments were compared in conscious pigs dehydrated for 48 hr prior to a 37% blood volume hemorrhage. Pigs randomized to treatment were resuscitated with equivalent sodium loads of either HSD (4 ml/kg) or LR (33.3 ml/kg) following the 60-min hemorrhage. Dehydration resulted in a 7-8% body weight loss. Mortality through 180 min of recovery was 1/13 (7.7%) for HSD, 1/11 (9.1%) for LR, and 4/8 (50%) for a group of dehydrated and untreated controls (DC). HSD and LR solutions elicited similar heart rate, cardiac output, arterial pressure, and oxygen transport responses in the recovery period following hemorrhage and resuscitation. HSD was as effective as LR in expanding plasma volume in dehydrated hemorrhaged pigs. Serum chemistries provided no evidence for a sustained systemic toxicity from HSD treatment. These findings support low-volume HSD resuscitation of hemorrhagic shock in moderately dehydrated subjects.

Clinical significance of coronary vascular adaptations to exercise training.

Coronary vascular adaptations to exercise training have been extensively studied at the microscopic level in animals and correlated with direct and indirect measurements of myocardial blood flow in patients with coronary artery disease. Animals have permitted more extensive study. These findings have generally supported an increased blood flow to the myocardium with exercise training. However, consistent positive structural and functional adaptations to training have not been observed in large animals. Clinical studies have been limited by methodological problems related to techniques for detecting ischemia and measuring myocardial blood flow and the variability in exercise stimulus. Well-established ischemia and high-intensity, long-duration training were the factors that promoted vascular growth in exercising patients with coronary artery disease. Animals studies also have demonstrated the necessity for myocardial ischemia to be present to induce coronary collateral development with exercise training. Optimal promoters of vascular growth in patients with coronary disease may consist of pharmacological interventions combined with exercise training.

Effects of dobutamine and arbutamine on regional myocardial function in a porcine model of myocardial ischemia.

OBJECTIVES: The present study was performed to determine the mechanisms for catecholamine-induced wall motion abnormalities and to compare the diagnostic efficacy of two catecholamines: arbutamine and dobutamine. BACKGROUND: Catecholamine stress echocardiography is used to induce regional wall motion abnormalities for the detection of coronary artery disease, but the mechanism by which these abnormalities occur is unknown. METHODS: Ten pigs were instrumented with left circumflex coronary artery ameroid constrictors, sonomicrometers to measure transmural wall thickening in the left circumflex (ischemic) and left anterior descending (control) coronary artery beds and a pressure gauge to measure left ventricular pressure and its first derivative (dP/dt). Myocardial blood flow was measured by microspheres. RESULTS: At 38 +/- 6 days (mean +/- SEM) after surgery, percent wall thickening was normal at rest in both beds but abnormal in the left circumflex coronary artery bed during atrial pacing. These findings were associated with reduced myocardial blood flow in the ischemic bed during atrial pacing. Dobutamine infusion increased percent wall thickening, with no differences between the two beds (p = 0.63). In contrast, arbutamine infusion increased percent wall thickening only in the nonischemic bed, with no effect on percent wall thickening in the ischemic bed (p = 0.03). Although the endocardial/epicardial blood flow ratio tended to be reduced in the left circumflex artery bed during catecholamine infusion (p = 0.07), both agents were similar in this effect. Despite differences in function between the beds, there was no difference in transmural myocardial blood flow between the two beds during catecholamine infusion. When examined at matched metabolic demands, arbutamine elicited greater differences in percent wall thickening than dobutamine between the two beds (p < 0.01). CONCLUSIONS: Arbutamine was able to provoke regional differences in function in a manner superior to dobutamine. This occurred independently of altered transmural myocardial blood flow or differences in hemodynamic effects between the agents. Differences in their inotropic properties may be important in explaining their different effects on ischemic myocardium.

Factors associated with vasopressin release in exercising swine.

This study examined the effect of dynamic exercise on vasopressin release in the miniswine and factors that may elicit this response (n = 15). Thus lysine vasopressin (LVP), the catecholamines epinephrine and norepinephrine (EPI and NE), plasma renin activity (PRA), and plasma volume, Na+, and osmolality were measured before and during treadmill running at work intensities of 60, 80, and 100% of each swine's maximal heart rate reserve (HRR). LVP increased in a progressive manner similar to that of humans, ranging from 5.9 +/- 0.4 pg/ml before exercise to 30.1 +/- 4.5 pg/ml during maximal exercise. EPI, NE, and PRA [an index of angiotensin II (ANG II) activity] demonstrated a pattern of response comparable to LVP. Although these hormones can influence the release of LVP, only PRA displayed a strong correlation with LVP (r = 0.84). When ANG II synthesis was blocked (captopril, 1-3 mg/kg, intra-atrial injection) during exercise (80% HRR), plasma LVP was reduced from 9.9 +/- 0.6 to 7.5 +/- 0.6 pg/ml (P < 0.05). In addition, moderate-to-strong correlations were found between plasma concentrations of LVP and plasma osmolality (r = 0.79) and body temperature (r = 0.78). Plasma LVP also correlated with decreases in plasma volume (r = 0.84). These data suggest that the miniswine model is a good one for studying vasopressin effects during exercise and that ANG II appears to be a particularly strong stimulus for the release of this hormone.

Regional myocardial downregulation of the inhibitory guanosine triphosphate-binding protein (Gi alpha 2) and beta-adrenergic receptors in a porcine model of chronic episodic myocardial ischemia.

Regional myocardial ischemia is associated with increased levels of adenosine and norepinephrine, factors that may alter activation of the beta-adrenergic receptor (beta AR)-G protein-adenylyl cyclase pathway in the heart. We have used the ameroid constrictor model to determine whether alterations in myocardial signal transduction through the beta AR-G protein-adenylyl cyclase pathway occur in the setting of chronic episodes of reversible ischemia. Pigs were instrumented with ameroid occluders placed around the left circumflex coronary artery. 5 wk later, after ameroid closure, flow and function were normal in the ischemic bed, but flow (P = 0.001) and function (P < 0.03) were abnormal when metabolic demands were increased. The ischemic bed showed a reduction in myocardial beta AR number (P < 0.005). Despite regional downregulation of myocardial beta AR number, adenylyl cyclase activity was similar in the ischemic and control beds. Quantitative immunoblotting showed that the cardiac inhibitory GTP-binding protein, Gi alpha 2, was decreased in the ischemic bed (P = 0.02). In contrast, the cardiac stimulatory GTP-binding protein, Gs alpha, was increased in endocardial sections from the ischemic bed (P = < 0.05). Decreased Gi alpha 2 content was associated with decreased inhibition of adenylyl cyclase. Reduced Gi alpha 2 content, in conjunction with increased Gs alpha content in the endocardium, may provide a means by which adrenergic activation is maintained in the setting of chronic episodic myocardial ischemia.

Effects of feeding on muscle blood flow during prolonged exercise in miniature swine.

Eight exercise-trained miniature swine were studied during prolonged treadmill runs (100 min) under fasting and preexercise feeding conditions. Each animal ran at identical external work loads that corresponded to 65% of the heart rate reserve (210-220 beats/min) for the two exercise bouts. Cardiac outputs and stroke volumes were higher and heart rates lower for fed than for fasting runs (P less than 0.05). Preexercise feeding did not alter oxygen consumption, core temperature, mean arterial pressure, and arterial-mixed venous oxygen difference during prolonged exercise; however, mixed venous lactate concentration was lower at end exercise than during fasting conditions (1.2 vs. 2.6 mM, P less than 0.05). Microsphere measurements of regional blood flow revealed significantly higher total gastrointestinal flow (23%) for fed than for fasting conditions. Throughout the exercise bout, blood flow to the biceps femoris, semitendinosus, and tibialis anterior muscles was lower in fed than in fasted animals (P less than 0.05). Combined hindlimb muscle blood flow averaged 15 ml.min-1.100 g-1 (18%, P less than 0.05) lower under feeding than fasting run conditions. These findings provide further evidence that cardiovascular reflexes originate in the gut after feeding to increase cardiac output and redistribute a portion of the blood flow away from active muscle to the gastrointestinal tract during prolonged exercise.

Plateau in muscle blood flow during prolonged exercise in miniature swine.

Cardiovascular, metabolic, and thermoregulatory responses were studied in eight male miniature swine during a prolonged treadmill run. Each animal underwent 8-10 wk of exercise training, thoracic surgery, and 3 wk of retraining before the experimental run. This regimen enabled the animals to run at 65% of the heart rate range (210-220 beats/min) for approximately 100 min. Skin wetting and a fan were used to cool the pigs during the run. Regional blood flow was significantly altered with the onset of exercise; however, hindlimb muscle and total gastrointestinal blood flow were unchanged throughout the exercise period. Compared with 5-min values, heart rate and cardiac output were significantly elevated by 17 beats/min and 31 ml.min-1.kg-1 at 60 min and by 20 beats/min and 33 ml.min-1.kg-1 at end exercise, respectively. Core temperatures increased between 5 and 30 min of exercise (39.4 vs. 39.9 degrees C) but then remained unchanged to the end of exercise. Mean arterial pressure, O2 consumption, and blood lactate did not change during the exercise bout. These data indicate that limiting increases in core temperature during prolonged exercise was associated with a plateau in active muscle blood flow.

Effects of chronic dobutamine administration on the response to acute exercise in dogs.

The effects of chronic dobutamine administration on haemodynamic and metabolic responses to submaximal and maximal exercise were studied in dogs. Dobutamine was infused at a rate of 40 micrograms/kg min-1, 2 h day-1, 5 days week-1 for a period of 6 weeks. Acute infusion of dobutamine for 1 h increased heart rate by 73 +/- 30 beats min-1 and cardiac output by 143 +/- 141 ml/min kg-1, reduced mean arterial blood pressure by 12 +/- 10 mmHg and arterial-venous O2 difference by 1.5 +/- 1 vol%. Maximal oxygen consumption, heart rate, stroke volume, cardiac output and arterial-venous O2 difference were unchanged after 6 weeks of treatment. Reductions in heart rate at rest and during submaximal exercise following chronic dobutamine treatment were small and significant only at the lowest exercise level studied. Mixed venous lactate concentrations measured at rest, during submaximal and maximal exercise and at 2 min of recovery were not different after dobutamine treatment. Chronic dobutamine infusion did not change the citrate synthase activity in the lateral gastrocnemius muscle. These results suggest that chronic dobutamine therapy in healthy dogs does not produce aerobic training responses.