Alpha 1-adrenergic tone does not influence the transmural distribution of myocardial blood flow during exercise in dogs with pressure overload left ventricular hypertrophy

Animal exercise studies in cardiovascular research: Current knowledge and optimal design-A position paper of the Committee on Cardiac Rehabilitation, Chinese Medical Doctors' Association

Growing evidence has demonstrated exercise as an effective way to promote cardiovascular health and protect against cardiovascular diseases However, the underlying mechanisms of the beneficial effects of exercise have yet to be elucidated. Animal exercise studies are widely used to investigate the key mechanisms of exercise-induced cardiovascular protection. However, standardized procedures and well-established evaluation indicators for animal exercise models are needed to guide researchers in carrying out effective, high-quality animal studies using exercise to prevent and treat cardiovascular diseases. In our review, we present the commonly used animal exercise models in cardiovascular research and propose a set of standard procedures for exercise training, emphasizing the appropriate measurements and analysis in these chronic exercise models. We also provide recommendations for optimal design of animal exercise studies in cardiovascular research, including the choice of exercise models, control of exercise protocols, exercise at different stages of disease, and other considerations, such as age, sex, and genetic background. We hope that this position paper will promote basic research on exercise-induced cardiovascular protection and pave the way for successful translation of exercise studies from bench to bedside in the prevention and treatment of cardiovascular diseases.

Coronary vasoconstrictor responses are attenuated in young women as compared with age-matched men

Recent work in humans suggests coronary vasoconstriction occurs with static handgrip with a time course that suggests a sympathetic constrictor mechanism. These findings are consistent with animal studies that suggest this effect helps maintain transmural myocardial perfusion. It is known that oestrogen can attenuate sympathetic responsiveness, however it is not known if sympathetic constrictor responses vary in men and women. To examine this issue we studied young men (n = 12; 28 ± 1 years) and women (n = 14; 30 ± 1 years). Coronary blood flow velocity (CBV; Duplex Ultrasound), heart rate (ECG) and blood pressure (BP; Finapres) were measured during static handgrip (20 s) at 10% and 70% of maximum voluntary contraction. Measurements were also obtained during graded lower body negative pressure (LBNP; activates baroreflex-mediated sympathetic system) and the cold pressor test (CPT; a non-specific sympathetic stimulus). A coronary vascular resistance index (CVR) was calculated as diastolic BP/CBV. Increases in CVR with handgrip were greater in men vs. women (1.25 ± 0.49 vs. 0.26 ± 0.38 units; P < 0.04) and CBV tended to fall in men but not in women (−0.9 ± 0.9 vs. 1.7 ± 0.8 cm s−1; P < 0.01). Changes in CBV with handgrip were linked to the myocardial oxygen consumption in women but not in men. CBV reductions were greater in men vs. women during graded LBNP (P < 0.04). Men and women had similar coronary responses to CPT (P = n.s.). We conclude that coronary vasoconstrictor tone is greater in men than women during static handgrip and LBNP.

Evidence Favoring the Use of an α 2 -Selective Vasopressor Agent for Cardiopulmonary Resuscitation

Background— Both α 1 - and β-adrenergic agonists increase the severity of global myocardial ischemic injury. We hypothesized that combined β- and α 1 -adrenergic blockade would improve initial resuscitation and postresuscitation myocardial and neurological functions. We further hypothesized that the resulting α 2 -actions of relatively brief duration would favor improved functions compared with the more prolonged effect of nonadrenergic vasopressin.

Methods and Results— Three groups of 5 male domestic pigs weighing 37±3 kg were investigated. Ventricular fibrillation was untreated for 7 minutes before the start of precordial compression, mechanical ventilation, and attempted defibrillation. Animals were randomized to receive central venous injections of equipressor doses of (1) epinephrine, (2) epinephrine in which both α 1 - and β-adrenergic effects were blocked by previous administration of prazosin and propranolol, and (3) vasopressin during CPR. All but 1 animal were successfully resuscitated. After injection of epinephrine, significantly better cardiac output and fractional area change, together with lesser increases in troponin I, were observed after α 1 - and β-adrenergic blockade. Postresuscitation neurological function was also improved after α 1 - and β-block in comparison with unblocked epinephrine and after vasopressin.

Conclusions— Equipressor doses of epinephrine, epinephrine after α 1 - and β-adrenergic blockade, and vasopressin were equally effective in restoring spontaneous circulation after prolonged ventricular fibrillation. However, combined α 1 - and β-adrenergic blockade, which represented a predominantly selective α 2 -vasopressor effect, resulted in improved postresuscitation cardiac and neurological recovery.

Effect of NO on transmural distribution of blood flow in hypertrophied left ventricle during exercise

When exercise in the presence of a coronary artery stenosis results in subendocardial ischemia, administration of a nitric oxide (NO) donor increases subendocardial blood flow, whereas NO synthesis blockade worsens subendocardial hypoperfusion. Because left ventricular hypertrophy (LVH) is also associated with subendocardial hypoperfusion during exercise, this study tested the hypothesis that alterations of NO availability can similarly influence subendocardial blood flow in the hypertrophied heart. Studies were performed in seven dogs in which ascending aortic banding resulted in an 80% increase in LV weight. Myocardial blood flow was measured with microspheres during treadmill exercise that increased heart rates to 216 +/- 8 beats/min. During control exercise, mean myocardial blood flow in animals with LVH was similar to that in historic controls, but the ratio of subendocardial to subepicardial blood flow was lower in animals with hypertrophy (0.88 +/- 0.07) than in controls (1.36 +/- 0.08; P < 0.05). Blockade of NO synthesis with NG-nitro-L-arginine (L-NNA; 1.5 mg/kg ic) caused no change in heart rate or LV systolic pressure during exercise. Furthermore, L-NNA did not worsen subendocardial hypoperfusion during exercise. Intracoronary infusion of nitroglycerin (0.4 microgram. kg-1. min-1) did not significantly alter either mean blood flow or the transmural distribution of perfusion during exercise in the hypertrophied hearts. Thus, unlike the subendocardial underperfusion that occurs when a stenosis limits coronary blood flow, alterations of NO availability did not alter subendocardial hypoperfusion in the hypertrophied hearts.

Effect of treadmill exercise on transmural distribution of blood flow in hypertrophied left ventricle

Pressure-overload left ventricular (LV) hypertrophy (LVH) is associated with increased vulnerability to subendocardial hypoperfusion during exercise. Abnormal perfusion could be the result of failure of the coronary vessels to grow in proportion to the degree of myocyte hypertrophy or could be due to increased extravascular forces acting on the intramural coronary vasculature. This study assessed the contribution of extravascular forces by examining the effect of exercise on the distribution of myocardial blood flow when coronary vasomotor tone was abolished with a maximal vasodilating dose of intracoronary adenosine. One year after ascending aortic banding in six dogs, the LV-to-body weight ratio was 7.80 +/- 0.38 g/kg compared with 4.57 +/- 0.20 g/kg in nine normal dogs (P < 0.01). Under awake resting conditions blood flow in LVH hearts increased from 1.17 +/- 0.27 ml . min-1 . g-1 during basal conditions to 5.78 +/- 1.06 ml . min-1 . g-1 during adenosine (at a coronary pressure of 100 +/- 6 mmHg), whereas in normal dogs blood flow increased from 1.22 +/- 0.17 to 5.26 +/- 0.71 ml . min-1 . g-1 (at a coronary pressure of 62 +/- 4 mmHg). At rest the transmural distribution of blood flow during adenosine was not different between hypertrophied and normal hearts, with subendocardial-to-subepicardial (Endo-to-Epi) blood flow ratios of 1. 01 +/- 0.09 and 1.14 +/- 0.13, respectively (P = not significant). During adenosine infusion, treadmill exercise to produce heart rates of 200-220 beats/min caused redistribution of blood flow away from the subendocardium that was much more marked in LVH (Endo-to-Epi blood flow ratio = 0.35 +/- 0.04) than in normal hearts (Endo-to-Epi blood flow ratio = 0.76 +/- 0.09, P < 0.05 vs. LVH). In comparison with normal, the exaggerated decrease in subendocardial blood flow produced by exercise in LVH hearts resulted from abnormally increased extravascular compressive forces, including a greater decrease in diastolic duration and an increase in LV end-diastolic pressure.