Autonomic control of skeletal muscle vasodilation during exercise

Buckwalter, John B., Patrick J. Mueller, and Philip S. Clifford. Autonomic control of skeletal muscle vasodilation duringexercise. J. Appl. Physiol. 83(6):2037-2042, 1997.Despite extensive investigation, the control ofblood flow during dynamic exercise is not fully understood. The purposeof this study was to determine whether -adrenergic or muscarinicreceptors are involved in the vasodilation in exercising skeletalmuscle. Six mongrel dogs were instrumented with ultrasonic flow probeson both external iliac arteries and with a catheter in a branch of onefemoral artery. The dogs exercised on a treadmill at 6 miles/h whiledrugs were injected intra-arterially into one hindlimb. Isoproterenol(0.2 µg) or acetylcholine (1 µg) elicited increases in iliac bloodflow of 89.8 ± 14.4 and 95.6 ± 17.4%, respectively, withoutaffecting systemic blood pressure or blood flow in the contralateraliliac artery. Intra-arterial propranolol (1 mg) or atropine (500 µg)had no effect on iliac blood flow, although they abolished theisoproterenol and acetylcholine-induced increases in iliac blood flow.These data indicate that exogenous activation of -adrenergic ormuscarinic receptors in the hindlimb vasculature increases blood flowto dynamically exercising muscle. More importantly, because neitherpropranolol nor atropine affected iliac blood flow, we conclude that-adrenergic and muscarinic receptors are not involved in the controlof blood flow to skeletal muscle during moderate steady-state dynamicexercise in dogs.

     

Neuromuscular regulation and metabolism during exercise

This article reviews current evidence regarding neuromuscular regulation and metabolism during exercise. Particular emphases are given on the relationship between motor unit (MU) activity, including single MU analysis results and spinal alpha-motoneuron excitability, and cardio-respiratory response and blood lactate during dynamic exercise. In addition, a close physiological link between muscle energy metabolism and excitation-contraction processes (failure of one will affect the extent of the other) is summarized in the light of recent nuclear magnetic resonance (NMR) studies and results of neuromuscular disorder patients.     

Clinical application of pacemakers in atrial tachyarrhythmias

The diagnostic programmes of modern pacemakers have increased our knowledge of atrial tachyarrhythmias (ATAs) in chronically paced patients. These programmes also support the evaluation of the effects of pharmacological treatment of ATAs. The success of interruption and/or prevention of ATAs with pacemakers depends strongly on the diagnostic accuracy and the properties of the pacing algorithms, their individual programming and the site and configuration of the pacing leads. Atrial septum pacing can be beneficial in patients with paroxysmal atrial fibrillation and prolonged P wave duration. Recent large-scale studies on preventive and interruptive atrial pacing of ATAs show modestly positive or no results. Therefore, atrial pacing therapy for ATAs should be considered cautiously, serving as an adjuvant to pharmacological treatment rather than as a primary intervention. This also applies for pacing interventions for ATAs in cardiac resynchronisation therapy. The pacemaker algorithms for the detection of ATAs and atrial lead configuration are crucial for the success of pacemaker-mediated prevention or interruption of ATAs. The success of these interventions is dependant on future improvements of pacemaker technology. (Neth Heart J 2008;16 (suppl 1): S20-S24.)     

Cardiovascular regulation during head-out water immersion exercise

Head-out water immersion is known to increase cardiac filling pressure and volume in humans at rest. The purpose of the present study was to assess whether these alterations persist during dynamic exercise. Ten men performed upright cycling exercise on land and in water to the suprasternal notch at work loads corresponding to 40, 60, 80, and 100% maximal O2 consumption (VO2max). A Swan-Ganz catheter was used to measure right atrial pressure (PAP), pulmonary arterial pressure (PAP), and cardiac index (CI). Left ventricular end-diastolic (LVED) and end-systolic (LVES) volume indexes were assessed with echocardiography. VO2max did not differ between land and water. RAP, PAP, CI, stroke index, and LVED and LVES volume indexes were significantly greater (P less than 0.05) during exercise in water than on land. Stroke index did not change significantly from rest to exercise in water but increased (P less than 0.05) on land. Arterial systolic blood pressure did not differ between land and water at rest or during exercise. Heart rates were significantly lower (P less than 0.05) in water only during the two highest work intensities. The results indicate that indexes of cardiac preload are greater during exercise in water than on land.     

Heat regulation during exercise with controlled cooling

During heavy sustained exercise, when sweating is usually needed to dissipate the extra metabolic heat, controlled cooling caused heat loss to match total heat production with little sweating. The total heat produced and metabolic rate were varied independently by having subjects walk uphill and down. Heat loss was measured directly with a suit calorimeter; other measurements included metabolic energy from respiratory gas exchange and body temperatures. Thermoregulatory sweating was minimized by adjusting cooling in the calorimeter suit. Heat loss rose to match total heat, not metabolic rate, and there was a slow rise in rectal temperature. In the absence of major thermoregulatory response rectal temperature correlated most closely with total heat; it also correlated with the relative oxygen cost of exercise. Heat flow or heat content appeared to be the controlled variable and body temperature rise a secondary event resulting from thermal transport lag.     

Neural regulation of the cardiovascular system during exercise

Neural components important in control of the cardiovascular system during exercise can be divided into central nervous system (CNS) components and peripheral components. CNS components would include the cerebral cortex, cerebellum, medullary region of the brain stem, and the spinal cord. Peripheral components would include the efferent limbs of the autonomic nervous system and afferent fibers carrying information to the CNS. The neural pathways involved in the control of cardiovascular system during exercise and the relationship between the various neural components have been actively pursued in the last few years. Several new studies suggest that information arising from the active muscles and the cardiovascular system itself may be important in the control of the cardiovascular system during exercise. The cerebellum may play a modulating role in the cardiovascular response. The information from the peripheral afferent fibers, the cerebellum, and the cerebral cortex is integrated in the brain to result in overall neural control. Exercise training probably modifies the central integration of information and modifies the cardiovascular response to exercise and other stresses.     

Hormonal regulation of potassium shifts during graded exhausting exercise

Serum potassium, aldosterone and insulin, and plasma adrenaline, noradrenaline and cyclic adenosine 3':5'-monophosphate (cAMP) concentrations were measured during graded exhausting exercise and during the following 30 min recovery period in six untrained young men. During exercise there was an increase in concentration of serum potassium (4.74 mmol.l-1, SEM 0.12 at the end of exercise vs 3.80 mmol.l-1, SEM 0.05 basal, P less than 0.001), plasma adrenaline (2.14 nmol.l-1, SEM 0.05 at the end of exercise vs 0.30 nmol.l-1, SEM 0.02 basal, P less than 0.001), plasma noradrenaline (1.10 nmol.l-1, SEM 0.64 at the end of exercise vs 1.50 nmol.l-1, SEM 0.05 basal, P less than 0.001), serum aldosterone (0.92 nmol.l-1, SEM 0.14 at the end of exercise vs 0.36 nmol.l-1, SEM 0.05 basal, P less than 0.01), and plasma cAMP (35.4 nmol.l-1, SEM 2.3 at the end of exercise vs 21.4 nmol.l-1, SEM 4.5 basal, P less than 0.05). While concentrations of serum potassium, plasma adrenaline and cAMP returned to their basal levels immediately after exercise, those of plasma noradrenaline and serum aldosterone remained elevated 30 min later (1.90 nmol.l-1, SEM 0.01, P less than 0.01; and 0.85 nmol.l-1, SEM 0.12, P less than 0.01, respectively). Serum insulin concentration did not change during exercise (6.47 mlU.l-1, SEM 0.58 at the end of exercise vs 5.47 mlU.l-1, SEM 0.41 basal, NS) but increased significantly (P less than 0.02) at the end of the recovery period (7.12 mlU.l-1, SEM 0.65).(ABSTRACT TRUNCATED AT 250 WORDS)     

Autonomic denervation for the treatment of atrial fibrillation

The influence of the autonomic nervous system (ANS) on triggering and perpetuation of atrial fibrillation (AF) is well established. Ganglionated plexi (GP) ablation achieves autonomic denervation by affecting both the parasympathetic and sympathetic components of the ANS. An anatomic approach for GP ablation at relevant atrial sites appears to be safe, and improves the results of PV isolation in patients with paroxysmal and persistent AF. GP ablation can be accomplished endocardially or epicardially, ie, during the maze procedure or thoracoscopic approaches. Further experience is needed to assess the clinical value of this promising technique.     

Recent advances in temperature regulation during exercise in humans

This paper describes first the dynamics of heat transfer from active muscle to the body core and then the physiological regulatory mechanisms that act to modify the rates of heat transfer from core to skin and from skin to environment. After this, nonthermal factors influencing the regulatory mechanisms are described, emphasizing the importance of body fluid status and its influence on the temperature regulatory mechanisms. The control of cutaneous vasomotor and venomotor tone is the shared effector loop of both the blood pressure and temperature regulatory systems; during exercise these systems interact, with the former system predominating when mutually exclusive demands exist. The importance of blood volume is emphasized again in a final discussion of the effects of improved physical condition on the temperature regulatory system.     

Acid-base regulation during exercise and recovery in humans.

Arterial pH, PCO2, standard bicarbonate, lactate, and ventilation were measured with a high sampling density during rest, exercise, and recovery in normal subjects performing upright cycle ergometer exercise. Three 6-min constant-work exercise tests (moderate, heavy, and very heavy) were performed by each subject. We found a small respiratory acidosis during the moderate-intensity exercise and an early respiratory acidosis followed by a metabolic acidosis for the heavy- and very-heavy-intensity exercise. During recovery, arterial pH rapidly returned to the preexercise value for the moderate-intensity work. However, arterial pH decreased further during the first 2 min of recovery for the heavy- and very-heavy-intensity work, before a slower return toward the resting values. We conclude that arterial acidosis is the consistent arterial pH reaction for moderate-, heavy-, and very-heavy-intensity cycle ergometer exercise in humans and that this acidosis is blunted but not eliminated by the ventilatory response. During recovery, the return to resting arterial pH and PCO2 and standard bicarbonate appears to be determined by the rate of lactate decline.