Stephen E. DiCarlo

Sinoaortic denervation prevents postexercise reductions in arterial pressure and cardiac sympathetic tonus

Arterial pressure, cardiac sympathetic tonus (ST), and heart rate (HR) are reduced after a single bout of dynamic exercise in spontaneously hypertensive rats (SHR). To test if the arterial baroreflex is required for these postexercise responses, intact (n = 9) and sinoaortic-denervated (SAD) rats (n = 5) were chronically instrumented with an arterial catheter for the measurement of arterial pressure and HR and for the infusion of cardiac autonomic antagonists. Five days after instrumentation, cardiac ST and parasympathetic tonus (PT) were determined under two experimental conditions (no exercise and postexercise). SAD rats did not alter no-exercise cardiac ST (intact 47 +/- 3 vs. SAD 50 +/- 3 beats/min); however, no-exercise PT was reduced (intact -24 +/- 2 vs. SAD -4 +/- 4 beats/min, P < 0.05). Acute exercise reduced arterial pressure (postexertional hypotension, -20 +/- 3 mmHg, P < 0.05), cardiac ST (no exercise 47 +/- 3 vs. postexercise 24 +/- 3 beats/min, P < 0.05), and PT (no exercise -24 +/- 2 vs. postexercise -11 +/- 2 beats/min, P < 0.05) in intact SHR. In contrast, SAD prevented postexercise reductions in arterial pressure and cardiac ST (no exercise 50 +/- 3 vs. postexercise 59 +/- 7 beats/min). Furthermore, SAD had no effect on postexercise PT (no exercise -4 +/- 4 vs. postexercise -7 +/- 4 beats/min). These results demonstrate that the arterial baroreflex is required for the reduction in arterial pressure and cardiac ST that occurs in SHR after a single bout of dynamic exercise.Arterial pressure, cardiac sympathetic tonus (ST), and heart rate (HR) are reduced after a single bout of dynamic exercise in spontaneously hypertensive rats (SHR). To test if the arterial baroreflex is required for these postexercise responses, intact ( n = 9) and sinoaortic-denervated (SAD) rats ( n = 5) were chronically instrumented with an arterial catheter for the measurement of arterial pressure and HR and for the infusion of cardiac autonomic antagonists. Five days after instrumentation, cardiac ST and parasympathetic tonus (PT) were determined under two experimental conditions (no exercise and postexercise). SAD rats did not alter no-exercise cardiac ST (intact 47 ± 3 vs. SAD 50 ± 3 beats/min); however, no-exercise PT was reduced (intact -24 ± 2 vs. SAD -4 ± 4 beats/min, P < 0.05). Acute exercise reduced arterial pressure (postexertional hypotension, -20 ± 3 mmHg, P < 0.05), cardiac ST (no exercise 47 ± 3 vs. postexercise 24 ± 3 beats/min, P < 0.05), and PT (no exercise -24 ± 2 vs. postexercise -11 ± 2 beats/min, P < 0.05) in intact SHR. In contrast, SAD prevented postexercise reductions in arterial pressure and cardiac ST (no exercise 50 ± 3 vs. postexercise 59 ± 7 beats/min). Furthermore, SAD had no effect on postexercise PT (no exercise -4 ± 4 vs. postexercise -7 ± 4 beats/min). These results demonstrate that the arterial baroreflex is required for the reduction in arterial pressure and cardiac ST that occurs in SHR after a single bout of dynamic exercise.

Onset of exercise increases lumbar sympathetic nerve activity in rats

We hypothesized that lumbar sympathetic nerve activity (LSNA) increases at the onset of whole-body dynamic exercise in the rat. To test this hypothesis, we recorded LSNA, heart rate (HR), and arterial pressure (AP) at rest and during a graded exercise test in six adult rats. Rats were instrumented with arterial and venous catheters and recording electrodes around the lumbar sympathetic trunk. Following recovery, each rat ran continuously on a hand-driven or motorized treadmill at 6 m.min-1, 12 m.min-1, and 18 m.min-1 on a 10% grade for approximately 3 min at each workload. Before exercise, mean arterial pressure (MAP), HR, and LSNA averaged 108 +/- 4 mm Hg, 385 +/- 20 bpm, and 100%, respectively. As hypothesized, all variables increased abruptly and dramatically at the onset of treadmill exercise. For example, MAP (117 +/- 5 mm Hg), HR (450 +/- 15 bpm), and LSNA (225 +/- 19%) all increased significantly within the first 25 s of treadmill running at 6 m.min-1. As the exercise continued, there was a progressive increase in HR; however, MAP plateaued at 6 m.min-1 and LSNA plateaued at 12 m.min-1. Since LSNA increased at the onset of whole-body dynamic exercise in the rat, we suggest that the increase in LSNA at the onset of exercise is mediated by a central (feed forward) mechanism.

Acute Exercise Attenuates Cardiac Autonomic Regulation in Hypertensive Rats

Dynamic exercise may be used as a safe, therapeutic approach to reduce sympathetic nerve activity at rest and thus may be beneficial for individuals with hypertension. Therefore, we tested the hypothesis that a single bout of mild to moderate dynamic exercise would decrease cardiac sympathetic tonus at rest. We designed two experimental protocols to test this hypothesis in male spontaneously hypertensive rats. In protocol 1 (n = 6) cardiac sympathetic tonus and parasympathetic tonus were determined before and after a single bout of dynamic exercise. We developed protocol 2 (n = 5) to determine the component of the autonomic nervous system responsible for the postexercise reduction in heart rate. Rats were instrumented with catheters inserted into the descending aorta for measurements of arterial pressure, mean arterial pressure, and heart rate and into the jugular vein for infusion of drugs. A single bout of mild to moderate dynamic treadmill exercise (12 m/min, 10% grade for 42 +/- 1 minutes, representing approximately 74% to 79% of maximal heart rate) resulted in a postexercise reduction in mean arterial pressure (163 +/- 7 to 149 +/- 5 mm Hg; P < .05). Associated with the postexercise hypotension was a reduction in sympathetic and parasympathetic tonus (47 +/- 12% and 71 +/- 12%, respectively). The reduction in heart rate during the early recovery phase was due to a withdrawal of sympathetic tonus, because beta 1-adrenergic receptor blockade significantly enhanced the postexercise reduction in heart rate, and muscarinic-cholinergic receptor blockade did not affect the postexercise decrease in heart rate until 20 minutes after exercise.

Postexertional hypotension: A brief review

A statistically and clinically significant reduction in arterial pressure occurs following a single bout of low intensity, short duration aerobic exercise in individuals with hypertension. The reduction in arterial pressure is most often associated with a decrease in total peripheral resistance, peripheral sympathetic nerve activity, and plasma norepinephrine concentration; however, cardiac output is often elevated without a change in heart rate. The purpose of this article is to review the literature and propose a model that analyzes the components contributing to the postexercise reduction in arterial pressure. The model implicates an attenuated vascular response to catecholamine stimulation and an enhanced inhibitory cardiopulmonary reflex response. Recent evidence has shown that a single bout of dynamic exercise significantly attenuated the vasoconstrictor response to phenylephrine in an isolated aortic ring preparation and in the intact conscious rabbit and rat. This suggests that the ability of the ...

Diabetes Reduces Growth and Body Composition More in Male Than in Female Rats

Food restriction and/or starvation has a consistently greater and more permanent effect on physical growth in males than in females. Because diabetes may be viewed as being analogous to starvation, we tested the hypothesis that diabetes would reduce growth more in male than in female rats. Diabetes was induced with streptozotocin (65-125 mg/kg IP) at 3 weeks of age in 7 female and 10 male Lewis rats. Body weight (BW) and blood glucose (bGlc) were measured over the following 8 weeks. Subsequently, animals were assessed for body (ano-nasal; ANL) and bone length (tibia; TBL) and chemically analyzed for body composition. Results were compared to age-matched controls (male = 11; female = 9). A 2-way factorial analysis of covariance (ANCOVA), with body weight as the covariate, was used to test for statistical significance for the effects of gender and diabetes on body composition (fat and protein mass) and linear growth because control males and females had significantly different body weights. There were no significant differences in bGlc between genders. However, males had a greater decrease from controls in BW (-45% vs. -13%), protein (-48% vs. -11%), fat (-89% vs. -65%), TBL (-13% vs. 0%), and ANL (-17% vs. -5%) compared to females. In addition, males had a greater absolute decrease from controls in protein (-40 g vs. -5 g) and fat (-39 g vs. -23 g) mass. These results suggest that male rats are more susceptible than females to the deleterious effects of diabetes on linear growth and body composition.

MODEL DEMONSTRATING RESPIRATORY MECHANICS FOR HIGH SCHOOL STUDENTS

We wanted to develop educational materials appropriate for the high school student which would present physiological concepts in an innovative way. The impetus was in response to the lack of physiology educational materials appropriate for the high school level. To this end, we developed an exercise that presents the physiological basis for respiratory mechanics. The materials were designed to engage students in interactive learning and to stimulate interest for future science study. The emphasis of the exercise was the construction of a model that could be built by high school students to demonstrate respiratory mechanics. The use of models to present complex materials has been shown to be an effective medium for science learning. Our exercise contains directions for an inexpensive, easy-to-build model, as well as many supplemental teaching tools. Questions are interspersed throughout the text and at the end of the laboratory experience to facilitate the learning process. Answers are provided to the questions. Students and teachers alike are challenged to build, manipulate, and discuss their experience during the investigation of respiratory mechanics.

Vagal afferents reflexly inhibit exercise in conscious rats.

Activation of vagal afferents reflexly inhibited locomotion induced by stimulation of the mesencephalic locomotor region in decerebrate cats. However, this reflex has not been tested in intact mammals. Therefore, the purpose of this study was to test the hypothesis that vagal afferent stimulation would inhibit somatomotor activity in the intact conscious rat. Six Sprague-Dawley rats were chronically instrumented with carotid arterial and femoral venous catheters and electromyogram (EMG) electrodes inserted into the biceps femoris muscle. Cardiac autonomic efferent blockade [atropine methyl bromide (14 mg.kg-1, i.v.) and metoprolol (14 mg.kg-1, i.v.)] and alpha-adrenergic receptor blockade [phenoxybenzamine (5 mg.kg-1, i.v.)] was achieved to prevent bradycardia and hypotension. Vagal afferents were stimulated (phenyl-biguanide 2.5 and 5.0 micrograms.kg-1 i.v.) during steady state exercise (9.0 m.min-1, 10% grade). Phenyl-biguanide decreased exercise EMG activity 30 +/- 6% and 54 +/- 10% in a dose dependent manner without significantly altering mean arterial pressure or heart rate. We speculate that this reflex may serve as a negative feedback mechanism to indirectly reduce myocardial oxygen demands during exercise.

Enhanced cardiopulmonary reflex inhibition of heart rate during exercise

We tested the hypothesis that the reflex inhibition of heart rate (HR) during mechanical (acute bolus injection of 0.5% and 2% of estimated blood volume) and chemical (phenylbiguanide, PBG, 2.5 and 5 micrograms.kg-1) stimulation of cardiopulmonary receptors would be enhanced during exercise. Rats were instrumented with arterial and venous catheters. The reflex response to mechanical (N = 7) and chemical (N = 8) stimulation of cardiopulmonary receptors was examined at rest and during exercise (6 m.min-1, 10% grade). A two-way analysis of covariance (ANCOVA) with repeated measures was used to test for differences in the reflex regulation of HR at rest vs exercise. HR was used as the covariate because exercise significantly increased baseline HR. There was no significant treatment effect (rest vs exercise) for the reflex inhibition of HR during mechanical stimulation. However, the two-way ANCOVA revealed a significant treatment effect (rest vs exercise) for the reflex inhibition of HR during chemical stimulation. The reflex decreases in HR were enhanced (-delta 23 +/- 8 vs -delta 133 +/- 47 and -delta 208 +/- 40 vs -delta 374 +/- 10 bpm at 2.5 and 5 micrograms.kg-1, respectively). These data suggest that factors associated with exercise enhanced the cardiopulmonary reflex inhibition of heart rate during chemical stimulation.