Search PubMed⌕ Search

Biomedical subjects

J H Mitchell

Publications and source records attributed to J H Mitchell.

At least 145 records · Page 8Linked to original sources
PubMed1985

Dynamic exercise training in foxhounds. I. Oxygen consumption and hemodynamic responses.

Ten foxhounds were studied during maximal and submaximal exercise on a motor-driven treadmill before and after 8-12 wk of training. Training consisted of working at 80% of maximal heart rate 1 h/day, 5 days/wk. Maximal O2 consumption (VO2max) increased 28% from 113.7 +/- 5.5 to 146.1 +/- 5.4 ml O2 X min-1 X kg-1, pre- to posttraining. This increase in VO2max was due primarily to a 27% increase in maximal cardiac output, since maximal arteriovenous O2 difference increased only 4% above pretraining values. Mean arterial pressure during maximal exercise did not change from pre- to posttraining, with the result that calculated systemic vascular resistance (SVR) decreased 20%. There were no training-induced changes in O2 consumption, cardiac output, arteriovenous O2 difference, mean arterial pressure, or SVR at any level of submaximal exercise. However, if post- and pretraining values are compared, heart rate was lower and stroke volume was greater at any level of submaximal exercise. Venous lactate concentrations during a given level of submaximal exercise were significantly lower during posttraining compared with pretraining, but venous lactate concentrations during maximal exercise did not change as a result of exercise training. These results indicate that a program of endurance training will produce a significant increase in VO2max in the foxhound. This increase in VO2max is similar to that reported previously for humans and rats but is derived primarily from central (stroke volume) changes rather than a combination of central and peripheral (O2 extraction) changes.

Animals↗
PubMed1985

Pressor reflex evoked by muscular contraction: contributions by neuraxis levels.

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is one important model of cardiovascular adjustments during static exercise. The central nervous system (CNS) structures mediating this reflex have remained largely obscure. Therefore, we examined the contribution of selected levels of the neuraxis in mediating the pressor reflex evoked by muscular contraction from stimulation of ventral roots. Decerebrate cats exhibited larger pressor reflexes than those found in intact alpha-chloralose-anesthetized cats, a difference more apparent at low (5 Hz or repeated twitch) rather than at high (50 Hz or tetanic) stimulus frequencies. Although a depressor response to 5-Hz stimulation was observed in the intact anesthetized cats, it appeared to be primarily due to anesthetic level, since a depressor response was not observed in decerebrate animals (nonanesthetized). Cerebellectomy produced no changes in the reflexes of the decerebrate animal. Further transection of the neuraxis (caudal to the midcollicular level) attenuated the exercise pressor reflex. The spinal cat demonstrated slight evidence of exercise pressor reflex activity. These results provide clarification as to representation of this pressor reflex within the CNS and establish the reflex's characteristics at several levels of neuraxis integration.

Anesthesia↗
PubMed1985

Hindlimb muscular contraction reflexly decreases total pulmonary resistance in dogs.

We have previously shown that contraction of the gracilis muscles of anesthetized dogs reflexly relaxes tracheal smooth muscle. We have also found that electrical stimulation of these afferents decreases total pulmonary resistance (TPR), a calculation that provides a functional index of airway caliber. Despite these findings, we have yet to show that muscular contraction reflexly decreases TPR. Therefore, in 11 alpha-chloralose-anesthetized dogs, we contracted the hindlimb muscles by electrically stimulating the L6-L7 ventral roots while measuring TPR breath by breath. We found that static contraction decreased TPR from 12.6 +/- 1.1 to 10.4 +/- 0.9 cmH2O X l-1 X s (P less than 0.05). This decrease was reflex in origin because it was prevented by section of the spinal roots innervating the working hindlimb. Repetitive twitch contractions (5 Hz) also reflexly decreased TPR, but the effect was smaller than that evoked by static contraction. The reflex decreases in TPR evoked by contraction were unaffected by propranolol but were abolished by atropine. We conclude that muscular contraction dilates the airways by a reflex mechanism whose efferent arm consists of a withdrawal of cholinergic input to airway smooth muscle.

Airway Resistance↗
PubMed1984

Activation of caudal brainstem cell groups during the exercise pressor reflex in the cat as elucidated by 2-[14C]deoxyglucose.

Cell groups of the caudal brainstem were labeled with 2-[14C]deoxyglucose during the pressor response evoked by contraction of hindlimb muscles (exercise pressor reflex). The nuclear groups which were labeled in excess of control levels included: the lateral reticular nucleus, the inferior olive (medial accessory olive), and the lateral tegmental field (adjacent to the lateral reticular nucleus).

Animals↗
PubMed1984

Effects of static and rhythmic twitch contractions on the discharge of group III and IV muscle afferents.

Although both static and rhythmic twitch contractions of the hindlimb muscles of anaesthetised cats have been shown to reflexly evoke pressor responses, the increase in arterial pressure evoked by the former type of contraction has been shown to be substantially larger than that evoked by the latter. We have therefore recorded the impulse activity of single group III and IV muscle afferents, whose activation reflexly increases arterial pressure, while we both statically and rhythmically twitch-contracted the triceps surae muscles of anaesthetised cats. We found that group III afferents (n = 17) discharged significantly more impulses in response to static contraction than in response to rhythmic contraction. By contrast, group IV afferents (n = 18) fired approximately the same number of impulses in response to the two types of contraction. In addition, we found that many of the group III but only a few of the group IV afferents displayed discharge properties suggestive that these afferents were mechanoreceptors. We conclude that the discharge of group III afferents are likely to be responsible for the difference in the magnitudes of the reflex pressor responses evoked by static and rhythmic contraction.

Animals↗
PubMed1984

Employees' choice of a health plan and their subsequent satisfaction.

This article describes the reasons given by employees for choosing between an indemnification -type health insurance plan (Blue Cross/Blue Shield) and a health maintenance organization (group health plan). A population of 1,003 matched pairs was determined and questionnaires were sent to 1,861 individuals remaining in the company, with a response rate of 74%. Many of the reasons listed as important for joining one or the other plan were alike for both groups. However, the members of the group health plan felt that preventive care and regular checkups were more important than did the Blue Cross/Blue Shield members. The latter group felt that free choice of physician was very important, whereas the subscribers to the group health plan did not place such importance on this factor. Both groups were satisfied with their care, although there were some expected differences.

Adult↗
PubMed1984

Cardiovascular effects of haemorrhagic shock in spleen intact and in splenectomized dogs.

Cardiac performance was evaluated during haemorrhagic shock in 27 dogs with spleens intact, 24 splenectomized, and 23 splenectomized transfused dogs that were given a volume of packed red blood cells simulating splenic contraction. Contractile changes were evaluated by calculating dP/dt at 20 mmHg developed pressure (dP/dt DP20), and by relating stroke work to left ventricular end-diastolic volume measured by biplane cinefluorography. Although heart rate increased comparably during early shock, cardiac output, stroke volume, maximal dP/dt, dP/dt DP20, and arterial blood pressure decreased more in splenectomized and splenectomized transfused dogs than in those with spleens intact. During shock dP/dt DP20 was more depressed in the splenectomized and splenectomized transfused dogs than in those with spleens intact. In addition, an increase in left ventricular end-diastolic volume was accompanied by an increase in left ventricular stroke work in dogs with spleens intact. In contrast, stroke work remained depressed in both splenectomized groups despite increased left ventricular volume. Progressive acidosis and decreased left ventricular blood flow were similar in all dogs during haemorrhage. The greater reduction in left ventricular performance during haemorrhagic shock in the splenectomized and splenectomized transfused dogs was not related to excess lactate, changes in plasma volume, or red blood cell mass. Decreased left ventricular performance, despite improved ventricular filling, indicates greater cardiac dysfunction during haemorrhagic shock. This study suggests that, in dogs, the spleen maintains left ventricular performance during haemorrhage by mechanisms other than autotransfusion.

Animals↗
PubMed1984

Arterial pressure responses to increasing interstitial potassium in hindlimb muscle of dogs.

Static contraction of hindlimb skeletal muscle is known to increase reflexly arterial pressure and heart rate. Potassium is known to be released by the working muscle and is thought to activate the afferents responsible for the reflex cardiovascular responses to muscular contraction. However, it is not known whether potassium, at interstitial concentrations within the range observed during static contraction, is capable of stimulating these afferents. Thus we injected potassium into the gracilis artery of chloralose-anesthetized dogs while we measured interstitial potassium concentrations in the gracilis muscle with potassium-selective electrodes. In 16 dogs, we found that potassium injections, which increased interstitial potassium concentrations by 4.7 +/- 0.3 mM, increased mean arterial pressure by 18 +/- 3 mmHg and heart rate by 12 +/- 8 beats/min; cutting the obturator nerve abolished these increases. These heart rate and blood pressure responses were of short duration (20 +/- 7 s), even though interstitial potassium remained elevated for a period of several minutes. In 5 of the 16 dogs, static contraction of the gracilis muscle for 60 s increased interstitial potassium concentration by 4.3 +/- 0.3 mM. Our data are consistent with the hypothesis that potassium plays a role in causing the reflex cardiovascular responses to static muscular contraction.

Animals↗
PubMed1984

Effect on arterial pressure of rhythmically contracting the hindlimb muscles of cats.

Although static contraction of the hindlimb muscles of anesthetized cats is known to reflexly increase arterial pressure and heart rate, the cardiovascular effects of rhythmic contractions of these muscles is unclear. To help clarify this issue, we determined, in chloralose-anesthetized cats, the effects on arterial pressure and heart rate of rhythmically contracting the hindlimb muscles at a frequency of 5 Hz. In addition, we determined the effect of rhythmic contractions on the impulse activity of group III and IV muscle afferents whose activation is known to increase cardiovascular function. We found that rhythmic contractions increased arterial pressure (from 108 +/- 8 to 134 +/- 9 mmHg; P less than 0.05) and heart rate (from 192 +/- 13 to 208 +/- 10 beats/min; P less than 0.05) in 10 cats and decreased arterial pressure (from 107 +/- 8 to 93 +/- 9 mmHg; P less than 0.05) but did not change heart rate in 9 other cats. The increases were reflex, because they were prevented by cutting the spinal roots innervating the contracting hindlimb. The decreases, however, were not reflex, because they persisted after spinal root section. The differences in the arterial pressure responses to rhythmic contractions may have been partly due to individual differences in the level of anesthesia, because in three cats the pressor responses to this maneuver were converted to depressor responses after giving the cats additional chloralose. Rhythmic contractions of the triceps surae muscles stimulated 8 of 10 group III afferents and 9 of 16 group IV afferents. We conclude that rhythmic contraction is capable of reflexly increasing cardiovascular function in cats provided that the effect is not depressed by anesthesia.

Afferent Pathways↗
PubMed1984

Oxygen consumption and hemodynamic responses during graded treadmill exercise in the dog.

A description is given of a technique that provides a relatively simple means by which O2 consumption and hemodynamic variables can be measured in exercising dogs. We used a multistage submaximal treadmill test to study the responses of 10 foxhounds to dynamic exercise. They were also studied during maximal treadmill exercise. O2 consumption increased from 16.3 +/- 1.7 ml O2 X min-1 X kg-1 at rest to 92.9 +/- 9.7 ml O2 X min-1 X kg-1 at a work load of 6.4 km/h, 20% grade and to 111.9 +/- 9.6 ml O2 X min-1 X kg-1 during maximal exercise. Cardiac output (CO) increased from 6.11 +/- 0.45 l/min at rest to 16.91 +/- 1.46 and 17.66 +/- 0.60 l/min at 6.4 km/h, 20% grade and maximal exercise, respectively. Arteriovenous O2 difference increased from 5.8 +/- 0.3 vol% at rest to 12.0 +/- 0.4 and 13.2 +/- 0.7 vol% at 6.4 km/h, 20% grade and maximal exercise, respectively. Heart rate (HR) increased from 116 +/- 7 beats/min at rest to 250 +/- 8 beats/min at 6.4 km/h, 20% grade and to 278 +/- 6 beats/min during maximal exercise. O2 uptake, CO, and arteriovenous O2 difference increased with the onset of exercise, appeared to level at lower work intensities (6.4 km/h, 4 and 8% grade), and increased significantly at each of the higher work intensities (6.4 km/h, 12, 16, and 20% grade). Additionally, we observed linear relationships between O2 consumption and HR (HR = 1.35 X VO2 + 120.5; r = 0.87; P less than 0.001) and between O2 consumption and CO (CO = 5.91 X VO2 + 216.6; r = 0.96; P less than 0.001). Further, the linear relationship between O2 consumption and CO demonstrated in the present study is similar to that observed in humans.

Animals↗
PubMed1984

Clinical exercise performance.

The authors discuss key concepts of exercise performance as they relate to the practice of clinical medicine. These concepts include types of exercise, muscle physiology, maximal oxygen uptake, myocardial oxygen demand and supply, and neural regulation of cardiovascular response to exercise.

Blood Pressure↗
PubMed1984

The role of maximal oxygen uptake in exercise performance.

The maximal ability to deliver oxygen to the tissues of the body establishes the upper limit of endurance performance; however, the ability of the skeletal muscles to utilize a high oxygen load for a sustained period of time is also of great importance. The fatigue that limits endurance is due to a local limitation of oxygen or substrate, which leads to excessive anaerobic metabolism or decreased energy production. The peripheral adaptation from specific and intense training may further improve endurance performance.

Aerobiosis↗
PubMed1983

Stimulation of pancreatic afferents reflexly activates the cardiovascular system in cats.

Chemical stimulation of afferents from the stomach and gallbladder has been shown reflexly to activate the cardiovascular system. It is not known, however, whether stimulating afferents from the pancreas evoke similar reflex activity. Therefore we recorded the cardiovascular responses in cats anesthetized with methoxyflurane, while we applied capsaicin (200 micrograms/ml) and bradykinin (0.001-1,000 micrograms/ml) to the surface of the pancreas. Topically applying these algesic substances evoked cardiovascular responses that included increases in systemic arterial pressure, heart rate, left ventricular dP/dt at 40-mmHg developed pressure and systemic vascular resistance. Bilateral vagotomy at the level of the diaphragm did not diminish the cardiovascular responses evoked by capsaicin or bradykinin. In contrast, removal of the celiac and superior mesenteric ganglia abolished the cardiovascular responses demonstrated previously when capsaicin or bradykinin was applied to the pancreas. We conclude that afferent endings in the pancreas can be stimulated reflexly to increase cardiovascular function in cats. This reflex activation represents a potential mechanism for eliciting the cardiovascular changes observed during acute pancreatitis, particularly the marked vasoconstriction that may lead to renal failure.

Afferent Pathways↗
PubMed1983

Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats.

Static contraction of the hindlimb muscles, induced by electrical stimulation of the ventral roots, reflexly increases arterial blood pressure and heart rate. Although stimulation of groups III and IV muscle afferents is believed to cause these reflex increases, the responses of these afferents to a level of static contraction that increases arterial pressure have not yet been determined. Therefore, in barbiturate-anesthetized cats, afferent impulses arising from endings in the gastrocnemius muscle were recorded from the L7 or S1 dorsal roots, while the cut peripheral end of the L7 ventral root was stimulated. In addition, the effects of capsaicin (100-200 micrograms) and bradykinin (25 micrograms) on the activity of the groups III and IV afferents stimulated by contraction were examined. Contraction of the gastrocnemius muscle to a level equal to or greater than that needed to cause a pressor response stimulated 12 of 19 (63%) group III afferents and 13 of 19 (68%) group IV afferents. However, the discharge patterns of the group III afferents stimulated by contraction were very different from those of the group IV fibers. No relationship was found between those fibers stimulated by contraction and those stimulated by chemicals. Our results suggest that although both groups III and IV muscle afferents contribute to the reflex cardiovascular increases evoked by static exercise, group III fibers were likely to be stimulated by the mechanical effects of muscular contraction, whereas at least some group IV fibers were likely to be stimulated by the metabolic products of muscular contraction.

Afferent Pathways↗