A model for demonstration of reversal of impairment of endothelium-dependent relaxation in the cholesterol-fed rabbit

This investigation was undertaken to determine whether it was possible to restore endothelium-dependent relaxation (EDR) in the cholesterol-fed rabbit model of atherosclerosis following discontinuation of the cholesterol. New Zealand white rabbits, approximately 8 weeks of age, were randomized into (i) control group (9 animals fed a standard rabbit diet) and (ii) experimental group (27 animals: fed the same diet supplemented with 2.5% cholesterol). The experimental animals were restored to the standard diet after 3 weeks. EDR to acetylcholine (-9.0 to -5.0 log mol/L) was examined in the experimental animals at 3, 7, and 15 weeks after commencement of the study (n = 9 at each stage) and the nine control animals examined after 7 weeks. At the end of 7 weeks, EDR to acetylcholine (-6.0 log mol/L) was significantly (p less than 0.05) impaired in the experimental group (34.3 +/- 3.8%) compared with that in the control group (79.8 +/- 3.0%). The loss of EDR was not apparent in the experimental group at 3 weeks (relaxation: 81.7 +/- 4.7%). At the end of 15 weeks, the EDR was significantly restored in the experimental group (relaxation: 63.6 +/- 5.1%). These findings demonstrate that it is possible to reverse the loss of EDR that occurs with cholesterol feeding in the rabbit by limiting the period of exposure to a high cholesterol diet.

Long term follow-up after isolated aortic valve replacement

Between January 1982 and June 1989, 102 consecutive patients (25 women and 77 men) who had isolated aortic valve replacements were reviewed. The overall early operative mortality was 5%. The deaths were related mainly to pump failure and endocarditis. The early operative mortality in younger patients (less than 70 years old) was 3.5% and in the elderly (70 or older) 11%. The overall late mortality was 4%. All of the survivors except three (follow-up by telephone) were seen in follow-up by cardiologists. The mean follow-up was 40 +/- 26 months (range four to 89). Eighty-seven per cent of all patients or 92% of the survivors in the younger population did well following the operation. In the elderly patients, the results were less favorable (P less than 0.025). The overall cumulative survival was 92% at one year and 89% at five years. The actuarial event-free percentage of survivors was 86% at one year and 74% at five years. Aortic valve replacement can be accomplished in symptomatic patients with an acceptable operative mortality. Long term follow-up of these patients showed functional improvement and low mortality.

Risk stratification after percutaneous transluminal coronary angioplasty

Approximately 20-30% of patients who undergo elective percutaneous transluminal coronary angioplasty (PTCA) require a second angioplasty within 12 months. A significant proportion of patients develop clinical cardiac events during the first year following the initial procedure. The present investigation was undertaken to establish a statistical model for predicting such events. The study group consisted of 100 patients who underwent elective PTCA at the University of Alberta Hospital. All patients were prescribed nifedipine (10 mg tid) and aspirin (325 mg daily) in addition to other medications determined by the attending cardiologist. The patients were reviewed 10 weeks after the procedure and again at the end of 1 year. The follow-up was completed on 96 patients. Within the first year, forty-five experienced cardiac events (1 death, 5 myocardial infarctions, 4 bypass surgeries, 22 repeat PTCAs). These events occurred in 29 patients. An additional 16 patients experienced significant anginal symptoms. A statistical model based upon the patients' perception of symptoms immediately after the procedure, history of hypertension, vessel subjected to PTCA, ejection fraction pre-PTCA, and occurrence of intimal dissection during PTCA was used to identify patients likely to develop cardiac events. Overall, the model classified 72% of the patients (with and without events). Such a statistical model could be used to identify patients who should be subjected to an enhanced degree of cardiologic surveillance in a rehabilitation program.

Reflex Effects of Pulmonary Venous Congestion: Role of Vagal Afferents

The rapidly adapting receptors of the lung are stimulated by changes in extravascular fluid space of the large airways. The natural stimulus to these receptors appears to be a fluid flux from the vasculature. These receptors are probably involved in certain reflexes associated with left ventricular dysfunction.

Morphology of presumptive rapidly adapting receptors in the rat bronchus

The present investigation was undertaken in rats to determine whether sensory nerves exist in apposition to the bronchial microvessels which may function as rapidly adapting receptors (RAR). The primary and secondary bronchi on both sides were removed and processed for light and electron microscopy. Nerves were frequently found in relation to venules external to the muscle coat of bronchi. They comprised myelinated axons which ended individually as non-myelinated convoluted terminals enclosed within a loose capsule of attenuated cells. Serial sections showed that these terminals were not related to ganglion cells. Cervical vagal section and injection of HRP-WGA into the nodose ganglion provided corroborative evidence of the sensory nature of these terminals. Vagal section caused degenerative changes in the encapsulated nerve terminals in the bronchial walls and horseradish peroxidase labelling was demonstrable in such terminals. Moreover, immunocytochemical studies demonstrated the presence of calcitonin gene regulated peptide and substance P in these structures. It is suggested that they comprise the RAR. Encapsulated nerve terminals were not found in the epithelial layer, in the submucous coat or in the muscularis of bronchi.

Reflex changes in tracheal smooth muscle tone during high-frequency oscillation

We examined the effect of high-frequency oscillatory ventilation (HFOV) on tracheal smooth muscle tension and upper airway resistance in anesthetized dogs. The animals were ventilated via a low tracheostomy by HFOV or conventional intermittent positive pressure ventilation (IPPV) with and without added positive end-expiratory pressure (PEEP). The transverse muscle tension of the trachea above the tracheostomy was measured and found to be lower during HFOV when compared with IPPV or IPPV with PEEP. When both vagi were cooled to 8 degrees C to interrupt afferent traffic from the lungs, there was no longer any difference between the modes of ventilation. In a second series of experiments, the airflow resistance of the upper airway above the tracheostomy was measured (Ruaw). During HFOV, Ruaw was significantly lower than during either IPPV or IPPV with PEEP. We conclude that HFOV induces a relaxation of tracheal smooth muscle and a reduction of upper airway resistance through a vagally mediated mechanism.

Action of histamine on the rapidly adapting airway receptors in the dog

The effects of histamine on the activity of rapidly adapting receptors (RAR) of the airways were investigated in anesthetized dogs. With bolus injections given into the right atrium, the threshold dose of histamine required for the excitation of RAR (n = 7) was 0.82 microgram/kg (+1.33/-0.51, geometric mean). With increasing doses of histamine, a dose-response relationship was seen in the activity of RAR. Obstruction of the lymphatic drainage from the lungs reduced the threshold dose to histamine (i.e., shifted the dose-response curve to the left significantly). This change in the dose-response relationship was not accompanied by a corresponding change in the relationship of histamine dose to airway pressures recorded before and after lymphatic obstruction. Against a background of pulmonary venous congestion produced by partial obstruction of the mitral valve, subthreshold doses of histamine stimulated the RAR (n = 4). The excitatory effect of histamine on RAR was found to be abolished by the administration of the H1 receptor antagonist diphenhydramine but not by the H2 receptor antagonist cimetidine. Intravenous infusion of histamine (0.4 microgram.kg-1.min-1) for a period of 10 min increased the RAR activity (n = 6) significantly without producing detectable changes in airway mechanics. The results indicate that contraction of the smooth muscle of the airways may not be a prerequisite for the excitation of RAR, especially at low doses. It is suggested that some of the effects of histamine on RAR are mediated by a local expansion of the extravascular fluid caused by an increase in the permeability of the bronchial vasculature.

Persistent impairment of endothelium-dependent relaxation to acetylcholine and progression of atherosclerosis following 6 weeks of cholesterol feeding in the rabbit

The synthesis and (or) release of endothelium-dependent relaxant factor released by acetylcholine is impaired in New Zealand white rabbits fed an atherogenic diet. Experiments were designed to investigate whether the synthesis and (or) release of the endothelium-dependent relaxant factor from rabbit aortas are restored after reversal from an atherogenic diet to a non-atherogenic diet. Atherosclerosis was induced by feeding a diet containing lipids and 2% cholesterol for 6 weeks. Rabbits were sacrificed after 6 weeks on the atherogenic diet and 36 weeks after return to a standard laboratory diet. Synthesis and (or) release of the factor from the thoracic aorta was assayed using a bioassay system. The relaxant responses produced in the assay tissue were impaired both in the acute stage and after 36 weeks on non-atherogenic food. This impaired relaxation is probably due to a persistent functional abnormality in the aortic endothelium resulting in the failure to synthesize and (or) release endothelium-dependent relaxation factor 36 weeks after induction of atherosclerosis.

The influence of high-velocity circuit resistance training on VO2max and cardiac output

In order to investigate the influence of high-velocity circuit resistance training on maximal aerobic power, maximal stroke volume and cardiac output, and blood lactate removal during recovery, 16 habitually active males were blocked on initial VO2max into either training or control groups. The training group completed two (weeks 1 and 2) or three (weeks 3-6) circuits of 10 variable-resistance hydraulic exercise stations at an exercise: relief ratio of 1:2 on alternate days over six weeks. Angular velocities of movement were maintained at approximately 3.1 rad.s-1. Following training, the VO2max was increased (p less than .01) from 4.32 to 4.68 1.min-1. Maximal stroke volume was increased (p less than .05) from 120 to 129 mL and heart rate response to an absolute submaximal exercise load was decreased (p less than .05) from 153 to 146 beats.min-1. As well, enhanced (p less than .01) removal of lactate from the blood was observed during recovery from exhausting exercise. No changes were observed for control subjects. These results indicate that positive alterations in aerobic and cardiovascular function may be achieved consequent to high-velocity circuit resistance training.

Plasmapheresis affects responses of slowly and rapidly adapting airway receptors to pulmonary venous congestion in dogs

1. The effects of plasmapheresis on the responses of rapidly adapting receptors (RARs) and slowly adapting receptors (SARs) of the airways to pulmonary venous congestion were examined in dogs anaesthetized with alpha-chloralose. Pulmonary venous congestion was produced in a graded manner by partial obstruction of the mitral valve sufficient to raise the mean left atrial pressure by 5, 10 and 15 mmHg. Plasmapheresis was performed by withdrawing 10% of blood volume twice. 2. Both RARs (n = 11) and SARs (n = 5) responded to pulmonary venous congestion by increasing their activities. The responses of the former were proportionately greater. 3. After plasmapheresis which reduced the concentration of plasma proteins by 12.3 +/- 1.0%, the responses of the RARs to pulmonary venous congestion were enhanced significantly. There was no significant change in the responses of SARs. 4. In another set of six RARs, the effects of graded pulmonary venous congestion were investigated twice with an interval of 45 min between the two observations. No significant differences were noted between the two responses. 5. Collection of lymph from the tracheobronchial lymph duct (n = 6) showed that after plasmapheresis, there was an increase in the control lymph flow. In addition, the lymph flow was enhanced during pulmonary venous congestion (mean left atrial pressure increased by 10 mmHg). 6. It is suggested that a natural stimulus for the excitation of the RAR is a function of the fluid fluxes in the pulmonary extravascular space.

Effect of pulmonary venous congestion on respiratory rate in dogs

1. The effect of pulmonary venous congestion on the respiratory rate was examined in dogs anaesthetized with alpha-chloralose. The study was done on both spontaneously breathing and artificially ventilated animals. Pulmonary venous congestion was produced by partial obstruction of the mitral valve sufficient to raise the left atrial pressure by 5 mmHg. 2. In artificially ventilated dogs, pulmonary venous congestion increased significantly the activity in phrenic nerves. Both the number of bursts/min and the total number of impulses/min increased. However, there was no significant change in the number of impulses/burst. 3. In spontaneously breathing dogs, pulmonary venous congestion produced a significant increase in the frequency of breathing with a significant shortening of the inspiratory and expiratory durations. 4. Cooling of the cervical vagi to 8-9 degrees C abolished both the above responses. 5. Pulmonary venous congestion (left atrial pressure +5 mmHg) stimulated the rapidly adapting receptors of the airways. This effect was abolished by cooling the ipsilateral vagus proximally to 8-9 degrees C. 6. It is concluded that pulmonary venous congestion increases the respiratory rate reflexly in dogs. The afferent pathway for this reflex response resides in the vagus and the rapidly adapting receptors are likely to be the receptors involved in this response.

Exercise testing after myocardial infarction: relative values of the low level predischarge and the postdischarge exercise test

This study was undertaken to compare the relative values of the low level predischarge exercise test and the postdischarge (6 weeks) symptom-limited test in 518 consecutive patients admitted with an acute myocardial infarction. Of the patients who did not develop significant ST segment depression or angina during the predischarge test, the symptom-limited test also remained negative in 91.5 and 91.9% of the patients, respectively. Similar results were obtained with ST segment elevation and the systolic blood pressure response during the two exercise tests with only 2.1 and 11.4% changing from normal to abnormal, respectively. Discriminant function analysis was done to predict the occurrence of coronary events (unstable angina, reinfarction, cardiac failure, cardiac death) with use of the data from the exercise tests together with other clinical and investigational data. The jackknife method correctly classified 71.9 and 71.4% of the patients with the data from the predischarge exercise test and symptom-limited test, respectively. Combining the data from the two tests improved the overall predictive accuracy to only 75.0%. It is concluded that the routine performance of a symptom-limited test 6 to 8 weeks after infarction does not reveal any significant additional information in those patients who have undergone a predischarge low level exercise test. Thus the 6 to 8 week test should be restricted to selected patients after myocardial infarction.

Exercise training after anterior Q wave myocardial infarction: importance of regional left ventricular function and topography

To determine whether the extent of left ventricular dysfunction and the degree of shape distortion can predict outcome in survivors of moderate-sized anterior Q wave myocardial infarction who are undergoing exercise training, these variables were measured by two-dimensional echocardiography before and after 12 weeks of a low level exercise training program starting 15 weeks after infarction in 13 patients (7 in group 1 and 6 in group 2) and 12 weeks apart in 24 matched control patients without training. By the end of training, the functional class score had increased in group 2 (from 2.25 to 2.67, p less than 0.005) but had not changed in group 1. Further discrimination of groups 1 and 2 was provided by an initial asynergy (akinesia or dyskinesia, or both) less than 18% or greater than or equal to 18%. Compared with group 1, group 2 had greater initial asynergy (32 versus 6%, p less than 0.001), expansion index (asynergic/normal endocardial segment length: 1.8 versus 1.6, p less than 0.025) and peak shape distortion index (12.2 versus 1.0 mm, p less than 0.005) but lower ejection fraction (43 versus 59%, p less than 0.05) and thinning ratio (asynergic/normal wall thickness: 0.61 versus 0.74, p less than 0.05). These variables did not change with training in group 1. However, in group 2, training caused significant increase in asynergy (from 32 to 40%, p less than 0.05), expansion index (from 1.8 to 2.0, p less than 0.01) and peak shape distortion (from 12.2 to 20.9 mm, p less than 0.05) associated with a decrease in thinning ratio (from 0.61 to 0.51, p less than 0.001) and ejection fraction (from 43 to 30%, p less than 0.005). Initial values for these variables were similar for corresponding control groups but did not change over the 12 weeks. Thus, patients with greater than or equal to 18% left ventricular asynergy on the initial echocardiogram showed more shape distortion, expansion and thinning before exercise training and developed further functional and topographic deterioration with training.

Reflex tracheal contraction during pulmonary venous congestion in the dog

1. The effect of pulmonary venous congestion on tracheal tone was studied in dogs anaesthetized with alpha-chloralose. Pulmonary venous congestion was produced by partial obstruction of the mitral valve to increase left atrial pressure by 10 mmHg. Tracheal tone was measured in vivo by an isometric force displacement method. 2. Tracheal tone increased by 6.3 +/- 0.3 g from a control level of 91.6 +/- 2.8 g when left atrial pressure was increased by 10.5 +/- 0.3 mmHg. This response was abolished by cooling the cervical vagi to 8 degrees C at a point caudal to the origin of the superior laryngeal nerves. Also, sectioning the superior laryngeal nerves abolished this increase in tracheal tone. 3. Afferent activity recorded from rapidly adapting receptors of the airways increased significantly during pulmonary venous congestion. This increase in activity was abolished by cooling the vagi caudal to the recording site to 8-9 degrees C. 4. Administration of propranolol (0.5 mg/kg) failed to abolish this increase in tracheal tone while atropine (3 mg/kg) did so. 5. Stimulation of left atrial receptors without an increase in left atrial pressure and stimulation of right atrial receptors with and without increases in right atrial pressure did not cause any change in tracheal tone. 6. It is suggested that pulmonary venous congestion is associated with a reflex increase in tracheal tone, the afferent limb of which is formed by pulmonary receptors discharging into myelinated fibres in the cervical vagi and the efferent limb by parasympathetic cholinergic fibres in the superior laryngeal nerves. The afferent receptors are likely to be the rapidly adapting receptors. This reflex may be of importance in the development of the respiratory symptoms associated with left ventricular failure.

Short-term cardiovascular adaptations to vertical head-down suspension

Inversion devices have been advocated as means of therapy for patients with low back pain. The present investigation was undertaken to determine the cardiovascular responses to vertical head-down suspension with such a device. Eight men were monitored for three minutes in the upright and supine position, and for ten minutes in the vertical head-down position. Stroke volume (SV), heart rate (HR), cardiac output (QT), segmental arm blood flow (ABF), and segmental leg blood flow (LBF) were measured by impedance plethysmography. Moving from the upright to the supine positions resulted in an SV increase from 93.1 +/- 18.8 to 138.2 +/- 28.2 ml.beat-1 (p less than 0.01). During the first minute of suspension a further increase in SV to a maximum of 178.6 +/- 45.0 ml.beat-1 was noted. By the end of the suspension period, SV had declined to 159.6 +/- 34.2 ml.beat-1. Resumption of the upright position was associated with a reduction in SV to 94.3 +/- 15.8 ml.beat-1. Changes in QT across the supine and vertical head-down positions reflected the changes in SV. The vertical head-down position was also associated with a gradual reduction in both ABF and LBF. These data suggest that the primary effect of inverted suspension was a transient increase in venous return, which requires an effective ventricular response. An element of caution should be exercised in using these devices to avoid aggravating concurrent pathologic conditions.

Stimulation of rapidly adapting pulmonary stretch receptors by pulmonary lymphatic obstruction in dogs

The effects of pulmonary lymphatic obstruction and pulmonary venous congestion on the activities of slowly adapting receptors (SAR) and rapidly adapting receptors (RAR) of the airways were examined in anaesthetized, artificially ventilated dogs. In 11 out of 12 RAR (12 dogs) examined, pulmonary lymphatic obstruction for a period of 20 min produced a sustained significant increase in activity without a significant change in peak airway pressure and dynamic compliance. The activity remained significantly elevated even after the pulmonary lymphatic obstruction was released. This stimulus was without effect on the SAR (n = 5 dogs). Pulmonary venous congestion alone increased the RAR activity (n = 7 dogs) significantly without producing significant changes in airway mechanics. Lymphatic obstruction, when superimposed upon congestion, did not produce a further significant increase in activity. In four dogs the effect of pulmonary venous congestion (left atrial pressure increased from 7.6 +/- 1.7 to 16.3 +/- 2.7 mmHg) (1 mmHg = 133.3 Pa) on pulmonary lymphatic flow was examined. The procedure caused a significant increase in lymph flow. These results suggest that in the dog, the RAR activity is influenced by changes in the pulmonary extravascular space.

Behaviour of canine pulmonary vagal afferent receptors during sustained acute pulmonary venous pressure elevation

1. The effects of an acute sustained increase in pulmonary venous pressure induced by partial obstruction of the mitral valve on the activity of the four types of pulmonary receptors, namely, slowly adapting, rapidly adapting, pulmonary C-fibre and bronchial C-fibre receptors, were studied in the dog. 2. Fifteen slowly adapting receptors, eleven rapidly adapting receptors and nine bronchial C-fibre receptors showed significant sustained increases in activity when stimulated by the elevated left atrial pressure by 9.4 +/- 0.2 mmHg for 15 min. Nine pulmonary C-fibre receptors did not show a significant increase (six of these nine receptors increased their activity in response to the stimulus). 3. When the left atrial pressure was increased in graded steps of 5 mmHg for 5 min each up to 15 mmHg, a significant graded response was found in all of seven slowly adapting receptors, five rapidly adapting receptors and five bronchial C-fibre receptors. The five pulmonary C-fibre receptors examined also showed increases, but the changes were not statistically significant. 4. In response to stimulation by the elevated left atrial pressure, increases in activity occurred within 1 min of application of the stimulus in all the receptors and returned to control levels within 1 min of removal of this stimulus. 5. It is concluded that in the dog, pulmonary vagal receptors are influenced by small increases in pulmonary venous pressure induced by partial obstruction of the mitral valve. The changes appeared to be greatest in the case of rapidly adapting receptors. The physiological significance of these responses remains to be investigated.

Absence of effect of calcium antagonists on endothelium-dependent relaxation in rabbit aorta

The effect of chronic feeding of New Zealand White rabbits with nicardipine (60 mg kg-1 daily for 5 weeks) on the endothelium-dependent relaxation (EDR) to acetylcholine (ACh) was examined in vitro. The effect of acute exposure to nicardipine and diltiazem (10 mumol l-1) in the tissue bath was also examined. A bioassay system for endothelium-dependent relaxation factor (EDRF) in which a rabbit aortic ring with endothelium removed was used as recipient and a segment of rabbit aorta with endothelium as donor (producing EDRF in response to ACh) was developed. This system enabled the effect of nicardipine on the synthesis/release and on the relaxation to EDRF to be studied separately. The maximum relaxations to ACh in control and nicardipine-fed animals were 43.6 +/- 5.5 and 53.8 +/- 6.7% (mean +/- s.e. mean) of the contractile response to noradrenaline (NA, 1 mumol l-1) (n = 6, P greater than 0.05). Similarly the EDR to ACh was not significantly altered by acute exposure (30 min) to nicardipine or diltiazem. The maximum relaxations without and with nicardipine were 32.4 +/- 4.2% and 28.0 +/- 3.1% of the contraction to NA (1 mumol l-1) (n = 11, P greater than 0.05). The corresponding data for diltiazem were 42.1 +/- 5.7 and 36.4 +/- 7.3% respectively (n = 11, P greater than 0.05). Both calcium antagonists inhibited the contraction induced by potassium (100 mmol l-1). Nicardipine and diltiazem in concentrations of 100 mumol l-1 reduced the potassium-induced contraction to 33.0 +/- 9.0% and 53.8 +/- 6.7% of control respectively (n = 6, P less than 0.05). In the bioassay experiments the infusion of nicardipine on (a) the recipient tissue only and (b) the donor and the recipient tissue had no significant effect on the relaxant response observed in the recipient tissue when superfused with Krebs-bicarbonate buffer containing ACh via the donor tissue (n = 6, P greater than 0.05). These results indicate that nicardipine and diltiazem had no significant effect on synthesis/release and the relaxant response to EDRF in the rabbit aorta. Thus the translocation of Ca2+ accompanying the EDR to ACh in the rabbit aorta is likely to utilize Ca2+ channels not blocked by these calcium antagonists.

Potentiation of the responses to transmural nerve stimulation by alpha-agonists: a possible role in vivo

This study was undertaken to assess the effects of exogenous alpha-agonists on the effector response to transmural nerve stimulation in canine saphenous vein rings. The response to a fixed train (5 s duration) of transmural nerve stimulation (8 Hz, 0.3 ms, 9 V) applied every 5 min was determined in the control state and in the presence of subthreshold (for contraction) concentrations of noradrenaline, adrenaline, clonidine, and methoxamine. The maximum potentiations achieved by the three drugs were 246.2 +/- 36.9, 220.5 +/- 38.8, 384.3 +/- 78.7, and 353.3 +/- 68.0%, respectively. The potentiation observed was significantly inhibited by indomethacin (10(-6) mol/L) and propranolol (5 X 10(-6) mol/L). Both indomethacin and propranolol potentiated the response to transmural nerve stimulation. The potentiation of the responses to transmural nerve stimulation by alpha-agonists suggests that, presynaptic alpha 2-inhibition by circulating catecholamines is likely to be of limited biological significance in modulating the effector responses in the canine saphenous vein.

Endothelium-dependent relaxation and experimental atherosclerosis in the rabbit aorta

This study was undertaken to determine whether the production or release of the endothelium-dependent relaxatory factor is impaired in atherosclerotic New Zealand White rabbits. Atherosclerosis was induced by feeding a diet containing 2% cholesterol for 6 weeks. The production or release of endothelium-dependent relaxatory factor was assayed as follows. A 5-cm length of aorta donor was perfused with Krebs-bicarbonate buffer and the perfusate drained over a deendothelialized ring of recipient aorta set up for recording isometric tension. The recipient was precontracted with norepinephrine (0.2 mumol/L) in the perfusate. When acetylcholine was added to the perfusate, the recipient relaxed in a dose-dependent manner. This assay was used to compare the relaxatory responses produced in recipient rings by adding acetylcholine to donors from atherosclerotic and control rabbits. The relaxation produced by atherosclerotic donors were smaller than those generated by control donors (16.5 +/- 4.9 vs. 32.7 +/- 5.3%; n = 10, p less than 0.05). It is suggested that in atherosclerotic rabbits the ability of aortic endothelium to produce or release endothelium-dependent relaxatory factor is impaired.

Effect of calcium antagonists on adrenergic mechanisms in canine saphenous veins

Experiments were designed to investigate the effect of two calcium antagonists, diltiazem and nicardipine (concentration range: 10(-7)-10(-4) M), on the contractile responses to transmural nerve stimulation, exogenous noradrenaline and tyramine in isolated canine saphenous vein rings. Both diltiazem and nicardipine inhibited the contractile response to transmural nerve stimulation in a non-competitive, concentration-dependent manner. At a concentration of 10(-4) M, diltiazem and nicardipine inhibited the maximum contractile response to transmural nerve stimulation to 0.8 +/- 0.8% and 20 +/- 10% of control respectively. Effects of diltiazem and nicardipine (up to 10(-4)M) on the contractile response to exogenous noradrenaline were minimal. The only significant difference observed was a 30% depression of the maximum contractile response with a shift in ED50 at high concentrations of nicardipine. Diltiazem (up to 10(-4) M) had no significant effect on concentration-effect curves for tyramine. Nicardipine inhibited the response to tyramine in a non-competitive manner with the maximum response depressed to 46% of control at 10(-4) M-nicardipine. Release of [3H]noradrenaline during transmural nerve stimulation was reduced by both calcium antagonists in a concentration-dependent manner. However, release of [3H]noradrenaline produced by the indirect sympathomimetic agent tyramine was not significantly inhibited by nicardipine. These experiments suggest that the calcium antagonists diltiazem and nicardipine inhibit the contractile response to transmural nerve stimulation in the canine saphenous vein predominantly by inhibiting the release of endogenous noradrenaline. However, nicardipine appears to have an additional post-synaptic inhibitory effect on the responses to exogenous as well as endogenous noradrenaline.

Atrial reflexes during high frequency oscillatory ventilation

High frequency oscillatory ventilation (HFOV) is a new method of artificial ventilation which has been advocated for use in critically ill individuals. It alters the discharge in pulmonary stretch receptors (SAR) from a phasic to a continuous pattern. Since some cardiovascular neurones in the medulla are influenced by the discharge from SAR, experiments were undertaken to determine whether the reflexes from the left atrial (volume) receptors (LAR) were influenced by HFOV. The reflex increases in heart rate and urine flow which result from activation of the (LAR) were examined during both intermittent positive pressure ventilation (IPPV) and HFOV. In five dogs, the increase in heart rate was 23.9 +/- 4.3 and 24.5 +/- 5.4 beats/min during IPPV and HFOV, respectively. In six dogs the response of an increase in urine flow was examined and this response also was not altered by HFOV. It is concluded that the integrity of these reflexes was unaffected by HFOV in the anesthetized dog model.

Cholesterol feeding impairs endothelium-dependent relaxation of rabbit aorta

Experiments were designed to assess the effect of cholesterol feeding on the endothelium-mediated relaxation of the rabbit aorta to acetylcholine. Age-matched male New Zealand white rabbits were fed either a 2% cholesterol diet or standard rabbit chow. The animals were anaesthetized with sodium pentobarbitone and sacrificed after 4 and 8 weeks on these diets. Rings were prepared from the proximal thoracic aorta and examined in tissue baths. These rings were contracted first with norepinephrine (-6 log mol/L) and acetylcholine was added to demonstrate the endothelium-mediated relaxation. The endothelium-dependent relaxation was significantly less in aortas from rabbits fed the 2% cholesterol diet than in aortas from animals fed the conventional diet. This impairment of relaxation was apparent after both 4 and 8 weeks of cholesterol feeding. In both groups of animals no relaxation was seen in rings from which the endothelium was removed. These results show that cholesterol feeding leads to an impairment of endothelium-mediated relaxation of the rabbit aorta to acetylcholine.

Interaction of left atrial receptors and carotid sinus baroreceptors on heart rate in the dog

This study was undertaken to determine the influence of 1) the left atrial receptors (LA) on the ability of the carotid sinus baroreceptors (CS) to regulate heart rate and 2) the CS on the reflex increase in heart rate mediated by the LA. The LA were stimulated by stretching the pulmonary vein-atrial junctions in dogs anesthetized with alpha-chloralose. Aortic pressure was controlled, and the pressure in the CS was regulated. Stimulus-response curves were obtained relating heart rate to pressures in the CS, in the control state, and during stimulation of LA (6 dogs). Factorial analysis revealed that LA exerted a significant influence on heart rate (P less than 0.01). Next the CS pressure was set at mid, low, and high levels and the LA stimulated. It was found that the effect on heart rate was greatest at the mid setting (+19.3 +/- 2.9 beats/min) and least at the low setting of the pressure in the CS (+0.9 +/- 0.5, 11 dogs). Sympathetic blockade attenuated significantly the response in the mid setting of the CS pressure and left intact the response at the high setting of the pressure in the CS. It is concluded that there is a significant interaction between these two reflexes.

Lack of effect of exercise on platelet activation and platelet reactivity

The effects of brief maximal exertion on platelet activation and reactivity have been studied in normal subjects. Although initial studies in seven subjects showed apparent exercise-induced platelet activation and enhanced platelet reactivity, these findings could not be confirmed in 13 subjects studied subsequently. There was no change in the platelet aggregate ratio, platelet fluorescent granule number or the plasma platelet factor 4 (PF4) or beta-thromboglobulin, although transient and significant increases in the platelet count and plasma heparin neutralizing activity (HNA) occurred. These results were reproducible in subjects studied more than once. It is postulated that in vitro platelet activation, most likely associated with blood collection, explained the initial results. It is concluded from the subsequent studies that in normal subjects brief maximal exercise causes neither platelet activation nor altered platelet reactivity but does cause a transient increase in the platelet count. The increase in HNA with exertion, without any accompanying increase in the PF4, demonstrates that these assays measure different substances and that the increase in HNA following exertion is most unlikely to be derived from platelets.

Physical training with minimal hospital supervision of patients after coronary artery bypass surgery

The effect of a program of rehabilitative training involving minimal hospital supervision was investigated in a group of 30 patients who had undergone coronary artery bypass surgery (CABPS). Fifteen of these undertook the training program while the remainder, who undertook a self-regulated program of activity, served as controls. It was found that the surgery per se had significantly improved both the effort tolerance and the highest rate pressure products generated. In the 15 surgical patients who trained we observed a further improvement in work capacity and in the highest rate pressure products achieved. These findings suggest that the effects of training and coronary revascularization are additive and that an effective training program can be instituted with minimal hospital supervision in selected patients who have undergone CABPS.

Platelet activation caused by cardiac catheter blood collection, and its prevention

The study of platelet changes occurring across the coronary circulation is important in the investigation of the platelet's role in ischemic heart disease. It requires blood sampling through cardiac catheters. This could activate platelets and alter the results of tests of platelet activation and reactivity. This study was designed to examine this problem and to devise satisfactory methods for obtaining blood for platelet studies through long catheters. Blood collected through catheters introduced with a guide-wire had a much higher plasma heparin neutralising activity (HNA), platelet factor 4(PF4) and beta-thromboglobulin (beta TG) than peripheral venous blood, and lower platelet count(PC). Blood collected through catheters introduced via a sheath, and kept filled with anticoagulant/antiplatelet solution until blood sampling, gave results similar to peripheral venous blood for the PC, platelet aggregate ratio, platelet fluorescent granule count, and for plasma HNA, PF4 and beta TG. It is concluded that platelets are activated during blood collection through cardiac catheters; however, with appropriate precautions, blood which is satisfactory for platelet studies can be obtained.

Effects of combined carotid chemoreceptor and atrial receptor stimulation on renal blood flow in anaesthetized dogs

In dogs anaesthetized with chloralose and artificially ventilated, carotid chemoreceptors were stimulated by changing the perfusate of the vascularly isolated carotid bifurcations from arterial to venous blood. Left atrial receptors were stimulated by distending balloons in two pulmonary vein-left atrial junctions and in this left atrial appendage. The left renal blood flow was measured by an electromagnetic flow meter at a constant systemic (renal) arterial pressure in preparations in which heart rate changes were prevented by administration of propranolol hydrochloride (0.5 mg kg-1) and atropine sulphate (0.4 mg kg-1). Muscular movement was prevented by gallamine triethiodide (0.2 mg kg-1). Stimulation of left atrial receptors resulted in a significant increase (P less than 0.001) in renal blood flow of 5.6 +/- 0.88 ml min-1 100 g-1 renal mass from a control of 223 ml min-1 100 g-1 renal mass. The responses were abolished by cooling the cervical vagus nerves to 6-8 degrees C. Stimulation of carotid chemoreceptors, by perfusion of the carotid bifurcations by venous blood, caused a decrease in renal blood flow of 20 +/- 6.9 ml min-1 100 g-1 renal mass from 224 ml min-1 100 g-1 renal mass. Stimulation of left atrial receptors during venous perfusion of carotid chemoreceptors resulted in an increase in renal blood flow of 10.9 +/- 1.82 ml min-1 100 g-1 renal mass from 208 ml min-1 100 g-1 renal mass. These results show that atrial receptors and chemoreceptors can interact in their effects on renal blood flow.

Effects of high-frequency oscillatory ventilation on vagal and phrenic nerve activities

This study was undertaken to define the mechanism for the respiratory inhibition observed during high-frequency oscillatory ventilation (HFOV). The effects of HFOV on the activities of single units in the vagus (Vna) and phrenic nerves (Pna) were examined in pentobarbital-anesthetized dogs. The animals were either ventilated by intermittent positive-pressure ventilation (IPPV) with and without positive end-expiratory pressure (PEEP), or by HFOV at a frequency of 25 Hz and pump displacement volume of 3 ml/kg. In 13 vagal units the Vna was much higher during HFOV than during IPPV or airway occlusion at a matched airway pressure. Ten units in the phrenic nerves were examined, and Pna (expressed as bursts/min) was attenuated by HFOV in all of them. In four of them, the effect of cooling the vagi to 8-10 degrees C on Pna was examined, and it was found that HFOV failed to alter the Pna. We conclude that 1) HFOV stimulates the pulmonary vagal afferent fibers continuously and to a degree greater than that due to static lung inflation and increased airway pressure and 2) the increased vagal activity during HFOV probably causes phrenic nerve activity inhibition.

Effect of stimulating right atrial receptors on renal blood flow

This investigation was undertaken to determine the effect of stretching the superior vena caval--right atrial (SVC-RA) junction and the right atrial appendage on blood flow to the kidney (RBF) and to establish whether any changes observed were influenced by the input from the baroreceptors in the carotid sinus. The experiments were performed on seven dogs, anaesthetized with alpha-chloralose. The systemic arterial (i.e., renal perfusion) pressure was held constant. At a carotid sinus pressure (CSP) of 59.0 +/- 1.2 mmHg (1 mmHg = 133.322 Pa), the RBF increased from 218 +/- 16.1 to 231.7 +/- 18.4 mL/min per 100 g renal mass (p less than 0.025). At a CSP of 88.0 +/- 3.5 mmHg, the RBF increased from 230.1 +/- 19.2 to 237.1 +/- 19.2 mL/min per 100 g renal mass (p less than 0.05). At a CSP of 137 +/- 3.7 mmHg there were no significant changes in RBF. These responses were abolished by cutting (four dogs) or cooling the vagi (one dog only). In a subsidiary investigation it was shown that stretching the SVC-RA junction activated receptors in the endocardial surface of the right atrium which discharged into myelinated fibres in the vagi, having an average conduction velocity of 8.1 m/s (range 3.8-15). It is concluded that stimulation of right atrial receptors increases the RBF and that this response is influenced by the input from the baroreceptors in the carotid sinus.

Influence of carotid sinus pressure on atrial receptors and renal blood flow

Stimulation of the atrial receptors results in an increase in renal blood flow. The present investigation was undertaken to determine whether this response was modulated by the input from the baroceptors in the carotid sinus. The experiments were performed on dogs anesthetized with chloralose. The systemic arterial pressure was held constant. The carotid sinuses were perfused at 62 +/- 1.3, 95 +/- 2.7, and 145 +/- 8.3 mmHg. The atrial receptors were stimulated by distension of small balloons positioned at the left pulmonary vein-atrial junctions and the left atrial appendage. At a carotid sinus pressure of 62 +/- 1.3 mmHg, the blood flow increased from 182 +/- 8.5 to 199 +/- 8.9 ml . min-1 . 100 g-1 renal mass. At a carotid sinus pressure of 95 +/- 2.7 mmHg, the blood flow increased from 202 +/- 9.6 to 209 +/- 10.4 ml . min-1 . 100 g-1 renal mass. At a carotid sinus pressure of 145 +/- 8.3 mmHg, the blood flow increased from 237 +/- 13.0 to 239 +/- 12.5 ml . min-1 . 100 g-1 renal mass. The first two responses alone were statistically significant. The response at a carotid sinus pressure of 62 +/- 1.3 mmHg was abolished by cutting or cooling the cervical vagi to 8--10 degrees C. It is concluded that stimulation of the left atrial receptors produces a reflex increase in blood flow to the kidney, and this response is modulated by the input from the carotid sinus baroceptors.

Transducer properties of atrial receptors in the dog after 60 min of increased atrial pressure

This investigation was undertaken to examine the effect of a period of increased atrial pressure lasting 60 min on the ability of atrial receptors to transduce changes in atrial pressure. The experiments were performed on dogs anesthetized with pentobarbitone. Action potentials were recorded from branches of the cervical vagi and the atrial pressure was increased by distending a balloon in the lumen of the left atrium. Twelve receptors were examined in 14 dogs. For each receptor, stimulus–response curves relating the action potentials generated (number per cardiac cycle) to the mean left atrial pressure (centimetres of H2O) were obtained under three experimental conditions: (a) during an initial control period, (b) after 60 min of increased atrial pressure (i.e. 10–12 cm H2O), and (c) 60 min after restoration of the atrial pressure to its control value. It was found that in all 12 units the ability of the receptors to transduce changes in atrial pressure was impaired after 60 min of increased atrial pressure, i.e., the number of action potentials generated by a receptor for the same atrial pressure was smaller than that in the control period. This effect was partially restored in the 12 units 60 min after restoration of the atrial pressures to their respective control values.In four units the stimulus–response curves were repeated 60 min after obtaining the initial control response. It was found that there was no significant difference in each pair of curves. It is concluded that relatively short periods of increased atrial pressure impair transducer properties of the atrial receptors.

Effect of stretching the superior vena cava on heart rate in rats

The effect on the heart rate of stretching the superior vena cava 3-4 mm above its entrance to the right atrium was examined in rats anesthetized with chloralose. The superior vena cava was stretched 41 times in 16 rats. There was a mean increase of 24.1 +/- 1.9 (SE) beats/min. This response was abolished (6 rats) by sympathetic blockade (with propranolol hydrochloride and bretylium tosylate) and by bilateral vagosympathectomy (5 rats). Application of lidocaine to the cervical vagi also abolished the response in a reversible manner (4 rats). In five rats, histological examination of the superior vena cava revealed an abundance of nerves, which formed a net in the stretched region. Complex unencapsulated nerve endings similar to those demonstrated in the dog and cat were not found.

Effect of changes in the red cell volume on the carbon dioxide titration curve in vivo in the rat

Acute in vivo CO2 titration curves were performed on rats anaesthetized with pentobarbitone. The slope of the in vivo CO2 titration curve in the rat was found to be similar to that previously reported in the dog and in man. Removal of approximately 30% of the haemoglobin of the body did not influence significantly the slope of the in vivo CO2 titration curve in the rat.

The role of renal nerves in the diuresis and natriuresis caused by stimulation of atrial receptors

The diuretic response to stimulation of left atrial receptors by distending balloons located either at the pulmonary vein-left atrial junctions or in the body of the left atrium, was studied in anaesthetized dogs in which on kidney had been surgically denervated. In these dogs diuretic and natriuretic responses to stimulation of atrial receptors were obtained in the denervated and the intact kidney; both responses were significantly greater in the intact kidney than in the denervated kidney. In the denervated kidney the natriuretic response showed a significant correlation to increases in heart rate resulting from stimulating of atrial receptors. It is concluded that the renal nerves contribute significantly to the increases in urine flow and sodium excretion in response to stimulation of atrial receptors. In the denervated kidney, the diuresis appeared to be mediated by a humoral mechanism and the natriuresis was associated with concomitant increases in heart rate.

The nature of atrial receptors responsible for the increase in urine flow caused by distension of the left atrium in the dog

In dogs anaesthetized with alpha-chloralose the effects of distension of a large balloon in the lumen of the left atrium on the discharge of action potentials in vagal fibres and on urine flow were studied with the cervical vagi cooled at 18 and 12 degrees C. Distension of the balloon in seven dogs resulted in an increase in urine flow. Cooling the cervical vagi to 18 degrees C reduced the response to 70% of that obtained at 37 degrees C, and at 12 degrees C the response was reduced to 28%. In a second group of dogs, the effect of distension of the balloon on atrial receptors which discharged into myelinated nerve fibres of the vagi, i.e. Paintal type A and type B receptors, was examined. The increase in the activity of these receptors was reduced to 68% when the vagus nerve was cooled to 18 degrees C and was reduced further to 25% at 12 degrees C., In a third group of dogs, the effect of distension of the balloon on receptors which discharged into non-myelinated nerve fibres in the vagi was examined. Cooling of the cervical vagi also reduced the evoked increase in activity in these fibres; this reduction in activity occurred over a wider range of temperature, unlike the effect of cooling on the response in myelinated vagal fibres. It is concluded that the increase in urine flow caused by distension of a balloon in the left atrium is mediated solely by the Paintal-type atrial receptors which discharge into the myelinated fibres in the vagi.

Physical training after heart valve replacement

A controlled trial was undertaken to examine the efficacy of physical training in patients recovering from the replacement of a single heart valve. Patients were allocated to a test or control group two weeks after operation. Each patient performed a submaximal exercise test at entry, and 12 and 24 weeks after this test. The Canadian Air Force exercise programme was undertaken by the test group, while the control group continued normal activities for the 24 weeks between the first and last exercise group. A regression line of submaximal heart rate on oxygen consumption was calculated from the data of each exercise test in each patient. Alterations in this line were used as an "index" of changes in "cardiorespiratory fitness". The individual results showed a consistent improvement in "cardiorespiratory fitness" over the first 12 weeks in both groups. Only patients in the test group continued to improve between 12 and 24 weeks. Thus the exercise programme modified the recovery of "cardiorespiratory fitness" after operation. Results in patients who developed clinical complications, and were excluded from the trial, predicted a deteriorating clinical condition. This finding suggested that sequential exercise tests are of value after cardiac surgery.

Physical fitness training in patients

Patients recovering after heart valve surgery are not in an optimum state of 'cardiorespiratory fitness'. To examine this proposition a controlled trial of physical training was undertaken in patients recovering from the replacement of a single heart valve. Patients were allocated to a test or control group two weeks after operation. Each patient performed a submaximal exercise test at entry, and twelve and twenty-four weeks after this test. The Canadian Air Force exercise programme was undertaken by the test group, while the control group continued normal activities for the twenty-four weeks between the first and last exercise test. A regression line of submaximal heart rate on oxygen consumption was calculated from the data of each exercise test in each patient. Alterations in this line were used as an 'index' of changes in 'cardiorespiratory fitness'. The individual results showed a consistent improvement in 'cardiorespiratory fitness' over the first 12 weeks in both groups. Only patients in the test group continued to improve between 12 and 24 weeks. Thus the exercise programme modified the recovery of 'cardiorespiratory fitness' after operation. A physical fitness rehabilitation programme may help these patients gain maximum benefit from correctiv surgery.

"Summation" of the increase in heart rate from stimulation of atrial receptors

In dogs anaesthetized with chloralose, application of stimuli which are likely to activate left atrial (L.A.) and right atrial (R.A.) receptors (complex unencapsulated endings) has been shown to result in an increase in heart rate. The present investigation was undertaken to determine whether the response elicited by the application of one stimulus (i.e., to the left atrium) could be enhanced by the application of a second stimulus (i.e., to the right atrium) in the same animal.The L.A. receptors were stimulated by distending a small balloon at the right upper pulmonary vein-L.A. junction and the R.A. receptors by "expanding" a spherical wire cage positioned at the superior vena caval (S.V.C.)-R.A. junction. Pressures in the S.V.C., R.A., L.A., and femoral artery were measured and the electrocardiogram monitored.In eight dogs stimulation of L.A. receptors resulted in an increase in heart rate (H.R.) of 18.5 beats/min (SEM 6.0; N = 23). In the same animals stimulation of R.A. receptors resulted in an increase in H.R. of 14.6 beats/min (SEM 2.0; N = 25). Application of both stimuli simultaneously resulted in an increase of 32.2 beats/min (SEM 8.0; N = 13). In four dogs propranolol hydrochloride (0.5 mg/kg) markedly diminished the response. In three dogs the response was abolished by bretylium tosylate (10 mg/kg).It is concluded that the increase in H.R. resulting from the application of these two stimuli could be "summated" and these findings support the proposition that the receptors in the two atria act as a functional entity.

Effects of physical training on myocardial ischaemia in patients with coronary artery disease

The effects of a simple physical training programme were investigated in a prospective and randomised trial in patients with stable angina pectoris using a modified exercise test. Twenty-four patients with ischaemic heart disease and an ischaemic response to conventional exercise electrocardiography were randomised into two groups: 12 patients took part in a training programme and 12 patients were allocated to a control group (no training). Exercise testing was performed sequentially at entry to the study and six months afterwards. At both studies we determined the heart rate at the same level of ischaemic ST segment depression (HR/ST threshold), the duration of the test, and relation of heart rate to the exercise load. The HR/ST threshold increased only in patients who underwent the exercise programme, suggesting indirectly that training resulted in the ability to do more work and attain a higher degree of myocardial oxygen consumption at the same level of myocardial ischaemia. In addition, training led to an increase in the duration of the test and to a reduction in heart rate at any level of submaximal exercise load. It is concluded that physical training in anginal patients results in an enhancement of myocardial oxygen availability.

Increased sensitivity of ADH bio-assay in rats by change in diet

1. Water-loaded rats anaesthetized with ethanol were used for the detection of antidiuretic hormone (ADH). 2. Litter-mates were divided into groups and placed on different diets during the fourth week of life directly after weaning, and maintained on their respective diets for approximately 1 month and their sensitivity to vasopressin was determined. 3. The rats maintained on a diet of only vegetable or vegetable plus casein supplement showed a greater sensitivity to exogenous vasopressin than rats maintained on a standard laboratory diet (pellets). 4. It is proposed that the quantitative relationship between ADH and urine flow is dependent upon the previous history of water consumption.

Acid-base changes after cardiorespiratory arrest in the dog

1. To assess the non-respiratory acid-base disturbance which results from cardiorespiratory arrest experiments were performed on anaesthetized dogs, and circulatory arrest was established either by inducing ventricular fibrillation or by complete occlusion of the great veins. 2. In the first series of experiments (group A) the period of cardiorespiratory arrest was limited to 2.0-3.5 min. It was shown that the nonrespiratory component of the ensuing acid-base disturbance as expressed by the non-respiratory pH was small: in the nine episodes of circulatory arrest after occlusion of the great veins there was a mean reduction in the non-respiratory pH or 0.04 pH unit (range 0.025-0.055), and in seven episodes of ventricular fibrillation the mean reduction was 0.024 pH unit (range 0-0.055). 3. In the second series of experiments (group B) the period of cardiorespiratory arrest was varied from 3.0 to 11.0 min. It was found that the degree of non-respiratory acidaemia was correlated with the duration of arrest but the magnitude of these changes was not large.

Diagnostic value of routine exercise testing in hospital patients with angina pectoris

A group of patients with angina pectoris were investigated prospectively using a simple and well-recognised exercise test protocol, and S-T segment displacements during exercise were correlated with the results of coronary arteriography. Definitive exercise results in 100 patients correlated well with findings on coronary angiograms: 69 out of 70 patients with positive exercise test results had significant coronary disease on angiography, while 23 out of 30 with negative exercise test results had no significant coronary disease. In six patients a period of physical training changed indeterminate initial tests to definitive ones after the training. These results show that the use of a widely available exercise test and a light physical training programme, with rigid adherence to the protocol, can reduce the overall demand on coronary arteriography.

A comparison of the oxygen consumption/body weight relationship obtained during submaximal exercise on a bicycle ergometer and on a treadmill

It is widely accepted that the relationship between oxygen consumption and body weight obtained during exercise on a bicycle ergometer differs from that obtained during treadmill walking. Experimental evidence to support this claim is lacking. To examine this difference a group of subjects (body weight 41--81 kg) undertook a predetermined level of submaximal exercise on a bicycle ergometer and a treadmill. Oxygen consumption was measured in a steady state at rest (i.e. sitting on the bicycle ergometer and standing on the treadmill) and during the two modes of exercise. A significant positive correlation between oxygen consumption and body weight was obtained under all four conditions of measurement. At rest the two regression lines did not differ in slope or elevation. During exercise the slope and the elevation of the line obtain from treadmill walking were significantly greater than from bicycle ergometer exercise. The 'metabolic cost' of bicycle ergometer exercise, (Vo2 during exercise--V02 at rest), showed no significant correlation with body weight. In contrast, there was a significant positive correlation during walking. It is suggested that these differences have arisen due to a different proportion of the total body weight supported by the subject in the two forms of exercise.

Effect of the Canadian Air Force training programme on a submaximal exercise test

Validation of the submaximal heart rate/oxygen consumption relationship as an index of 'cardiorespiratory fitness' requires the demonstration of systematic alterations in this relationship concomitant with interventions designed to alter physical fitness. To fulfil those criteria a longitudinal training/de-training study was undertaken. Previously sedentary adult subjects undertook the Canadian Airforce 5BX-XBX exercise programme. Submaximal exercise tests were performed before and after training, and following several weeks cessation of training. A regression line of submaximal heart rate on submaximal oxygen consumption was calculated from the data of each submaximal exercise test. Alterations in the regression lines were examined for each subject individually by testing statistically for difference in slope and elevation between any pair of lines. Subjects who undertook the training/de-training study demonstrated significant systematic alterations in the elevation of the regression lines concomitant with periods of training and de-training. The reproducibility of the submaximal heart rate/oxygen consumption relationship was examined in two additional groups of subjects. Group A repeated a submaximal test on 3 or 4 successive days; Group B were tested before and after 16 weeks of normal activity. Subjects in Group A demonstrated non significant, random alterations in the regression lines on repeated testing and subjects in Group B demonstrated random, though on occasion significant, alterations in the regression lines. The elevation of the submaximal heart rate/oxygen consumption relationship is therefore a valid index for detecting sequential changes in 'cardiorespiratory fitness' in individual subjects.

Diuresis from left atrial receptors: effect of plasma on the secretion of the Malpighian tubules of Rhodnius prolixus

1. Stimulation of left atrial receptors by distension of a balloon in the lumen of the left atrium of anaesthetized dogs was shown to result in an increase in urine flow. Plasma samples obtained from these dogs during control periods and during periods of stimulation were applied to the Malpighian tubules of Rhodnius prolixus. 2. It was found that the tubules suspended in test plasma secreted at a significantly lower rate than those suspended in control plasma. 3. These differences were also evident in extracts of plasma prepared using the solvent n-butanol. 4. Cutting or cooling the cervical vagi abolished these differences along with the increase in urine flow. It is argued that this preparation of the Malpighian tubule of Rhodnius prolixus could be used as a means of detecting the diuretic agent responsible for the increase in urine flow.

The nature of the atrial receptors responsible for a reflex increase in heart rate in the dog

1. In dogs anaesthetized with chloralose, small latex balloons were positioned at the left pulmonary vein-atrial junctions so as to stretch this region. By recording action potentials from slips of the cervical vagi it was established that distension of these balloons stimulated receptor endings in the atrial endocardium which discharged into the myelinated branches of the vagi i.e. Paintal type A, type B and Intermediate type receptors. 2. In other dogs, cooling the cervical vagus in steps of 2 degrees C reduced this response in vagal myelinated fibres. With twelve receptors the response to distension was reduced by 30% when the vagus was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished at 8--12 degrees C. 3. In a third group of dogs, distension of balloons at the pulmonary vein-atrial junctions resulted in a reflex increase in heart rate. Cooling the cervical vagi in these dogs in stages to 8 degrees C reduced this increase in heart rate. In nine dogs the response was reduced by 20% when the vagi was cooled to 16 degrees C, by 70% when cooled to 12 degrees C and abolished between 12 and 8 degrees C. 4. In a fourth group of dogs, distension of balloons at the pulmonary vein-atrial junctions was shown also to activate receptor endings in the atria which discharged into non-myelinated branches of the Vagi. In twelve receptors, cooling the cervical vagus in steps of 2 degrees C reduced this evoked increase in activity in non-myelinated fibres. These responses were abolished over a wide range of temperature unlike the responses observed above. 5. It is concluded that the increase in heart rate which follows distension of balloons at the pulmonary vein-atrial junctions is mediated solely by the Paintal-type receptors which discharge into the myelinated fibres in the vagi.