Effects of feeding on thermoregulatory behaviours and gut blood flow in white sturgeon (Acipenser transmontanus) using biotelemetry in combination with standard techniques.

The effects of thermoregulatory behaviours on gut blood flow in white sturgeon Acipenser transmontanus before and after feeding was studied using a blood flow biotelemetry system in combination with a temperature preference chamber. This is the first study to look at cardiovascular responses to feeding in white sturgeon, and also the first time behavioural tests in fish have been combined with recordings of cardiac output, heart rate, cardiac stroke volume and gut blood flow. The results showed strong correlations between gut blood flow and temperature choice after feeding (R(2)=0.88+/-0.03, 6-8 h postprandially and R(2)=0.89+/-0.04, 8-10 h postprandially) but not prior to feeding (R(2)=0.11+/-0.05). Feeding did not affect the actual temperature preference (18.4+/-0.7 degrees C before feeding, 18.1+/-0.7 degrees C, 6-8 h postprandially and 17.5+/-0.5 degrees C, 8-10 h postprandially). Fish instrumented with a blood flow biotelemetry device, and allowed to move freely in the water, had a significantly lower resting heart rate (37.3+/-0.26 beats min(-1)) compared with the control group that was traditionally instrumented with transit-time blood flow probes and kept in a confined area in accordance with the standard procedure (43.2+/-2.1 beats min(-1)). This study shows, for the first time in fish, the correlation between body temperature and gut blood flow during behavioural thermoregulation.

Jugular venous pooling during lowering of the head affects blood pressure of the anesthetized giraffe.

How blood flow and pressure to the giraffe's brain are regulated when drinking remains debated. We measured simultaneous blood flow, pressure, and cross-sectional area in the carotid artery and jugular vein of five anesthetized and spontaneously breathing giraffes. The giraffes were suspended in the upright position so that we could lower the head. In the upright position, mean arterial pressure (MAP) was 193 +/- 11 mmHg (mean +/- SE), carotid flow was 0.7 +/- 0.2 l/min, and carotid cross-sectional area was 0.85 +/- 0.04 cm(2). Central venous pressure (CVP) was 4 +/- 2 mmHg, jugular flow was 0.7 +/- 0.2 l/min, and jugular cross-sectional area was 0.14 +/- 0.04 cm(2) (n = 4). Carotid arterial and jugular venous pressures at head level were 118 +/- 9 and -7 +/- 4 mmHg, respectively. When the head was lowered, MAP decreased to 131 +/- 13 mmHg, while carotid cross-sectional area and flow remained unchanged. Cardiac output was reduced by 30%, CVP decreased to -1 +/- 2 mmHg (P < 0.01), and jugular flow ceased as the jugular cross-sectional area increased to 3.2 +/- 0.6 cm(2) (P < 0.01), corresponding to accumulation of approximately 1.2 l of blood in the veins. When the head was raised, the jugular veins collapsed and blood was returned to the central circulation, and CVP and cardiac output were restored. The results demonstrate that in the upright-positioned, anesthetized giraffe cerebral blood flow is governed by arterial pressure without support of a siphon mechanism and that when the head is lowered, blood accumulates in the vein, affecting MAP.

A novel, fully implantable, multichannel biotelemetry system for measurement of blood flow, pressure, ECG, and temperature.

Biotelemetry provides high-quality data in awake, free-ranging animals without the effects of anesthesia and surgery. Although many biological parameters can be measured using biotelemetry, simultaneous telemetric measurements of pressure and flow have not been available. The objective of this study was to evaluate simultaneous measurements of blood flow, pressure, ECG, and temperature in a fully implantable system. This novel system allows the measurement of up to four channels of blood flow, up to three channels of pressure, and a single channel each of ECG and temperature. The system includes a bidirectional radio-frequency link that allows the implant to send data and accept commands to perform various tasks. The system is controlled by a base station decoder/controller that decodes the data stream sent by the implant into analog signals. The system also converts the data into a digital data stream that can be sent via ethernet to a remote computer for storage and/or analysis. The system was chronically implanted in swine and alligators for up to 5 wk. Both bench and in vivo animal tests were performed to evaluate system performance. Results show that this biotelemetry system is capable of long-term accurate monitoring of simultaneous blood flow and pressure. The system allows, within the room, recordings, since the implant transmission range is between 6 and 10 m, and, with a relay, backpack transmission distance of up to 500 m can be achieved. This system will have significant utility in chronic models of cardiovascular physiology and pathology.

Coronary-bronchial blood flow and airway dimensions in exercise-induced syndromes.

1. We have an incomplete understanding of integrative cardiopulmonary control during exercise and particularly during the postexercise period, when symptoms and signs of myocardial ischaemia and exercise-induced asthma not present during exercise may appear. 2. The hypothesis is advanced that baroreflex de-resetting during exercise recovery is normally associated with (i) a dominant sympathetic vasoconstrictor effect in the coronary circulation, which, when associated with obstructive coronary disease, may initiate a potentially positive-feedback cardiocardiac sympathetic reflex (variable myocardial ischaemia with symptoms and signs); and (ii) a dominant parasympathetic bronchoconstrictor effect in the presence of bronchovascular dilatation, which, when associated with raised mediator release in the bronchial wall, reinforces the tendency for airway obstruction (variable dyspnoea results). 3. There is a need for new techniques to examine hypotheses concerning autonomic control, during and after exercise, of the coronary and bronchial circulations and the dimensions of airways. Accordingly, a new ultrasonic instrument has been designed named an 'Airways Internal Diameter Assessment (AIDA) Sonomicrometer'. It combines pulsed Doppler flowmetry with transit-time sonomicrometry of airway circumference and single-crystal sonomicrometry of airway wall thickness. Initial evaluation suggests it is relatively easy to apply during thoracotomy in recovery animals. The component devices are linear and will measure target variables with excellent accuracy. 4. In anaesthetized sheep, intubated with controlled ventilation, intravenous isoproterenol causes large increases in bronchial blood flow, a fall in arterial pressure and a reduction in airway circumference. This may reflect the dominant action of reflex vagal activity over direct beta-adrenoceptor inhibition of bronchial smooth muscle, the reflex source being baroreflex secondary to the fall in arterial pressure. These findings provide insight into the integrative mechanisms underlying the paradoxical negative effects sometimes observed when beta-adrenoceptor agonists are used in asthma.

Naloxone reverses inhibitory effect of electroacupuncture on sympathetic cardiovascular reflex responses.

Acupuncture and electroacupuncture (EA) have been used in traditional Chinese medicine to treat a wide range of diseases and conditions, including angina pectoris and myocardial infarction. In a feline model of reflex-induced reversible myocardial ischemia, electrical stimulation of the median nerves to mimic EA (Neiguan acupoint) significantly improved ischemic dysfunction, secondary to an inhibitory effect of EA on reflex pressor effects evoked by bradykinin (BK). The central mechanism of EA's inhibitory effect in this model is unknown. Accordingly, in alpha-chloralose-anesthetized cats, BK (10 micrograms/ml) was applied to the gallbladder to elicit a cardiovascular reflex response that significantly (P < 0.05) increased arterial blood pressure and heart rate; normalized systolic wall thickening (%WTh) of the left ventricle, measured by ultrasonic single-crystal sonomicrometer, increased by 31 +/- 11% (P < 0.05). After ligation of a side branch of the left anterior descending coronary artery, the reflex pressor response to BK resulted in a significant decrease of %WTh (-32 +/- 6%) in the ischemic region. When bilateral EA of the Neiguan acupoints was performed, the pressor response to BK was inhibited and regional myocardial function was significantly improved (+19 +/- 20%). The inhibitory effects of EA on blood pressure and %WTh were reversed by intravenous injection of naloxone (0.4 mg/kg; n = 9) or microinjection of naloxone (10 nM in 0.1 microliter/site; n = 14) into the rostral ventrolateral medulla (rVLM). Thus %WTh with intravenous naloxone was reduced to -13 +/- 29% (P<0.05) during stimulation of the gallbladder. Our results indicate that the inhibitory effect of EA on the BK-induced pressor response and the consequent improvement of ischemic dysfunction is dependent on the activation of opioid receptors, specifically receptors located in the rVLM.

Reversal of reflex-induced myocardial ischemia by median nerve stimulation: a feline model of electroacupuncture.

BACKGROUND: Acupuncture is reported to reduce myocardial ischemia, arrhythmias, and hypertension. To investigate the physiological mechanisms underlying these observations, a model of reflex-induced, reversible myocardial ischemia was developed to test the effects of median nerve stimulation as a surrogate for electroacupuncture. METHODS AND RESULTS: Chloralose-anesthetized cats were instrumented to measure arterial blood pressure, left ventricular pressure, left ventricular dP/dt, heart rate, left anterior descending (LAD) coronary blood velocity, and regional wall motion. The LAD artery either was partially occluded or a small diagonal branch was ligated. Subsequently, transient reflex activation of the cardiovascular system was evoked by application of bradykinin (typically 1 microg/mL) to the gallbladder, which significantly increased myocardial oxygen demand (double product), left ventricular dP/dt, and coronary blood velocity and caused ischemia-induced regional dysfunction, evidenced by significant (P<.05) reduction in normalized wall thickening (10.7+/-4.2% versus -23.6+/-2.9%; control versus ischemia; n=7). However, when median nerves were stimulated with low frequency (5 Hz) to mimic electroacupuncture, bradykinin-induced change in normalized wall thickening was significantly improved (-23.6+/-2.9% versus 9.8+/-4.9%; ischemia versus median nerve stimulation, P<.05) and remained augmented > or = 1 hour. Results were similar in partial and complete occlusion groups. Significant improvement in wall thickening was associated with unchanged increment of coronary blood velocity and significantly diminished increments of double product and diastolic blood pressure. CONCLUSIONS: These results suggest that stimulation of the median nerve to mimic electroacupuncture diminishes regional myocardial ischemia triggered by a sympathetically mediated increase in cardiac oxygen demand. The mechanism of this effect is related to reduction in cardiac oxygen demand, secondary to a diminished pressor response. These data provide the first documentation of the physiological mechanisms underlying the possible beneficial effect of electroacupuncture in the context of restricted coronary blood flow and augmented myocardial oxygen demand.

Spatiotemporal aspects of sympathetic C-fiber afferent activity in pressor reflex during abdominal ischemia.

Activation of abdominal sympathetic visceral afferents during ischemia elicits excitatory cardiovascular reflexes. The present study examined the time course and discharge patterns of activation of ischemically sensitive sympathetic C-fiber afferents, and then the relationship between summated afferent activity and the pressor reflex induced by prolonged abdominal ischemia was determined. Single-unit activity of abdominal C-fiber afferents was recorded from the right thoracic sympathetic chain of anesthetized cats during 30 min of ischemia. The reflex pressor response to abdominal ischemia was induced by occlusion of celiac and superior mesenteric arteries. Of 68 C-fiber afferents studied, 36 (approximately 53%) were activated during 30 min of ischemia, whereas the activity of the remaining 32 were not altered. Onset latencies of 36 C-fiber afferents activated by ischemia ranged from 1.0 to 17.4 min with an average of 6.1 +/- 0.8 min. The majority of activated afferents manifested a bursting pattern of discharge activity as ischemia was prolonged beyond 10 min. Summated response of activated afferents, but not individual afferent activity, was related closely to the reflex pressor response during 30 min of ischemia. These results suggest that both recruitment of sufficient numbers of C-fiber afferents and adequate discharge frequency of afferents constitute an encoding mechanism for the pressor reflex during abdominal ischemia.

Hypoxia does not directly stimulate ischemically sensitive abdominal visceral afferents during ischemia.

Abdominal ischemia activates ischemically sensitive sympathetic visceral afferents and evokes reflex excitation of the cardiovascular system. These afferents respond to ischemic metabolites, including lactic acid, bradykinin, prostaglandins, and reactive oxygen species. Severe hypoxemia also has been shown to activate these afferents. It is not known, however, if the regional tissue hypoxia induced by abdominal ischemia directly or indirectly activates ischemically sensitive visceral afferents. To determine the role of tissue hypoxia in activation of ischemically sensitive abdominal afferents, continuous single-unit activity of ischemically sensitive abdominal sympathetic C-fiber afferents (conduction velocity = 0.51-1.48 m/s) and regional tissue PO2, measured by a polarographic oxygen electrode in the porta hepatis, duodenum, or pancreas, were recorded simultaneously in anesthetized cats before and during 10-15 min of ischemia. Abdominal ischemia rapidly decreased regional tissue PO2 from 161 +/- 10 to 8 +/- 2 mmHg (P < 0.01) within an interval of 136 +/- 12 s. By contrast, after longer latency (399 +/- 24 s, P < 0.01 vs. PO2 interval), the activity of these afferents increased from 0.06 +/- 0.01 to 0.33 +/- 0.07 imp/s (P < 0.01). Furthermore, the activity of ischemically sensitive afferents gradually increased throughout ischemia with peak activity (0.68 +/- 0.14 imp/s) occurring at 600 +/- 39 s, although tissue PO2 remained constant. There was no correlation between the changes of tissue PO2 and discharge activity of these afferents (r = -0.428, P = 0.144). These data suggest that tissue hypoxia induced by abdominal ischemia is not directly responsible for activation of ischemically sensitive sympathetic visceral afferents but likely acts in an indirect fashion by promoting formation of other metabolic products capable of activating these nerve endings.

An ultrasonic system for measurement of absolute myocardial thickness using a single transducer.

We have developed an ultrasonic instrument that can measure absolute regional myocardial wall motion throughout the cardiac cycle using a single epicardial piezoelectric transducer. The methods in place currently that utilize ultrasound to measure myocardial wall thickness are the transit-time sonomicrometer (TTS) and, more recently, the Doppler echo displacement method. Both methods have inherent disadvantages. To address the need for an instrument that can measure absolute dimensions of myocardial wall at any depth, an ultrasonic single-crystal sonomicrometer (SCS) system was developed. This system can identify and track the boundary of the endocardial muscle-blood interface. With this instrument, it is possible to obtain, from a single epicardial transducer, measurement of myocardial wall motion that is calibrated in absolute dimensional units. The operating principles of the proposed myocardial dimension measurement system are as follows. A short duration ultrasonic burst having a frequency of 10 MHz is transmitted from the piezoelectric transducer. Reflected echoes are sampled at two distinct time intervals to generate reference and interface sample volumes. During steady state, the two sample volumes are adjusted so that the reference volume remains entirely within the myocardium, whereas half of the interface sampled volume is located within the myocardium. After amplification and filtering, the true root mean square values of both signals are compared and an error signal is generated. A closed-loop circuit uses the integrated error signal to continuously adjust the position of the two sample volumes. We have compared our system in vitro against a known signal and in vivo against the two-crystal TTS system during control, suppression (ischemia), and enhancement (isoproterenol) of myocardial function. Results were obtained in vitro for accuracy (> 99%), signal linearity (r = 0.99), and frequency response to heart rates > 450 beats/min, and in vivo data were acquired for end-systolic dimension (r = 0.99), end-diastolic dimension (r = 0.99), and percent wall thickness (r = 0.99). Both in vitro and in vivo tests indicate that the SCS functions identically to the two-crystal TTS. Use of the SCS allows measurement of absolute wall thickness and hence myocardial function, for both acute and chronically instrumented animal studies, with minimal or no trauma to myocardium.

Biotelemetry of cardiovascular hemodynamic measurements in miniswine.

A three-channel biotelemetry system has been designed and implemented for use in a chronically instrumented animal model of cardiovascular disease. A dual-channel ultrasonic transit-time micrometer allows monitoring of left-ventricular wall motion for the regions perfused by the left circumflex and left anterior descending coronary arteries. A third channel provides left ventricular blood pressure obtained from a high-fidelity blood pressure transducer implanted in the left ventricle. The biotelemetry system features a high voltage dc-dc converter for improved ultrasonic signal strength, a time-to-voltage converter that is highly immune to synchronization frequency variations, low power consumption (approx. 100 mW), small size (4 x 6 x 12 cm), and low weight (approx. 200 g). This three-channel system has enabled our laboratory to continuously monitor untethered animals for 24-h periods. Data obtained from this miniature biotelemetry system can be utilized to quantify myocardial oxygen demand and regional left-ventricular wall thickening.

Chronic reduction of myocardial ischemia does not attenuate coronary collateral development in miniswine.

BACKGROUND: Myocardial ischemia is considered to be a possible stimulus for development of the coronary collateral circulation. We therefore hypothesized that chronic reduction of myocardial oxygen demand to lessen ischemia would attenuate coronary collateral development over an 8-week period using left circumflex coronary artery (LCx) ameroid-induced constriction in pigs. METHODS AND RESULTS: Collateral development was assessed by myocardial blood flow (radioactive microspheres) and left ventricular regional function (sonomicrometer dimension gauges). beta-Adrenoceptor blockade with propranolol (160 or 320 mg b.i.d.p.o.) was initiated in 15 animals 1 day after surgery. Compared with 16 untreated animals, beta-adrenoceptor antagonism was documented in the treated group by 1) pharmacological stimulation with isoproterenol, 2) physiological stimulation during graded treadmill exercise, and 3) repeated long-term biotelemetry recordings of oxygen demand (heart rate and blood pressure) and regional myocardial function. In addition to pharmacological and physiological verification of beta-blockade, biotelemetry showed that, compared with the untreated animals, propranolol significantly reduced the daily number, individual duration, and severity of events representing myocardial dysfunction. This suggests that in the beta-blocked group, little if any ischemia was present throughout the first 5 weeks when collateral growth occurs. Transmural myocardial blood flow (expressed as a ratio of flow in the LCx region to the nonoccluded region of the left ventricle) and systolic wall thickening in the LCx region were determined at rest and during treadmill exercise (240 beats per minute) 31-38 days (5 weeks) and 60-67 days (8 weeks) after surgery. Propranolol was withdrawn 3 days before flow and function determinations and was resumed immediately after testing. Blood flow ratios at 5 weeks decreased similarly from rest to exercise in the untreated (0.83 +/- 0.04 to 0.60 +/- 0.05, p less than 0.05) and beta-blockade group (0.82 +/- 0.09 to 0.57 +/- 0.10, p less than 0.05). Systolic wall thickening from rest to exercise was attenuated to the same degree in the untreated (59 +/- 6% to 38 +/- 6%, p less than 0.05) and beta-blockade group (50 +/- 8% to 30 +/- 5%, p less than 0.05). Similar flow and function responses were observed in both groups at 8 weeks. CONCLUSIONS: We conclude that growth and development of the coronary collateral circulation measured functionally during exercise at 90% of maximal heart rate is unrelated to the extent and duration of myocardial ischemia in this model.

Analysis of beat-to-beat cardiovascular hemodynamic variables obtained from long-term biotelemetry.

Manual methods of large volume data storage, retrieval, and analysis are difficult, time consuming, and present numerous opportunities for calculation errors. We have designed and implemented a comprehensive computer-based system for performing these functions. Development of this system was necessary since left ventricular (LV) blood pressure and two regional LV wall thickness measurements were obtained during long-term extracorporeal biotelemetry of miniswine for 24-h periods. During a single recording period over 100,000 individual cardiac cycles were recorded on analog tape and later analysed for determination of global myocardial oxygen demand and regional myocardial function. In addition, custom designed software was developed to determine the extent and duration of myocardial dysfunction. Batch file commands enabled the customized software to operate without prompting by the user thus optimizing the time usage of the computer, and the computer based data acquisition and analysis system. Although this system was designed specifically for analysing cardiovascular hemodynamic variables, it is flexible and can be applied to other experimental applications.