Central venous pressure and mean circulatory filling pressure in the dogfish Squalus acanthias: adrenergic control and role of the pericardium

Subambient central venous pressure (Pven) and modulation of venous return through cardiac suction (vis a fronte) characterizes the venous circulation in sharks. Venous capacitance was estimated in the dogfish Squalus acanthias by measuring the mean circulatory filling pressure (MCFP) during transient occlusion of cardiac outflow. We tested the hypothesis that venous return and cardiac preload can be altered additionally through adrenergic changes of venous capacitance. The experiments involved the surgical opening of the pericardium to place a perivascular occluder around the conus arteriosus. Another control group was identically instrumented, but lacked the occluder, and was subjected to the same pharmacological protocol to evaluate how pericardioectomy affected cardiovascular status. Routine Pven was negative (-0.08+/-0.02 kPa) in control fish but positive (0.09+/-0.01 kPa) in the pericardioectomized group. Injections of 5 microg/kg body mass (Mb) of epinephrine and phenylephrine (100 microg/kg Mb) increased Pven and MCFP, whereas isoproterenol (1 microg/kg Mb) decreased both variables. Thus, constriction and relaxation of the venous vasculature were mediated through the respective stimulation of alpha- and beta-adrenergic receptors. Alpha-adrenergic blockade with prazosin (1 mg/kg Mb) attenuated the responses to phenylephrine and decreased resting Pven in pericardioectomized animals. Our results provide convincing evidence for adrenergic control of the venous vasculature in elasmobranchs, although the pericardium is clearly an important component in the modulation of venous function. Thus active changes in venous capacitance have previously been underestimated as an important means of modulating venous return and cardiac performance in this group.     

Alveolar pressure measurement in open-chest rats.

In open-chest rats, alveolar pressure was measured with alveolar capsules connected via pliable tubing to inductive pressure transducers. By means of the interrupter technique during constant-flow inflation, it was possible to determine pulmonary static elastance (Est,L) and tissue and airway resistances (Rdiff,L and Rinit,L, respectively). In eight anesthetized paralyzed mechanically ventilated rats, 118 measurements of Rdiff,L and Est,L were performed over a wide range of flows and tidal volumes. There was excellent agreement between the data calculated using transpulmonary pressures and those computed using capsule pressures, the latter being measured at different points of the lung. In another group of rats studied under the same experimental conditions, two capsules were simultaneously placed on different pulmonary lobes. No regional differences in pulmonary mechanics could be detected in either experiment. In addition, alveolar pressure could also be measured accurately by a catheter inserted into lung parenchyma.     

Factors affecting the accuracy of esophageal balloon measurement of pleural pressure in dogs.

Simultaneous measurement of esophageal and tracheal pressures during an occluded inspiratory effort was used to assess the accuracy of the esophageal balloon for measuring pleural pressure in dogs. Esophageal balloons were inserted in five mongrel dogs, and an occlusion test was performed with the balloon tip 5, 10, 15, 20, and 25 cm above the esophageal sphincter; at lung volumes of functional residual capacity (FRC) and FRC + 600 ml; and in supine and right- and left-side lying postures. The protocol was repeated in paralyzed animals. This time the occlusion test was performed by injecting air into a plethysmograph to change the body surface pressure, simulating pressure changes produced by respiratory efforts in spontaneously breathing animals. In 47% of the tests in spontaneously breathing dogs, the slope of esophageal vs. tracheal pressure varied greater than 10% from unity. After paralysis the slope did not vary greater than 5% from unity under any circumstance. These data indicate that the poorer performance of the occlusion test in nonparalyzed dogs is due to active tension in the walls of the esophagus and stress induced in the intrathoracic soft tissues by the descent of the diaphragm during a breathing effort.     

A modified technique for tail cuff pressure measurement in unrestrained conscious rats

In physiological experiments, it is essential to measure arterial pressure (AP) and heart rate (HR) in animals. Tail cuff pressure (TCP) measurement using photoelectric volume oscillometry has been commonly used. We designed a new technique for continuous measurement of AP and HR in conscious, unrestrained rats. This is based on the observation that fixation of the rat's tail with tape keeps the animal in position without struggling. The animal is free to move its body. To test the accuracy of this new technique, Sprague-Dawley rats underwent four AP and HR measurement techniques. These included a new unrestrained method (UR), which was compared to the following three methods: traditional restrained TCP method with restrainer, direct monitoring of AP and HR with femoral artery catheterization and a combination of photoelectric volume oscillometry (with body heating to 37 degrees C) and femoral arterial recording. The results show that the modified UR measurement provides accurate data on AP and HR. This method obtains a lower value of HR and similar mean AP when compared to direct monitoring from femoral arterial catheterization. Accordingly, the modified unrestrained TCP measurement can be used in conscious rats as a noninvasive method.     

Influence of pregnancy on mean systemic filling pressure and the cardiac function curve in guinea pigs.

To assess the degree of circulatory fullness and to evaluate the influence of peripheral and cardiac factors in the regulation of cardiac output during pregnancy, the following studies were conducted using pentobarbital-anesthetized, open-chest nonpregnant and late term pregnant guinea pigs. Mean circulatory filling pressure was taken as the equilibrium pressure when the pulmonary artery was constricted. Total vascular compliance was assessed by +/- 5-mL changes in blood volume performed while this constriction was maintained. A separate group of guinea pigs was prepared with a pulmonary artery electromagnetic flow probe and right atrial catheter. Rapid infusion of saline was used to increase right atrial pressure while the cardiac output was determined. Pregnancy was characterized by the following changes relative to nonpregnant controls: 51Cr-labelled RBC blood volume increased from 55 +/- 3 to 67 +/- 3 mL/kg; mean circulatory filling pressure increased from 7.1 +/- 0.2 to 8.0 +/- 0.5 mmHg (1 mmHg = 133.322 Pa); right atrial pressure decreased from 3.4 +/- 0.2 to 2.1 +/- 0.3 mmHg; and cardiac output increased from 71.8 +/- 3.9 to 96.8 +/- 3.3 mL.min-1.kg-1. Total vascular compliance was not changed (2.1 +/- 0.1 mL.kg-1.mmHg-1) and most of the expanded blood volume was accommodated as unstressed volume. The cardiac function curve was shifted upwards in pregnant animals. The resistance to venous return, as determined from the slope of the venous return curves, was not changed. These data suggest that the circulation of the pregnant guinea pig is slightly overfilled.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of mean arterial pressure in rats by a tail-cuff method with sensitive photoelectric sensors

We evaluated a recently developed tail-cuff apparatus containing sensitive photoelectric sensors in the measurement of arterial pressure in rats. A total of 48 male Wistar rats were used in the study. The indirect mean arterial pressure (MPi) was determined from the cuff pressure when the pulse volume oscillations were maximal. To create sufficiently large pulse volume oscillations, we heated the rats for about 12 minutes at 38 degrees C prior to recording the pressure. The heating increased the mean arterial pressure by an average of 4 +/- 2 mmHg, as indicated by direct measurement of pressure. Three different sizes of cuffs were tested, with the result that the indirect measurements were nearly identical to those obtained directly when an appropriate cuff size was selected. The MPi was well correlated with the direct measurement of mean arterial pressure before (r = 0.918, p less than 0.001) and during (r = 0.903, p less than 0.001) elevation of arterial pressure via norepinephrine infusion. These results indicate that the MPi determined at maximum pulse volume oscillations coincides fairly well with the true mean arterial pressure.     

Catecholamines increase lung edema clearance in rats with increased left atrial pressure.

During hydrostatic pulmonary edema, active Na(+) transport and alveolar fluid reabsorption are decreased. Dopamine (DA) and isoproterenol (ISO) have been shown to increase active Na(+) transport in rat lungs by upregulating Na(+)-K(+)-ATPase in the alveolar epithelium. We studied the effects of DA and ISO in isolated rat lungs with increased left atrial pressure (Pla = 15 cmH(2)O) compared with control rats with normal Pla (Pla = 0). Alveolar fluid reabsorption decreased from control value of 0.51 +/- 0.02 to 0.27 +/- 0.02 ml/h when Pla was increased to 15 cmH(2)O (P < 0.001). DA and ISO increased the alveolar fluid reabsorption back to control levels. Treatment with the D(1) antagonist SCH-23390 inhibited the stimulatory effects of DA (0.30 +/- 0.02 ml/h), whereas fenoldopam, a specific D(1)-receptor agonist, increased alveolar fluid reabsorption in rats exposed to Pla of 15 cmH(2)O (0.47 +/- 0.04 ml/h). Propranolol, a beta-adrenergic-receptor antagonist, blocked the stimulatory effects of ISO; however, it did not affect alveolar fluid reabsorption in control or DA-treated rats. Amiloride (a Na(+) channel blocker) and ouabain (a Na(+)-K(+)-ATPase inhibitor), either alone or together, inhibited the stimulatory effects of DA. Colchicine, which disrupts the cellular microtubular transport of ion-transporting proteins to the plasma membrane, inhibited the stimulatory effects of DA, whereas the isomer beta-lumicolchicine did not block the stimulatory effects of DA. These data suggest that DA and ISO increase alveolar fluid reabsorption in a model of increased Pla by regulating active Na(+) transport in rat alveolar epithelium. The effects of DA and ISO are mediated by the activation of dopaminergic D(1) receptors and the beta-adrenergic receptors, respectively.     

The influence of carotid sinus pressure on plasma atrial natriuretic peptide in anaesthetized rabbits

The effect of changes in carotid sinus perfusion pressure on plasma immunoreactive atrial natriuretic peptide (IR-ANP) was examined in anaesthetized rabbits, and the role of arterial pressure in mediating the changes in IR-ANP was assessed. Plasma IR-ANP was significantly greater (101.7 +/- 24.3 pg ml-1) when carotid sinus pressure was 60 mmHg than when it was 160 mmHg (27.1 +/- 8.6 pg ml-1). Mean arterial pressure (MAP) was significantly greater when carotid sinus pressure was controlled at 60 mmHg compared to when it was 160 mmHg, but right atrial pressure (RAP) was not significantly different at the two carotid sinus pressures. The administration of hexamethonium attenuated the changes in MAP and heart rate (HR) which occurred in response to alterations in carotid sinus pressure, and abolished the change in plasma IR-ANP. The results suggest that an inverse relationship exists between carotid sinus pressure and plasma IR-ANP, and that the release of ANP in response to a reduction of carotid sinus pressure is mediated by the associated haemodynamic changes.     

Development of a noninvasive technique for the measurement of intracranial pressure.

Intracranial pressure (ICP) dynamics are important for understanding adjustments to altered gravity. Previous flight observations document significant facial edema during exposure to microgravity, which suggests that ICP is elevated during microgravity. However, there are no experimental results obtained during space flight, primarily due to the invasiveness of currently available techniques. We have developed and refined a noninvasive technique to measure intracranial pressure noninvasively. The technique is based upon detecting skull movements of a few micrometers in association with altered intracranial pressure. We reported that the PPLL technique has enough sensitivity to detect changes in cranial distance associated with the pulsation of ICP in cadavera. In normal operations, however, we place a transducer on the scalp. Thus, we cannot rule out the possibility that the PPLL technique picks up cutaneous pulsation. The purpose of the present study was therefore to show that the PPLL technique has enough sensitivity to detect changes in cranial distance associated with cardiac cycles in vivo.     

Hypoventilation after acute phrenicotomy of the urethane anaesthetized rats.

The aim of this study was to explore the mechanism resulting in hypoventilation in rats with denervated diaphragm. Bilateral cervical phrenicotomy (PX) was performed in 15 male rats anaesthetized with urethane (1.3 g/kg i.p.); other 8 rats were sham operated (SX). Ventilation, PaCO2 and the integrated EMG of the external intercostal muscles (iEMG) were measured before and after the surgery, at regular intervals, up to 4 hours postoperatively. During the 4 hours after PX there was a progressive decrease in minute ventilation and an increase in PaCO2 compared with the control values and with that in the SX rats. The increase in PaCO2 was accompanied by an increase in the peak amplitude of the iEMG to 155 +/- 18% of control values after PX and to 228 +/- 33% 4 hours later. Despite the augmented EMG activity tidal volume gradually decreased. The iEMG of the intercostal muscles, however, did not reach a maximum because the shortlasting stimulation of breathing by acute hypercapnia and hypoxia as the result of added dead space (0.5 ml) increased the iEMG still further. These results indicate that both the central and peripheral mechanisms contribute to hypoventilation in anaesthetized rats with denervated diaphragm.     

Capsaicin inhibits the pentagastrin-stimulated gastric acid secretion in anaesthetized rats with acute gastric fistula.

The effect of capsaicin on basal and pentagastrin-stimulated gastric acid secretion was investigated in the urethane anaesthetized acute gastric fistula rat. Gastric acid secretion was measured by flushing of the gastric lumen with saline every 15 min or by continuous gastric perfusion. Capsaicin given into the rat stomach at 120 ng x mL(-1) prior to pentagastrin (25 microg x kg(-1), iv) reduced gastric acid secretory response to pentagastrin by 24%. Intravenous (iv) capsaicin (0.5 microg x kg(-1)) did not reduce the pentagastrin-stimulated gastric acid secretion. After topical capsaicin desensitization (3 mg x mL(-1)), basal gastric acid secretion and that in response to pentagastrin (25 microg x kg(-1), intraperitonaeally) was unaltered compared with the control group. Data indicate that topical capsaicin inhibits gastric acid secretion stimulated with pentagastrin in anaesthetized rats.     

Protection of adrenocortical activity by dietary casein in ether anaesthetized rats

Adrenal delta5-3beta-hydroxysteroid dehydrogenase (delta5-3beta-HSD) activity and serum corticosterone level were significantly higher in rats fed with 5% casein or 4% albumin diets after 1 hr of ether anaesthetic stress as compared to the controls, 5% casein and 20% casein (equivalent to 4% albumin) respectively. Ether anaesthesia to 20% casein fed rats caused no change in adrenal delta5-3beta-HSD activity and serum corticosterone level when compared with controls fed 20% casein diet. The results suggest that high milk protein diet may prevent acute stress effects by protecting adrenocortical activity. The present investigation opens up a new area of management of stress.     

Time course of release of atrial natriuretic peptide in the anaesthetized dog

In 12 chloralose anaesthetized dogs plasma concentration of immunoreactive atrial natriuretic peptide (IR-ANP) was measured using a radioimmunoassay. Plasma IR-ANP was 74 +/- 4.8 pg/mL (mean +/- SE) and increased by 39 +/- 4.1 pg/mL when left atrial pressure was increased by 10 cm H2O during partial mitral obstruction. Observation of the time course of the changes in IR-ANP during atrial distension showed that IR-ANP was increased within 2 min of atrial distension and declined after atrial distension, with a half-time of 4.5 min. The time course of the changes in IR-ANP was unaffected by vagotomy or administration of atenolol. Maximum electrical stimulation of the right ansa subclavia failed to produce any change in IR-ANP. IR-ANP was higher in coronary sinus plasma than in femoral arterial plasma confirming that the heart was the source of the IR-ANP. The results support the hypothesis that IR-ANP is released from the heart by a direct effect of stretch of the atrial wall rather than by a neural or humoral mechanism involving a reflex from atrial receptors.