Canine Circulation Research Articles

Pharmacokinetics of recombinant human lipocalin-type prostaglandin D synthase/β-trace in canine

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is identical to β-trace, a major protein in human cerebrospinal fluid (CSF), and acts as both a PGD 2-producing enzyme and as an extracellular transporter for lipophilic ligands. In this study, we investigated the pharmacokinetics of recombinant human L-PGDS (rh-L-PGDS) in canines. After an intravenous bolus injection of rh-L-PGDS, the serum concentration decreased bi-exponentially with a half-life of the terminal line phase of 0.77 h, which was markedly shorter than that of other proteins with the same molecular weight as that of rh-L-PGDS. The distribution volume was 55.4 ml/kg, which was close to the volume of canine circulation plasma, indicating that the administrated rh-L-PGDS was distributed mainly in the blood. Only 10.3% of the administered rh-L-PGDS was excreted to the urine, suggesting that rh-L-PGDS was actively degraded within the body. After an intrathecal injection, the peak serum concentration of rh-L-PGDS was observed at 4–5 h. The area under the plasma concentration–time curve obtained for 12 h after the intrathecal injection was one third of the value for 3 h after the intravenous injection, suggesting that at least one third of the intrathecally injected rh-L-PGDS shifted to the blood.

Haemodynamic action of B‐type natriuretic peptide substantially outlasts its plasma half‐life in conscious dogs

1. The objective of the present study was to determine the plasma half-life of B-type natriuretic peptide (BNP) in conscious dogs after intravenous administration and to compare this with its haemodynamic effects. In six chronically instrumented dogs, plasma BNP concentrations were measured under basal conditions, during a constant infusion of canine BNP-32 (10 pmol/kg per min; 25 min) to steady state and at nominated time points up to 75 min after stopping the infusion. Concomitant, continuous measurements of mean arterial blood pressure (MAP), heart rate (HR), central venous pressure (CVP) and mesenteric blood flow (MBF) were obtained. 2. Baseline plasma BNP levels were 15.0 +/- 2.3 fmol/mL and rose approximately 10-fold to 159 +/- 23 fmol/mL after 20-25 min BNP infusion. When the infusion was turned off, plasma BNP levels declined in a biphasic manner, with an initial half-life of 1.57 +/- 0.14 min and a terminal half-life of 301 +/- 85 min. The metabolic clearance rate of BNP was 2.29 +/- 0.34 L/min. 3. The infusion of BNP reduced MAP (approximately 10%), CVP (approximately 65%) and MBF (approximately 25%), whereas haematocrit (approximately 4%) and mesenteric vascular resistance (MVR) increased (approximately 40%; all P < 0.05). Plasma BNP levels returned to baseline by 20 min after BNP infusion had been stopped, whereas none of the haemodynamic variables returned to normal by this time. Mean arterial pressure returned to resting levels within 10-15 min after plasma BNP returned to normal. However, CVP, haematocrit and MBF remained substantially below baseline values for more than 20 min after circulating BNP levels had returned to pre-infusion levels. Of these, only mesenteric vascular changes were returned to baseline within 60 min of plasma BNP levels normalizing. 4. These results demonstrate that the removal of BNP from the canine circulation is rapid, similar to observations made regarding the metabolism of circulating atrial natriuretic peptide in dogs. The half-life of BNP in dogs was shorter than that in rats, sheep or humans. However, the haemodynamic actions of BNP substantially outlasted its plasma half-life. Whether this disparity in plasma level and haemodynamic activity of BNP reflects long-lasting activation of second messenger systems or slow recovery from the hydraulic changes at the capillary level, reflected in the haematocrit and CVP, remains to be answered.

Effects of inhibition of nitric oxide formation on the regulation of coronary blood flow in anesthetized dogs.

In 11 open-chest dogs with a flowmeter on the left circumflex artery, L-NMMA, a selective inhibitor of nitric oxide-formation, was subselectively infused into the left circumflex artery at a rate of 2.5 mg/ml (ml/min) to avoid systemic hemodynamic effects. The coronary blood flow at normal arterial blood pressure was similar prior to and during L-NMMA infusion. However, when the arterial blood pressure was raised by inflating a balloon in the descending aorta, the nitric oxide suppression induced a dramatic increase in coronary vascular resistance by almost 40% compared to control conditions without L-NMMA infusion at identically elevated arterial blood pressure. L-NMMA induced a significant downward shift and flattening of the pressure-flow relation over a pressure range from 60-150 mmHg. Peak hyperemic coronary flow after 20-s transient coronary occlusion was similar prior to and during L-NMMA infusion, but the duration of the hyperemic flow response was significantly shortened during L-NMMA infusion indicating exaggerated constriction after hyperemic stimulus. The EDRF/nitric oxide-system plays an important role for the regulation of coronary blood flow by counteracting autoregulatory constrictor responses to increased driving pressure and shear stress in the intact canine circulation.

A Mathematical Model of the Canine Circulation

A lumped-parameter model of the canine closed-loop circulation has been developed. The model consists of the following components: the right heart (atrium and ventricle), pulmonary circulation (pulmonary arteries, capillaries, and veins), left heart (atrium and ventricle), systemic circulation (aorta, systemic arteries, systemic veins, and the vena cavae). The passive elements, the arteries and veins, are represented by their resistive, compliant, and inertial properties.In addition to the mathematical description of the arteries and veins of both the systemic and pulmonary circulations, this model makes use of the time-varying elastance (instantaneous pressure-volume relationship) to describe the nature of the source pumps in the intact circulation. Using elastance functions adopted for both the left and right atria and ventricles, the model produces realistic left and right atrial and ventricular pressure and volume waveforms. Model parameters associated with the systemic and pulmonary circulations have been estimated so that the pressure waveforms at various points in the circulation are in agreement with data from animal experiments. The model can generate realistic pressure waveforms on a beat-to-beat basis and yields good estimates of mean blood pressures, cardiac output and ejection fraction for a normal dog. Furthermore, the model allows the description and specification of blood volume distribution within the entire closed circulation, and the changes in blood volumes can be monitored dynamically. We conclude that our model provides a useful theoretical framework for analyzing experimental studies of the circulatory system.

Retrograde coronary sinus perfusion provides non-homogeneous myocardial blood flow.

The ability of retrograde cardioplegia to protect the right ventricle has been questioned. Canine myocardial circulation was assessed by infusing colored microspheres through the coronary sinus. The relative flow index (RFI), a normalized measure of tissue blood flow, was determined for 76 sections of myocardium. Three distinct flow regions were evident from these measurements. A paucity of blood flow through some basal sections of the right ventricle (RFI = 0.23 +/- 0.19) was found to be significantly different (p < 0.005) from regions of the heart with normal flow (RFI = 1.12 +/- 0.06). Sections from the right ventricular apex demonstrated augmented flow (RFI = 3.72 +/- 1.18). These data indicate that retrograde coronary perfusion provides nonuniform flow and under some conditions may provide inadequate perfusion to portions of the right ventricle.

Laser Doppler velocimetry measurement of canine mucosal circulation response to bronchial artery reperfusion after lung transplantation.

Left lung transplantation accompanied by reestablishment of the circulation of the bronchial arteries by simultaneous transplantation of part of the aorta was performed on seven dogs. Mucosal circulation of the bronchi at the anastomosis was evaluated using laser Doppler velocimetry (LDV). Since the donor lung must maintain blood supply through the bronchial arteries, the donor aorta from its ascending portion to the descending portion below the outlet for the last bronchial artery was retained with the lung graft. The distal end of the left brachiocephalic artery was sutured to the side of the recipient aorta when the blood supply to the bronchial arteries was to be restored. The LDV values before reperfusion of the bronchial arteries were 20 to 65% (mean: 42.8%) of the preoperative value. Since the LDV values increased to 36 to 125% (mean: 79.9%) of the control, blood flow to the bronchial mucosa increased evidently. Four out of seven dogs died one to two days after the operation due to bleeding and operative stress. One dog was sacrificed on the 4th postoperative day and another on the 16th. The last dog died 40 days after surgery. The unfavorable course of the LDV values in these three dogs suggest that the method used does not ensure an adequate blood supply to the bronchial arteries in the long term.

Norepinephrine release during autoregulatory escape: effects of alpha 2-receptor blockade.

The partial recovery that intestinal blood flow undergoes during continued sympathetic nerve stimulation is termed autoregulatory escape. This study tested two hypotheses that might explain escape: 1) diminishing norepinephrine (NE) release during sustained stimulation and 2) an alpha 2-receptor-mediated competition between local and neural control mechanisms. The rates of NE release before and during stimulation of the perivascular sympathetic nerves were determined by measuring blood flow in isolated loops of canine small intestine and assaying the concentrations of NE in arterial and venous blood. The presence of functional alpha 2-receptors was demonstrated by clonidine injections, and the effects of alpha 2-receptor blockade were studied during yohimbine infusions. The time course of NE release was inconsistent with a cause-effect relationship; NE release was greatest during the phase when resistance had already escaped. Deliberately altering NE release by changing the stimulus duration did not affect escape. The study demonstrated 1) that diminished NE release during continued sympathetic stimulation does not occur and cannot account for escape, 2) that resistance vessels in the canine intestinal circulation possess functional alpha 2-receptors which are responsible for part of the vasoconstriction caused by sympathetic stimulation, 3) that blockade of presynaptic alpha 2-receptors significantly enhanced NE release during the initial 30-s period but not during the escape phase, and 4) that alpha 2-receptor blockade enhances autoregulatory escape. Altogether these findings indicate that the postsynaptic alpha 2-receptors on intestinal resistance vessels deserve further investigation as the possible site at which local and neural mechanisms compete to influence vascular resistance.

Atrial natriuretic factor and sympathetic nervous output in response to volume shifts in the immature canine circulation

This study evaluated the immature circulation's response to acute shifts in intravascular volume with respect to atrial natriuretic factor and plasma catecholamines. Serial measurements were performed on thirteen beagle puppies during volume expansion with a saline and albumin solution followed by volume contraction with furosemide. Atrial natriuretic factor correlated with right ( r=.73 , p<0.001) and left ( r=.62 , p<0.001) atrial pressures and increased to much greater levels than previously reported for mature animals. Simultaneously, 10 puppies had a progressive decrease in plasma norepinephrine over the 60-minute infusion ( p<0.05) while two puppies demonstrated a marked increase between the 30- and 60-minute samples. Furosemide increased urine output and reversed the hormonal changes caused by volume expansion. Thus a greatly augmented output of atrial natriuretic factor occurs in the immature canine circulation in response to increased atrial and pulmonary pressures, while sympathetic output remains unchanged or falls with increasing intravascular volume until a critical decrease in cardiac output triggers a catecholamine surge.

Computer simulation of the mechanically-assisted failing canine circulation

A model of the cardiovascular system is presented. The model includes representations of the left and right ventricles, a nonlinear multielement model of the aorta and its main branches, and lumped models of the systemic veins and the pulmonary circulation. A simulation of the intra-aortic balloon pump and representations of physiological compensatory mechanisms are also incorporated in the model. Parameters of the left ventricular model were set to simulate either the normal or failing canine circulation. Pressure and flow waveforms throughout the circulation as well as ventricular pressure and volume were calculated for the normal, failing, and assisted failing circulation. Cardiac oxygen supply and consumption were calculated from the model. They were used as direct indices of cardiac energy supply and utilization to assess the effects of cardiac assistance.

Independent release of peptide YY (PYY) into the circulation and ileal lumen of the awake dog

Peptide YY (PYY) is a recently discovered polypeptide found in the endocrine cells of the distal small intestine, colon, and rectum. This study was designed to evaluate the postprandial release of PYY into the peripheral blood and the ileal lumen in response to different ingested nutrients. Six adult conditioned mongrel dogs had 25-cm distal ileal Thiry-Vella segments surgically constructed. After a 2-week postoperative recovery period the animals were fed meals consisting of protein (2 g/kg), fat (5 g/kg), or glucose (1.5 g/kg). Circulating and ileal intraluminal PYY concentrations were measured by a sensitive radioimmunoassay developed in our laboratory. Significant increases in circulating PYY levels were seen only after the fat meal in dogs. Ileal intraluminal PYY concentrations were significantly greater than those concentrations seen in the peripheral blood. A significant rise in ileal intraluminal PYY was seen only after the glucose meal without concomitant changes in circulating PYY. These findings document independent release mechanisms of PYY into the circulation and ileal lumen of the awake dog. We believe that the release of PYY into the canine circulation and into the ileal lumen after feeding may be dependent on the type of nutrient ingested and may serve to regulate the diverse physiologic roles of PYY in the gastrointestinal tract.

Fibronectin coating of an experimental PTFE vascular prosthesis

Because of the known inefficiency of endothelial cell seeding of vascular prostheses, some investigators have pretreated their grafts with fibronectin in an effort to increase endothelial cell adhesion. However, the most efficient method of coating vascular prostheses with fibronectin has not yet been determined and more importantly, the durability of this coating following restoration of pulsatile perfusion has not yet been established. This experiment measured the adsorption of fibronectin which was perfused at three different concentrations through an experimental, highly porous PTFE vascular graft. Most efficient coating of the prosthetic appeared to occur with a 250 mg/ml concentration of fibronectin perfused through the graft for 60 min. Grafts so prepared were subsequently studied in a pulsatile perfusion system which mimicked the canine circulation. Fifteen additional fibronectin-coated grafts were also interposed in the canine carotid artery. Flow was restored, grafts were removed at varying intervals, and fibronectin disappearance was measured. In both the in vitro and in vivo models, three distinct phases of fibronectin dissociation were seen: an initial rapid loss which resulted in an average 30.5% retention of the initial concentration of fibronectin following 30 min of perfusion; a second phase, which occurred from 30 to 120 min, demonstrated an average 2.5% loss of fibronectin per hour; and a final phase, from 2 to 24 hr, during which no significant additional loss of fibronectin was seen. This study describes a new method by which PTFE vascular prostheses can be effectively coated by perfusing them with physiologic concentrations of fibronectin and demonstrates that the resulting bond between fibronectin and prosthetic is stable following restoration of pulsatile flow.

Measurement of peak systolic elastance in intact canine circulation with servo pump.

Peak systolic elastance (Emax) was measured in the intact canine circulation by means of a new experimental technique. In this technique the heart is isolated from the circulation during a single systole and subjected to controlled ventricular loads. An electropneumatic aortic occluder is used to isolate the ventricle, and a servo-controlled syringe pump is used to control the ventricular load. Because the experimental load is applied for a single heartbeat only, ventricular function can be measured without the interference of regulatory feedback mechanisms. In eight dogs, weighing 17-42 kg, the relationship between changes in endsystolic pressure and volume was determined from the single-beat application of purely compliant loads. The end-systolic relations were linear, and their slope, Emax, was inversely related to weight. The observed relation between Emax and body weight allows comparisons to be made between different preparations in which Emax has been determined. Values of Emax obtained from the single-beat preparation were found to be 27-74% above those reported in isolated heart preparations and nearly identical to those reported for in vivo or denervated in situ preparations.

Microsphere passage through intestinal circulation: via shunts or capillaries?

Significant quantities of 9-micron microspheres (20-30%) are not trapped in the intestine following intracardiac or intra-arterial injection, but reach venous blood. Some investigators propose that the passage of 9-micron spheres measures blood flow through noncapillary connections. Because frequency distributions of intestinal capillary diameters and 9-micron spheres overlap, microspheres could simply pass through capillaries. Therefore, we developed simple probabilistic models to predict both the size distribution and the percentage of injected spheres [9 +/- 1 (SD) micron] that should appear in venous blood. Chief assumptions in models are that microsphere delivery and sphere diameter are independent and that microspheres pass through capillaries of equal or larger size. The passage predicted by the models was consistent with values in canine intestinal circulation, demonstrating that passage through capillaries [7.38 +/- 1.4 (SD) micron] adequately accounts for spheres in venous blood. Because the diameters of nominal 9-micron spheres were distributed too narrowly to show a marked sieving effect on passage through the intestinal circulation, we also injected microspheres varying from 5 to 20 micron in diameter. This mixture demonstrated a marked sieving effect. The predicted frequency distribution for microsphere diameters in venous blood agreed with the observed distribution. Our models demonstrate that the passage of 9-micron spheres through capillaries, rather than through "shunts," adequately accounts for the appearance of spheres in venous blood and suggests that the frequency distribution of venous microspheres can provide an in vivo method for estimating the frequency distribution of intestinal capillary diameters.

歯髄電気刺激の脳血流, 脳代謝に及ぼす影響

歯髄電気刺激の脳血流, 脳代謝に及ぼす影響

Myocardial blood flow and function during gradual coronary occlusion in awake dog.

The purpose of this study was to correlate dimensions and transmural blood flow in a segment of myocardium supplied by a coronary artery undergoing gradual closure. Nine adult dogs were instrumented with an electromagnetic flow probe, pneumatic occluder, and an ameroid constrictor on the circumflex coronary artery. Ultrasonic crystals were implanted 10 mm from the external surface of the left ventricle in a segment perfused by the circumflex artery. Regional blood flow was determined with 7- to 10-microns radiolabeled microspheres. Data were collected in the awake state at rest before closure began (control), during partial closure, and immediately after total closure. Seven of the nine animals were studied after occlusion during treadmill exercise. During both partial and total closure at rest the rate and extent of systolic shortening as well as the transmural blood flow were unchanged from control. During treadmill exercise mean flow increased. However, flow was redistributed away from the inner two layers causing deterioration in both the rate and extent of shortening of this segment. These data suggest that, although regional myocardial function and flow can be maintained at rest by the immature canine collateral circulation, these parameters are impaired markedly during augmented flow with exercise.

Cardiovascular responses to PGI2 (prostacyclin) in the dog.

Arachidonic acid (AA) produces characteristic hemodynamic changes in the canine circulation. These responses are blocked by prostaglandin (PG) synthetase inhibitors, indicating that AA and its nonprostanoic metabolites are not vasoactive. The hemodynamic effects of the cyclic endoperoxides, thromboxanes, and PGD2, PG2, and PGF2a differ from those produced by AA. PGI2, a newly identified product of AA, is reported to relax arterial strips. However, its cardiac and systemic effects are unknown. In 12 open-chest, anesthetized dogs, PGI2 (0.25-5.0 microgram/kg) produced a dose-related decrease in systemic arterial pressure (BP) and myocardial contractile force (MCF). In five left ventricular bypass preparations, PGI2 produced only a slight decrease in MCF at all doses, whereas the BP decreases were parallel to those in the intact preparation. AA, PGD2, PGE2, and PGI2 were administered in random order by bolus intravenous injections in approximately equidepressor doses to intact dogs. BP fell with each agent (AA, 300 microgram/kg, -25 percent; PGD2, 5 microgram/kg, -26 percent; PGE2, 5 microgram/kg, -26 percent PGI2, 0.5 microgram/kg, -26 percent). The vasodepressor action of PGI2 was approximately 10 times greater than that of PGD2 and PGE2. Pulmonary arterial pressure (PAP) rose significantly with PGD2 and PGE2 (AA, -1 percent; PGD2 +66 percent; PGE2, +20 percent; PGI2, -1 percent). Only PGE2 had a significant effect on MCF (AA, +7 percent; PGD2, +5 percent, PGE2, +20 percent; PGI2, -0.3). At this dose, PGI2 resembles AA in that it has little effect on either PAP or MCF. Of all known AA metabolites the response to PGI2 most closely resembles that of exogenous AA in the dog.

Adrenergic mechanisms in canine gastric circulation

The effects of adrenergic stimulation and blockade on the gastric circulation were studied in anesthetized dogs. Blood flow through the right and left gastric artery was measured electromagnetically. Norepinephrine and isoproterenol were injected intra-arterially and intravenously before and after alpha- and beta-adrenergic blockade. Isoproterenol caused vasodilation of both right and left gastric circulations and this effect was attenuated by beta blockade. Epinephrine and norepinephrine induced constriction followed by dilation in both circulations. The constrictor components were attenuated or abolished by alpha-adrenergic blockade and the dilator components were attenuated by beta-adrenergic blockade. The right and left gastric vascular beds demonstrated quantitatively different responses to the same dose of each adrenergic amine. The left gastric circulation had a greater vasodilator response than did the right gastric circulation. These data support the classical concepts that epinephrine and norepinephrine are "mixed" adrenergic agonists and isoproterenol is a "pure" beta-adrenerigic agonist. The data further suggest that there is a differential in beta-adrenergic receptor distribution with the left gastric vasculature demonstrating greater dilator responses than the right.

Adrenergic mechanisms in canine hindlimb circulation.

Epinephrine dilates or constricts canine hindlimb vasculature depending upon the dose; however, norepinephrine causes only vasoconstriction. Since both agonists interact with alpha and beta adrenergic receptors, one might expect norepinephrine to cause vasodilation under appropriate circumstances. This study evaluates the actions of intra-arterial injections (10−3 to 10° [og kg−1) of norepinephrine, epinephrine, isoproterenol, and phenylephrine on canine hindlimb circulation. Femoral arterial blood flow was measured with an electromagnetic flowmeter. Arterial and central venous pressures were recorded. Low doses of each agonist significantly increased flow (P < 0-05). Higher doses of the amines all reduced flow except isoproterenol which remained a dilator. Alpha adrenergic blockade converted all constrictor responses to vasodilation except that observed with the highest dose of norepinephrine. Beta adrenergic blockade, in general, abolished dilator responses and augmented constrictor responses. These data indicate that the vasoactive properties of these adrenergic amines are dose dependent. The classical alpha adrenergic agonists are all vasodilators at a low dose due to their interaction with beta adrenergic receptors.

Generation of a carboxyl-terminal fragment of bovine parathyroid hormone by canine renal plasma membranes

Incubation of 125I-bovine parathyroid hormone with purified canine renal plasma membranes resulted in the generation of a carboxyl-terminal fragment of the labelled hormone. This fragment appears similar to that found previously in the human and bovine circulation and in the canine circulation after infusion of intact bovine parathyroid hormone.

Pacing induced maximal stress in the normal canine circulation.

Lactate production by the myocardium does not occur during pacing induced tachycardia in a normal circulation unless coronary perfusion pressure drops to unphysiological levels concomitant with maximum oxygen extraction.