André Simard

Tracer Oxygen Distribution Is Barrier-Limited in the Cerebral Microcirculation

The kinetics of tracer oxygen distribution in the brain microcirculation of the awake dog were investigated with the multiple indicator dilution technique. A bolus containing 51Cr-labeled red blood cells, previously totally desaturated and then resaturated with [18O]2 (oxygen), 125I-albumin, 22Na, and [3H]water, was injected into the carotid artery, and serial anaerobic blood samples were collected from the sagittal sinus over the next 30 seconds. The outflow recovery curves were analyzed with a distributed-in-space two-barrier model for water and a one-barrier model for oxygen. The analysis provided an estimate of flow per gram brain weight as well as estimates for the tracer water and oxygen rate constants for blood-to-brain exchange and tracer oxygen parenchymal sequestration. Flow to tissue was found to vary between different animals, in concert with parallel changes in oxygen consumption. The 18O2 outflow curves showed an early peak, coincident with and more than half the magnitude of its vascular reference curve (labeled red blood cells), whereas the [3H]water curve increased abruptly to a low-in-magnitude curve at low flow values and to a small early peak at high flow values. Analysis indicates that the transfers of both 18O2 and [3H]water indicators from blood to brain are barrier-limited, with the former highly so because of the large red blood cell capacity for oxygen, and that the proportion of the tracer oxygen returning to the circulation from tissue is a small fraction of the total tracer emerging at the outflow.

Kinetics of endothelin-1 binding in the dog liver microcirculation in vivo

Endothelin-1 (ET-1) is a 21-amino acid peptide produced by vascular endothelial cells that acts as a potent constrictor of hepatic sinusoids. Hepatic binding of tracer 125I-labeled ET-1 was investigated in anesthetized dogs with the multiple-indicator dilution technique with simultaneous measurements of unlabeled immunoreactive ET-1 plasma levels. Despite 80% binding to albumin, tracer 125I-ET-1 was avidly extracted by the liver, with only 15 ± 6% of the peptide surviving passage through the organ. Exchange of ET-1 between plasma and binding sites, probably located on the surface of liver cells, was quantitatively described by a barrier-limited, space-distributed variable transit time model. Reversible and irreversible parallel binding sites were found. Reversible and irreversible plasma clearances of unbound 125I-ET-1 were 0.084 ± 0.033 ml ⋅ s-1 ⋅ g liver-1 and 0.17 ± 0.09 ml ⋅ s-1 ⋅ g liver-1, respectively, and the dissociation rate constant for reversible binding was 0.24 ± 0.12 s-1. The specific ETA receptor antagonist BMS-182874 did not modify binding to either site. The nonspecific ETA/ETBantagonist LU-224332 dose-dependently reduced irreversible binding only. ET-1 levels in the hepatic vein were significantly lower than in the portal vein but were not different from those in the hepatic artery. The ratio between hepatic vein and portal vein levels (0.64 ± 0.31) was considerably higher than survival fractions, suggesting a substantial simultaneous release of newly synthesized or stored ET-1 by the liver. These results demonstrate both substantial clearance and production of ET-1 by the intact liver. Hepatic ET-1 clearance is mediated by the ETB receptor, with the presence of reversible, nonspecific ET-1 binding at the liver surface.

Reduction in hepatic endothelin-1 clearance in cirrhosis

Circulating endothelin-1 (ET-1) levels are increased in cirrhosis. The liver is an important site for circulating ET-1 clearance through the ET(B) receptor. We evaluated ET-1 kinetics in cirrhosis to determine if a reduced liver clearance contributes to this process. Cirrhosis was induced by carbon tetrachloride in rats. Hepatic ET-1 clearance was measured in isolated perfused livers using the single bolus multiple indicator-dilution technique. Plasma ET-1 levels doubled in cirrhosis from 0.49+/-0.04 fmol/ml (mean+/-S.E.M.) to 1.0+/-0.18 fmol/ml ( P <0.01). Liver ET-1 extraction was reduced from 81+/-1% (mean+/-S.E.M.) in controls to 50+/-6% in cirrhosis ( P <0.01). Kinetic modelling revealed a major irreversible binding site for ET-1 that is blocked by the selective ET(B) receptor antagonist BQ788 and a minor non-specific reversible binding site that cannot be blocked with BQ788 or the selective ET(A) antagonist BQ123. Reduced hepatic clearance correlated with the biochemical markers of cirrhosis, portal vein perfusion pressure ( r =-0.457; P <0.001) and the increase in ET-1 levels ( r =-0.462; P =0.002). Immunohistofluorescence with specific anti-(ET(B) receptor) antibodies revealed a preponderant distribution of ET(B) receptors on hepatic stellate cells, which was increased in cirrhosis. We conclude that cirrhosis reduces ET-1 clearance probably by capillarization of hepatic sinusoids and reduced access to ET(B) receptors. This relates to the severity of cirrhosis and may contribute to the increase in circulating ET-1 levels.

Effect of sternotomy and extracorporeal circulation on pulmonary neutrophil kinetics in pigs

AbstractPulmonary margination of neutrophils may contribute to lung damage after extracorporeal circulation for cardiac surgery. We evaluated single–pass pulmonary neutrophil kinetics using the multiple indicator–dilution technique in control pigs (n = 10), after sternotomy alone (sterno, n = 10) or after 30 min of observation following a period of 90 min extracorporeal circulation (n = 7). Blood neutrophils increased in the control and sterno groups (p < 0.05) but remained unchanged in the extracorporeal circulation group. The transfer coefficient for neutrophil margination from the circulating to the lung–marginated pool (kc–m) and pulmonary neutrophil clearance (Clc–m) were similar between the three groups. There was an inverse correlation between kc–m and the degree of lung tissue perfusion evaluated from the tracer–accessible extravascular lung water (r = –0.54, p < 0.01). There was no arterio–venous gradient of neutrophils in any of the groups, suggesting a dynamic equilibrium of the margination/demargination processes.We conclude that extracorporeal circulation does not significantly modify single pass pulmonary neutrophil kinetics 30 min after reperfusion. The rate of neutrophil margination to the tracer–accessible lung tissue suggests that lung tissue de–recruitment is associated with increased neutrophil margination.

In-vivo measurement of coronary Angiotension-Converting Enzyme (ACE) activity in humans

The relationship between atherosclerosis and ACE activity is unclear.We tried to establish whether the endothelium bound coronary ACE activity correlates with coronary atherosclerotic burden. Methods. A single bolus indicator-dilution experiment was performed in 10 patients. Trace amounts of the specific ACE substrate 3H-benzoyI-Phe-Ala-Pro (3H-BPAP) and of the myocardial interstitial space reference 14C-sucrose were injected in the left main coronary artery. Serial samples were obtained from the coronary sinus. Plasma concentrations of residual substrate and end-product (3H-BP) were measured. Mean transit time of each tracer and myocardial plasma flow (~) were determined. The curves were modelled to compute the rate constant for 3H-BPAP hydrolysis by ACE (Kh). Coronary angiograms were scored for atherosclerotic burden according to 2 previously described techniques to obtain the plaque area index (PAl)and Sullivans extent score (SES). Spearman correlation test was used to calculate p values. Results. The atherosclerotic burden extent scores correlated well with each other (r= 0.82; p=0.002), There was a weak correlation between K h and measures of atherosclerotic burden (r =0.4; p=0.22). Conclusion. The indicator-dilution technique is a safe tool to quantify endothelium bound coronary ACE activity. The weak correlation between endothelium bound coronary ACE activity and extent scores suggests that tissue ACE may play a more important pathophysiologic role in the disease process, March 6, 2002 ABSTRACTS Hypertension, Vascular Disease, and Prevention 267A