Mechanism of glucocorticoid-mediated reversal of inhibition of Cl(-)/HCO(-)(3) exchange during chronic ileitis

In the normal ileum, coupled NaCl absorption occurs via the dual operation of Na(+)/H(+) and Cl(-)/HCO(-)(3) exchange on the brush-border membrane (BBM) of villus cells. In a rabbit model of chronic small intestinal inflammation we determined the cellular mechanism of inhibition of NaCl absorption and the effect of steroids on this inhibition. Cl(-)/HCO(-)(3) but not Na(+)/H(+) exchange was reduced in the BBM of villus cells during chronic ileitis. Cl(-)/HCO(-)(3) exchange was inhibited secondary to a decrease in the affinity for Cl(-) rather than an alteration in the maximal rate of uptake of Cl(-) (V(max)). Methylprednisolone (MP) stimulated Cl(-)/HCO(-)(3) exchange in the normal ileum by increasing the V(max) of Cl(-) uptake rather than altering affinity for Cl(-). MP reversed the inhibition of Cl(-)/HCO(-)(3) exchange in rabbits with chronic ileitis. However, MP alleviated the Cl(-)/HCO(-)(3) exchange inhibition by restoring the affinity for Cl(-) rather than altering the V(max) of Cl(-) uptake. These data suggest that glucocorticoids mediate the alleviation of Cl(-)/HCO(-)(3) exchange inhibition in chronically inflamed ileum by reversing the same mechanism that was responsible for inhibition of this transporter rather than exerting a direct effect on the transporter itself, as was the case in normal ileum.

Effect of chronic inflammation on ileal short-chain fatty acid/bicarbonate exchange

Short-chain fatty acids (SCFA) have been demonstrated to at least partially ameliorate chronic intestinal inflammation. However, whether and how intestinal SCFA absorption may be altered during chronic intestinal inflammation is unknown. A rabbit model of chronic ileitis produced by coccidia was used to determine the effect of chronic inflammation on ileal SCFA/HCO(-)(3) exchange. SCFA/HCO(-)(3) exchange was present in the brush-border membrane (BBM) of villus but not crypt cells from normal rabbit ileum. An anion-exchange inhibitor, DIDS, significantly inhibited SCFA/HCO(-)(3) exchange. Extravesicular Cl(-) did not alter the uptake of SCFA, suggesting that SCFA/HCO(-)(3) exchange is a transport process distinct from Cl(-)/HCO(-)(3) exchange. In chronically inflamed ileum, SCFA/HCO(-)(3) exchange was also present only in BBM of villus cells. The exchanger was sensitive to DIDS and was unaffected by extravesicular Cl(-). However, SCFA/HCO(-)(3) exchange was significantly reduced in villus cell BBM vesicles (BBMV) from chronically inflamed ileum. Kinetic studies demonstrated that the maximal rate of uptake of SCFA, but not the affinity for SCFA, was reduced in chronically inflamed rabbit ileum. These data demonstrate that a distinct SCFA/HCO(-)(3) exchange is present on BBMV of villus but not crypt cells in normal rabbit ileum. SCFA/HCO(-)(3) exchange is inhibited in chronically inflamed rabbit ileum. The mechanism of inhibition is most likely secondary to a reduction in transporter numbers rather than altered affinity for SCFA.

Corticosteroids reverse the inhibition of Na-glucose cotransport in the chronically inflamed rabbit ileum

In a rabbit model of chronic ileal inflammation, we previously demonstrated inhibition of Na-glucose cotransport (SGLT-1). The mechanism of inhibition was secondary to a decrease in the number of cotransporters and not solely secondary to an inhibition of Na-K-ATPase or altered affinity for glucose. In this study, we determined the effect of methylprednisolone (MP) on SGLT-1 inhibition during chronic ileitis. Treatment with MP almost completely reversed the reduction in SGLT-1 in villus cells from the chronically inflamed ileum. MP also reversed the decrease in Na-K-ATPase activity in villus cells during chronic ileitis. However, MP treatment reversed the SGLT-1 inhibition in villus cell brush-border membrane vesicles from the inflamed ileum, which suggested an effect of MP at the level of the cotransporter itself. Kinetic studies demonstrated that the reversal of SGLT-1 inhibition by MP was secondary to an increase in the maximal velocity for glucose without a change in the affinity. Analysis of immunoreactive protein levels of the cotransporter demonstrated a restoration of the cotransporter numbers after MP treatment in the chronically inflamed ileum. Thus MP treatment alleviates SGLT-1 inhibition in the chronically inflamed ileum by increasing the number of cotransporters and not solely secondary to enhancing the activity of Na-K-ATPase or by altering the affinity for glucose.

Mechanism of inhibition of Na+-bile acid cotransport during chronic ileal inflammation in rabbits

In the chronically inflamed ileum, unique mechanisms of alteration of similar transport processes suggest regulation by different immune-inflammatory mediator pathways. In a rabbit model of chronic ileitis, we previously demonstrated that Na+-glucose cotransport was inhibited by a decrease in the cotransporter numbers, whereas Na+-amino acid cotransport was inhibited by a decrease in the affinity for the amino acid. In this study, we demonstrated that Na+-bile acid cotransport was reduced in villus cells from the chronically inflamed ileum. In villus cell brush-border membrane vesicles from the chronically inflamed ileum, Na+-bile acid cotransport was reduced as well, suggesting a direct effect at the cotransporter itself. Kinetic studies demonstrated that Na+-bile acid cotransport was inhibited by both a decrease in the affinity as well as a decrease in the maximal rate of uptake of the bile acid. Analysis of steady-state mRNA and immunoreactive protein levels of the Na+-bile acid cotransporter also demonstrated some reduction during chronic ileitis. Thus, in the chronically inflamed ileum, the mechanisms of inhibition of Na+-glucose, Na+-amino acid, and Na+-bile acid cotransport are different. These data suggest that different cotransporters are uniquely altered either secondary to their intrinsic differences or by different immune-inflammatory mediators during chronic ileitis.

Unique mechanism of inhibition of Na+-amino acid cotransport during chronic ileal inflammation

In the chronically inflamed ileum, unique mechanisms of alteration of transport processes suggest regulation by different immune-inflammatory mediator pathways. We previously demonstrated that Na+-glucose cotransport in the chronically inflamed ileum was inhibited by a decrease in cotransporter number without a change in glucose affinity. The aim of this study was to determine the alterations in Na+-amino acid cotransport in chronically inflamed ileum produced by coccidial infection in rabbits. [3H]alanine uptake was performed in cells and vesicles by rapid filtration. In villus cells from chronically inflamed ileum, Na+-K+-ATPase was reduced 50% and Na+-alanine cotransport was also reduced (5.8 +/- 1.2 in normal and 1.4 +/- 0.5 nmol/mg protein in inflamed; n = 6, P < 0.05). [3H]alanine uptake in brush-border membrane vesicles was reduced in chronically inflamed ileum (73.2 +/- 1.2 in normal and 21.5 +/- 3.2 pmol/mg protein in inflamed; n = 3, P < 0.05), suggesting a direct effect on the cotransporter itself. Na+-amino acid cotransport in chronically inflamed ileum was inhibited by a decrease in affinity without a change in the maximal rate of uptake, and unaltered steady-state mRNA levels also suggested that the number of cotransporters was unchanged. Thus the mechanisms of inhibition of Na+-amino acid cotransport and Na+-glucose cotransport in chronically inflamed ileum are different. These observations suggest that different immune-inflammatory mediators may regulate different transport pathways during chronic ileitis.

Pulmonary infections in hospitalized patients with cirrhosis

We have further characterized pulmonary infections by bronchoalveolar lavage in hospitalized patients with cirrhosis. Sixty-seven consecutive patients admitted to the Ohio State University Medical Center from 1992 to 1995 with liver disease who underwent bronchoscopy with bronchoalveolar lavage were identified. Twenty-one patients with cirrhosis and pneumonia were further analyzed. During the same period, we consecutively identified 23 patients without liver disease or immunosuppression, 19 patients with HIV infections, and 30 patients with cancer or pharmacologic immunosuppression who had bronchoscopy with bronchoalveolar lavage for pneumonia. These groups were included in these analyses as a control and immunosuppressed controls, respectively. Bronchoscopy isolated respiratory pathogens in 16 patients (76.2%) with cirrhosis and 6 patients (26.1%) in the control group (p = 0.002). Fungal organisms were most commonly found in patients with cirrhosis although several patients had more than one organism identified. The control group had mostly bacterial pathogens; the immunosuppressed controls were commonly infected with opportunistic organisms. Six (85.7%) of 7 patients with cirrhosis and fungal pneumonia died of their respiratory illness. Hospitalized patients with cirrhosis commonly have opportunistic pulmonary infections; diagnostic bronchoscopy and empiric antifungal therapy should be considered in those who do not respond to antibiotics.

Mechanism of inhibition of Na+-glucose cotransport in the chronically inflamed rabbit ileum

In a rabbit model of chronic ileal inflammation, we previously demonstrated that coupled NaCl absorption was reduced because of an inhibition of Cl-/HCO3- but not Na+/H+ exchange on the brush-border membrane (BBM) of villus cells. In this study we determined the alterations in Na+-stimulated glucose [Na+-O-methyl-D-glucose (Na+-OMG)] absorption during chronic ileitis. Na+-OMG uptake was reduced in villus cells from the chronically inflamed ileum. Na+-K+-adenosinetriphosphatase (ATPase), which provides the favorable Na+ gradient for this cotransporter in intact cells, was found to be reduced also. However, in villus cell BBM vesicles from the inflamed ileum Na+-OMG uptake was reduced as well, suggesting an effect at the level of the cotransporter itself. Kinetic studies demonstrated that Na+-OMG uptake in the inflamed ileum was inhibited by a decrease in the maximal rate of uptake for OMG without a change in the affinity. Analysis of steady-state mRNA and immunoreactive protein levels of this cotransporter demonstrates reduced expression. Thus Na+-glucose cotransport was inhibited in the chronically inflamed ileum, and the inhibition was secondary to a decrease in the number of cotransporters and not solely secondary to an inhibition of Na+-K--ATPase or altered affinity for glucose.

Effect of chronic inflammation on electrolyte transport in rabbit ileal villus and crypt cells

The effect of chronic inflammation on electrolyte transport in rabbit ileal villus and crypt cells was determined with the use of a rabbit model of chronic ileitis. In both cells, Na+/H+ exchange was monitored by following recovery from an acid load, and Cl-/HCO3- exchange was monitored by following recovery from an alkaline load. In villus cells, recovery from an acid load was not affected; however, recovery from an alkaline load was slowed. These data suggest that chronic inflammation inhibits Cl-/HCO3- exchange in villus cells. In contrast, in crypt cells, recovery from an alkaline load was unaffected, whereas recovery from an acid load was accelerated. These data suggest that chronic inflammation stimulates Na+/H+ exchange in crypt cells. Inhibition of Cl-/HCO3- exchange in villus cells would be expected to inhibit coupled NaCl absorption, which occurs by the coupling of brush-border membrane (BBM) Na+/H+ and Cl-/HCO3- exchange. Stimulation of Na+/H+ exchange in crypt cells, known to be present only on the basolateral membrane, alkalinizes the cell. This alkalinization may stimulate BBM Cl-/HCO3- exchange, resulting in HCO3- secretion. Thus these unique alterations in transporter activity suggest that different endogenous immune-inflammatory mediators may have differing effects on specific transporters in villus and crypt cells in the chronically inflamed ileum.

Radiologic and manometric study of the gastroesophageal junction in dysphagia aortica

This article reports radiologic and manometric findings in dysphagia aortica, with particular attention to the gastroesophageal (GE) junction. Records of three patients, ages 70-78 years, with clinical/radiologic dysphagia aortica were compared to those in control groups. Subsequently, manometric findings of such vascular compression were sought in 10 consecutive patients > or = 65 years old with dysphagia. The three patients with dysphagia aortica had radiologic/endoscopic evidence for compression at the GE junction. Manometric studies, performed in two of them, showed evidence at the GE junction for superimposed rhythmic contractions at 60-72/min (maximum amplitudes, 35 mm Hg), consistent with vascular compression. One patient had marked elevation of "sphincter" pressure to 110 mm Hg and "poor relaxation" of the "sphincter." One of 10 patients with dysphagia had rhythmic contractions of 20 mm Hg; a barium study subsequently showed aortic compression at the GE junction. There are characteristic manometric findings that may help to identify symptomatic vascular compression of the esophagus in the elderly.

Mechanism of intestinal absorption. Effect of clonidine on rabbit ileal villus and crypt cells

In intact tissue studies, intestinal absorptogogues stimulate NaCl absorption that occurs via the dual operation of Na:H and Cl:HCO3 exchanges on the brush border membrane (BBM) of villus cells. To determine the cellular mechanism of action of an intestinal absorptogogue, the effect of clonidine was determined on Na:H and Cl:HCO3 exchange in rabbit ileal villus and crypt cells. Using 2,7-bis(carboxyethyl)-5,6-carboxy-fluorescein we have previously shown that recovery from an acid load occurs via Na:H exchange, whereas recovery from an alkaline load occurs via Cl:HCO3 exchange in both cells. In villus cells, the rate of recovery from a propionate-induced alkaline load was not altered by clonidine. However, clonidine stimulated recovery from an acid load induced by NH4Cl, Na removal, or amiloride. These data suggest that clonidine stimulates Na:H exchange in villus cells. In crypt cells, the rate of recovery from a propionate-induced alkaline load was also not altered by clonidine. However, in crypt cells, unlike the villus cells, clonidine inhibited recovery from an acid load induced by NH4Cl, Na removal, or amiloride. These data suggest that clonidine inhibits Na:H exchange in crypt cells. Stimulation of Na:H exchange on the BBM of villus cells would be expected to stimulate coupled NaCl absorption (which occurs by coupling of Na:H and Cl:HCO3 exchange). Inhibition of Na:H in crypt cells, known to be present only on the basolateral membrane, will acidify the cell and may inhibit Cl:HCO3 exchange on the BBM, resulting in the inhibition of HCO3 secretion.

Selective coronary angiography in intracardiac tumors

Cardiac catheterization and selective coronary angiography were performed in ten patients with intra-cardiac tumors [left atrial myxoma (7), right atrial myxoma (1), angiosarcoma of right heart (1), and right ventricular tumor (1)]. The patient with angiosarcoma had characteristic hemodynamics suggesting cardiac compression. The coronary arteriographic finding included: neovascularization (8); filling defect due to emboli (1); and displacement of coronary artery (1). Tumor neovascularization from branches of the left circumflex or right coronary artery was invariably observed in patients with myxoma. We conclude that invasive studies are safe, provide additional information of academic interest, and occasionally aid in the diagnosis.

Selective coronary angiography via antegrade venous route in congenital cyanotic heart disease

An antegrade venous technique was utilised to perform selective coronary angiography in cyanotic infants and children. The procedure was successful in 88% (37/42) cases and excellent quality angiograms were recorded. The importance of proper catheter selection and details of the technique are discussed.

Value of haemodynamic studies during pericardiocentesis in a cardiac catheterisation laboratory

Pericardiocentesis was performed in 20 patients using a 7F diagnostic catheter in a cardiac catheterisation laboratory under fluoroscopic control, with haemodynamic and electrocardiographic monitoring. This technique offers several advantages over the bedside technique which utilises a sharp needle. The technique utilised for percutaneous aspiration is safe and simple and allows complete drainage of fluid. The haemodynamic monitoring before and after drainage aids in detecting cases of effusive constriction.

Mechanism of intestinal secretion: effect of cyclic AMP on rabbit ileal crypt and villus cells

Cyclic AMP-dependent secretagogues such as cholera toxin inhibit the coupled absorption of Na+ and Cl- and stimulate the secretion of HCO3- and Cl- in the ileum. Aside from Cl- secretion, little is known about the mechanism of these cyclic AMP-mediated effects. We therefore determined the effect of forskolin, an agent known to increase intracellular cyclic AMP by stimulation of adenylyl cyclase, on Na+/H+ and Cl-/HCO3- exchange in isolated crypt and villus cells from rabbit ileum. Forskolin increased cyclic AMP in the villus cells and decreased intracellular pH. The effect of forskolin on pH in villus cells was HCO3- independent, Na+ dependent, and amiloride sensitive. Further, the rate of recovery from an acid load was decreased by forskolin. These data suggest that increasing cyclic AMP inhibits Na+/H+ exchange in villus cells. In crypt cells also, forskolin increased cyclic AMP; however, forskolin increased intracellular pH in these cells. The effect of forskolin in crypt cells was also HCO3- independent, Na+ dependent, and amiloride sensitive. However, the rate of recovery from an acid load was increased by forskolin, the opposite effect of that seen in villus cells. These data suggest that increasing cyclic AMP in crypt cells stimulates Na+/H+ exchange. Inhibition of Na+/H+ exchange on the brush border membrane in villus cells would be expected to inhibit coupled NaCl absorption (which occurs by coupling of Na+/H+ and Cl-/HCO3- exchange). Stimulation of Na+/H+ exchange in crypt cells, present only on the basolateral membrane, alkalinizes the cell, which would be expected to stimulate HCO3- secretion by stimulating the Cl-/HCO3- exchanger on the brush border membrane. Thus, these results provide a mechanism for some of the previously unexplained in vivo and in vitro effects of cyclic AMP on ileal electrolyte transport.

Concurrent double balloon valvotomy for combined rheumatic mitral and tricuspid stenosis

We expanded the application of percutaneous balloon valvotomy (PBV) to 4 adults (age 14 to 30 years, average 22.2 years) with combined rheumatic mitral and tricuspid stenosis. Double balloon dilatation reduced the transmitral gradient from 17.36 +/- 3.54 to 5.52 +/- 0.89 (P less than 0.025) and transtricuspid gradient from 12.65 +/- 2.67 to 3.67 +/- 0.95 (P less than 0.025). Mitral and tricuspid valve area increased from 0.73 +/- 0.20 to 2.57 +/- 0.67 (P less than 0.005) and from 0.77 +/- 0.24 to 2.67 +/- 0.24 cm2 (P less than 0.005), respectively. The procedures were well tolerated, with no significant increase in valvular regurgitation or left to right shunt across the atrial septum. The excellent symptomatic and haemodynamic benefits are sustained at 3-24 months follow-up. It is concluded that combined dilatation of stenotic valves by double balloon technique can emerge as an alternative to surgery in selected patients with polyvalvar rheumatic heart disease.

pH regulation in ileum: Na(+)-H+ and Cl(-)-HCO3- exchange in isolated crypt and villus cells

Current evidence suggests that intestinal crypt and villus cells have different functions in electrolyte transport. To study the regulation of transporters, we isolated and separated these two cell types. This was accomplished by sequential collection of enterocytes from rabbit ileal loops incubated with buffered solutions of calcium chelators. Alkaline phosphatase and thymidine kinase activity, sodium-glucose cotransport, and morphological criteria were used to determine cell separation. Cell viability was evaluated with trypan blue exclusion, leucine incorporation into protein, and morphological features. The role of Na(+)-H+ and Cl(-)-HCO3- exchange in the regulation of intracellular pH was analyzed using an intracellular pH sensitive dye, BCECF. Removal of external Na+ or the addition of amiloride resulted in acidification of both crypt and villus cells. Removal of Cl- or the addition of DIDS resulted in alkalinization of both cell types. The cells could be acidified with NH4Cl, and recovery from this acid load was dependent on Na+ and inhibited by amiloride. Similarly, the cells could be alkalinized with propionate and recovery was Cl- dependent and DIDS sensitive. These data are consistent with the presence of Na(+)-H+ and Cl(-)-HCO3- exchange in both crypt and villus cells. Both exchanges appear to be involved in the regulation of basal pH as well as in recovery from alterations in intracellular pH. Having demonstrated the presence of Na(+)-H+ and Cl(-)-HCO3- exchange activity in both crypt and villus cells, we can now use these cells to determine the regulation of these exchangers by intracellular second messengers.

Mechanism of intestinal secretion. Effect of serotonin on rabbit ileal crypt and villus cells

To determine the mechanism of action of an intestinal secretagogue, serotonin, we have isolated crypt and villus cells and demonstrated Na:H and Cl:HCO3 exchange activity using the intracellular pH-sensitive fluorescent dye, 2,7-bis (carboxy-ethyl)-5,6-carboxy-fluorescein. Serotonin alkalinized both crypt and villus cells. Alkalinization in villus cells was HCO3 dependent and Na independent. In contrast, alkalinization in crypt cells was HCO3 independent and Na dependent. In villus cells, recovery from an alkaline load induced by Cl removal, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid or propionate pulse, known to occur via the Cl:HCO3 exchange, is inhibited by serotonin. In contrast, in crypt cells, recovery from an acid load induced by Na removal, amiloride and NH4Cl pulse, known to occur via Na:H exchange, is stimulated by serotonin. These data suggest that serotonin is inhibiting Cl:HCO3 exchange in villus cells and stimulating Na:H exchange in crypt cells. These effects of serotonin would be expected to inhibit coupled Na and Cl absorption by villus cells and stimulate HCO3 secretion by crypt cells in the intact ileum.

Recurrent myocardial infarction in a young patient with normal coronary arteries

A 33 year old patient with two acute myocardial ischaemic episodes (one, a probable inferior wall infarction and the second, a definitive Q wave anterior wall infarction) within a span of 6 months, with angiographically documented normal coronary arteries, is reported. Coronary artery spasm studies performed using ergonovine during the second coronary angiography, could not provoke spasm. The mechanism of recurrent infarction remains unclear.

[3H]Methotrexate loss from the rat brain following enhanced uptake by osmotic opening of the blood-brain barrier

Right brain regions of anesthetized rats were loaded with [3,5,7-3H]methotrexate ([3H]MTX) or with [14C]sucrose by infusing the tracers into the right carotid artery, after the blood-brain barrier had been opened by right carotid infusion of a hypertonic arabinose solution. During the 6 hr following the procedure, the [3H]MTX concentration in 7 right-sided brain regions, when normalized to the plasma concentration integral during tracer infusion, fell, with an average half-time of 4.8 hr as compared to less than 20 min for the initial rate of loss [14C]sucrose. Right-left brain concentration differences 3 hr after treatment were statistically significant (p less than 0.05) for [3H]MTX but not for [14C]sucrose. The results indicate that intracerebral [3H]MTX is lost more slowly than is intracerebral [14C]sucrose, possibly because [3H]MTX enters brain cells, whereas [14C]sucrose remains largely extracellular.

Drug entry into and distribution within brain and cerebrospinal fluid: [14C]urea pharmacokinetics

A four-compartment model was derived to analyze drug exchange among cerebral capillary plasma, cerebrospinal fluid (CSF), and the brain extracellular and intracellular (or bound) compartments. Equations that were derived incorporated the factor of cerebral blood flow. They were fit by nonlinear least squares to measured brain, plasma, and CSF (when available) concentrations of [14C]urea in the rat, in response to a step increase in plasma concentration, to intravenous infusion, or to a bolus injection of tracer. Best-fit values for the transfer constants were consistent among the three administrative regimens and agreed with published values, when available. Expressions also were derived and numerically evaluated for the lower limit of the brain extracellular space, for half times of brain [14C]urea uptake, and for the steady-state brain/plasma distribution volume. The model should make it possible to use transfer constants obtained for a given drug from one study (e.g., constant plasma concentration) to predict brain concentrations from measured plasma concentrations in other acute or chronic studies.

Regional cerebral blood flow during development and ageing of the rat brain

Regional cerebral blood flow (rCBF) in the conscious Fischer-344 rat was measured in 14 brain regions at 5 different ages. rCBF increased significantly (P less than 0.05) in most anterior brain regions between 1 and 3 months of age, but not in phylogenetically more primitive regions from the mid- and hindbrain that may have matured prior to 1 month of age. rCBF tended to increase or remain constant between 3 and 12 months, and rose significantly in the frontal lobe. Between 12 and 24 months of age, rCBF declined by an average of 17 per cent and fell significantly in 5 brain regions, mainly from the posterior brain, and in some, possibly in relation to partial functional deafferentation. There were no statistically significant changes in rCBF between 24 and 34 months of age. rCBF and local cerebral glucose utilization (LCGU) do not follow identical time courses during development and maturation of the rat brain. A fall in LCGU between 3 and 12 months of age, when rCBF remains constant or tends to rise, may reflect increased sensitivity of the cerebrovascular bed to metabolic factors which regulate cerebral blood flow.

Effects of immobilization stress on regional cerebral blood flow in the conscious rat

Immobilization stress of conscious, normotensive, freely breathing 10-month-old Wistar-Kyoto rats produced an overall decline in regional cerebral blood flow (rCBF), as measured with [14C]iodoantipyrine, except at the frontal lobe. In 14 brain regions, rCBF fell by an average of 14.3% after 5 min of immobilization and by 11.9% after 15 min. Immobilization stress also stimulated hyperventilation and thereby reduced PaCO2. The slope relating rCBF to PaCO2 averaged 1.5 ml 100 g-1 min-1 mm Hg-1 in 9 significantly affected regions. The findings suggest that rCBF declines during immobilization stress because of cerebrovascular constriction caused by a reduction in PaCO2. Comparison of the average slope with published values in indicates furthermore that were PaCO2 to remain unchanged during immobilization, rCBF would increase by at most 20%.