In vitro metabolism of a rifamycin derivative by animal and human liver microsomes, whole blood and expressed human CYP3A isoform

1. In vitro metabolism of a rifamycin derivative, benzoxazinorifamycin KRM-1648, was studied using mouse, rat, guinea pig, dog, monkey and human liver microsomes. 30-Hydroxy-KRM-1648 (M2) was produced in mouse, dog, monkey and human microsomes. 25-Deacetyl-KRM-1648 (M1) was produced in dog and human microsomes, but not in mouse or monkey microsomes. Neither M1 nor M2 was detected in rat or guinea pig microsomes. 2. In dog and human liver microsomes the formation of M2 was dependent on NADPH, but the formation of M1 was not. 3. In vitro metabolism of the parent compound was studied in whole blood in some species. Only M1 was detected in mouse and rat blood, and not in dog and human blood. 4. These findings demonstrated that the metabolite pattern in dog resembled that in man, and suggested that the 30-hydroxylation of KRM-1648 was mediated by cytochrome P450, but that the 25-deacetylation was not. 5. Among the ten recombinant human P450 isoforms used, only the cell lysates including CYP3A3 and CYP3A4 catalysed the M2 formation from KRM-1648.

Bilateral giant intracavernous carotid artery aneurysms mimicking a cavernous sinus neoplasm--case report

An 84-year-old female presented with bilateral giant intracavernous carotid artery aneurysms manifesting as right total ophthalmoplegia and trigeminal nerve paresis. Computed tomography and magnetic resonance imaging showed the two aneurysms as one fused mass, so the initial diagnosis was a parasellar tumor occupying the bilateral cavernous sinuses. The correct diagnosis required cerebral angiography. Considering her age, no surgical or interventional treatment was given. Eight months later, her right eye movement partially recovered and she had no further symptoms. There was no definite causative factors other than aging in this case.

Effects of nilvadipine (a dihydropyridine-type calcium entry blocker) on cerebral blood flow in acute experimental brain ischemia in rats

The purpose of the present study was to examine the effect of nilvadipine (a dihydropyridine-type calcium entry blocker) on the cerebral circulation, experimental models of cerebral ischemia were used to measure abrupt changes in the cortical cerebral blood flow (CBF) at the 'penumbra' (the boundary region between the anterior cerebral artery (ACA) and the middle cerebral artery (MCA)). Left middle cerebral artery occlusion (MCAO) was performed in Sprague--Dawley rats, and then 8 or 16 micrograms kg-1 of nilvadipine was administered to two groups of the rats. The former group was referred to as the 8-MCAO(+) group (n = 8), and the latter group, the 16-MCAO(+) group (n = 8). To a control group (n = 8), only the solvent of nilvadipine was administered. After the administration in each group, CBF and the mean arterial blood pressure (MAP) were continuously measured for 60 min. The MAP in both the 8-MCAO(+) and 16-MCAO(+) groups was significantly lower than in the control group for 25 min after nilvadipine administration (Wilcoxon 2 sample test, p < 0.05). Also, the CBF in the 16-MCAO(+) group was significantly lower (about 20%) for 55 minutes than that in the control group. On the other hand, the CBF in the 8-MCAO(+) group three minutes after nilvadipine administration was not significantly lower than in the control group (Wilcoxon 2 sample test). Our findings confirmed that nilvadipine (8 micrograms kg-1) maintained CBF while lowering the blood pressure in the experimental model of acute cerebral ischemia.

Cerebral venous angiomas associated with hemorrhagic lesions. Their MRI manifestations

Using two 1.5-T magnetic resonance (MR) scanners, 82 venous angiomas (VAs) were imaged. There were 13 (16%) VAs associated with hemorrhagic lesions. Ten of the 13 hemorrhagic lesions were considered hematomas in the subacute or chronic stage, caused by VAs or coexisting cavernous hemangiomas. Two of the 13 were subacute intracerebral hematomas; the remaining one was a sequela of a hemorrhagic venous infarct. After analysis of our data, it was concluded that infratentorial VAs and deeply draining supratentorial VAs in relatively young adults, especially females, are relatively frequently associated with intracerebral hemorrhagic lesions. MR imaging proved useful for diagnosing VAs and associated hemorrhagic lesions.

Contrasting effect of isoflurane on drug metabolism: decreased type I and increased type II substrate metabolism in guinea pig liver microsomes

Inhalation anaesthetics might affect perioperative drug elimination by altering drug distribution, hepatic blood flow or drug metabolism. The in vitro effects of isoflurane on aniline hydroxylation and aminopyrine N-demethylation were investigated with guinea pig liver microsomes to assess the role of isoflurane on oxidative drug metabolism through the hepatic mixed-function oxidase system. p-Aminophenol and formaldehyde were measured spectrophotometrically as metabolic products of aniline hydroxylation and aminopyrine N-demethylation, respectively, where the reaction mixture consisted of a microsomal suspension, NADPH, aminopyrine or aniline, with or without isoflurane. The rate of cytochrome P-450 reduction by NADPH affected in the presence of isoflurane was investigated by spectrometric measurement of the CO-cytochrome P-450 complex formation at various times. Due to the addition of isoflurane, the Vmax values for aniline hydroxylation evidently increased except in high isoflurane concentration (3.33 mM) and for aminopyrine N-demethylation the value was significantly low only in the presence of a high isoflurane concentration, whereas the K(m) values significantly decreased in aniline hydroxylation and increased in aminopyrine N-demethylation, and isoflurane also accelerated the rate of cytochrome P-450 reduction by NADPH. These results reflect the inhibition of aminopyrine N-demethylation and activation of aniline hydroxylation in the presence of isoflurane as a consequence of isoflurane-accelerated cytochrome P-450 reduction by NADPH and/or drug-enzyme binding properties, and may have implications on the metabolism of perioperatively administered drugs during isoflurane anaesthesia.

Altered cerebrovascular response to a potassium channel opener in hypertensive rats

We examined whether the effect of Y-26763, an ATP-sensitive potassium channel opener, on cerebral blood flow is altered in stroke-prone spontaneously hypertensive rats (SHRSP) and, if altered, whether long-term antihypertensive treatment with cilazapril, an angiotensin-converting enzyme inhibitor, is capable of preventing the change. Cerebral blood flow during intracarotid infusion of Y-26763 was measured in anesthetized SHRSP and normotensive Wistar-Kyoto rats (WKY) as control. Y-26763 increased cerebral blood flow in a dose-dependent manner in WKY, and glibenclamide, a selective inhibitor of ATP-sensitive potassium channels, inhibited the Y-26763-induced increase in cerebral blood flow. In contrast, the response to Y-26763 in SHRSP was significantly impaired compared with that in WKY. Antihypertensive treatment with cilazapril lowered blood pressure toward normal and prevented the impaired response in cerebral blood flow to Y-26763 in SHRSP. These findings suggest that (1) ATP-sensitive potassium channels contribute to the regulation of cerebral blood flow in rats, (2) the response to an ATP-sensitive potassium channel opener is markedly diminished in hypertensive rats, and (3) the altered response to an ATP-sensitive potassium channel opener during chronic hypertension can be prevented by long-term antihypertensive treatment.

Renal transplantation in cats with chronic renal failure

Renal transplantation was performed on 6 cats with chronic renal failure. Clinical signs and the blood chemistry findings (BUN, Cr, IP) improved one week after renal transplantation. Renal anemia also improved 3-4 weeks after surgery. Two cases had the same complications, hydronephrosis and hydroureter within 1 or 2 weeks of surgery, evidenced by marked elevations of BUN and Cr, suggesting that ureteral obstruction at the site of ureterocystostomy is the main complication of renal transplantation in cats. As a new resolution for this problem, we transplanted the ureter with the intact ureteral opening of the bladder from the donor in the remaining 4 cats. Ureteral obstruction was not observed in them, thus the procedure was considered to be useful for avoiding this complication. Two cats died of pneumonia and other complications within 3 months. There were no acute rejections or side effects from the immunosuppressant during the observation period in the remaining four cats.

Myocardial integrated ultrasonic backscatter in patients with old myocardial infarction: comparison with radionuclide evaluation

The purpose of our study was to clarify whether the abnormalities in integrated backscatter may be used to assess myocardial viability in patients with old myocardial infarction by comparing these integrated backscatter parameters with conventional radionuclide and echocardiographic estimates of myocardial viability. Two myocardial integrated backscatter parameters, the magnitude of cyclic variation in integrated backscatter and the myocardial integrated backscatter calibrated with the power of Doppler signals from the blood along the same ultrasound beam (calibrated myocardial integrated backscatter), were measured in 21 normal persons and 33 patients with old anteroseptal myocardial infarction. Calibrated myocardial integrated backscatter was higher and the magnitude of cyclic variation in integrated backscatter was lower in the infarct septum compared with the septum of normal subjects. Percent thallium uptake, as assessed in scintigraphic images taken at rest or after reinjection, correlated well with the calibrated myocardial integrated backscatter (r = -0.72, p < 0.01) and more weakly but significantly with the magnitude of cyclic variation in integrated backscatter (r = 0.55, p < 0.05) in 16 of 33 patients. The measurement of calibrated myocardial integrated backscatter, in addition to the magnitude of cyclic variation of integrated backscatter, may likely be valuable in the noninvasive assessment of myocardial viability.

K-ras and p53 mutations in stage I gallbladder carcinoma with an anomalous junction of the pancreaticobiliary duct

BACKGROUND

An anomalous junction of the pancreaticobiliary duct (AJPBD) was thought to be an important risk factor for gallbladder carcinoma in Japan. In this report, we compared K-ras and p53 mutations in Stage I gallbladder carcinomas (GC) of patients with AJPBD with those in patients without AJPBD:

METHODS

We examined 6 GC of patients with AJPBD and 20 GC of patients without AJPBD: Immunohistochemistry was performed for p53 protein. K-ras and p53 mutations were examined using genomic DNA extracted from the cancer regions. The methods of polymerase chain reaction (PCR) single strand conformation polymorphism analysis were performed for mutations in exons 5-8 of p53. The methods of PCR restriction fragment length polymorphism were performed for mutation in codon 12 of K-ras.

RESULTS

p53 positivity was 67% in GC of patients with AJPBD and 65% in GC of patients without AJPBD: p53 mutations were found in exons 7 and 8 in GC of patients with AJPBD and in exons 5, 6, and 7 in GC of patients without AJPBD: The incidence of K-ras mutation in GC of patients with AJPBD (50%) was greater than that in patients without AJPBD (6%) (P < 0.05).

CONCLUSIONS

These results suggest that K-ras mutation may be important in the early stage of carcinogenesis of the gallbladder mucosa with AJPBD, and that p53 mutations may also contribute to the early stage of carcinogenesis of the gallbladder mucosa, regardless of AJPBD:

Impaired action of levcromakalim on ATP-sensitive K+ channels in mesenteric artery cells from spontaneously hypertensive rats

The purpose of the present study was to test the hypothesis that properties of ATP-sensitive K+ channels are altered in arterial smooth muscle cells of hypertensive rats. Using a patch-clamp technique, we compared effects of a K+ channel opener, levromakalim, on membrane currents in mesenteric artery cells from adult Wistar Kyoto rats (WKY) and age-matched spontaneously hypertensive rats (SHR) treated or not treated with hydralazine. Blood pressure was significantly higher in SHR than in WKY or hydralazine-treated SHR. Levcromakalim evoked a time-independent and voltage-insensitive current in a dose-dependent manner in the whole-cell clamp configuration. The reversal potential of the evoked current depended on extracellular K+ concentration. Application of 3 micromol/L glibenclamide, a specific blocker of ATP-sensitive K+ channels, abolished the levcromakalim-evoked current; however, the current was unaffected by either 1 mmol/L tetraethylammonium or 0.3 micromol/L charybdotoxin. These results suggest that the levcromakalim-evoked current was carried through ATP-sensitive K+ channels. In SHR cells, the maximal slope conductance of the levcromakalim-evoked current, normalized by cell capacitance, was decreased, and the dose-response curve was shifted to the right compared with WKY cells. The levcromakalim action was not impaired in cells from hydralazine-treated SHR. In conclusion, the action of levcromakalim on ATP-sensitive K+ channels in SHR mesenteric artery muscle cells was impaired compared with WKY cells. This impairment was corrected by long-term antihypertensive treatment.

Temporal changes in regional end-diastolic wall thickness early after reperfusion in acute anterior myocardial infarction: relation to myocardial viability and vascular damage

We investigated early temporal changes in end-diastolic wall thickness (EDWT) of the infarcted myocardium in 46 patients with reperfused anterior acute myocardial infarction in relation to myocardial viability. Two-dimensional echocardiography was performed on days 1 and 2 of acute myocardial infarction, and the EDWT of the anterior segment was measured in the short-axis view. Patients were divided into three groups on the basis of day 1 to day 2 ratio of EDWT: the ratio < or = 0.85 as group A (n = 13), >0.85 but < or = 1.15 as group B (n = 23), and >1.15 as group C (n = 1 0). Left ventricular functional improvement was significantly better in group B than in groups A and C. Substantial size of "no reflow" phenomenon was observed only in groups A (n = 9, 69%) and C (n = 6, 60%). The frequency of transient ST re-elevation after reperfusion was the highest in group C (70%), and left ventricular expansion was observed at day 2 only in group A. We conclude that changes in the EDWT of the infarct segment early after reperfusion, either decreases or increases, are related to irreversibly damaged myocardium. A decrease in EDWT and concomitant ventricular expansion may be related to impaired myocardial perfusion. An increase in EDWT after reperfusion may be caused by accelerated myocardial and microvascular damage after reperfusion.

High frequency of p53 gene mutation in adenocarcinomas of the gallbladder

p53 mutations in adenocarcinomas of the gallbladder were analyzed by deoxynucleotide sequencing of the gene. Of 23 cases, 16 (70%) harbored 18 missense mutations in exon 5, 6, or 8 of the p53 gene. The characteristics of the p53 mutation spectrum observed in gallbladder carcinomas were (a) frequent mutations at an A:T pair [10 (55%) of 18 mutations], (b) high transversion incidence [12 (66%) of 18 mutations], and (c) only one mutation at the CpG site. The immunohistochemical study revealed that 36 (55%) of 65 cases showed an abnormal accumulation of p53 immunoreactivity, and 12 (52%) of 23 cases had p21 expression. No statistical correlation was observed between p53 and p21 immunoreactivity. These results suggest that p53 mutations may confer the carcinogenesis of the adenocarcinoma of the gallbladder with the specific mutation spectrum of p53.

Myocardial perfusion patterns related to thrombolysis in myocardial infarction perfusion grades after coronary angioplasty in patients with acute anterior wall myocardial infarction

BACKGROUND

Epicardial coronary flow is occasionally reduced even after coronary intervention despite the absence of vessel obstruction in patients with acute myocardial infarction. Our aim was to clarify the cause and outcomes of radiocontrast slow filling in patients with reperfused acute anterior myocardial infarction by assessing microvascular damage with the use of myocardial contrast echocardiography (MCE) and functional outcomes.

METHODS AND RESULTS

We carefully reviewed the cineangiograms of 86 patients who achieved coronary revascularization within 12 hours of the onset and underwent MCE before and soon after recanalization with the intracoronary injection of sonicated microbubbles. Antegrade coronary flow after recanalization was graded by two observers based on Thrombolysis in Myocardial Infarction (TIMI) trial flow grades. Left ventricular ejection fraction was measured on the day of infarction and 1 month later. TIMI grade 2 was observed in 18 patients (21%), and the other 68 patients manifested TIMI grade 3 after recanalization. All patients with TIMI 2 showed substantial MCE no reflow, whereas only 11 patients (16%) with TIMI 3 showed MCE no reflow. Functional improvement was worse in patients with TIMI 2 than in those with TIMI 3 (TIMI 2, 38 +/- 8% versus 40 +/- 8%, P = NS [acute versus late]; TIMI 3, 44 +/- 13% versus 55 +/- 13%, P < .001). Among patients with TIMI 3, significant functional improvement was observed only in patients with MCE reflow (MCE reflow, 46 +/- 13% versus 57 +/- 12%, P < .001; MCE no reflow, 35 +/- 11% versus 45 +/- 12%, P = NS).

CONCLUSIONS

Despite no obstructive lesion of the vessel, TIMI 2 is caused by advanced microvascular damage and is a highly specific, although not sensitive, predictor of poor functional outcomes in patients with acute myocardial infarction. TIMI 3 does not necessarily indicate myocardial salvage, and detection of MCE no reflow in these patients is particularly useful for the prediction of functional outcome.

Hepatitis B virus carrier status linked to autoimmune hepatitis

We describe a hepatitis B virus carrier who satisfied the criteria of autoimmune hepatitis proposed by the International Autoimmune Hepatitis Group. A 43-year-old Japanese female showed human leukocyte antigen typing including DR4 in addition to hypergammaglobulinemia, presence of autoantibodies, and liver histology suggestive of autoimmune hepatitis. Moreover, the predominant presence of hepatitis B core antigen in nuclei rather than in cytoplasm of hepatocytes suggested less of a possibility of liver cell damage related to hepatitis B virus infection. She completely responded to immunosuppressive therapy and no clinical or biochemical relapse has been recognized to date.

Effects of the addition of hyaluronate segments with different chain lengths on the viscoelasticity of hyaluronic acid solutions

The effects of the addition of sodium hyaluronate segments (sHA) with different chain lengths on the viscoelasticity of high molecular weight sodium hyaluronate (HA) aqueous solutions were studied. The additive effects of sHA depended on the chain length of sHA. Shorter sHA decreased storage (G') and loss (G") moduli, whereas longer sHA increased both moduli. In physiological saline, the effects of the addition of sHA depended on the chain length of sHA, as in aqueous solutions, except that shorter segments did not decrease both moduli. The effects of the addition of monosaccharides, i.e., N-acetyl glucosamine (GlcNAc) and sodium glucuronate (GlcANa), were also studied. Although GlcNAc increased G' and G", GlcANa decreased both moduli. A shift factor "A," which depended on chain-length effects, a sugar effect, and a salt effect, was developed to explain these additive effects quantitatively.

[A familial case of Gorlin syndrome]

An 11-year-old girl was admitted to Chiba University Hospital because of the infection of jaw cysts. She had many pits on her palms and soles, basal cell nevus, and typical facial features to Gorlin syndrome (nevoid basal cell carcinoma syndrome). Her 14-year-old sister and 44-year-old father also had similar clinical symptoms, thus they were diagnosed as having a familial Gorlin syndrome. Recently, the father had a complication of maxillary cancer. Colony formation tests after X-ray and ultraviolet irradiation in skin fibroblasts derived from these sisters revealed no remarkable hypersensitivity to these agents. It is important to make an early diagnosis and a proper management in Gorlin syndrome, which has cancer predisposition.

[Measurement of the plasma transforming growth factor-beta 1 (TGF-beta 1) level in patients of gastric carcinoma--compared with the serum IAP level and the lymphocyte subsets (CD3, CD4, CD8)]

We evaluated the clinical significance of the plasma TGF-beta 1 by enzyme-linked immunosorbent assay (ELISA) in 40 patients with gastric carcinoma before operation who were hospitalized at our department between August 1992 and March 1993. Moreover, we examined the correlation of the immunosuppressive acidic protein (IAP) in the serum with the plasma TGF-beta 1 level. The lymphocyte subsets were analyzed with monoclonal antibodies (CD3, CD4, CD8) and compared with the plasma TGF-beta 1 level. Results 1) The plasma TGF-beta 1 was significantly high in patients with advanced stage of gastric carcinoma (with respect to the depth invasion) and poorly differentiated adenocarcinoma (histological type) (p < 0.01). When the carcinoma was macroscopically found to be advanced, TGF-beta 1 was higher in the invasive type than in the noninvasive type (p < 0.05). 2) We found a significant correlation between the plasma TGF-beta 1 and IAP in the serum (n = 25, r = 0.677, p < 0.01). 3) The lymphocyte fraction of CD3, CD4 was decreased in patients with a high level of TGF-beta 1, and was related with the plasma TGF-beta 1 level. Thus, TGF-beta 1 was presumed to be deeply associated with in the growth and progression of gastric carcinoma and related to systemic immunosuppressive reaction.

Isoflurane enhances dechlorination of carbon tetrachloride in guinea-pig liver microsomes

Effect of isoflurane on the dechlorination of carbon tetrachloride to chloroform was investigated in the guinea-pig liver microsomes. Under anaerobic conditions, chloroform is produced from carbon tetrachloride through the microsomes in the presence of NADPH, and such production of chloroform was increased by the addition of isoflurane. The K(m) for the production of chloroform from carbon tetrachloride was decreased to 86% by isoflurane compared with the control; however the maximum velocity of chloroform production was also decreased to 50%. The formation of the 445 nm band in the mixture of reduced cytochrome P-450 and carbon tetrachloride, and cytochrome P-450 reduction by NADPH were both accelerated by isoflurane, without alteration of NADPH-cytochrome c reductase activity. These results indicate that trichloromethyl radical, an intermediate product of carbon tetrachloride, easily combines to the haeme part of cytochrome P-450, whereas the protein part combines to isoflurane after being reduced by NADPH, which results in acceleration of carbon tetrachloride dechlorination under a lower concentration of carbon tetrachloride. These results may have implications for other drugs that are administered during isoflurane anaesthesia.

Pathology and cellular kinetics of gallbladder with an anomalous junction of the pancreaticobiliary duct

OBJECTIVES

Anomalous junction of the pancreaticobiliary duct (AJPBD) is thought to be an important risk factor for gallbladder carcinoma in Japan. In this report, we examine histopathology and cellular kinetics of gallbladder mucosae of patients with AJPBD and the possible risk of gallbladder carcinoma.

METHODS

We examined 62 gallbladders from patients with AJPBD (group A), 16 gallbladder carcinomas from patients with AJPBD (group B), 60 gallbladder carcinomas from patients without AJPBD (group C), and six normal gallbladders from patients without AJPBD (group D). Histopathology, mucosal heights, and proliferative cell nuclear antigen-labeling index were obtained from routinely processed tissue specimens.

RESULTS

The incidence of hyperplastic changes in group A and in the noncancerous regions (NCRs) of group B was greater than in the NCRs of group C (p < 0.05). The incidence of dysplastic changes in the NCRs of group B was greater than in the NCRs of group C (p < 0.05). The mucosal heights in group A and in the NCRs of group B were higher than in the NCRs of group C (p < 0.05). A high proliferative cell nuclear antigen-labeling index was observed in group A and in the NCRs of group B, where hyperplastic changes were frequently observed.

CONCLUSIONS

These results suggest that a sequence of hyperplastic changes with a corresponding increase in cellular kinetics with progression through dysplasia to carcinoma may be important in carcinogenesis in gallbladders of patients with AJPBD. AJPBD itself may be a possible risk for gallbladder carcinoma.

[Clinical characteristics and therapeutic strategy for acute myocardial infarction in the elderly]

Many randomized studies have revealed that reperfusion therapy is an epochmaking treatment for acute myocardial infarction (AMI). However, it is no clear whether it is equally beneficial in the elderly or not. In this study, we elucidated the clinical characteristics and effectiveness of reperfusion therapy and discussed the optimal treatment for AMI in the elderly. The study population comprised 1,891 consecutive patients with AMI. The reinfarction rates and the rates of Killip class III or IV on admission increased with age. The mortality was significantly higher in the older subgroups. In the patients with first AMI within 24 hours of the onset and who underwent emergency catheterization, those accompanied by hypercholesterolemia or with habitual smoking were significantly fewer in the older group. Although the Q-wave MI rate, the peak CPK level and the reperfusion rate were no different, the low cardiac output condition, multi vessel disease and short-term mortality were significantly greater in the older group. The patients over 80-year-old were subdivided into three groups; those treated conventionally (G-C), those treated with intracoronary thrombolysis (G-T) and those treated with direct PTCA (G-A). The overall mortality did not differ among the three groups. However, in patients hospitalized after 1990, the mortality in G-A was significantly lower than in G-C. The prevalence of bleeding complications between G-A and G-T did not differ. The patients in G-A showed greater improvement of the left ventricular wall motion and lower incidence of postinfarctional angina than other groups. Reperfusion therapy by direct PTCA appears to be the optimal strategy for treatment of elder patients with AMI.

[General anesthesia for expandable endotracheobronchial metal stent insertion]

We experienced eight cases of general anesthesia for endotracheobronchial expandable metal stent insertion. Awake intubation was performed following NLA induction. Anesthesia was maintained by inhalational anesthetics and fentanyl was added under spontaneous breathing assisted manually. Two patients showed a marked hemodynamic change and hypoxia due to endotracheobronchial bleeding and suction. Six patients received EMS insertion with no trouble. We conclude that endotracheobronchial bleeding is an important factor to be watched for during anesthetic management for EMS insertion.

Changes in arterioles, arteries, and local perfusion of the brain stem during hemorrhagic hypertension

Cerebral arterioles have been regarded as the primary sites of autoregulatory responses, whereas the role of large arteries in the cerebral autoregulation is poorly understood. The goal of this study was to determine in vivo whether the basilar artery and its primary branches act as resistance vessels under hypotensive conditions by simultaneously measuring their diameters and local brain stem blood flow with laser-Doppler flowmetry. In 10 anesthetized rats, blood flow to the brain stem was well maintained during stepwise hemorrhagic hypotension when mean arterial blood pressure fell from 116 +/- 3 to 50 mmHg and decreased gradually between 50 and 30 mmHg. Diameter of the basilar artery (n = 10) and its large branches (n = 22), measured through an open cranial window, increased by 10% from the baseline value at 50 mmHg and reached their maximum at 30 mmHg (314 +/- 9 from 244 +/- 6 mum, and 149 +/- 4 from 117 +/- 3 mum, respectively). Small branches (n = 15) dilated to a larger extent compared with the larger arteries throughout hypotension and reached the maximum at 30 mmHg (69 +/- 3 from 48 +/- 2 mum). Below 30 mmHg, there was a steep fall in blood flow and reduction in diameter of all-sized arteries. Thus small vessels contribute to reductions in cerebrovascular resistance throughout the entire autoregulatory-range in the brain stem circulation. Large arteries, such as the basilar artery and its branches, also contribute to reductions in cerebrovascular resistance around the lower limits of cerebral blood flow autoregulation and may thus play a significant role in maintaining blood flow to the brain stem during severe systemic hypotension.