Hypertension and insulin resistance: role of peroxisome proliferator-activated receptor gamma

1. Insulin resistance has been highlighted as a common causal factor for hypertension, hyperlipidaemia, diabetes mellitus and obesity, all of which are recognized to occur simultaneously, and a distinct clinical entity is defined as 'multiple risk factor syndrome'. 2. Recently, a new class of antidiabetic agents, thiazolidinediones (TZD) has been developed and has been shown to improve insulin resistance by binding and activating a nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma. 3. cDNA of rat PPAR gamma 1 and gamma 2 were cloned and gene regulation of PPAR gamma in rat mature adipocytes was examined. Hydrogen peroxide, an oxygen radical, which is recognized to be the common intracellular signal for multiple risk factors, potently down-regulated PPAR gamma mRNA expression in rat mature adipocytes. 4. Tumour necrosis factor (TNF)-alpha, which is considered to play a role in obesity-induced non-insulin-dependent diabetes mellitus and to augment oxidative stress, also suppressed PPAR gamma expression. 5. Thiazolidinediones dose-dependently recovered TNF-alpha-induced down-regulation of PPAR gamma mRNA expression. 6. The modulation of PPAR gamma expression by TZD can be one mechanism for the improvement of insulin resistance by TZD. 7. Vascular tone and remodelling are controlled by several vasoactive autocrine/paracrine factors produced by endothelial cells in response to several vascular injury stimuli, including hypertension. The PPAR gamma gene transcript was detected in cultured endothelial cells. 8. The administration of TZD stimulated the endothelial secretion of type-C natriuretic peptide, which is one of the natriuretic peptide family and is demonstrated by us to act as a novel endothelium-derived relaxing peptide. 9. Concomitantly, TZD significantly suppressed the secretion of endothelin, a potent endothelium-derived vasoconstricting peptide. 10. Thiazolidinediones can affect vascular tone and growth by modulating the production of endothelium-derived vasoactive substances to influence occurrence and progression of hypertension and atherosclerosis.

New members of uncoupling protein family implicated in energy metabolism

1. The regulation of energy metabolism involves food intake and energy expenditure. Mitochondrial uncoupling proteins (UCP) are implicated in energy expenditure. 2. cDNA of a homologue of UCP highly expressed in rat skeletal muscle, UCP-3, is isolated and sequenced. Rat UCP-2 cDNA is also isolated and sequenced. 3. Rat UCP-3 cDNA probe hybridized two bands, a major band at 2.5 kb and a minor band at 2.8 kb in rat tissues. The mRNA was expressed at the highest level in the skeletal muscle, and moderately in the interscapular brown adipose tissue (BAT). Only weak signals were detected in the epididymal white adipose tissue (WAT) and the heart. Rat UCP-2 cDNA probe hybridized a 1.7 kb band detected widely in the whole body, especially abundant in the lung and the spleen. In contrast to the UCP-3 gene expression, the UCP-2 gene expression was expressed at substantial levels in the WAT and only at slight levels in the skeletal muscle and BAT. 4. The UCP-3 gene expression is augmented two-fold in the gastrocnemius muscle from rats fed a high-fat diet (P < 0.05). The UCP-3 mRNA levels remained unchanged in the interscapular BAT, and epididymal WAT. The levels of the UCP-2 gene expression are augmented significantly in the epididymal WAT (1.6-fold; P < 0.05), while no significant increase is observed in the gastrocnemius muscle and interscapular BAT.

Down regulation of peroxisome proliferator-activated receptorgamma expression by inflammatory cytokines and its reversal by thiazolidinediones

AIMS/HYPOTHESIS

Previous studies show that inflammatory cytokines play a part in the development of insulin resistance. Thiazolidinediones were developed as insulin-sensitizing drugs and are ligands for the peroxisome proliferator-activated receptory (PPARgamma). We hypothesized that the anti-diabetic mechanism of thiazolidinediones depends on the quantity of PPARgamma in the insulin resistant state in which inflammatory cytokines play a part.

METHODS

We isolated rat PPARgamma1 and gamma2 cDNAs and examined effects of various cytokines and thiazolidinediones on PPARgamma mRNA expression in rat mature adipocytes.

RESULTS

Various inflammatory cytokines, such as tumour necrosis factor-alpha (TNF-alpha), interleukin-1alpha (IL-1alpha), IL-1beta, IL-6 and leukaemia inhibitory factor decreased PPARgamma mRNA expression. In addition, hydrogen peroxide, lysophosphatidylcholine or phorbol 12-myristate 13-acetate also decreased the expression of PPARgamma. The suppression of PPARgamma mRNA expression caused by 10 nmol/l of TNF-alpha was reversed 60% and 55% by treatment with 10(-4) mol/l of troglitazone and 10(-4) mol/l of pioglitazone, respectively. The suppression of glucose transporter 4 mRNA expression caused by TNF-alpha was also reversed by thiazolidinediones. Associated with the change of PPARgamma mRNA expression, troglitazone improved glucose uptake suppressed by TNF-alpha.

CONCLUSION/INTERPRETATION

Our study suggests that inflammatory cytokines could be factors that regulate PPARgamma expression for possible modulation of insulin resistance. In addition, we speculate that the regulation of PPARgamma mRNA expression may contribute to the anti-diabetic mechanism of thiazolidinediones.

Nicotinamide inhibits sodium-dependent phosphate cotransport activity in rat small intestine

BACKGROUND

We recently reported that the administration of niceritorol (a nicotinic acid derivative which improves lipid metabolism and peripheral circulation, and is used for the treatment of hyperlipidaemia and impaired peripheral circulation) to patients with hyperphosphataemia undergoing dialysis decreased the serum phosphate (Pi) concentration. We found that this was due to an acceleration of faecal Pi excretion by niceritrol.

METHODS

Intestinal brush border membrane vesicles (BBMVs) were prepared from rat jejunum, and the Na+-dependent and Na+-independent Pi transport activities in these vesicles were measured. In addition, the functional Pi transporter from rat small intestine was injected in Xenopus oocytes, and the effect of nicotinamide on the levels of its expression were measured by northern blotting.

RESULTS

The Na+-dependent component was significantly decreased in the BBMVs isolated from rats treated with nicotinamide, while the Na+-independent component was not changed. Kinetic studies demonstrated that the decreased activity was due to reduction of the Vmax value and not an elevation of the Km values. When poly(A)+RNA from rats treated with nicotinamide was microinjected into Xenopus oocytes, the Pi transport activity was significantly decreased compared with that in the control animals. In addition, there were no significant changes in Na/Pi cotransporters and activators, but the vitamin D receptor mRNA level was reduced to 80% of the control level.

CONCLUSIONS

These observations suggest that nicotinamide may regulate the expression of a major functional Na/Pi cotransporter in the rat small intestine.

Comparison of bioelectrical impedance analysis and dual energy X-ray absorptiometry for assessment of body composition in children

BACKGROUND

There are a variety of methods for assessing body composition. Bioelectrical impedance analysis (BIA) is an easy and non-invasive technique, but has limitations in underweight and overweight subjects. Few reports have investigated the validity of BIA in children. In this report, the characteristics of BIA, especially in overweight and underweight children, are assessed and the results are compared with those of dual energy X-ray absorptiometry (DXA), as a reliable method for assessing body composition.

METHODS

Determination of the fat-free mass (FFM), body fat content and percent body fat (%fat) was carried out using both BIA and DXA. The subjects (60 males and 44 females) were divided into five groups according to the percentage of ideal bodyweight (%IBW). Ten obese children, who were treated with exercise and a low-energy diet for 1 month, were also enrolled in this study.

RESULTS

The %fat, FFM and body fat content showed a close correlation when measured by BIA and DXA with the correlation coefficients being 0.90, 0.95, and 0.95, respectively. In the underweight group, the %fat value determined by BIA tended to be greater than that determined by DXA, while in the overweight group, the BIA value was lower than the DXA value. The same trend was also seen in obese children before and after therapy with exercise and diet.

CONCLUSION

Bioelectrical impedance analysis seems to be a reasonable method for daily clinical use, but attention should be paid to the interpretation of %fat values in underweight and overweight children.

Effects of peptidase inhibitors, [D-Ala2, Met5]-enkephalinamide and antiserum to methionine-enkephalin microinjected into the caudal periaqueductal gray on morphine withdrawal in rats

We examined the involvement of enkephalins in the caudal periaqueductal gray (cPAG) in morphine withdrawal in rats. Rats were treated with increasing doses of morphine (20-30 mg/kg/day, s.c., for 5 days) to develop morphine dependence. Morphine withdrawal was induced by naloxone (5 mg/kg, s.c.) 24 hr after the final morphine injection. The level of preproenkephalin (PPE) mRNA in the cPAG was estimated by quantitative in situ hybridization. PPE mRNA in the cPAG was increased 4-24 hr after naloxone in morphine-treated rats. A mixture of peptidase inhibitors (0.5 microl of a solution of amastatin, captopril and phosphoramidon, 3 x 10(-3) M each) microinjected into the cPAG suppressed morphine withdrawal (a decrease in the number of jumping, chin rubbing, paw rubbing and teeth chattering). Antiserum to methionine-enkephalin (1:10 dilution) microinjected into the cPAG did not significantly aggravate morphine withdrawal with or without the mixture of peptidase inhibitors. However, [D-Ala2, Met5]-enkephalinamide (20 nmol), an enkephalin analog, injected into the cPAG decreased the number of jumping without any influence on the other withdrawal signs. These results suggest that the increase in enkephalins in the cPAG may participate in the alleviation of morphine withdrawal (jumping behavior).

Cross polarization for 1H NMR image contrast in solids

A novel 1H imaging method for solids, yielding images reflecting 1H-13C dipolar interactions through cross relaxation time TIS, is presented. Phase-alternating multiple-contact cross polarization (PAMC CP) was incorporated into the magic-echo frequency-encoding imaging scheme; the PAMC CP sequence may partly but efficiently destroy the initial 1H magnetization depending on the TIS values. A theory describing the effects of the PAMC CP sequence was developed, which was used for the assessment of the sequence as well as the analysis for the experimental results. It was demonstrated that the TIS-weighted 1H image and the TIS mapping for a phantom, constituted of adamantane and ferrocene, can distinguish these compounds clearly. Copyright 1998 Academic Press.

Enzyme-linked immunosorbent assay for serological survey of equine arteritis virus in racehorses

To examine antibodies against equine arteritis virus (EAV), an enzyme-linked immunosorbent assay (ELISA) using purified virus antigen was developed. The results of ELISA were compared with those of serum neutralization (SN) tests. The ELISA absorbance values and the SN titers in sera collected weekly from EAV-infected horses showed a similar pattern. The ELISA could detect antibody to EAV in horses experimentally infected with not only a homologous virus strain, which was used as the ELISA antigen, but also a heterologous strain. Using the ELISA, serum samples collected in 1996 from racehorses in three prefectures (Hokkaido, Ibaraki, and Shiga) were examined and there was no evidence of recent EAV infection among these racehorse populations in Japan. The ELISA should be a simple and highly specific method for rapid screening of EAV infection in racehorses.

[A surgical case for concomitant coronary artery disease and ASO with advanced colon cancer]

We report a successful surgical case of concomitant severe coronary artery disease and ASO with advanced sigmoid colon cancer. This patient underwent two-stage operation. Resection of the sigmoid colon was carried out first, then CABG and aortofemoral bypass was carried out simultaneously. The surgical strategy of a patient with coronary artery disease and malignant neoplasm is still controvertial, however, it should be decided considering the severity and the symptoms of both disease. In this case, ascending aorta was used as a donor artery for aortofemoral bypass. This technique will bring those benefits, i.e., consecutive operation procedures in same operating field, obtaining abundant blood flow to lower limb and safe IABP catheter insertion from subcutaneously tunneled bypass graft.

Low bone mineral density in a case of mosaicism Klinefelter syndrome: rapid response to testosterone therapy

Male hypogonadism has been recognized as one of the major causes of secondary osteoporosis, but most cases seem to be left undiagnosed. We report a 54-year-old case of mosaicism Klinefelter syndrome lacking typical clinical features such as tall stature or low intelligence, who was found to have marked decrease in lumbar bone mineral density (BMD: 0.686 g/cm2) during treatment of diabetes mellitus. In investigation for etiologies of secondary osteoporosis, he was diagnosed as having mosaicism Klinefelter syndrome (XXY/XY/XX). Although he was infertile, he lacked typical clinical features of Klinefelter syndrome. Testosterone replacement was started, which resulted in an increase in BMD up to 0.712 g/cm2 two months after the initiation of therapy. The fact that BMD increased shortly after the initiation of testosterone replacement therapy in the present case supported a beneficial effect of testosterone on BMD, as recently suggested in idiopathic hypogonadotropic hypogonadism. Although the present case was diagnosed as having mosaicism Klinefelter syndrome by investigating etiologies for osteoporosis, it may be stressed that male hypogonadism, in general, should be adequately suspected in the presence of infertility and from the findings of physical examination.

Physiologic shear stress suppresses endothelin-converting enzyme-1 expression in vascular endothelial cells

Shear stress dilates blood vessels and exerts an antiproliferative effect on vascular walls. These effects are ascribed to shear stress-induced, endothelium-derived vasoactive substances. Endothelin-converting enzymes (ECEs), the enzymes that convert big endothelin-1 (ET-1) to ET-1, have recently been isolated and the corresponding proteins have been termed ECE-1 and ECE-2. Furthermore, two isoforms of human ECE-1 have been demonstrated and termed ECE-1 alpha and ECE-1 beta. In this study, to elucidate the role of ECE-1 under shear stress we examined the effect of physiologic shear stress on the mRNA expression of ECE-1 and ET-1 in cultured bovine carotid artery endothelial cells (BAECs) and human umbilical veins (HUVECs), and also ECE-1 alpha mRNA expression in HUVECs. ECE-1 mRNA expression was significantly downregulated by shear stress in 24 h, both in BAECs and HUVECs, in a shear stress intensity-dependent manner. The expression of ECE-1 alpha mRNA was also attenuated by shear stress in HUVECs. ET-1 mRNA expression showed a concordant decrease with ECE-1 mRNA expression. These results suggest that shear stress-induced gene regulation of ET-1 and ECE-1 mRNA expression can contribute to the decrease of ET-1 peptide level by shear stress.

Oxidative stress suppresses the endothelial secretion of endothelin

To address endothelial function on vascular walls exposed to oxidative stress, we investigated the effect of oxidative stress on the secretion of endothelin-1 (ET-1) from cultured bovine carotid artery endothelial cells (BAECs). Concentrations of ET-1 in the media were measured by a specific radioimmunoassay and ET-1 mRNA expression was estimated by Northern blot analysis. Treatment of BAECs with 0.5-2.0 mM H2O2 for 3 h suppressed both ET-1 secretion and ET-1 mRNA expression in a dose-dependent manner compared to control. Attenuation of ET-1 mRNA expression by H2O2 was revealed to take place at the transcriptional level. The addition of NG-nitro-L-arginine-methyl ester (L-NAME) 10 microns, a specific nitric oxide synthase inhibitor, had no effect on H2O2-induced suppression of ET-1 mRNA expression. Suppression of ET secretion under oxidative stress observed in the present study is proposed to be a compensatory mechanism of endothelial cells to inhibit vasoconstriction and proliferation during oxidative stress.

Effect of neurotropin on the binding of high molecular weight kininogen and Hageman factor to human umbilical vein endothelial cells and the autoactivation of bound Hageman factor

Bradykinin is generated by activation of the plasma kallikrein-kinin (K-K) cascade and contributes to the symptoms of allergic reactions and the perception of pain. Neurotropin is a biological material obtained from inflamed rabbit skin inoculated with vaccinia virus, which is widely used clinically in Japan as an effective agent for these disorders. Factor XII (FXII) and high molecular weight kininogen (HK), two critical constituents of the plasma K-K cascade, bind to endothelial cells, and bound FXII is autoactivated in the presence of zinc ions. We have investigated the effects of Neurotropin on the interactions of FXII and HK with endothelial cells. Neurotropin inhibited the binding of both proteins to cultured human umbilical vein endothelial cells (HUVEC) and inhibited autoactivation of FXII upon HUVEC in a concentration-dependent manner. These data suggest that the ameliorating effects of Neurotropin in allergic disorders and pain syndromes may be related to this ability to inhibit activation of the K-K cascade and, consequently, the formation of bradykinin.

Time course of morphine withdrawal and preproenkephalin gene expression in the periaqueductal gray of rats

We have previously reported the increase of preproenkephalin (PPE) mRNA in the caudal periaqueductal gray (PAG) of rats during morphine withdrawal. In this study, it was further evidenced that PPE mRNA in the caudal PAG was not increased by various kinds of stressor, suggesting that the increase in PPE mRNA in the caudal PAG is specific to morphine withdrawal. In order to investigate the physiological significance of the increase of PPE mRNA in the caudal PAG, we compared the time course of the increase of PPE mRNA in the caudal PAG with that of naloxone-precipitated or spontaneous morphine withdrawal signs. The increase of plasma corticosterone (PCS: 52 and 52 microg/100 ml; control group, 18 and 15 microg/100 ml) and body weight loss (-6 and -9%; control group, 0 and -1%) were observed but PPE mRNA increase was not detected 1 and 2 h after naloxone in morphine treated rats. PPE mRNA increased by 37 to 56%, while PCS elevation and body weight loss gradually diminished 4 h to 2 days after naloxone. A total of 12 h after spontaneous withdrawal, PCS was prominently increased (51 microg/100 ml; control group, 12 microg/100 ml), but body weight and PPE mRNA were not affected. One day after spontaneous withdrawal, PCS elevation (38 microg/100 ml; control group, 8 microg/100 ml) and body weight loss (-5%; control group, +3%) were observed and PPE mRNA also increased by 42%. Two to 3 days after the final morphine injection, PCS recovered to control level and body weight loss gradually disappeared, while PPE mRNA was still increased by 74 to 46%. These results suggest that PPE gene expression in the caudal PAG is stimulated in the recuperative phase of these morphine withdrawal signs.

Effect of heparin on hemagglutination by equine arteritis virus

Heparin inhibited hemagglutination (HA) by equine arteritis virus (EAV) as well as did HA by Aujeszky's disease virus (ADV), but failed to inhibit HA by parainfluenza virus type 3 (PIV-3). The minimal concentration of heparin required to inhibit 8 HA U of EAV was 0.1 U/ml. In addition, most EAV hemagglutinin was retained by heparin acrylic beads, as was ADV hemagglutinin, but was not PIV-3 hemagglutinin. Mouse erythrocytes failed to combine with the HA inhibitory factor of heparin. However, mouse erythrocytes treated with heparinase had greatly reduced agglutinability by EAV. All these findings suggest that a heparin-like molecule on the surface of mouse erythrocytes serves as the virus-cell receptor.

A case of sarcoid reaction associated with papillary thyroid carcinoma

Sarcoid reaction, a granulomatous lesion similar to those seen in sarcoidosis, has been reported to be associated with various disorders. Here we describe a 54-year-old woman, who was diagnosed with sarcoid reaction associated with papillary carcinoma of the thyroid. Her history included total thyroidectomy with radical neck dissection for a papillary carcinoma of the thyroid. She was found to have a right subclavian mass. Dissection of the mass was performed for the diagnosis of metastatic papillary carcinoma to the lymph node, but the pathological examination showed granuloma without caseation as well as metastasis to the lymph node. Polymerase chain reaction (PCR) of the specimen excluded a possibility of Mycobacterium infection. There was no supporting evidence for systemic sarcoidosis in this patient; the patient showed no skin, eye, or lung lesions, or bilateral hilar lymphadenopathy, and she did not show increase in serum gamma-globulin or in plasma angiotensin-converting enzyme (ACE) levels, or increased CD4/CD8 ratio of lymphocytes obtained from bronchoalveolar lavage. These findings suggest that the present case had sarcoid reaction associated with papillary carcinoma. Although sarcoid reaction has been reported to be associated with various malignancies, only five cases, to our knowledge, are reported in the literature, which were associated with papillary carcinoma.

Cloning of rat uncoupling protein-3 and uncoupling protein-2 cDNAs: their gene expression in rats fed high-fat diet

In order to elucidate energy balance in the skeletal muscle, we cloned cDNA of a homologue of uncoupling protein (UCP) from rat skeletal muscle. We also cloned rat UCP-2 cDNA from rat brown adipose tissue (BAT). The UCP cloned from rat skeletal muscle showed 57% and 72% identity with rat UCP-1 and UCP-2. The mRNA was expressed abundantly in the skeletal muscle, moderately in the BAT, and slightly in the white adipose tissue (WAT) with a major band at 2.5 kb and a minor band at 2.8 kb, while the UCP-2 gene expression was widely detected in the whole body with substantial levels in the WAT and with slight levels in the skeletal muscle and BAT. The rat UCP cloned in the present study showed 86% identity with the recently cloned human UCP-3, which was also expressed abundantly in the skeletal muscle with a signal of 2.4 kb. Therefore, the rat UCP was considered to be rat UCP-3. In rats fed high-fat diet the UCP-3 gene expression was augmented 2-fold in the skeletal muscle while UCP-2 mRNA levels were increased significantly (1.6-fold) in the epididymal WAT. Augmented expression of UCPs may provide defense against high-fat induced obesity and impairment of glucose metabolism.

Antinuclear antibodies in children with chronic nonspecific complaints

Children who are chronically complaining nonspecific symptoms such as headache, fatigue, abdominal pain, and low grade fever are commonly seen in daily pediatric outpatient clinics. Some of them are unable to go to school and are diagnosed as school refusal by physicians or educational staff. On the other hand, there are children who do not fulfill any criteria of collagen diseases and whose anti-nuclear antibodies (ANA) are found to be positive. Some of these children have chronic nonspecific complaints. We prospectively studied the prevalence of ANA in children who visited a pediatric outpatient clinic because of chronic nonspecific complaints. Surprisingly, 74 out of 140 symptomatic children (52.4%) were positive for ANA, while only 5 out of 82 healthy control children (6.1%) were positive (p < 0.0001). 39 of 74 ANA positive patients (52.1%) have low ANA titers < or = 1:80, nevertheless 36 patients (47.9%) have high ANA titers > or = 1:160. ANA fluorescent patterns were homogeneous and speckled in 75.3%, speckled in 17.6% and others in 6.8%. ANA positive patients tended to have general fatigue and low grade fever, while gastrointestinal problems such as abdominal pain and diarrhea and orthostatic dysregulation symptoms were commonly seen in ANA negative patients. Children who were unable to go to school more than 1 day a week were seen significantly more in ANA positive patients than in negative patients. Autoantibody analysis using Western immunoblot revealed that 26 out of 63 ANA positive sera (41.3%) had antibodies to the 62 kD protein which had not been previously noticed. These data suggest that autoimmune mechanism may play a role in childhood chronic nonspecific symptoms. We therefore propose a new disease entity of the autoimmune fatigue syndrome in children. When chronically complaining children visit a pediatric out-patient clinic, immunological approaches should be considered before they are discriminated as school refusal or having psychogenic disorders.

Venereal infection of mares by equine arteritis virus and use of killed vaccine against the infection

Venereal infection with equine arteritis virus (EAV) was established in each of seven mares by inoculation via the cervix with 20 ml of viral suspension (> or = 8 x 10(6) plaque-forming units; PFU), following treatment with prostaglandin and oestradiol. A dose of < or = 8 x 10(5) PFU produced infection in only five of eight mares. Serum neutralizing antibody developed in mares manifesting clinical signs of equine viral arteritis (EVA), and a weak antibody was detectable in one apparently healthy mare inoculated with 8 x 10(5) PFU. Virus isolation was demonstrated not only in the buffy coat but also in nasal swabs of infected mares. EAV was isolated frequently from the body tissues of the mares (killed 10 to 34 days post-inoculation) up to day 12, but rarely from the reproductive tissues later than day 12. The virus persisted longest in the splenic and deep inguinal lymph nodes, followed by the spleen and internal iliac lymph nodes. Four mares immunized with a killed vaccine for EVA showed no clinical disease after venereal challenge with EAV; the virus was recovered from the buffy coat of three mares and from the nasal swab of one of them, but not from the remaining animal.

Hemagglutination with equine arteritis virus

Equine arteritis virus (EAV) grown on RK13 cell cultures was tested for hemagglutination (HA) with erythrocytes from a variety of species at 4 degrees C, room temperature and 37 degrees C. HA was observed at all temperatures with erythrocytes from mouse and chicken but not with those of cattle, horse, rabbit, guinea pig, mongolian gerbil, goose or chick embryo. Chickens showed an individual variation in agglutinability of their erythrocytes, requiring selection of birds to obtain erythrocytes for HA. The HA activity was enhanced by treatment of virus materials with Tween 80 followed by treatment with ether. The HA reaction was inhibited by specific antiserum. Higher HA-inhibiting (HI) antibody titers were obtained by the incubation of serum-HA antigen mixture at 4 degrees C for 24 hr. HI antibody titers of individual horse sera showed a significant positive correlation with their neutralizing antibody titers.