Effects of renin-angiotensin system blockade and dietary salt intake on left ventricular hypertrophy in Dahl salt-sensitive rats

We studied the effects of chronic blockade of the renin-angiotensin system on hypertension and cardiac left ventricular hypertrophy (LVH) in Dahl salt-sensitive (DS) rats given a high-salt or low-salt diet. [Experiment 1] Twelve-week-old male DS rats were fed an 8% NaCl diet and received the angiotensin II receptor (AT1) antagonist, candesartan (3 mg/kg/d), the angiotensin converting enzyme inhibitor enalapril (30 mg/kg/d), or vehicle for 6 wk after 3 wk of 8% salt-loading. Neither candesartan nor enalapril with concomitant high salt-loading attenuated the blood pressure (BP) elevation. LVH was also not attenuated significantly by these treatments. [Experiment 2] After 8 wk of 8% salt-loading, the rats were given a 0.3% NaCl diet and concurrently received candesartan, enalapril, or vehicle for 5 wk. Switching from the high-salt to low-salt diet significantly decreased BP and left ventricular mass in the vehicle-treated animals. Both candesartan and enalapril normalized BP during salt-depletion; the blockade of the renin-angiotensin system produced an additive reduction in LVH. These findings suggest that sodium intake and hemodynamic load, but not the renin-angiotensin system, may be major determinants of the development of LVH in DS rats.

Polymerase chain reaction analysis of the Y chromosome long arm in azoospermic patients: lack of the Y chromosome recognition motif (YRRM1) gene

We analyzed DNA from a patient with azoospermia whose Y chromosome was cytogenetically normal. A total of 16 loci on the Y chromosome long arm were examined: 15 loci between DYS7E and DYZ1, and the Y chromosome RNA recognition motif (YRRM1) locus, a candidate gene for the azoospermic factor AZF. We did not detect the YRRM1 gene in this patient. This finding supports the theory that YRRM1 is an essential gene for spermatogenesis.

[Cerebrovascular diseases in patients with arteriosclerotic aortic aneurysm and arteriosclerosis obliterans studied by carotid ultrasonography and by computed tomography of the brain]

To estimate the incidence of cerebrovascular diseases in arteriosclerotic aortic aneurysm (AA) and arteriosclerosis obliterans (ASO) and their characteristics, 92 patients with AA and 102 patients with ASO were studied with carotid ultrasonography and brain computed tomography (CT), and were compared with 49 patients with hypertension. The mean ages of the patients were 70 to 72 years old and all were male. Hypertension was common in the AA group: diabetes and cigarette smoking were common in the ASO group. Carotid plaque lesions seen on ultrasonography were significantly more common in the AA (66%) and ASO (85%) groups than in the patients with hypertension (39%). The mean carotid diameter was significantly greater in the AA group than in the other two groups. The mean wall thicknesses in the AA and ASO groups were greater than in the patients with hypertension. Computed tomography showed that low-density areas were also common in the AA group (56%) and ASO (53%) groups than in the patients with hypertension (24%). Most of the low-density areas were thought to be lacunae or dilated perivascular spaces in the subinsula and putamen. Moderately and highly severe periventricular lucencies were also common in the AA group. These findings indicate that carotid changes, seen in AA and ASO, reflected the characteristics of arterial lesions, and that arteriolosclerotic lesions were common in patients with AA and ASO. Therefore, cerebrovascular diseases should be taken into account in the management of patients with AA and ASO.

[A case of primary malignant lymphoma of the bladder]

The patient was a 34-year-old male. He visited a community hospital complaining of macroscopic hematuria and pollakiuria. Cystoscopic examination demonstrated a bladder tumor. Transurethral resection of the bladder tumor (TUR-Bt) was performed. Histological examination disclosed malignant lymphoma (non-Hodgkin's lymphoma, mixed type). The tumor was classified into the B cell type by the immunohistological staining with surface antigens. He was referred to St. Marianna University, School of Medicine for chemotherapy. Pelvic computed tomography (CT) after admission demonstrated a tumor with a wide pedicle located in the vesicle triangle extending to the posterolateral wall of the bladder. No abnormalities were found in other organs. After establishment of the diagnosis of primary bladder malignant lymphoma, 6 courses of chemotherapy (adriamycin, vincristine, cyclophosphamide, prednisolone, etoposide, methotrexate) were performed. The tumor disappeared completely on imaging studies after chemotherapy. Biopsy of the bladder disclosed no abnormal tissues. No evidence of recurrence or metastasis was found 5 years after chemotherapy.

Differential inhibition of Na+/Ca2+ exchanger isoforms by divalent cations and isothiourea derivative

We compared the properties of three mammalian Na+/Ca2+ exchanger isoforms, NCX1, NCX2, and NCX3, by analyzing the effects of Ni2+ and other cations as well as the recently identified inhibitor isothiourea derivatives on intracellular Na+-dependent 45Ca2+ uptake into CCL-39 (Dede) fibroblasts stably expressing each isoform. All these NCX isoforms had similar affinities for the extracellular transport substrates Ca2+ and Na+. Ni2+ inhibited 45Ca2+ uptake by competing with Ca2+ for the external transport site, with 10-fold less affinity in NCX3 than in NCX1 or NCX2. Ni2+ and Co2+ were most efficient in such discrimination of NCX isoforms, although their inhibitory potencies were less than those of La3+ and Cd2+. The monovalent cation Li+ stimulated 45Ca2+ uptake rate by all NCX isoforms similarly with low affinity, although the extent of stimulation was somewhat smaller in NCX1. On the other hand, the isothiourea derivative KB-R7943 was threefold more inhibitory to NCX3 than to NCX1 or NCX2. Thus distinct differences in the kinetic and pharmacological properties were detected between NCX3 and the other two isoforms.

Streptozotocin-induced diabetic cynomolgus monkey is a model of hypertriglyceridemia with low high-density lipoprotein cholesterol

Hypertriglyceridemia with low high-density lipoprotein (HDL) cholesterol is a risk factor of cardiovascular disease. We attempted to create an animal model of hypertriglyceridemia with low HDL cholesterol by intravenously injecting 30 mg/kg body weight streptozotocin (STZ) to cynomolgus monkeys. This induced hypoinsulinemia and resulted in a decrease in postheparin plasma lipoprotein lipase (LPL) activity and LPL enzyme mass, reduction of plasma HDL cholesterol and elevation of triglycerides. Low HDL cholesterol subsequently caused a reduction of HDL2b cholesterol, while hypertriglyceridemia caused an elevation of very low-density lipoprotein (VLDL) triglyceridemia. Apolipoprotein CII, a co-factor of LPL, was not affected by STZ administration. These results show that hypertriglyceridemia with low HDL cholesterol results from a reduction of LPL activity without affecting apolipoprotein CII after STZ administration. The STZ-induced diabetic cynomolgus monkey is a model of hypertriglyceridemia with low HDL cholesterol, and may be potentially beneficial for studying atherosclerosis caused by hypertriglyceridemia with low HDL cholesterol.

Influence of inoculation site of combined oil-adjuvanted vaccine on the antibody response in chickens

Inactivated oil-adjuvanted vaccines for ND, IB, and IC serotypes A and C (OILVAX NB2AC) have been marketed from 1993. In the outdoors, various inoculation sites have been used in chickens because to make the inoculation procedure easier. We examined whether differences would be obtained in the antibody response according to the inoculation sites; the subcutaneous inoculation into the back of the neck for OILVAX use, and the thigh, lower thigh, breast and shoulder muscle for the possible application of the inoculation outdoors. The clear order was not found in the NDV-HI titer, IBV-SN titer against Nerima or TM-86 among the inoculation sites used during the examination period. The IC serotypes A and C HI titers did vary among the inoculation sites; the subcutaneous inoculation produced the highest antibody titer, and high antibody titers were observed in the order of lower thigh muscle > or = thigh muscle > breast muscle > or = shoulder muscle inoculation.

Na+/Ca2+ exchanger overexpression impairs calcium signaling in fibroblasts: inhibition of the [Ca2+] increase at the cell periphery and retardation of cell adhesion

We examined the Ca2+ handling property and cell function of CCL39 fibroblasts highly overexpressing the cardiac isoform (NCX1) of Na+/ Ca2+ exchanger. In NCX1 transfectants in 146 mM Na+, ionomycin, alpha-thrombin or thapsigargin only produced a small transient increase in [Ca2+]i compared to the large increase seen in control cells, although resting [Ca2+]i was not significantly different between these cells. In Na+-free medium, in contrast, the [Ca2+]i responses in NCX1 transfectants and control cells stimulated with these agents were not different, indicating that the Ca2+ content of the intracellular store(s) does not decrease on NCX1 transfection. The expression levels of the endoplasmic reticulum and plasma membrane Ca2+-ATPases, and thrombin- or serum-stimulated cell growth were not altered in NCX1 transfectants. The latter finding suggests that Ca2+ signaling in the nucleus is not impaired appreciably. On fluorescence imaging and confocal microscopy, we found that [Ca2+] did not increase in the peripheral cytoplasm of these cells treated with alpha-thrombin in Na+-containing medium. In these NCX1 transfectants, activation of the plasma membrane Ca2+-activated K+ channels by thrombin or ionomycin was markedly suppressed, and the integrin-mediated adhesion to substrate was significantly delayed compared with control cells. NCX1-overexpressing CCL39 cells thus seem to be a good model with which we can study the Ca2+-regulated membrane processes under physiologically relevant conditions.

Regulation of the Na+/H+ exchanger in fibroblasts overexpressing the Na+/Ca2+ exchanger

To assess the role of Ca2+ in regulation of the Na+/H+ exchanger (NHE1),we used CCL-39 fibroblasts overexpressing the Na+/Ca2+ exchanger (NCX1). Expression of NCX1 markedly inhibited the transient cytoplasmic Ca2+ rise and long-lasting cytoplasmic alkalinization (60-80% inhibition) induced by alpha-thrombin. In contrast, coexpression of NCX1 did not inhibit this alkalinization in cells expressing the NHE1 mutant with the calmodulin (CaM)-binding domain deleted (amino acids 637-656), suggesting that the effect of NCX1 transfection involves Ca2+-CaM binding. Expression of NCX1 only slightly inhibited platelet-derived growth factor BB-induced alkalinization and did not affect hyperosmolarity- or phorbol 12-myristate 13-acetate-induced alkalinization. Downregulation of protein kinase C (PKC) inhibited thrombin-induced alkalinization partially in control cells and abolished it completely in NCX1-transfected cells, suggesting that the thrombin effect is mediated exclusively via Ca2+ and PKC. On the other hand, deletion mutant study revealed that PKC-dependent regulation occurs through a small cytoplasmic segment (amino aids 566-595). These data suggest that a mechanism involving direct Ca2+-CaM binding lasts for a relatively long period after agonist stimulation, despite apparent short-lived Ca2+ mobilization, and further support our previous conclusion that Ca2+- and PKC-dependent mechanisms are mediated through distinct segments of the NHE1 cytoplasmic domain.

[Magnetization transfer imaging of periventricular white matter lesions in patients with multi-infarct dementia]

Using magnetization transfer (MT) imaging, we studied the underlying pathological conditions of periventricular hyperintense (PVH) white matter changes seen on T2-weighted MR images of patients with multi-infarct dementia. Twenty-two patients with multiple lacunar infarcts and PVH lesions, including 11 with dementia (diagnosed as multi-infarct dementia) and 11 without dementia, and 10 control subjects (with multiple lacunes, but no PVH lesion) were studied using the MT technique. MT ratios (MTRs) were calculated for PVH lesions (normal-appearing frontal white matter in controls) and the genu of the corpus callosum. Signal intensities on T2-weighted images in PVH lesions of patients were significantly higher than those in normal-appearing white matter of controls, while there were no significant differences in signal intensity in the genu of the corpus callosum among the dementia, non-dementia and control groups. However, MTRs in patients with PVH lesions were significantly lower than those in controls, and MTRs in demented patients were significantly lower than those in non-demented patients. Moreover, MTRs in the genu of the corpus callosum of demented patients were significantly lower than in those in non-demented patients and controls. MTRs in PVH lesions and the genu of the corpus callosum significantly correlated with Hasegawa's dementia scale score. These results suggest that there is some difference in histopathologic changes of PVH lesions between demented and non-demented patients and that the pathological substrate in the corpus callosum may play a role in inducing cognitive decline. Studies with MT imaging may allow the characterization of different pathological conditions that cannot be visualized by conventional MRI.

Factors affecting longevity

In view of the fact that hypercholesterolemia occurs in 31.4%, hypertension in 16.7% and the smoking rate is 58.8% in males (Table 8), risk factors are not low. Despite this, we Japanese preserve a leading position regarding longevity. I hope that I have provided some evidence supporting the proposal that apparently not only a low fat intake but other factors including genetic make up and a relatively high antioxidant intake contribute to our longevity.

Effect of 20 days' bed rest on the reverse cholesterol transport system in healthy young subjects

OBJECTIVES

To study the effects of 20 days of bed rest on HDL cholesterol, lipoprotein lipase, hepatic triglyceride lipase, cholesterol ester transfer protein and lecithin-cholesterol acyltransferase.

DESIGN

A 20-day intervention study.

SETTING

Makita general hospital.

SUBJECTS

Five male and five female healthy participants, mean age 20.4 years, range 19-24 years.

INTERVENTIONS

Twenty days of bed rest.

MAIN OUTCOME MEASURES

Lipid, lipoprotein, lipoprotein lipase, hepatic triglyceride lipase, cholesterol ester transfer protein and lecithin-cholesterol acyltransferase.

RESULTS

Fasting HDL, HDL2 and HDL3 cholesterol levels decreased from 1.748 to 1.404 mmol L(-1) (P < 0.01), from 0.807 to 0.628 mmol L(-1) (P < 0.01) and from 0.939 to 0.784 mmol L(-1) (P < 0.05), respectively, while VLDL triglyceride levels increased from 0.365 to 0.754 mmol L(-1) (P < 0.05). Plasma post-heparin lipoprotein lipase activity decreased from 0.494 to 0.418 micromol mL(-1) h(-1) (P < 0.01), but plasma post-heparin hepatic triglyceride lipase activity and cholesterol ester transfer protein activity did not change during bed rest. Lecithin-cholesterol acyltransferase activity increased from 72.5 to 84.8 nmol mL(-1) h(-1) (P < 0.001).

CONCLUSIONS

Twenty days of bed rest induced a decline in HDL cholesterol levels and an increase in VLDL triglyceride levels. When considering lipoprotein lipase, hepatic triglyceride lipase, cholesterol ester transfer protein and lecithin-cholesterol acyltransferase as factors in the decreased HDL cholesterol, the contribution of lipoprotein lipase is suggested.

Diffusion-weighted MR imaging of the hippocampus and temporal white matter in Alzheimer's disease

We investigated the changes in water diffusion in the hippocampus and the temporal white matter (the temporal stem) in eight patients with possible Alzheimer's disease (AD), 10 patients with probable AD, and 10 age-matched controls, using coronal diffusion-weighted magnetic resonance (MR) imaging. Apparent diffusion coefficients (ADCs) were derived for the three orthogonal axes and an index of diffusion anisotropy (IDA = ADC(max-min)/ADC(mean)) was then calculated. Although no significant differences were found in ADC and IDA values in the hippocampal body between controls and patients, vertical (superior-inferior) ADC values and ADC(mean) values in the temporal stem of patients with AD were significantly higher than those in controls, and IDA values were therefore significantly lower in patients with possible or probable AD than those in controls. Moreover, IDA values in the temporal stem were significantly correlated with the clinical severity. These results suggest that decreased fiber density, such as the disruption and loss of axonal membranes or myelin, occur early in the temporal stem, probably due to secondary degeneration related to grey matter pathology including the medial temporal lobe.

Diffusion-weighted magnetic resonance and magnetization transfer imaging in the assessment of ischemic human stroke

We compared characteristic features in ischemic stroke lesions from the hyperacute to the chronic stage on diffusion-weighted (DW) and magnetization transfer (MT) images with those on T2-weighted (T2W) images, and assessed changes in apparent diffusion coefficient (ADC), MT effect (MTe), and T2 ratios (infarct/normal) over time. DW images were particularly useful for detecting hyperacute infarcts within 6 hours of onset and in distinguishing acute lesions from chronic lesions. ADC ratios were lower within 7 days after onset and rose toward 1.0 in the subacute phase, becoming relatively isotense on ADC maps, but elevated thereafter. Although MTe ratios were unchanged or only subtly changed in the acute stage, they became significantly lower in the subacute and chronic stages. These combined magnetic resonance (MR) techniques were useful in the assessment of ischemic stroke and facilitated the determination of the age of cerebral infarct.

Ultrastructure of murine tumour cell lines defective in MHC class I expression before and after interferon-gamma treatment

Two tumour cell clones, 6D1 and 4C2 cells, which are defective both in the major histocompatibility gene complex (MHC) class I expression and in the endogenous antigen presentation, are recovered with interferon (IFN)-gamma treatment. The present study describes the ultrastructure of these cells by using scanning and transmission electron microscopy in relation to the effect of IFN-gamma treatment. The general morphology of these cells was found to be similar to each other and comparable to that of a tumour cell clone, 4A1 cells, of the same origin, normal in MHC class I expression; they exhibited a fibroblast-like appearance and had many blebs on all the cell surfaces, with desmosome-like junctions between cells. On IFN-gamma treatment, surface fine blebs appeared less, and mitochondria became more densely stained. Expression of MHC class I molecules on the cell surface was much higher in the IFN-gamma treated 6D1 and 4C2 cells than in untreated cells, when estimated by immunoelectron microscopy. The addition of an epitope peptide to these cells did not enhance the class I expression, which differed from other antigen presentation-defective cells such as RMA-S cells, nor change the cell surface morphology.

Aqueous solubilization of transmembrane peptide sequences with retention of membrane insertion and function

We recently reported that the peptide C-K4-M2GlyR mimics the action of chloride channels when incorporated into the apical membrane of cultured renal epithelial monolayers. C-K4-M2GlyR is one of a series of peptides that were prepared by the addition of lysine residues to the N- or C-terminus of the M2 transmembrane sequence of the brain glycine receptor. This study addresses how such modifications affect physical properties such as aqueous solubility, aggregation, and secondary structure, as well as the ability of the modified peptides to form channels in epithelial monolayers. A graded improvement in solubility with a concomitant decrease in aggregation in aqueous media was observed for the M2GlyR transmembrane sequences. Increases in short-circuit current (I(SC)) of epithelial monolayers were observed after treatment with some but not all of the peptides. The bioactivity was higher for the more soluble, less aggregated M2GlyR peptides. As described in our previous communication, sensitivity of channel activity to diphenylamine-2-carboxylate, a chloride channel blocker, and bumetanide, an inhibitor of the Na/K/2Cl cotransporter, was used to assess changes in chloride selectivity for the different assembled channel-forming peptides. The unmodified M2GlyR sequence and the modified peptides with less positive charge are more sensitive to these agents than are the more highly charged forms. This study shows that relatively insoluble transmembrane sequences can be modified such that they are easier to purify and deliver in the absence of organic solvents with retention of membrane association, insertion, and assembly.

Binding of influenza and paramyxoviruses to Group B Streptococcus with the terminal sialyl-galactose linkage

Using the virus-binding assay and scanning electron microscopy (SEM), influenza A and B type viruses and two paramyxoviruses, parainfluenza (Sendai) and mumps viruses, were found to bind to Group B Streptococcus (GBS), type Ia and II, with the terminal sialyl-galactose linkage, although some viruses detached during the sample processing for SEM, and mumps virus did not bind to GBSIa. Binding of viruses eluted from GBS at 37 degrees C depended on combination of virus and GBS. The biological significance of these findings is discussed.

Increased water diffusion in cerebral white matter in Alzheimer's disease

We investigated the changes in water diffusion in the cerebral white matter in 19 patients with Alzheimer's disease (AD), including 11 without and 8 with periventricular hyperintensity (PVH) lesions, using diffusion-weighted magnetic resonance imaging (MRI). The apparent diffusion coefficients in the anterior and posterior white matter were significantly higher in the 19 AD patients than in the 10 age-matched controls. The apparent diffusion coefficients were higher in patients with PVH than in those without. The anisotropic ratios, defined as diffusion restricted perpendicular to the direction of the nerve fibers, were significantly higher in AD patients, even in those without PVH, than in the controls. Our results suggest that mild myelin loss occurs in AD patients even in the apparently normal white matter. A definite loss of myelin and axons, including incomplete infarction, occurs in the white matter, as seen on T2-weighted images as PVH. Studies with diffusion-weighted MRI may allow the characterization of different pathological processes and enable the demonstration of underlying white matter lesions in AD that cannot be visualized by conventional MRI.

Differences in refractory-period response of canine subendocardium and subepicardium to bunazosin, an alpha1-adrenoceptor antagonist, and propranolol during myocardial ischemia

Our objective was to investigate the effects of alpha1- or beta-adrenoceptor blockers on endocardial and epicardial refractory-period changes during myocardial ischemia in alpha-chloralose-anesthetized dogs. The first and second diagonal branches of the left anterior descending coronary artery were ligated. The refractory period was determined by an S1-S2 extrastimulus method. Dogs were treated with the alpha1-blocker bunazosin (0.1-0.2 mg/kg, i.v.; n = 16), the beta-blocker propranolol (0.2 mg/kg, i.v.; n = 15), or saline (n = 11). Dogs that developed ventricular tachycardia/fibrillation (VT/VF) during the experiment were excluded from the statistical assessment in refractory periods. In all groups, coronary ligation produced a significant shortening of the refractory period of ischemic epicardial tissue (p < 0.05) but only minimal shortening of ischemic endocardial refractory periods, resulting in an increased difference in repolarization time between the endo- and epicardial sites. Treatment with bunazosin ameliorated this ischemia-related shortening of refractory periods at both the endo- and epicardial sites, with a greater effect seen epicardially (p < 0.05), resulting in values similar to those in the nonischemic tissue. Treatment with propranolol prolonged refractory periods more in the epicardial (p < 0.01) than in endocardial sites, exacerbating the disparity in the refractory period between the endo- and epicardial sites (p < 0.05). Propranolol also prolonged the refractory period of nonischemic tissue (p < 0.05 and p < 0.01 in endo- and epicardial sites, respectively), resulting in a significant difference between the ischemic and normal myocardium at the endocardial site (p < 0.05). Results suggest that the alpha1-blocker bunazosin reduces the refractory-period disparity between the ischemic and normal myocardium without increasing the disparity between the endo- and epicardial surfaces, whereas propranolol produces a greater disparity.

Effects of ethyl-all-cis-5,8,11,14,17-icosapentaenoate (EPA-E), pravastatin and their combination on serum lipids and intimal thickening of cuff-sheathed carotid artery in rabbits

The anti-arteriosclerotic effects of ethyl all-cis-5, 8, 11, 14, 17-icosapentaenoate (EPA-E), pravastatin and their combination in cuff-treated rabbits were investigated. EPA-E at 600 mg/kg, pravastatin at 50 mg/kg or their combination was orally administered once daily for 5 weeks, and each of the animals was sheathed with a cuff on the carotid artery 2 weeks after the beginning of drug administration. EPA-E, pravastatin and their combination significantly reduced serum total cholesterol compared to the control group. EPA-E also potently reduced serum triglyceride, while pravastatin only slightly reduced it. The combination of these two agents had the most potent effect on the level of serum triglyceride. Serum phospholipids were also reduced by these treatments in a similar fashion. At the end of treatment, diffuse intimal thickening was observed in the cuff-covered region in all animals in the control group, and the intima/media area ratio in this group was 0.293 +/- 0.038. Treatment with EPA-E alone tended to prevent the intimal thickening, and the intima/media area ratio was 0.209 +/- 0.058 (p = 0.094). This ratio was 0.287 +/- 0.048 (p = 0.902) when pravastatin was administered alone, indicating that it had no significant effect on intimal thickening. The ratio was 0.175 +/- 0.041 (p = 0.042) when both EPA-E and pravastatin were administered, indicating that this combination had a significant inhibitory effect on intimal thickening in the cuff-sheathed region. These findings suggest that combined treatment with EPA-E and pravastatin is more effective than respective monotherapies in lowering serum lipids and/or preventing an intimal thickening as events of atherogenesis.

Calmodulin-binding autoinhibitory domain controls "pH-sensing" in the Na+/H+ exchanger NHE1 through sequence-specific interaction

The calmodulin (CaM)-binding domain reduces the affinity of the Na+/H+ exchanger NHE1 for intracellular H+ by exerting an autoinhibitory function in quiescent cells. We replaced this domain (aa 637-656) with homologous segments from other NHE isoforms (NHE2 and 4) or functionally similar regions from other sources (Na+/Ca2+ exchanger, CaM-dependent protein kinase II, plasma membrane Ca2+-pump, or CaM-binding peptide Trp3). The NHE-1-, NHE2-, and NHE4-segments bound CaM with Kds of 16, 130, and 27 nM, respectively. These chimeric molecules were expressed in the exchanger-deficient cell PS120. NHE1 with incorporated NHE2-segment was activated in response to Ca2+-mobilizing agents ionomycin and thrombin resulting in an alkaline shift of the intracellular pH (pHi)-dependence of 22Na+ uptake, as was the case with the intact rat NHE2. In contrast, incorporation of the NHE4-segment or other CaM-binding segments induced a constitutive alkaline shift of pHi-dependence with concomitant abolishment of Ca2+-dependent activation, indicating that these segments could not function as an autoinhibitory domain in NHE1. Detailed analyses revealed that Leu639, Lys651 and Tyr652, conserved in the NHE1- and NHE2-segments, but not in the NHE4-segment, are important for the autoinhibition. Furthermore, 125I-labeled CaM-binding peptide from NHE1 was efficiently crosslinked to the NHE1 protein, suggesting that the inhibitory domain physically interacts with part(s) of the molecule. Together, these findings support the notion that the reduction of H+ affinity in Na+/H+ exchange occurs through a mechanism involving a highly sequence-specific interaction of the inhibitory domain with its putative acceptor in NHE1.