Effects of morphine on pentobarbital-induced responses in mu-opioid receptor knockout mice

Effects of morphine on the potentiation of pentobarbital-induced responses were investigated using mu-opioid receptor knockout mice. The duration of loss of righting reflex, hypothermia, and loss of motor coordination induced by pentobarbital were measured after pretreatment with either morphine or saline. Morphine pretreatment failed to show potentiation of both pentobarbital-induced loss of righting reflex and hypothermia in mu-opioid receptor knockout mice, while it significantly potentiated these responses in the wild-type controls. For motor incoordination test, morphine potentiated pentobarbital-induced motor incoordination in the wild-type mice. However, morphine may have opposite effects in the mu-opioid receptor knockout mice. These results demonstrate that synergism between morphine and pentobarbital is not detected in mu-opioid receptor knockout mice and that potentiation of pentobarbital-induced loss of righting reflex and hypothermia by morphine is mediated through mu-opioid receptor. It was interesting to note that pentobarbital-induced decrease in body temperature was less severe in mu-opioid receptor knockout mice than in wild-type mice.

Pulmonary phenotype of CCSP/UG deficient mice: a consequence of CCSP deficiency or altered Clara cell function?

Clara cell secretory protein (CCSP) is the most abundant secreted protein within airways of the lung. Moreover, CCSP levels are modulated in human lung disease, supporting a potentially important role for CCSP and/or Clara cells in lung homeostasis. However, in vivo roles for CCSP remain elusive. A popular hypothesis is that CCSP is a regulator of the inflammatory response. The purpose of this review is to provide an overview of the phenotype of CCSP null mice and relate this phenotype to proposed functions for the protein. Phenotypic analysis of mice homozygous for the CCSP-1 null allele of the CCSP gene (CCSP-/-1) revealed susceptibility to inhaled oxidant gases. Sensitivity of CCSP-/-1 mice to inhaled ozone is unrelated to alterations in antioxidant defenses, but is associated with increased cellular injury. Additional studies investigating inflammatory control in CCSP deficient mice found no differences between wild-type and CCSP-/-1 mice in their inflammatory response to low-dose inhaled endotoxin exposure, arguing against a role for CCSP in regulation of pulmonary inflammation. The findings among CCSP-/-1 mice of ultrastructural alterations to Clara cell secretory apparatus, with associated changes in airway lining fluid protein composition, demonstrate that the CCSP-/-1 genotype results in more complex changes to airways than CCSP deficiency per se. It can be concluded that CCSP does not regulate endotoxin-induced pulmonary inflammation. Moreover, CCSP-/-1 mice represent a valuable tool for probing functional roles for Clara cells in regulation of airway lining fluid composition and lung pollutant susceptibility.

Insight into the physiological function(s) of uteroglobin by gene-knockout and antisense-transgenic approaches

To determine the physiological function(s) of uteroglobin (UG), a steroid-inducible, homodimeric, secreted protein, we have generated transgenic mice that either are completely UG-deficient due to UG gene-knockout (UG-KO) or are partially UG-deficient due to the expression of UG antisense RNA (UG-AS). Both the UG-KO and UG-AS mice develop immunoglobulin A (IgA) nephropathy (IgAN), characterized by microhematuria, albuminuria, and renal glomerular deposition of IgA, fibronectin (Fn), collagen, and C3 complement. This phenotype of both UG-KO and UG-AS mice is virtually identical to that of human IgAN, the most common primary glomerulopathy worldwide. The molecular mechanism by which UG prevents this disease in mice appears to center around UG's interaction with Fn. Since Fn, IgA, and UG are present in circulation and high plasma levels of IgA-Fn complex have been reported in human IgAN, we sought to determine whether UG interacts with Fn and prevents Fn-Fn and/or IgA-Fn interactions, essential for abnormal tissue deposition of Fn and IgA. Our coimmunoprecipitation studies uncovered the formation of Fn-UG heteromers in vitro and these heteromers are detectable in the plasma of normal mice, but not UG-KO mice. Further, high plasma levels of IgA-Fn complex, a characteristic of human IgAN patients, were also found in UG-KO mice. Finally, coadministration of UG + Fn or UG + IgA to UG-KO mice prevented glomerular deposition of Fn and IgA, respectively. Our results define a possible molecular mechanism of IgAN and provide insight into at least one important physiological function of UG in maintaining normal renal function in mice.

The D2 receptor is critical in mediating opiate motivation only in opiate-dependent and withdrawn mice

According to the dual systems model for opiate reward, dopamine mediates opiate motivation when an animal is in a deprived motivational state (i.e. opiate-dependent and in withdrawal) and not when an animal is in a nondeprived state (i.e. previously drug-naive). To determine the role of the D2 dopamine receptor subtype in mediating opiate motivation, we examined the behaviour of N5 congenic D2 receptor knockout mice and their wild-type siblings in opiate-naive and opiate-dependent and withdrawn place conditioning paradigms. Opiate-naive D2 receptor knockout mice demonstrated acquisition of morphine-conditioned place preference but failed to acquire place preference when conditioned in the deprived state. We propose that D2 receptor function is critical in mediating the motivational effects of opiates only when the animal is in an opiate-dependent and withdrawn motivational state. These findings also underscore the important influence of the genetic background to a given phenotype, as evidenced by the observation that increasing the allelic contribution from the 129/SvJ strain abolishes morphine place preference in C57BL/6 wild-type mice.

Morphine tolerance and dependence in nociceptin/orphanin FQ transgenic knock-out mice

It has been hypothesized that morphine tolerance and dependence in mice following chronic exposure may reflect increased compensatory activity of antiopioid systems. The endogenous peptide nociceptin/orphanin FQ has been shown to have anti-opioid effects, for example antagonizing morphine analgesia. Moreover, chronic morphine administration increases synthesis of the peptide, and morphine tolerance and dependence can be attenuated or reversed by antagonists and agonists of the nociceptin/orphanin FQ receptor, respectively. The present study seeks to confirm a role for nociceptin/orphanin FQ in opioid tolerance and dependence by comparing morphine ED(50) values and naloxone-precipitated withdrawal jumping in mice homozygous (knock-out) and heterozygous for a null mutation of the Npnc1 gene encoding the nociceptin/orphanin FQ propeptide, and their wild type littermates, following chronic morphine exposure. Relative to morphine-naive control mice, significant rightward shifts in the morphine dose-response curve, resulting in increased morphine ED(50) values (approximately two to three-fold), was observed for all genotypes following three days of repeated systemic morphine injections. However, no differences between genotypes in the magnitude of tolerance were observed. In contrast, knock-out mice displayed significantly increased naloxone-precipitated withdrawal jumping relative to heterozygous and wild-type mice following implantation with a morphine pellet (25mg) for 72h. Use of nociception/orphaninFQ transgenic knock-out mice thus demonstrate the differential involvement of nociceptin/orphanin FQ in morphine tolerance and dependence.

Improved recovery after spinal cord injury in neuronal nitric oxide synthase-deficient mice but not in TNF-alpha-deficient mice

Wild-type mice and mice lacking nitric oxide synthase (NOS) of neuronal type or TNF-alpha were subjected to an extradural compression of the thoracic spinal cord. The functional outcome of the hind limbs was assessed by using a motor function score (MFS). The injury resulted in paraplegia of the hind limbs in wild-type mice at day 1 after injury. Gradual recovery was observed during the following 14 days. Injured NOS -/- animals had an improved hind limb motor function during the entire observation period compared to wild-type controls. The difference was statistically significant on day 10 (p < 0.022) and day 14 (p < 0.048) after injury. At the site of injury, there was a trend of gray matter preservation in NOS -/- mice, as measured by MAP2 staining (p < 0.077). Injured mice lacking TNF-alpha had the lowest motor score among all the groups on day 1. During the following period, they had motor scores similar to those of wild-type controls and there was no significant difference at any time point. TNF-alpha -/- animals showed a trend of decreased white matter preservation compared to wild-type animals (p < 0.097). Our study shows that after spinal cord injury, mice lacking NOS have a better functional ability of their hind limbs than controls with the same degree of injury. This would indicate that the functional outcome is influenced in a negative way in wild mice by the presence of NO. The degree of secondary damage to the spinal cord might be attenuated in NOS-deficient mice.

Postnatal development of perineuronal nets in wild-type mice and in a mutant deficient in tenascin-R

The extracellular matrix glycoprotein tenascin-R (TN-R), colocalizing with hyaluronan, phosphacan, and aggregating chondroitin sulphate proteoglycans in the white and grey matter, is accumulated in perineuronal nets that surround different types of neurons in many brain regions. To characterize the role of TN-R in the formation of perineuronal nets, we studied their postnatal development in wild-type mice and in a TN-R knock-out mutant by using the lectin Wisteria floribunda agglutinin and an antibody to nonspecified chondroitin sulphate proteoglycans as established cytochemical markers. We detected the matrix components TN-R, hyaluronan, phosphacan, neurocan, and brevican in the perineuronal nets of cortical and subcortical regions. In wild-type mice, lectin-stained, immature perineuronal nets were first seen on postnatal day 4 in the brainstem and on day 14 in the cerebral cortex. The staining intensity of these nets for TN-R, hyaluronan, phosphacan, neurocan, and brevican was extremely weak or not distinguishable from that of the surrounding neuropil. However, all markers showed an increase in staining intensity of perineuronal nets reaching maximal levels between postnatal days 21 and 40. In TN-R-deficient animals, the perineuronal nets tended to show a granular component within their lattice-like structure at early stages of development. Additionally, the staining intensity in perineuronal nets was reduced for brevican, extremely low for hyaluronan and neurocan, and virtually no immunoreactivity was detectable for phosphacan. The granular configuration of perineuronal nets became more predominant with advancing age of the mutant animals, indicating the continued abnormal aggregation of chondroitin sulphate proteoglycans complexed with hyaluronan. As shown by electron microscopy in the cerebral cortex, the disruption of perineuronal nets was not accompanied by apparent changes in the synaptic structure on net-bearing neurons. The regional distribution patterns and the temporal course of development of perineuronal nets were not obviously changed in the mutant. We conclude that the lack of TN-R initially and continuously disturbs the molecular scaffolding of extracellular matrix components in perineuronal nets. This may interfere with the development of the specific micromilieu of the ensheathed neurons and adjacent glial cells and may also permanently change their functional properties.

Altered pathological progression of diaphragm and quadriceps muscle in TNF-deficient, dystrophin-deficient mice

We have previously demonstrated a role for T cells in Duchenne muscular dystrophy (DMD) using the mdx mouse and have shown that T cell killing of dystrophic muscle can occur through perforin-dependent and perforin-independent mechanisms. In this investigation, we explore the possibility that one perforin-independent mechanism utilized by the T cells is cytokine-based killing, specifically by tumor necrosis factor (TNF). We tested this hypothesis by generating mice that are TNF-deficient and dystrophin-deficient (TNF-/mdx). Body mass and muscle mass of the TNF-/mdx mice were significantly less than TNF+/mdx mice at 8 weeks of age. Creatine kinase levels and overall muscle strength were unchanged. Histopathology measurements showed different results in the diaphragm and quadriceps muscles. These data suggest that removal of TNF in vivo in dystrophic mice has differential effects on diaphragm and quadriceps suggesting that TNF is an unfavorable target for immunotherapy for DMD.

Estrogen-regulated progestin receptors are found in the midbrain raphe but not hippocampus of estrogen receptor alpha (ER alpha) gene-disrupted mice

Estrogen and progesterone may modulate serotonergic function through intracellular receptors, alpha (ER alpha) and/or beta (ER beta), and the progestin receptor (PR). Studies in macaque and rat suggest species differences in steroid action. Presently, we examined the mouse. To identify whether ER alpha is involved in estrogen induction of PR in midbrain raphe, we studied the ER alpha gene-disrupted (alpha ERKO) mouse. The hippocampus was examined as another estrogen/progestin-sensitive brain area reported to express ER alpha, ER beta, and PR. Female and male homozygous alpha ERKO and wildtype mice were gonadectomized and given estradiol benzoate or vehicle. Dual-label immunocytochemistry was performed for PR or ER alpha and the serotonin-synthesizing enzyme, tryptophan hydroxylase (TPH). Cells exhibiting PR immunoreactivity (PR-ir) or ER alpha-ir were observed in dorsal and median raphe and hippocampus in both sexes. No ER alpha-ir cells were observed in alpha ERKO brains. In raphe, PR-ir or ER alpha-ir often colocalized with TPH-ir. Thus, estrogen and progesterone may directly modulate gene expression in select serotonergic neurons via ER alpha and PR in female and male mice. Estrogen significantly increased the number of PR-ir cells, and the percentage of PR-ir cells colocalizing TPH-ir in both raphe nuclei, regardless of sex and genotype. Although less among alpha ERKO mice, the significant estrogen induction of PRs implicates the involvement of another ER, perhaps ER beta. In hippocampus, distinct estrogen-induced PR-ir cells were observed only in wildtype animals, demonstrating an ER alpha-mediated event in this forebrain region. Collectively, these findings suggest that estrogen can regulate the expression of one gene (the PR) via multiple mechanisms, based upon brain region.

Onion-bulb formation after a single compression injury in the macrophage scavenger receptor knockout mice

Onion-bulb (OB) formation is often encountered in acquired neuropathies such as chronic inflammatory demyelinating polyradiculoneuropathy and diabetic neuropathy and is believed to require repeated injuries to peripheral nerves. Although this suggests that remaining damaged cell membranes, including myelin debris, might trigger OB formation, the molecular mechanism remains unclear. In this study, we were successful in producing many small OBs after a single compression injury to peripheral nerves of the knockout mice deficient of macrophage scavenger receptor class A (MSR-A). Although morphometry showed no difference in the average densities of the remaining myelinating fibers between wild-type and MSR-A knockout mice after the compression injury, there were more macrophages and myelin debris positive for oxidized-phosphatidylcholine in the nerves from the MSR-A knockout mice. We believe that OB formation was induced after a single compression injury as the result of delayed phagocytosis of myelin debris possessing oxidized lipids by MSR-A deficient macrophages. The present work shed light on the molecular mechanism of OB formation seen in chronic neuropathies and provided a model for further investigation.

Involvement of sst2 somatostatin receptor in locomotor, exploratory activity and emotional reactivity in mice

Somatostatin (SRIF) controls many physiological and pathological processes in the central nervous system but the respective roles of the five receptor isotypes (sst1-5) that mediate its effects are yet to be defined. In the present study, we attempted to identify functions of the sst2 receptor using mice with no functional copy of this gene (sst2 KO mice). In contrast with control 129Sv/C57Bl6 mice, sst2 mRNA was no longer detectable in the brain of sst2 KO mice; 125I-labeled Tyr0DTrp8-SRIF14 binding was also greatly reduced in almost all brain structures except for the hippocampal CA1 area, demonstrating that sst2 accounts for most SRIF binding in mouse brain. Invalidation of this subtype generated an increased anxiety-related behaviour in a number of behavioural paradigms, while locomotor and exploratory activity was decreased in stress-inducing situations. No major motor defects could be detected. sst2 KO mice also displayed increased release of pituitary ACTH, a main regulator of the stress response. Thus, somatostatin, via sst2 receptor isotype pathways, appears involved in the modulation of locomotor, exploratory and emotional reactivity in mice.

Role of the GABA(A) receptor gamma2 subunit in the development of gonadotropin-releasing hormone neurons in vivo

We have employed transgenic mouse models to examine the functional significance of the gamma2 subunit of the GABA(A) (gamma-aminobutyric acid) receptor to the correct development of gonadotropin-releasing hormone (GnRH) neurons in vivo. In the first experiment, the expression of gamma2 subunit protein by the GnRH phenotype was determined using transgenic mice in which GnRH gene sequences direct the expression of the LacZ reporter to the nucleus of the GnRH neurons. This greatly facilitates the immunocytochemical identification of non-nuclear-located antigens within GnRH neurons and revealed that approximately 25% of juvenile GnRH neurons were immunoreactive for the gamma2 subunit and that this increased to 40% in pubertal mice. In the second experiment, GnRH mRNA expression was examined in the brains of gamma2 subunit knockout mice (gamma2(0/0)) and their wild-type (gamma2+/+) littermates at embryonic day 15 and postnatal days (P) 0 and 11-16 using in situ hybridization. The distribution and numbers of cells expressing GnRH mRNA in gamma2+/+ and gamma2(0/0) mice were not found to differ at any age. However, the GnRH mRNA content of medial septal cells was significantly lower in gamma2(0/0) compared with gamma2+/+ mice at P11-16 (P<0.05) and the same trend was observed for preoptic area neurons. These results demonstrate that while the gamma2 subunit of the GABA(A) receptor is expressed by postnatal GnRH neurons, their embryonic development does not require a functional gamma2 subunit. In contrast, postnatal GnRH mRNA expression was found to be dependent upon signalling through the GABA(A) receptor.

p75(NGFR) and cholinergic neurons in the developing forebrain: a re-examination

The low-affinity nerve growth factor receptor (p75(NGFR)) apparently can mediate apoptosis in a variety of cells in vitro and in vivo. Previously, our laboratory suggested that p75(NGFR) induced apoptosis in a subpopulation of cholinergic forebrain neurons during postnatal development, i.e., the number of choline acetyltransferase (ChAT)-positive neurons in a control strain of mice decreased whereas it remained higher in p75(NGFR)-deficient (-/-) mice. Discrepancies with subsequent data sets in our laboratory caused us to thoroughly re-analyze the fate of these cholinergic medial septum and neostriatal neurons in new sets of p75(NGFR) -/- and two DNA control strains of mice during development. Between postnatal day (P)6 and P15 the number of ChAT-positive neurons detected in the medial septum of 129/Sv mice and Balb/c mice increased by approximately 64% and approximately 62%, respectively. This increase is contrary to previous reports from our laboratory and indicative of normal postnatal development (including an increase in ChAT-enzyme) of the cholinergic forebrain neurons. In p75(NGFR) -/- mice the number of ChAT-positive neurons in the medial septum remained constant between P6 and P15 and was approximately 31% and approximately 56% higher at P6 than 129/Sv and Balb/c mice, respectively. At P15 and adulthood, p75(NGFR) -/- mice had similar numbers of cholinergic neurons as control mice. In the developing neostriatum, the number of ChAT-positive neurons increased by approximately 56% between P6 and P15 and did not differ between p75(NGFR) -/- and control mice at any time. Analyses for apoptotic DNA fragmentation (TUNEL labeling) at P8 revealed no differences between p75(NGFR) -/- and control mice in 12 forebrain regions, including the septum and neostriatum. At all times, all mice had similar levels of acetylcholinesterase-positive cholinergic innervation of the molecular layer in the dorsal dentate gyrus. These findings suggest that the p75(NGFR) does not necessarily mediate apoptosis in medial septum or neostriatal cholinergic neurons during the postnatal time period. The discrepant results of the previous study are most likely due to a less rigorous application of criteria for data acquisition, including anatomical boundaries that define the nucleus.

Insight into prostaglandin, leukotriene, and other eicosanoid functions using mice with targeted gene disruptions

In recent years, there has been an exponential increase in the number of targeted gene disruptions performed in mice. At least 18 different gene knockouts have now been reported that have direct relevance to eicosanoid biology. These include genes that influence substrate availability (phospholipases), metabolism to eicosanoids (e.g., prostaglandin H synthases, lipoxygenases), and eicosanoid action (e.g., receptors for various prostaglandins). This minireview will outline the phenotype of these knockout mice and what has been learned about eicosanoid functions through use of this novel methodology.

Genetic approaches to studying norepinephrine function: knockout of the mouse norepinephrine transporter gene

Norepinephrine is an important chemical messenger in the nervous system. It regulates affective states, learning and memory, endocrine and autonomic functions. It has been implicated in depression, aggression, and addiction, as well as cardiac and thermal dysregulation. The norepinephrine transporter functions by uptaking norepinephrine back into the cell for cyclic use, and is a direct target of a number of antidepressants and psychostimulants. Functional deletion (knockout) of monamine transporters results in increases in extracellular levels of neurotransmitters, thereby prolonging their actions. For the norepinephrine transporter knockout mice, this altered state of the norepinephrine system should simulate the therapeutic effects of norepinephrine selective antidepressants and some of the effects of psychostimulants. Careful use of such an animal model can hopefully provide valuable insight into the multiple roles norepinephrine plays in normal and pathological physiology.

UNC-13 and neurotransmitter release

Genetic studies in mice, worms and flies indicate that the synaptic protein UNC-13 plays a central role in the regulation of vesicle fusion.

Effects of knockout of the protein kinase C beta gene on glucose transport and glucose homeostasis

The beta-isoform of protein kinase C (PKC) has paradoxically been suggested to be important for both insulin action and insulin resistance as well as for contributing to the pathogenesis of diabetic complications. Presently, we evaluated the effects of knockout of the PKCbeta gene on overall glucose homeostasis and insulin regulation of glucose transport. To evaluate subtle differences in glucose homeostasis in vivo, knockout mice were extensively backcrossed in C57BL/6 mice to diminish genetic differences other than the absence of the PKCbeta gene. PKCbeta-/- knockout offspring obtained through this backcrossing had 10% lower blood glucose levels than those observed in PKCbeta+/+ wild-type offspring in both the fasting state and 30 min after i.p. injection of glucose despite having similar or slightly lower serum insulin levels. Also, compared with commercially obtained C57BL/6-129/SV hybrid control mice, serum glucose levels were similar, and serum insulin levels were similar or slightly lower, in C57BL/6-129/SV hybrid PKCbeta knockout mice in fasting and fed states and after i.p. glucose administration. In keeping with a tendency for slightly lower serum glucose and/or insulin levels in PKCbeta knockout mice, insulin-stimulated 2-deoxyglucose (2-DOG) uptake was enhanced by 50-100% in isolated adipocytes; basal and insulin-stimulated epitope-tagged GLUT4 translocations in adipocytes were increased by 41% and 27%, respectively; and basal 2-DOG uptake was mildly increased by 20-25% in soleus muscles incubated in vitro. The reason for increased 2-DOG uptake and/or GLUT4 translocation in these tissues was uncertain, as there were no significant alterations in phosphatidylinositol 3-kinase activity or activation or in levels of GLUT1 or GLUT4 glucose transporters or other PKC isoforms. On the other hand, increases in 2-DOG uptake may have been partly caused by the loss of PKCbeta1, rather than PKCbeta2, as transient expression of PKCbeta1 selectively inhibited insulin-stimulated translocation of epitope-tagged GLUT4 in adipocytes prepared from PKCbeta knockout mice. Our findings suggest that 1) PKCbeta is not required for insulin-stimulated glucose transport; 2) overall glucose homeostasis in vivo is mildly enhanced by knockout of the PKCbeta gene; 3) glucose transport is increased in some tissues in PKCbeta knockout mice; and 4) increased glucose transport may be partly due to loss of PKCbeta1, which negatively modulates insulin-stimulated GLUT4 translocation.

Norepinephrine is required for leptin effects on gene expression in brown and white adipose tissue

Exogenous leptin enhances energy utilization in ob/ob mice by binding its hypothalamic receptor and selectively increasing peripheral fat oxidation. Leptin also increases uncoupling protein 1 (UCP1) expression in brown adipose tissue (BAT), but the neurotransmitter that mediates this effect has not been established. The present experiments sought to determine whether leptin regulates UCP1 expression in BAT and its own expression in white adipose tissue (WAT) through the long or short forms of leptin receptor and modulation of norepinephrine release. Mice lacking dopamine beta-hydroxylase (Dbh-/-), the enzyme responsible for synthesizing norepinephrine and epinephrine from dopamine, were treated with leptin (20 microg/g body weight/day) for 3 days before they were euthanized. UCP1 messenger RNA (mRNA) and protein expression were 5-fold higher in BAT from control (Dbh+/-) compared with Dbh-/- mice. Leptin produced a 4-fold increase in UCP1 mRNA levels in Dbh+/- mice but had no effect on UCP1 expression in Dbh-/-. The beta3-adrenergic agonist, CL-316,243 increased UCP1 expression and established that BAT from both groups of mice was capable of responding to beta-adrenergic stimulation. Similarly, exogenous leptin reduced leptin mRNA in WAT from Dbh+/- but not Dbh-/- mice. In separate experiments, leptin produced comparable reductions in food intake in both Dbh+/- and Dbh-/- mice, illustrating that norepinephrine is not required for leptin's effect on food intake. Lastly, db/db mice lacking the long form of the leptin receptor failed to increase UCP1 mRNA in response to exogenous leptin but increased UCP1 mRNA in response to CL-316,243. These studies establish that norepinephrine is required for leptin to regulate its own expression in WAT and UCP1 expression in BAT and indicate that these effects are likely mediated through the centrally expressed long form of the leptin receptor.

Mechanism of proximal tubule bicarbonate absorption in NHE3 null mice

NHE3 is the predominant isoform responsible for apical membrane Na(+)/H(+) exchange in the proximal tubule. Deletion of NHE3 by gene targeting results in an NHE3(-/-) mouse with greatly reduced proximal tubule HCO(-)(3) absorption compared with NHE3(+/+) animals (P. J. Schultheis, L. L. Clarke, P. Meneton, M. L. Miller, M. Soleimani, L. R. Gawenis, T. M. Riddle, J. J. Duffy, T. Doetschman, T. Wang, G. Giebisch, P. S. Aronson, J. N. Lorenz, and G. E. Shull. Nature Genet. 19: 282-285, 1998). The purpose of the present study was to evaluate the role of other acidification mechanisms in mediating the remaining component of proximal tubule HCO(-)(3) reabsorption in NHE3(-/-) mice. Proximal tubule transport was studied by in situ microperfusion. Net rates of HCO(-)(3) (J(HCO3)) and fluid absorption (J(v)) were reduced by 54 and 63%, respectively, in NHE3 null mice compared with controls. Addition of 100 microM ethylisopropylamiloride (EIPA) to the luminal perfusate caused significant inhibition of J(HCO3) and J(v) in NHE3(+/+) mice but failed to inhibit J(HCO3) or J(v) in NHE3(-/-) mice, indicating lack of activity of NHE2 or other EIPA-sensitive NHE isoforms in the null mice. Addition of 1 microM bafilomycin caused a similar absolute decrement in J(HCO3) in wild-type and NHE3 null mice, indicating equivalent rates of HCO(-)(3) absorption mediated by H(+)-ATPase. Addition of 10 microM Sch-28080 did not reduce J(HCO3) in either wild-type or NHE3 null mice, indicating lack of detectable H(+)-K(+)-ATPase activity in the proximal tubule. We conclude that, in the absence of NHE3, neither NHE2 nor any other EIPA-sensitive NHE isoform contributes to mediating HCO(-)(3) reabsorption in the proximal tubule. A significant component of HCO(-)(3) reabsorption in the proximal tubule is mediated by bafilomycin-sensitive H(+)-ATPase, but its activity is not significantly upregulated in NHE3 null mice.