Long-term regulation of ENaC expression in kidney by angiotensin II. (65/267)

We carried out semiquantitative immunoblotting of kidney to identify apical sodium transporter proteins whose abundances are regulated by angiotensin II. In NaCl-restricted rats (0.5 mEq Na/200 g BW/d), the type 1 angiotensin II receptor (AT1 receptor) antagonist, candesartan, (1 mg/kg of body weight per day SC for 2 days) markedly decreased the abundance of the alpha subunit of the epithelial sodium channel (ENaC). This subunit has been shown to be rate-limiting for assembly of mature ENaC complexes. In addition, systemic infusion of angiotensin II increased alphaENaC protein abundance in rat kidney cortex. The decrease in alphaENaC protein abundance in response to AT1 receptor blockade was associated with a fall in alphaENaC mRNA abundance (real-time RT-PCR), consistent with transcriptionally mediated regulation. The effect of AT1 receptor blockade on alphaENaC expression was not blocked by spironolactone, suggesting a direct role of the AT1 receptor in regulation of alphaENaC gene expression. Candesartan administration was also found to increase the abundances of the beta and gamma subunits. The increase in beta and gammaENaC protein abundance was not associated with a significant increase in the renal abundances of the corresponding mRNAs, suggesting a posttranscriptional mechanism. Immunocytochemistry confirmed the increase in beta and gammaENaC protein abundance and demonstrated candesartan-induced ENaC internalization in collecting duct cells. The results support the view that the angiotensin II receptor regulates ENaC abundance, consistent with a role for angiotensin II in regulation of collecting duct function.  (+info)

Ethanol inhibits alpha-amino-3-hydyroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function in central nervous system neurons by stabilizing desensitization. (66/267)

Ethanol actions on alpha-amino-3-hydyroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors were studied using voltage-clamp recordings from mouse cortical and hippocampal neurons. During whole-cell recordings ethanol (EtOH) inhibited AMPA receptor-mediated currents in a dose-dependent manner at concentrations from 10 to 500 mM. The steady-state component of AMPA-activated current was more sensitive to EtOH than the peak component. To examine the effect of EtOH on a well resolved peak current component, patches were excised from cultured cortical neurons, to which AMPA and EtOH were applied using a piezoelectric solution application system. Under this condition, the peak current was not inhibited significantly by EtOH. To further study possible mechanisms of EtOH inhibition, kainate and AMPA were used to evoke currents in the absence and presence of cyclothiazide. Ethanol inhibition was stronger when receptors were activated by low than high kainate concentrations. Cyclothiazide reduced inhibition by EtOH regardless of the agonist used to activate the receptor. Finally, EtOH inhibition was reduced in a point mutated (L497Y) GluRAi receptor that lacks desensitization. These findings suggest that EtOH inhibits AMPA receptors by stabilizing the desensitized state. Our results can explain some of the variation observed in EtOH inhibition in previous studies, and support the idea that physiologically relevant concentrations of EtOH can have a strong effect on AMPA receptor function.  (+info)

Development of circulatory-renal limitations to angiotensin-converting enzyme inhibitors identifies patients with severe heart failure and early mortality. (67/267)

OBJECTIVES: This study examined the hypothesis that patients who develop angiotensin-converting enzyme inhibitor intolerance attributable to circulatory-renal limitations (CRLimit) have more severe underlying disease and worse outcome. BACKGROUND: Although the renin-angiotensin system contributes to the progression of heart failure (HF), it also supports the failing circulation. Patients with the most severe disease may not tolerate inhibition of this system. METHODS: Consecutive inpatient admissions to the cardiomyopathy service of the Brigham and Women's Hospital between 2000 and 2002 were reviewed retrospectively for initial profiles, discharge medications, and documented reasons for discontinuation of angiotensin-converting enzyme inhibitors. Outcomes of death and transplantation were determined. RESULTS: Of the 259 patients, 86 were not on an angiotensin-converting enzyme inhibitor at discharge. Circulatory-renal limitations of symptomatic hypotension, progressive renal dysfunction, or hyperkalemia were documented in 60 patients (23%); other adverse effects, including cough, in 24 patients; and absent reasons in 2 patients. Compared with patients on angiotensin-converting enzyme inhibitors, patients with CRLimit were older (60 vs. 55 years; p = 0.006), with longer history of HF (5 vs. 2 years; p = 0.009), lower systolic blood pressure (104 vs. 110 mm Hg; p = 0.05), lower sodium (135 vs. 138 mEql/l; p = 0.002), and higher initial creatinine (2.5 vs. 1.2 mg/dl; p = 0.0001). Mortality was 57% in patients with CRLimit and 22% in the patients on angiotensin-converting enzyme inhibitors during a median 8.5-month follow-up (p = 0.0001). CONCLUSIONS: Development of CRLimit to angiotensin-converting enzyme inhibitor intolerance identifies patients with severe disease who are likely to die during the next year. New treatment strategies should be targeted to this population.  (+info)

Identification of a site in GluR1 and GluR2 that is important for modulation of deactivation and desensitization. (68/267)

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of ionotropic glutamate receptors consists of rapidly gating ion channels. Positive modulation of channel gating may slow gating kinetics through at least two distinct mechanisms, evidenced by the predominant slowing of either the rate of receptor desensitization or the rate of offset after agonist withdrawal (deactivation). This study compares the actions of two positive allosteric modulators [cyclothiazide, which modulates desensitization, and 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546), which modulates deactivation] in a mutant shown previously to impede modulation by cyclothiazide. These experiments test the hypothesis that the point mutation, GluR1(S493T), would also cause a loss of modulation by CX546. Wild-type GluR1 through -4 receptors were modulated by CX546, as assayed by the potentiation of steady-state currents in the Xenopus laevis oocyte expression system. CX546 potentiated steady-state currents of both splice isoforms of GluR1. Modulation by CX546 was completely abolished in GluR1(S493T) and its homolog, GluR2(S497T), although this mutation did not affect apparent agonist affinity in the absence of CX546. Thus, the GluR1(S493T) mutation has a similar impairment of modulation by either cyclothiazide or CX546, indicating that some residues at the subunit interface of glutamate receptors play an important role in channel deactivation and desensitization.  (+info)

Distinguishing between presynaptic and postsynaptic mechanisms of short-term depression during action potential trains. (69/267)

Short-term facilitation and depression have a profound influence on transmission at many glutamatergic synapses, particularly during trains of stimuli. A major component of these processes is postsynaptic receptor desensitization. Both presynaptic and postsynaptic mechanisms can contribute to synaptic efficacy, but it is often difficult to define their respective contributions. Blockers of desensitization such as cyclothiazide (CTZ) can be used, but many of these drugs have nonspecific effects on transmitter release, complicating attempts to define synaptic effectiveness under physiological conditions. We describe and validate a new method to minimize desensitization during trains of synaptic stimuli that is based on the low-affinity competitive glutamate receptor antagonists gamma-D-glutamylglycine or kynurenic acid. A computational model of AMPA receptor kinetics shows that the mechanism can be accounted for by simple competitive antagonism of AMPA receptors, where the rapid off-rate of the antagonist permits re-equilibration between blocked and unblocked pools during the interstimulus interval. Our results at the calyx of Held show that desensitization makes little contribution to synaptic depression at frequencies below 10 Hz, but at higher frequencies it makes an important contribution, with accumulating desensitization masking short-term facilitation and causing an underestimation of quantal content. This novel method of protection from desensitization is compatible with physiological studies but cannot be used in conjunction with CTZ. Although presynaptic vesicle depletion makes the dominant contribution to short-term depression, our results show that AMPA receptor desensitization contributes to the depression at auditory synapses after hearing onset and in a frequency-dependent manner.  (+info)

Desensitization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors facilitates use-dependent inhibition by pentobarbital. (70/267)

Although the mechanisms underlying the use-dependent inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) by barbiturates are not well understood, it has generally been assumed to involve open channel block. We examined the properties of the inhibition of AMPARs by the barbiturate pentobarbital (PB) in acutely isolated and cultured hippocampal neurons. PB caused a use- and concentration-dependent inhibition (IC50 = 20.7 microM) of AMPAR-mediated currents evoked by kainate. Contrary to the properties of an open channel blocker, the inhibition by PB developed with double exponential kinetics was reduced under conditions that favor the open channel state of AMPARs and was independent of membrane voltage. In addition, the inhibition was reduced at basic pH, indicating that the uncharged form of PB is active at AMPARs. Preventing AMPAR desensitization with cyclothiazide reduced the potency of inhibition by PB and prevented its trapping after the removal of agonist. PB preferentially reduced the steady-state (IC50 = 92.8 microM), rather than peak (IC50 > 1 mM) component of responses evoked by glutamate and accelerated the onset of desensitization in a concentration-dependent manner. Miniature excitatory postsynaptic currents recorded from cultured hippocampal neurons, the time course of which is minimally influenced by desensitization, are not inhibited by PB. The sensitivity of AMPAR-mediated synaptic responses to inhibition by PB therefore depends on the contribution of desensitization to these events. Our results suggest that PB does not act as an open channel blocker of AMPARs. Rather, the sensitivity, use dependence, and trapping of inhibition by PB are determined by AMPARs desensitization.  (+info)

The potential savings of using thiazides as the first choice antihypertensive drug: cost-minimisation analysis. (71/267)

BACKGROUND: All clinical practice guidelines recommend thiazides as a first-choice drug for the management of uncomplicated hypertension. Thiazides are also the lowest priced antihypertensive drugs. Despite this, the use of thiazides is much lower than that of other drug-classes. We wanted to estimate the potential for savings if thiazides were used as the first choice drug for the management of uncomplicated hypertension. METHODS: For six countries (Canada, France, Germany, Norway, the UK and the US) we estimated the number of people that are being treated for hypertension, and the proportion of them that are suitable candidates for thiazide-therapy. By comparing this estimate with thiazide prescribing, we calculated the number of people that could switch from more expensive medication to thiazides. This enabled us to estimate the potential drug-cost savings. The analysis was based on findings from epidemiological studies and drug trials, and data on sales and prescribing provided by IMS for the year 2000. RESULTS: For Canada, France, Germany, Norway, the UK and the US the estimated potential annual savings were US13.8 million dollars, US37.4 million dollars, US72.2 million dollars, US10.7 million dollars, US119.7 million dollars and US433.6 million dollars, respectively.  (+info)

General practitioners' management of hypertension in elderly patients. (72/267)

OBJECTIVE: To assess general practitioners' attitudes to the diagnosis and management of hypertension in elderly patients. DESIGN: Postal questionnaire to all general practitioners in Leicestershire. RESULTS: 360 of 451 general practitioners (80%) responded. 81% (292) reported rechecking an initially high blood pressure on two or three occasions before starting treatment, 56% (202) measured sitting blood pressure only, and just 28% (100) took sitting and standing levels. 36% (128) had no upper age limit for starting anti-hypertensive treatment; of the 58% (206) who did, the median was 80 (range 70-99) years. Blood pressure levels reported for starting treatment in patients aged 70-79 years were 180 (150-240)/106 (90-120) mm Hg. 34% of general practitioners (121) would not treat isolated systolic hypertension. The most popular first line treatment for an elderly hypertensive patient was a thiazide diuretic; only 17% of general practitioners (61) initially tried non-pharmacological methods. 34% (122) would continue anti-hypertensive treatment unchanged in the period immediately after stroke. CONCLUSIONS: The variation among general practitioners in the criteria for the measurement, diagnosis, and treatment of hypertension in elderly patients emphasises the need for clear management guidelines in this age group.  (+info)