Daniel G. Bichet

A Peptide Derived from a β2-Adrenergic Receptor Transmembrane Domain Inhibits Both Receptor Dimerization and Activation

One of the assumptions of the mobile receptor hypothesis as it relates to G protein-coupled receptors is that the stoichiometry of receptor, G protein, and effector is 1:1:1 (Bourne, H. R., Sanders, D. A., and McCormick, F. (1990) Nature 348, 125–132). Many studies on the cooperativity of agonist binding are incompatible with this notion and have suggested that both G proteins and their associated receptors can be oligomeric. However, a clear physical demonstration that G protein-coupled receptors can indeed interact as dimers and that such interactions may have functional consequences was lacking. Here, using differential epitope tagging we demonstrate that β2-adrenergic receptors do form SDS-resistant homodimers and that transmembrane domain VI of the receptor may represent part of an interface for receptor dimerization. The functional importance of dimerization is supported by the observation that a peptide derived from this domain that inhibits dimerization also inhibits β-adrenergic agonist-promoted stimulation of adenylyl cyclase activity. Moreover, agonist stimulation was found to stabilize the dimeric state of the receptor, while inverse agonists favored the monomeric species, which suggests that interconversion between monomeric and dimeric forms may be important for biological activity.

Pharmacological chaperones rescue cell-surface expression and function of misfolded V2 vasopressin receptor mutants

Over 150 mutations within the coding sequence of the V2 vasopressin receptor (V2R) gene are known to cause nephrogenic diabetes insipidus (NDI). A large number of these mutant receptors fail to fold properly and therefore are not routed to the cell surface. Here we show that selective, nonpeptidic V2R antagonists dramatically increase cell-surface expression and rescue the function of 8 mutant NDI-V2Rs by promoting their proper folding and maturation. A cell-impermeant V2R antagonist could not mimic these effects and was unable to block the rescue mediated by a permeant agent, indicating that the nonpeptidic antagonists act intracellularly, presumably by binding to and stabilizing partially folded mutants. In addition to opening new therapeutic avenues for NDI patients, these data demonstrate that by binding to newly synthesized mutant receptors, small ligands can act as pharmacological chaperones, promoting the proper folding and maturation of receptors and their targeting to the cell surface.

Radioimmunoassay of Plasma Arginine Vasopressin in Hyponatremic Patients with Congestive Heart Failure

HYPONATREMIA occurs frequently in patients with congestive heart failure who have moderate fluid intakes (1 to 3 liters per day).1 It is not clear whether persistent release of arginine vasopressin...

Potential Role of Increased Sympathetic Activity in Impaired Sodium and Water Excretion in Cirrhosis

In a study of 26 patients with cirrhosis, plasma norepinephrine concentrations were significantly higher in 19 patients who abnormally excreted an acute water load than in seven who excreted the load normally (8324 +/- 116 vs. 306 +/- 33 pg per milliliter; P less than 0.001). There was also a significant positive correlation between plasma levels of norepinephrine and arginine vasopressin after the water load, as well as a negative correlation between plasma norepinephrine and the percentage of the load excreted. A positive correlation between plasma norepinephrine and plasma renin activity, as well as between norepinephrine and aldosterone, was observed. In addition, there was a negative correlation between plasma norepinephrine and urinary sodium excretion. These findings indicate that increased sympathetic activity, as assessed by plasma levels of norepinephrine, correlates closely with sodium and water retention in cirrhotic patients and thus may be of pathogenetic importance.

Clinical practice guideline on diagnosis and treatment of hyponatraemia

Hyponatraemia, defined as a serum sodium concentration <135 mmol/l, is the most common disorder of body fluid and electrolyte balance encountered in clinical practice. It can lead to a wide spectrum of clinical symptoms, from subtle to severe or even life threatening, and is associated with increased mortality, morbidity and length of hospital stay in patients presenting with a range of conditions. Despite this, the management of patients remains problematic. The prevalence of hyponatraemia in widely different conditions and the fact that hyponatraemia is managed by clinicians with a broad variety of backgrounds have fostered diverse institution- and speciality-based approaches to diagnosis and treatment. To obtain a common and holistic view, the European Society of Intensive Care Medicine (ESICM), the European Society of Endocrinology (ESE) and the European Renal Association - European Dialysis and Transplant Association (ERA-EDTA), represented by European Renal Best Practice (ERBP), have developed the Clinical Practice Guideline on the diagnostic approach and treatment of hyponatraemia as a joint venture of three societies representing specialists with a natural interest in hyponatraemia. In addition to a rigorous approach to methodology and evaluation, we were keen to ensure that the document focused on patient-important outcomes and included utility for clinicians involved in everyday practice.

Role of Vasopressin in Abnormal Water Excretion in Cirrhotic Patients

Twelve stable cirrhotic patients with ascites received a 20 mL/kg water load. Seven patients had abnormal water excretion (27.3% +/- 5.4% of the water load in 5 hours) and a minimal urine osmolality of 262 mosmol/kg water. Five patients excreted 82.6% in 5 hours and had a minimal urine osmolality of 65 mosmol/kg water. Mean plasma arginine vasopressin values after water load were significantly higher in Group 1 (1.34 +/- 0.36 pg/mL) than in Group 2 (undetectable). An effective blood volume lower in Group 1 than Group 2 patients was suggested by a lower plasma albumin (2.5 versus 3.3 g/dL, p less than 0.02), a higher pulse rate (96 versus 72, p less than 0.001), a higher plasma renin activity (7.8 versus 1.5 ng/mL . h, p less than 0.005), a higher plasma aldosterone (66 versus 21 ng/dL, p less than 0.05), and a lower urinary sodium excretion (2.7 versus 14.2 meq Na/5 h, p less than 0.005). The results suggest that nonosmotic stimulation of vasopressin secondary to a decrease in effective blood volume is an important factor in the abnormal water excretion of cirrhosis.

Prognostic value of neurohumoral activation in patients with an acute myocardial infarction: effect of captopril.

OBJECTIVES This study attempted to evaluate whether neurohumoral activation at the time of hospital discharge in postinfarction patients helps to predict long-term prognosis and whether long-term therapy with the angiotensin-converting enzyme inhibitor captopril modifies this relation. BACKGROUND Neurohumoral activation persists at the time of hospital discharge in a large number of postinfarction patients. The Survival and Ventricular Enlargement (SAVE) study demonstrated that the angiotensin-converting enzyme inhibitor captopril improves survival and decreases the development of severe heart failure in patients with left ventricular dysfunction (left ventricular ejection fraction < or = 40%) but no overt postinfarction heart failure. METHODS In 534 patients in the SAVE study, plasma neurohormone levels were measured a mean of 12 days after infarction. Patients were then randomized to receive captopril or placebo and were followed up for a mean (+/- SD) of 38 +/- 6 months (range 24 to 55). The association between activation of plasma neurohormones at baseline and subsequent cardiovascular mortality or the development of heart failure was assessed with and without adjustment for other important prognostic factors. RESULTS By univariate analysis, activation of plasma renin activity and aldosterone, norepinephrine, atrial natriuretic peptide and arginine vasopressin levels were related to subsequent cardiovascular events, whereas epinephrine and dopamine levels were not. By multivariate analysis, only plasma renin activity (relative risk 1.6, 95% confidence interval [CI] 1.0 to 2.5) and atrial natriuretic peptide (relative risk 2.2, 95% CI 1.3 to 3.8) were independently predictive of cardiovascular mortality, whereas the other neurohormones were not. Only plasma renin activity and aldosterone, atrial natriuretic peptide and arginine vasopressin were independent predictors of the combined end points of cardiovascular mortality, development of severe heart failure or recurrent myocardial infarction. Except for 1-year cardiovascular mortality, the use of captopril did not significantly modify these relations. CONCLUSIONS Neurohumoral activation at the time of hospital discharge in postinfarction patients is an independent sign of poor prognosis. This is particularly true for plasma renin activity and atrial natriuretic peptide. Except for 1-year cardiovascular mortality, captopril does not significantly modify these relations.

Evidence for a third genetic locus for autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetically heterogeneous disease with loci on chromosomes 16p and 4q. It has a moderately high spontaneous mutation rate, although the relative frequency of such mutations at each gene locus is unknown. In studying genetic heterogeneity in the French-Canadian population, we identified a family in which a classical clinical presentation of ADPKD resulted from a mutation at a locus genetically distinct from either of the previously described loci for this disease. This suggests the existence of a third genetic locus for ADPKD.

Oral Tolvaptan Is Safe and Effective in Chronic Hyponatremia

Vasopressin antagonists increase the serum sodium concentration in patients who have euvolemia and hypervolemia with hyponatremia in the short term (</=30 days), but their safety and efficacy with longer term administration is unknown. SALTWATER was a multicenter, open-label extension of the Study of Ascending Levels of Tolvaptan in Hyponatremia (SALT-1 and SALT-2). In total, 111 patients with hyponatremia received oral tolvaptan for a mean follow-up of 701 days, providing 77,369 patient-days of exposure. All patients had hyponatremia at randomization in SALT-1 and SALT-2, and 85% continued to have hyponatremia at entry in SALTWATER. The most common adverse effects attributed to tolvaptan were pollakiuria, thirst, fatigue, dry mouth, polydipsia, and polyuria. Six drug-related adverse effects led to study discontinuation. The increase in serum sodium exceeded the desired 1 mmol/L per h at initiation in five patients. Hypernatremia (>145 mmol/L) led to discontinuation in one patient. Mean serum sodium increased from 130.8 mmol/L at baseline to >135 mmol/L throughout the observation period (P < 0.001 versus baseline at most points). Responses were comparable between patients with euvolemia and those with heart failure but more modest in patients with cirrhosis. In conclusion, prolonged administration of tolvaptan maintains an increased serum sodium with an acceptable margin of safety.

Urinary Excretion of Aquaporin-2 in Patients with Diabetes Insipidus

BACKGROUND Urine-concentrating ability is regulated by vasopressin. Recently, the specific water-channel protein of the renal collecting duct, known as aquaporin-2, was cloned. However, it is not certain whether this molecule is responsive to vasopressin. METHODS We measured the urinary excretion of aquaporin-2 and its response to vasopressin in 11 normal subjects and 9 patients with central or nephrogenic diabetes insipidus. The urine samples were collected during periods of dehydration and hydration and after the administration of vasopressin. Urine samples were analyzed for aquaporin-2 by the Western blot assay and immunogold labeling, and the amount of aquaporin-2 was determined by radioimmunoassay. RESULTS Aquaporin-2 was detectable in the urine in both soluble and membrane-bound forms. In the five normal subjects tested, the mean (+/- SE) urinary excretion of aquaporin-2 was 11.2 +/- 2.2 pmol per milligram of creatinine after a period of dehydration, and it decreased to 3.9 +/- 1.9 pmol per milligram of creatinine (P = 0.03) during the second hour after a period of hydration. In the six other normal subjects, an infusion of desmopressin (1-desamino-8-D-arginine vasopressin) increased the urinary excretion of aquaporin-2 from 0.8 +/- 0.3 to 11.2 +/- 1.6 pmol per milligram of creatinine (P < 0.001). The five patients with central diabetes insipidus also had increases in urinary excretion of aquaporin-2 in response to the administration of vasopressin, but the four patients with X-linked or non-X-linked nephrogenic diabetes insipidus did not. CONCLUSIONS Aquaporin-2 is detectable in the urine, and changes in the urinary excretion of this protein can be used as an index of the action of vasopressin on the kidney.