Osmoregulation of plasma vasopressin in diabetes mellitus with sustained hyperglycemia

NaX Channel Is a Physiological [Na+] Detector in Oxytocin- and Vasopressin-Releasing Magnocellular Neurosecretory Cells of the Rat Supraoptic Nucleus

The Scn7A gene encodes NaX, an atypical noninactivating Na+ channel, whose expression in sensory circumventricular organs is essential to maintain homeostatic responses for body fluid balance. However, NaX has also been detected in homeostatic effector neurons, such as vasopressin (VP)-releasing magnocellular neurosecretory cells (MNCVP) that secrete VP (antidiuretic hormone) into the bloodstream in response to hypertonicity and hypernatremia. Yet, the physiological relevance of NaX expression in these effector cells remains unclear. Here, we show that rat MNCVP in males and females is depolarized and excited in proportion with isosmotic increases in [Na+]. These responses were caused by an inward current resulting from a cell-autonomous increase in Na+ conductance. The Na+-evoked current was unaffected by blockers of other Na+-permeable ion channels but was significantly reduced by shRNA-mediated knockdown of Scn7A expression. Furthermore, reducing the density of NaX channels selectively impaired the activation of MNCVP by systemic hypernatremia without affecting their responsiveness to hypertonicity in vivo These results identify NaX as a physiological Na+ sensor, whose expression in MNCVP contributes to the generation of homeostatic responses to hypernatremia.SIGNIFICANCE STATEMENT In this study, we provide the first direct evidence showing that the sodium-sensing channel encoded by the Scn7A gene (NaX) mediates cell-autonomous sodium detection by MNCs in the low millimolar range and that selectively reducing the expression of these channels in MNCs impairs their activation in response to a physiologically relevant sodium stimulus in vitro and in vivo These data reveal that NaX operates as a sodium sensor in these cells and that the endogenous sensory properties of osmoregulatory effector neurons contribute to their homeostatic activation in vivo.

Rethinking Vasopressin: New Insights into Vasopressin Signaling and Its Implications

Vasopressin is a highly conserved peptide hormone that has been traditionally associated with water homeostasis. There is accumulating evidence in both humans and animal models that vasopressin is implicated in the regulation of metabolism. This review focuses on the effects that vasopressin exerts on the regulation of glucose and fatty acids with a particular emphasis on the potential repercussions of metabolic dysregulation in kidney disease.

Arginine vasopressin: Direct and indirect action on metabolism

From its identification and isolation in 1954, arginine vasopressin (AVP) has attracted attention, not only for its peripheral functions such as vasoconstriction and reabsorption of water from kidney, but also for its central effects. As there is now considerable evidence that AVP plays a crucial role in feeding behavior and energy balance, it has become a promising therapeutic target for treating obesity or other obesity-related metabolic disorders. However, the underlying mechanisms for AVP regulation of these central processes still remain largely unknown. In this review, we will provide a brief overview of the current knowledge concerning how AVP controls energy balance and feeding behavior, focusing on physiological aspects including the relationship between AVP, circadian rhythmicity, and glucocorticoids.

Association of Copeptin With Sense of Coherence in Individuals With Varying Degrees of Glucose Intolerance

OBJECTIVE

This study aimed to examine the association of serum copeptin levels, a surrogate marker of arginine-vasopressin secretion with sense of coherence (SOC) among individuals with varying degrees of glucose intolerance.

METHODS

The study was conducted in 120 age- and sex-matched individuals who were divided equally into three groups. Group A included individuals with normal glucose tolerance; group B, individuals with prediabetes (impaired glucose tolerance and/or impaired fasting glucose); and group C, individuals with newly detected diabetes mellitus (NDDM). SOC, perceived stress scale (PSS), copeptin, anthropometry, glycated hemoglobin, insulin, and salivary cortisol were measured in all study participants.

RESULTS

The SOC score was found to be significantly lower in group C compared with group A (p < .001) and group B (p = .006). The PSS score was found to be significantly higher in group C compared with group A (p = .002). No significant difference was found between PSS scores of groups B and C (p = .25). Copeptin levels were found to be significantly higher in group C compared with group A (p = .016). Copeptin levels in group C did not differ significantly from those in group B (p = .056). There was a significant negative correlation between serum copeptin levels and SOC in the NDDM group C (r = 0.31, p = .048) and overall (r = 0.19, p = .037). In multiple regression analysis, SOC emerged as the variable with the strongest association with 2-hour postprandial plasma glucose and glycated hemoglobin.

CONCLUSION

Individuals with NDDM displayed significantly higher serum copeptin levels that inversely correlated with SOC, a global measure of stress coping ability.

Osmotic diuresis by SGLT2 inhibition stimulates vasopressin-induced water reabsorption to maintain body fluid volume

Most of the filtered glucose is reabsorbed in the early proximal tubule by the sodium-glucose cotransporter SGLT2. The glycosuric effect of the SGLT2 inhibitor ipragliflozin is linked to a diuretic and natriuretic effect that activates compensatory increases in fluid and food intake to stabilize body fluid volume (BFV). However, the compensatory mechanisms that are activated on the level of renal tubules remain unclear. Type 2 diabetic Goto-Kakizaki (GK) rats were treated with vehicle or 0.01% (in diet) ipragliflozin with free access to fluid and food. After 8 weeks, GK rats were placed in metabolic cages for 24-hr. Ipragliflozin decreased body weight, serum glucose and systolic blood pressure, and increased fluid and food intake, urinary glucose and Na+ excretion, urine volume, and renal osmolar clearance, as well as urine vasopressin and solute-free water reabsorption (TcH2O). BFV, measured by bioimpedance spectroscopy, and fluid balance were similar among the two groups. Urine vasopressin in ipragliflozin-treated rats was negatively and positively associated with fluid balance and TcH2O, respectively. Ipragliflozin increased the renal membrane protein expression of SGLT2, aquaporin (AQP) 2 phosphorylated at Ser269 and vasopressin V2 receptor. The expression of SGLT1, GLUT2, AQP1, and AQP2 was similar between the groups. In conclusion, the SGLT2 inhibitor ipragliflozin induced a sustained glucosuria, diuresis, and natriuresis, with compensatory increases in fluid intake and vasopressin-induced TcH2O in proportion to the reduced fluid balance to maintain BFV. These results indicate that the osmotic diuresis induced by SGLT2 inhibition stimulates compensatory fluid intake and renal water reabsorption to maintain BFV.

Elevated copeptin, arterial stiffness, and elevated albumin excretion in adolescents with type 1 diabetes

OBJECTIVE

We sought to evaluate copeptin concentrations in adolescents with and without type 1 diabetes (T1D) and examine the associations between copeptin and measures of arterial stiffness and kidney dysfunction.

RESEARCH DESIGN AND METHODS

This analysis included 169 adolescents with T1D (12-19 years of age, 59% girls, mean HbA1c 9.0 ± 1.5% and diabetes duration of 8.6 ± 2.9 years), in addition to 61 controls without T1D. Arterial stiffness including carotid-femoral pulse wave velocity (CF-PWV), carotid-radial PWV (CR-PWV), augmentation index normalized to heart rate of 75 bpm (AIx@HR75), and brachial artery distensibility (BAD). Serum copeptin, urinary albumin-to-creatinine ratio (UACR), and estimated glomerular filtration rate (eGFR) by serum creatinine and cystatin C were also assessed.

RESULTS

Compared to controls, adolescents with T1D had higher median (Q1-Q3) copeptin (7.5 [5.2-11.3] vs 6.4 [4.8-8.3] pmol/L, P = .01), mean ± SD eGFR (121 ± 23 vs 112 ± 16 mL/min/1.73m² , P = .002) and lower BAD (7.1 ± 1.3 vs 7.2 ± 1.2%, P = .02). Adolescents with T1D in the in high tertile copeptin group (>9.1 pmol/L) had higher AIx@HR75 (10.7 ± 1.2 vs 5 ± 1.2, P = .001), CR-PWV (5.30 ± 1.0 vs 5.18 ± 1.0 m/s, P = .04), and UACR (12 ± 1 vs 8 ± 1 mg/g, P = .025) compared to those in low tertile (<5.8 pmol/L) after adjusting for age, sex, and eGFR. Copeptin inversely associated with CF-PWV independent of age, sex, eGFR, SBP, and HbA1c in T1D adolescents.

CONCLUSIONS

Our data demonstrate that elevated copeptin was associated with worse arterial stiffness in adolescents with T1D. These findings suggest that copeptin could improve CVD risk stratification in adolescents with T1D.

The role of blood volume in cardiac dysfunction and reduced exercise tolerance in patients with diabetes

Blood volume is an integral component of the cardiovascular system, and fundamental to discerning the pathophysiology of multiple cardiovascular conditions leading to exercise intolerance. Based on a systematic search of controlled studies assessing blood volume, in this Personal View we describe how hypovolaemia is a prevalent characteristic of patients with diabetes, irrespective of sex, age, and physical activity levels. Multiple endocrine and haematological mechanisms contribute to hypovolaemia in diabetes. The regulation of intravascular volumes is altered by sustained hyperglycaemia and hypertension. Chronic activation of endocrine systems controlling fluid homeostasis, such as the renin-angiotensin-aldosterone system and vasopressin axis, has a role in progressive kidney desensitisation and diabetic nephropathy. Furthermore, albumin loss from the intravascular compartment reduces the osmotic potential of plasma to retain water. Hypovolaemia also affects the loading conditions and filling of the heart in diabetes. The elucidation of modifiable volumetric traits will plausibly have major health benefits in the diabetes population.

Excitatory GABAergic Action and Increased Vasopressin Synthesis in Hypothalamic Magnocellular Neurosecretory Cells Underlie the High Plasma Level of Vasopressin in Diabetic Rats

Diabetes mellitus (DM) is associated with increased plasma levels of arginine-vasopressin (AVP), which may aggravate hyperglycemia and nephropathy. However, the mechanisms by which DM may cause the increased AVP levels are not known. Electrophysiological recordings in supraoptic nucleus (SON) slices from streptozotocin (STZ)-induced DM rats and vehicle-treated control rats revealed that γ-aminobutyric acid (GABA) functions generally as an excitatory neurotransmitter in the AVP neurons of STZ rats, whereas it usually evokes inhibitory responses in the cells of control animals. Furthermore, Western blotting analyses of Cl- transporters in the SON tissues indicated that Na+-K+-2Cl- cotransporter isotype 1 (a Cl- importer) was upregulated and K+-Cl- cotransporter isotype 2 (KCC2; a Cl- extruder) was downregulated in STZ rats. Treatment with CLP290 (a KCC2 activator) significantly lowered blood AVP and glucose levels in STZ rats. Last, investigation that used rats expressing an AVP-enhanced green fluorescent protein fusion gene showed that AVP synthesis in AVP neurons was much more intense in STZ rats than in control rats. We conclude that altered Cl- homeostasis that makes GABA excitatory and enhanced AVP synthesis are important changes in AVP neurons that would increase AVP secretion in DM. Our data suggest that Cl- transporters in AVP neurons are potential targets of antidiabetes treatments.

Urinary Excretion of Kidney Aquaporins as Possible Diagnostic Biomarker of Diabetic Nephropathy

Diabetic nephropathy (DN) is a microangiopathic complication of diabetes mellitus (DM) affecting one-third of diabetic patients. The large variability in the clinical presentation of renal involvement in patients with DM makes kidney biopsy a prerequisite for a correct diagnosis. However, renal biopsy is an invasive procedure associated with risk of major complications. Numerous studies aimed to identify a noninvasive biomarker of DN but, so far, none of these is considered to be sufficiently specific and sensitive. Water channel aquaporins (AQPs), expressed at the plasma membrane of epithelial tubular cells, are often dysregulated during DN. In this work, we analyzed the urine excretion of AQP5 and AQP2 (uAQP5 and uAQP2), via exosomes, in 35 diabetic patients: 12 normoalbuminuric with normal renal function (DM), 11 with proteinuric nondiabetic nephropathy (NDN), and 12 with histological diagnosis and classification of DN. ELISA and WB analysis independently showed that uAQP5 was significantly increased in DN patients. Interestingly, linear regression analysis showed a positive correlation between uAQP5 and the histological class of DN. The same analysis, focusing on uAQP2, showed comparable results. Taken together, these data suggest a possible use of AQP5 and AQP2 as novel noninvasive biomarkers to help in classifying the clinical stage of DN.

PGE2 receptor EP3 inhibits water reabsorption and contributes to polyuria and kidney injury in a streptozotocin-induced mouse model of diabetes

AIMS/HYPOTHESIS

The first clinical manifestation of diabetes is polyuria. The prostaglandin E2 (PGE2) receptor EP3 antagonises arginine vasopressin (AVP)-mediated water reabsorption and its expression is increased in the diabetic kidney. The purpose of this work was to study the contribution of EP3 to diabetic polyuria and renal injury.

METHODS

Male Ep 3 (-/-) (also known as Ptger3 (-/-)) mice were treated with streptozotocin (STZ) to generate a mouse model of diabetes and renal function was evaluated after 12 weeks. Isolated collecting ducts (CDs) were microperfused to study the contribution of EP3 to AVP-mediated fluid reabsorption.

RESULTS

Ep 3 (-/-)-STZ mice exhibited attenuated polyuria and increased urine osmolality compared with wild-type STZ (WT-STZ) mice, suggesting enhanced water reabsorption. Compared with WT-STZ mice, Ep 3 (-/-)-STZ mice also had increased protein expression of aquaporin-1, aquaporin-2, and urea transporter A1, and reduced urinary AVP excretion, but increased medullary V2 receptors. In vitro microperfusion studies indicated that Ep 3 (-/-) and WT-STZ CDs responded to AVP stimulation similarly to those of wild-type mice, with a 60% increase in fluid reabsorption. In WT non-injected and WT-STZ mice, EP3 activation with sulprostone (PGE2 analogue) abrogated AVP-mediated water reabsorption; this effect was absent in mice lacking EP3. A major finding of this work is that Ep 3 (-/-)-STZ mice showed blunted renal cyclooxygenase-2 protein expression, reduced renal hypertrophy, reduced hyperfiltration and reduced albuminuria, as well as diminished tubular dilation and nuclear cysts.

CONCLUSIONS/INTERPRETATION

Taken together, the data suggest that EP3 contributes to diabetic polyuria by inhibiting expression of aquaporins and that it promotes renal injury during diabetes. EP3 may prove to be a promising target for more selective management of diabetic kidney disease.

Is the association of serum sodium with mortality in patients with type 2 diabetes explained by copeptin or NT-proBNP? (ZODIAC-46)

BACKGROUND AND AIMS

Hyponatremia has been associated with an increased mortality risk in the general population. Diabetes is a condition predisposing for elevated levels of arginine vasopressin (AVP) and heart failure, both common causes of hyponatremia. These factors, however, are also associated with an increased mortality risk. We aimed to investigate whether serum sodium is associated with cardiovascular and all-cause mortality in type 2 diabetes and whether these associations could be explained by copeptin, a surrogate for AVP, or NT-proBNP, a marker for heart failure.

METHODS

Patients with type 2 diabetes participating in the observational ZODIAC study were included. Cox regression analyses were used to investigate the association of serum sodium with mortality.

RESULTS

We included 1068 patients (age 67 ± 12 years, 45% male, serum sodium 142 ± 3 mmol/L). After 15 years of follow-up, 519 patients (49%) died, with 225 cardiovascular deaths (21%). In univariable analyses, serum sodium, copeptin, and NT-proBNP were all significantly associated with cardiovascular and all-cause mortality. These associations remained significant after combination of these markers in a multivariable model. Serum sodium and NT-proBNP remained significantly associated with mortality after further adjustment for potential confounders, whereas copeptin lost significance after adjustment for SCr and ACR.

CONCLUSION

Low serum sodium was associated with an increased risk of cardiovascular and all-cause mortality in type 2 diabetes. Moreover, these associations were not explained by copeptin and NT-proBNP. Whether low serum sodium itself leads to poor outcome or is a marker for (unidentified) co-morbidity severity or use of specific medications remains to be elucidated.

Vasopressin activates Akt/mTOR pathway in smooth muscle cells cultured in high glucose concentration

Mammalian target of rapamycin (mTOR) complex is a key regulator of autophagy, cell growth and proliferation. Here, we studied the effects of arginine vasopressin (AVP) on mTOR activation in vascular smooth muscle cells cultured in high glucose concentration. AVP induced the mTOR phosphorylation in A-10 cells grown in high glucose, in contrast to cells cultured in normal glucose; wherein, only basal phosphorylation was observed. The AVP-induced mTOR phosphorylation was inhibited by a PI3K inhibitor. Moreover, the AVP-induced mTOR activation inhibited autophagy and increased thymidine incorporation in cells grown in high glucose. This increase was abolished by rapamycin which inhibits the mTORC1 complex formation. Our results suggest that AVP stimulates mTOR phosphorylation by activating the PI3K/Akt signaling pathway and, subsequently, inhibits autophagy and raises cell proliferation in A-10 cells maintained in high glucose concentration.

Copeptin, a surrogate marker for arginine vasopressin, is associated with cardiovascular and all-cause mortality in patients with type 2 diabetes (ZODIAC-31)

OBJECTIVE

Copeptin, a surrogate marker for arginine vasopressin, has been associated with cardiovascular (CV) events and mortality in patients with type 2 diabetes complicated by end-stage renal disease or acute myocardial infarction. For stable outpatients, these associations are unknown. Our aim was to investigate whether copeptin is associated with CV and all-cause mortality in patients with type 2 diabetes treated in primary care.

RESEARCH DESIGN AND METHODS

Patients with type 2 diabetes participating in the observational Zwolle Outpatient Diabetes Project Integrating Available Care (ZODIAC) study were included. Cox regression analyses with age as time scale were used to assess the relationship of baseline copeptin with CV and all-cause mortality.

RESULTS

We included 1,195 patients (age 67±12 years, 44% male). Median baseline copeptin concentration was 5.4 (interquartile range [IQR] 3.1-9.6) pmol/L. After a median follow-up of 5.9 (IQR 3.2-10.1) years, 345 patients died (29%), with 148 CV deaths (12%). Log2 copeptin was associated with CV (hazard ratio 1.17 [95% CI 0.99-1.39]; P=0.068) and all-cause mortality (1.22 [1.09-1.36]; P=0.001) after adjustment for age, sex, BMI, smoking, systolic blood pressure, total cholesterol to HDL ratio, duration of diabetes, HbA1c, treatment with ACE inhibitors and angiotensin receptor blockers, history of CV diseases, log serum creatinine, and log albumin to creatinine ratio; however, copeptin did not substantially improve risk prediction for CV (integrated discrimination improvement 0.14% [IQR -0.27 to 0.55%]) and all-cause mortality (0.77% [0.17-1.37%]) beyond currently used clinical markers.

CONCLUSIONS

We found copeptin to be associated with CV and all-cause mortality in patients with type 2 diabetes treated in primary care. Intervention studies should show whether the high CV risk in type 2 diabetes can be reduced by suppression of vasopressin, for example by reducing salt intake.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

The role of nitric oxide in the dysregulation of the urine concentration mechanism in diabetes mellitus

Uncontrolled diabetes mellitus results in osmotic diuresis. Diabetic patients have lowered nitric oxide (NO) which may exacerbate polyuria. We examined how lack of NO affects the transporters involved in urine concentration in diabetic animals. Diabetes was induced in rats by streptozotocin. Control and diabetic rats were given L-NAME for 3 weeks. Urine osmolality, urine output, and expression of urea and water transporters and the Na-K-2Cl cotransporter were examined. Predictably, diabetic rats presented with polyuria (increased urine volume and decreased urine osmolality). Although metabolic parameters of control rats were unaffected by L-NAME, treated diabetic rats produced 30% less urine and osmolality was restored. UT-A1 and UT-A3 were significantly increased in diabetic rat inner medulla. While L-NAME treatment alone did not alter UT-A1 or UT-A3 abundance, absence of NO prevented the upregulation of both transporters in diabetic rats. Similarly, AQP2 and NKCC2 abundance was increased in diabetic animals however, expression of these transporters were unchanged by L-NAME treatment of diabetes. Increased expression of the concentrating transporters observed in diabetic rats provides a compensatory mechanism to decrease solute loss despite persistent glycosuria. Our studies found that although diabetic-induced glycosylation remained increased, total protein expression was decreased to control levels in diabetic rats treated with L-NAME. While the role of NO in urine concentration remains unclear, lowered NO associated with diabetes may be deleterious to the transporters’ response to the subsequent osmotic diuresis.

Candesartan Differentially Regulates Epithelial Sodium Channel in Cortex Versus Medulla of Streptozotocin-Induced Diabetic Rats

Diabetes is associated with an activated renal renin-angiotensin-aldosterone system (RAAS) and it was shown that streptozotocin (STZ)-induced diabetic rats had increased whole kidney protein levels of the epithelial sodium channel subunits (α-, β- and γ-ENaC). However, the role of the RAAS on the regional, i.e., cortical versus medullary, regulation of ENaC is unclear. Male Sprague-Dawley rats were injected with STZ (intravenous, 65 mg/kg·bw, n=12/group). After 14 days, half of them received drinking water with candesartan (2 mg/kg·bw/day), an angiotensin-II type-1 receptor (AT1R) antagonist, for one week. In the medulla, i.e., inner stripe of the outer medulla (ISOM), base and/or tip of the inner medulla, immunoblotting revealed increased protein abundances of α1 Na-K-ATPase and ENaC subunits with diabetes (200-600% of controls), which were not reversed by candesartan. In fact, candesartan increased all ENaC subunits and α1 Na-K-ATPase in the ISOM and/or base in control rats. In contrast, in the cortex, diabetes did not increase these proteins. However, candesartan reduced cortical β- and γ-ENaC regardless of diabetic state. In summary, diabetes-induced increases in ENaC were seen preferentially in the medulla. These changes appeared to be due to a mechanism clearly distinct from AT1R activation, because they were not abolished by candesartan. In fact, candesartan treatment tended to increase some of these medullary proteins, perhaps in compensation for increased NaCl load. In contrast, cortical β- and γ-ENaC were reduced by candesartan regardless of diabetic state suggesting their regulation by AT1R at this site; however this did not appear to be a site of diabetes-induced ENaC up-regulation.

Relation between human vasopressin 1a gene variance, fat intake, and diabetes

BACKGROUND

Male arginine vasopressin 1a receptor knockout mice (V1aR(-/-)) display a phenotype of low triglycerides and high glucose concentrations and high-fat-diet-induced obesity and diabetes.

OBJECTIVE

We investigated whether genetic variation of the human arginine vasopressin 1A (AVPR1A) gene is associated with phenotypic features resembling those of the V1aR(-/-) mouse.

DESIGN

In a population-based cross-sectional study in southern Sweden, middle-aged individuals (n = 6,055) were examined in 1991-1994. Associations between 4 AVPR1A tag single nucleotide polymorphisms (rs1042615, rs10784339, rs7308855, and rs10747983) and diabetes status, glucose and triglyceride concentrations, and BMI were analyzed. Furthermore, rs1042615 was related to diabetes status, glucose, and triglycerides within sex-specific quartiles of dietary fat intake (Q1(Fat)-Q4(Fat)) and BMI (Q1(BMI)-Q4(BMI)).

RESULTS

Subjects carrying the T allele of rs1042615 had lower concentrations of triglycerides than did CC carriers (1.36 +/- 0.77 compared with 1.42 +/- 0.89 mmol/L; P = 0.014), especially in nondiabetic subjects (P = 0.001). Carriers of the rs1042615 T allele had higher fasting blood glucose (5.20 +/- 1.44 mmol/L compared with 5.12 +/- 1.22 mmol/L; P = 0.036) and a tendency toward an increased prevalence of diabetes (odds ratio: 1.22; 95% CI: 0.99, 1.51; P = 0.067) compared with CC carriers. The less common rs10784339, rs7308855, and rs10747983 were not consistently associated with metabolic variables. Among men, the rs1042615 T allele was associated with diabetes exclusively within Q4(Fat) (odds ratio: 2.22; 95% CI: 1.05, 4.71; P = 0.04) and Q4(BMI) (odds ratio: 1.81; 95% CI: 1.11, 2.93; P = 0.02).

CONCLUSION

The rs1042615 T allele is associated with features resembling the phenotype of the V1aR(-/-) mouse, including uncoupling of the usual direct relation between glucose and triglycerides and an increased prevalence of diabetes in subjects with a high fat intake or who are overweight.

The role of hypothalamic ingestive behavior controllers in generating dehydration anorexia: a Fos mapping study

Giving rats 2.5% saline to drink for 3-5 days simply and reliably generates anorexia. Despite having the neurochemical and hormonal markers of negative energy balance, dehydrated anorexic rats show a marked suppression of spontaneous food intake, as well as the feeding that is usually stimulated by overnight starvation or a 2-deoxy-d-glucose (2DG) challenge. These observations are consistent with a dehydration-dependent inhibition of the core circuitry that controls feeding. We hypothesize that this inhibition is directed at those neurons in the paraventricular nucleus and lateral hypothalamic area that constitute the hypothalamic "behavior controller" for feeding rather than their afferent inputs from the arcuate nucleus or hindbrain that convey critical feeding-related sensory information. To test this hypothesis, we mapped and quantified the Fos-immunoreactive response to 2DG in control and dehydrated rats drinking 2.5% saline. Our rationale was that regions showing an attenuated Fos response to 2DG in dehydrated animals would be strong candidates as the targets of dehydration-induced suppression of 2DG feeding. We found that the Fos response to combined dehydration and 2DG was attenuated only in the lateral hypothalamic area, with dehydration alone increasing Fos in the lateral part of the paraventricular nucleus. In the arcuate nucleus and those regions of the hindbrain that provide afferent inputs critical for the feeding response to 2DG, the Fos response to 2DG was unaffected by dehydration. Therefore, dehydration appears to target the lateral hypothalamic area and possibly the lateral part of the paraventricular nucleus to suppress the feeding response to 2DG.

Phenotypes developed in secretin receptor-null mice indicated a role for secretin in regulating renal water reabsorption

Aquaporin 2 (AQP2) is responsible for regulating the concentration of urine in the collecting tubules of the kidney under the control of vasopressin (Vp). Studies using Vp-deficient Brattleboro rats, however, indicated the existence of substantial Vp-independent mechanisms for membrane insertion, as well as transcriptional regulation, of this water channel. The Vp-independent mechanism(s) is clinically relevant to patients with X-linked nephrogenic diabetes insipidus (NDI) by therapeutically bypassing the dysfunctional Vp receptor. On the basis of studies with secretin receptor-null (SCTR(-/-)) mice, we report here for the first time that mutation of the SCTR gene could lead to mild polydipsia and polyuria. Additionally, SCTR(-/-) mice were shown to have reduced renal expression of AQP2 and AQP4, as well as altered glomerular and tubular morphology, suggesting possible disturbances in the filtration and/or water reabsorption process in these animals. By using SCTR(-/-) mice as controls and comparing them with wild-type animals, we performed both in vivo and in vitro studies that demonstrated a role for secretin in stimulating (i) AQP2 translocation from intracellular vesicles to the plasma membrane in renal medullary tubules and (ii) expression of this water channel under hyperosmotic conditions. The present study therefore provides information for at least one of the Vp-independent mechanisms that modulate the process of renal water reabsorption. Future investigations in this direction should be important in developing therapeutic means for treating NDI patients.

Galanin-like peptide gene expression in the hypothalamus and posterior pituitary of the obese fa/fa rat

We examined the galanin-like peptide (GALP) gene expression in the arcuate nucleus (ARC) and posterior pituitary (PP) in 6- and 18-week-old male obese fa/fa rats. GALP mRNA in the ARC in fa/fa rats was significantly decreased in 6- and 18-week-old and GALP mRNA in the PP in fa/fa rats was significantly increased in 18-week-old compared to lean Fa/? rats. Insulin treatment in hyperglycemic fa/fa rats partially reversed those changes. These results suggest that the GALP gene expression in fa/fa rats might be regulated in part by leptin-independent mechanisms.

Influence of plasma amino acid level on vasopressin secretion

OBJECTIVES

Vasopressin (VP) is known to be elevated in patients with diabetes mellitus (DM). While the influence of acute hyperglycemia has been ruled out, the mechanism or the osmotically active compound responsible for the increase in VP secretion is still not elucidated. Because the plasma level of several amino acids (AAs) is increased in DM, we evaluated whether AAs could represent an effective osmotic stimulus for VP secretion.

RESEARCH DESIGN AND METHODS

In a cross-over study, eight healthy volunteers randomly received an infusion of isotonic saline (control) or mixed AA solution, i.v., at a low or a high rate (2 or 4.5 mg/min/kg BW, respectively). Plasma VP (P(VP)) was measured for two hours before and three hours during AA or control infusion.

RESULTS

AA infusion induced a dose-dependent elevation in plasma AA concentration but did not alter P(VP). However, effective plasma osmolality (P(osm)) (osmolality minus urea concentration) remained unchanged because a concommittant fall in plasma sodium concentration (P(Na)), likely due to sodium-linked uptake of AA in peripheral cells, compensated for the rise in plasma AA.

CONCLUSION

The stability of effective P(osm) may explain the lack of change observed in P(VP). Because sodium is a very efficient stimulus for VP secretion, it may be assumed that the fall in P(Na) occurring during AA infusion should have reduced VP secretion and thus P(VP). In this setting, the stability of P(VP) suggests that AAs induced an increase in VP secretion which counterbalanced the fall attributable to the decrease in P(Na). In conclusion, in acute experiments, AAs seem to represent an effective stimulus for VP secretion, almost equally potent as sodium. Further studies are needed to evaluate their contribution to the high P(VP) seen in the chronic setting of DM.

Effects of hydration and hyperventilation on cortical complexity

The effects of hydration and hyperventilation on cortical complexity were investigated in a sample of 19 healthy volunteers in a double-blind placebo design using magnetoencephalographic recordings. The subjects were asked to abstain from the intake of liquids 18 h before the study. Spontaneous magnetoencephalograms (MEG) were recorded before and after drinking 750 ml water (WAT group: nine subjects) or saline solution (SAL group: ten subjects) with eyes closed and open and during hyperventilation (HV) with eyes open. The MEG data were analysed using both linear (spectral power) and non-linear (pointwise dimension and largest Lyapunov exponent) algorithms. The prediction that intake of water, because of induced cell swelling, will lead to an increased synchronization and a decreased complexity of the spontaneous MEG during hyperventilation was confirmed. Hyperventilation following the drinking condition produced an increase in all power spectra with a stronger increase of delta and theta power after drinking of water. This synchronization of spontaneous MEG is accompanied by a general significant decrease of cortical complexity, especially after water drinking. Moreover, cortical complexity was inversely related to delta and theta power and partly also to alpha power. The SAL and WAT groups showed different relations between alpha power and dimensional complexity during HV: whereas in the SAL group the correlations between these measures became more negative during HV, they reversed in the WAT group to become positive. The synchronizing effect of hyperventilation, leading to a decrease of cortical complexity, is related in the SAL group to delta, theta and alpha power, whereas in the WAT group only delta and theta activity contribute to a reduction of cortical complexity.

Compensatory increase in AQP2, p-AQP2, and AQP3 expression in rats with diabetes mellitus

Diabetes mellitus (DM) is associated with osmotic diuresis and natriuresis. At day 15, rats with DM induced by streptozotocin (n = 13) had severe hyperglycemia (27.1 +/- 0.4 vs. 4.7 +/- 0.1 mM in controls) and had a fivefold increase in water intake (123 +/- 5 vs. 25 +/- 2 ml/day) and urine output. Semiquantitative immunoblotting revealed a significant increase in inner medullary AQP2 (201 +/- 12% of control rats, P < 0.05) and phosphorylated (Ser(256)) AQP2 (p-AQP2) abundance (299 +/- 32%) in DM rats. Also, the abundance of inner medullary AQP3 was markedly increased to 171 +/- 19% of control levels (100 +/- 4%, n = 7, P < 0.05). In contrast, the abundance of whole kidney AQP1 (90 +/- 3%) and inner medullary AQP4 (121 +/- 16%) was unchanged in rats with DM. Immunoelectron microscopy further revealed an increased labeling of AQP2 in the apical plasma membrane of collecting duct principal cells (with less labeling in the intracellular vesicles) of DM rats, indicating enhanced trafficking of AQP2 to the apical plasma membrane. There was a marked increase in urinary sodium excretion in DM. Only Na(+)/H(+) exchanger NHE3 was downregulated (67 +/- 10 vs. 100 +/- 11%) whereas there were no significant changes in abundance of type 2 Na-phosphate cotransporter (128 +/- 6 vs. 100 +/- 10%); the Na-K-2Cl cotransporter (125 +/- 19 vs. 100 +/- 10%); the thiazide-sensitive Na-Cl cotransporter (121 +/- 9 vs. 100 +/- 10%); the alpha(1)-subunit of the Na-K-ATPase (106 +/- 7 vs. 100 +/- 5%); and the proximal tubule Na-HCO(3) cotransporter (98 +/- 16 vs. 100 +/- 7%). In conclusion, DM rats had an increased AQP2, p-AQP2, and AQP3 abundance as well as high AQP2 labeling of the apical plasma membrane, which is likely to represent a vasopressin-mediated compensatory increase in response to the severe polyuria. In contrast, there were no major changes in the abundance of AQP1, AQP4, and several major proximal and distal tubule Na(+) transporters except NHE3 downregulation, which may participate in the increased sodium excretion.

Effects of Opuntia megacantha leaves extract on renal electrolyte and fluid handling in streptozotocin (STZ)-diabetic rats

We previously demonstrated that Opuntia megacantha leaves' extracts can reduce blood glucose levels in diabetes mellitus. For O. megacantha leaves' extracts to have potential in the management of diabetes mellitus, it is necessary to establish its detailed effects on renal function since diabetes is associated with renal fluid and electrolyte disturbances. Therefore, the current study was designed to investigate the influence of the extracts on renal function in male diabetic Sprague-Dawley rats. Rats were made diabetic by an i.p. injection of streptozotocin (STZ, 60 mg/kg in citrate buffer). Vehicle injected animals acted as controls. Separate groups of nondiabetic and diabetic rats were orally administered O. megacantha leaves extracts (20 mg/100 g bw) or normal saline (0.1 ml x 100 g(-1) bw) daily for 5 weeks. Urine volume and total urinary outputs of Na+ and K+ were determined from 24 h samples. O. megacantha leaves' extracts significantly (p < 0.01) increased urinary Na+ output in diabetic and nondiabetic rats resulting in significantly (p < 0.01) low plasma concentration by comparison with untreated animals. Treatment with the extract significantly increased FE(Na+) and GFR in all groups. The urinary K+ outputs in nondiabetic was slightly lowered, but did not reach statistically significance. O. megacantha leaves' extracts did not alter plasma aldosterone and AVP concentrations in diabetic and nondiabetic rats in nondiabetic animals. It is concluded that O. megacantha leaves extracts modulate renal water and sodium handling. The mechanisms are not clear.

Vasopressin contributes to hyperfiltration, albuminuria, and renal hypertrophy in diabetes mellitus: study in vasopressin-deficient Brattleboro rats

Diabetic nephropathy represents a major complication of diabetes mellitus (DM), and the origin of this complication is poorly understood. Vasopressin (VP), which is elevated in type I and type II DM, has been shown to increase glomerular filtration rate in normal rats and to contribute to progression of chronic renal failure in 5/6 nephrectomized rats. The present study was thus designed to evaluate whether VP contributes to the renal disorders of DM. Renal function was compared in Brattleboro rats with diabetes insipidus (DI) lacking VP and in normal Long-Evans (LE) rats, with or without streptozotocin-induced DM. Blood and urine were collected after 2 and 4 weeks of DM, and creatinine clearance, urinary glucose and albumin excretion, and kidney weight were measured. Plasma glucose increased 3-fold in DM rats of both strains, but glucose excretion was approximately 40% lower in DI-DM than in LE-DM, suggesting less intense metabolic disorders. Creatinine clearance increased significantly in LE-DM (P < 0.01) but failed to increase in DI-DM. Urinary albumin excretion more than doubled in LE-DM but rose by only 34% in DI-DM rats (P < 0.05). Kidney hypertrophy was also less intense in DI-DM than in LE-DM (P < 0.001). These results suggest that VP plays a critical role in diabetic hyperfiltration and albuminuria induced by DM. This hormone thus seems to be an additional risk factor for diabetic nephropathy and, thus, a potential target for prevention and/or therapeutic intervention.

Effect of residual endogenous insulin secretion on the abnormal oxytocin response to hypoglycaemia in insulin-dependent diabetics

OBJECTIVES

Arginine-vasopressin (AVP) and oxytocin (OT) secretions are abnormally stimulated by hypoglycaemia in patients with IDDM. Since previous studies showed that AVP secretion is influenced by the persistence of residual endogenous insulin secretion, we wondered whether this factor also regulates OT secretion.

DESIGN

Case-control study: the OT response to insulin-induced hypoglycaemia was measured in normal and diabetic patients with or without residual endogenous insulin secretion.

SUBJECTS

Ten normal male subjects, 10 C-peptide positive (CpP) and 11 C-peptide negative (CpN) male diabetic patients.

PRELIMINARY STUDIES

plasma C-peptide levels were measured after intravenous administration of 1 mg glucagon. Insulin tolerance test (ITT): diabetics were studied after optimization of their metabolic status by 3 days of treatment with constant subcutaneous insulin infusion. CpP and CpN diabetics and normal controls were tested with an intravenous administration of 0.15 IU per kg body weight insulin. Blood samples for OT assay were taken just before the rapid injection of insulin (time 0) and at time 15, 30, 45 and 60 min.

RESULTS

The basal concentrations of OT were similar in all groups. Insulin induced a similar hypoglycaemic nadir in all groups at 30 min, even though diabetic groups showed a delayed recovery in blood glucose levels. The glycaemic pattern was similar in all diabetic patients. Hypoglycaemia-induced OT rise was significantly higher in the two diabetic groups than in the normal group. However, CpN patients showed significantly higher OT increments than CpP subjects.

CONCLUSIONS

These data indicate that a residual endogenous insulin secretion exerts a partial protective action against the hypothalamic-pituitary disorder affecting the OT secretory system in IDDM.