Renal handling of endogenous lithium in experimental diabetes mellitus in the rat

Impaired pressure natriuresis and non-dipping blood pressure in rats with early type 1 diabetes mellitus

KEY POINTS

Type 1 diabetes mellitus increases cardiovascular risk; hypertension amplifies this risk, while pressure natriuresis regulates long-term blood pressure. We induced type 1 diabetes in rats by streptozotocin injection and demonstrated a substantial impairment of pressure natriuresis: acute increases in blood pressure did not increase renal medullary blood flow, tubular sodium reabsorption was not downregulated, and proximal tubule sodium reabsorption, measured by lithium clearance, was unaffected. Insulin reduced blood glucose in diabetic rats, and rescued the pressure natriuresis response without influencing lithium clearance, but did not restore medullary blood flow. Radiotelemetry showed that diastolic blood pressure was increased in diabetic rats, and its diurnal variation was reduced. Increases in medullary blood flow and decreases in distal tubule sodium reabsorption that offset acute rises in BP are impaired in early type 1 diabetes, and this impairment could be a target for preventing hypertension in type 1 diabetes.

ABSTRACT

Type 1 diabetes mellitus (T1DM) substantially increases cardiovascular risk, and hypertension amplifies this risk. Blood pressure (BP) and body sodium homeostasis are linked. T1DM patients have increased total exchangeable sodium, correlating directly with BP. Pressure natriuresis is an important physiological regulator of BP. We hypothesised that pressure natriuresis would be impaired, and BP increased, in the early phase of T1DM. Male Sprague-Dawley rats were injected with streptozotocin (30-45 mg/kg) or citrate vehicle. After 3 weeks, pressure natriuresis was induced by serial arterial ligation. In non-diabetic controls, this increased fractional excretion of sodium from ∼1% to ∼25% of the filtered load (P < 0.01); in T1DM rats, the response was significantly blunted, peaking at only ∼3% (P < 0.01). Mechanistically, normal lithium clearance suggested that distal tubule sodium reabsorption was not downregulated with increased BP in T1DM rats. The pressure dependence of renal medullary perfusion, considered a key factor in the integrated response, was abolished. Insulin therapy rescued the natriuretic response in diabetic rats, restoring normal downregulation of tubular sodium reabsorption when BP was increased. However, the pressure dependence of medullary perfusion was not restored, suggesting persistent vascular dysfunction despite glycaemic control. Radiotelemetry showed that T1DM did not affect systolic BP, but mean diastolic BP was ∼5 mmHg higher than in non-diabetic controls (P < 0.01), and normal diurnal variation was reduced. In conclusion, functional impairment of renal sodium and BP homeostasis is an early manifestation of T1DM, preceding hypertension and nephropathy. Early intervention to restore pressure natriuresis in T1DM may complement reductions in cardiovascular risk achieved with glycaemic control.

Modeling oxygen consumption in the proximal tubule: effects of NHE and SGLT2 inhibition

The objective of this study was to investigate how physiological, pharmacological, and pathological conditions that alter sodium reabsorption (TNa) in the proximal tubule affect oxygen consumption (QO2 ) and Na(+) transport efficiency (TNa/QO2 ). To do so, we expanded a mathematical model of solute transport in the proximal tubule of the rat kidney. The model represents compliant S1, S2, and S3 segments and accounts for their specific apical and basolateral transporters. Sodium is reabsorbed transcellularly, via apical Na(+)/H(+) exchangers (NHE) and Na(+)-glucose (SGLT) cotransporters, and paracellularly. Our results suggest that TNa/QO2 is 80% higher in S3 than in S1-S2 segments, due to the greater contribution of the passive paracellular pathway to TNa in the former segment. Inhibition of NHE or Na-K-ATPase reduced TNa and QO2 , as well as Na(+) transport efficiency. SGLT2 inhibition also reduced proximal tubular TNa but increased QO2 ; these effects were relatively more pronounced in the S3 vs. the S1-S2 segments. Diabetes increased TNa and QO2 and reduced TNa/QO2 , owing mostly to hyperfiltration. Since SGLT2 inhibition lowers diabetic hyperfiltration, the net effect on TNa, QO2 , and Na(+) transport efficiency in the proximal tubule will largely depend on the individual extent to which glomerular filtration rate is lowered.

The role of Sgk-1 in the upregulation of transport proteins by PPAR-{gamma} agonists in human proximal tubule cells

BACKGROUND

Cellular sodium and water transport are dysregulated in diabetes mellitus. Synthetic peroxisome proliferator-activated receptor gamma (PPAR-gamma) agonists are currently used in the treatment of type 2 diabetes, but their use is limited by fluid retention. Recent data suggest that PPAR-gamma agonists stimulate distal tubular epithelial Na transport, potentially through the serine glucocorticoid kinase-1 (Sgk-1)-dependent regulation of the epithelial Na channel. We have recently demonstrated that Sgk-1 additionally regulates sodium reabsorption through the proximal tubular sodium hydrogen exchanger-3 (NHE3). However, the effects of PPAR-gamma agonists on Sgk-1, the water channel proteins aquaporins and on sodium transport in human proximal tubule cells (PTCs) have not previously been studied.

METHODS

PTCs were exposed to the PPAR-gamma agonists, pioglitazone and the more selective PPAR-gamma agonist L-805645 with and without the Sgk inhibitor (GSK650394A). PPAR-gamma, Sgk-1, NHE3, AQP 1 and 7 mRNA and protein expression were determined by semi-quantitative PCR and western blot. The Sgk-1-specific effect was determined using Sgk-1 siRNA.

RESULTS

Exposure of PTCs to 10 muM pioglitazone and 8 microM L-805645 increased the mRNA and protein expression of PPAR-gamma (P < 0.005), NHE3 and Sgk-1 (both P < 0.05). The expression of AQPs 1 and 7 was increased by pioglitazone and L-805645 (both P < 0.05). The increases in NHE3 and AQPs 1 and 7 were significantly reduced by pharmacological inhibition of Sgk and when cultures were exposed to Sgk-1-specific siRNA.

CONCLUSIONS

PPAR-gamma agonists enhanced the expression of NHE3, AQP 1 and 7 channels in human proximal tubule cells through Sgk-1-dependent pathways.

A comparison between endogenous and exogenous lithium clearance in the anaesthetized rat

The present study tests whether the endogenous renal lithium clearance (CLi) may be used as an alternative to the conventional method in order to avoid the adverse effects of exogenous lithium on renal function. Endogenous CLi) was measured by flameless atomic absorption spectrometry (AAS) in halothane anaesthetized, laparotomized rats and compared with values obtained after subsequent intravenous (i.v.) administration of LiCl. Endogenous plasma lithium concentration (PLi) was 0.30 +/- 0.06 mumol l-1. When measured by both conventional flame emission AAS and flameless AAS on the same samples, values for PLi did not differ significantly. Endogenous CL1/GFR was 0.195 +/- 0.01 (mean +/- SEM). Amiloride caused on significant change in either the endogenous CLi or 51Cr-EDTA clearance, a measure of glomerular filtration rate (GFR), but it caused a significant increase in CNa. Administration of exogenous Li+ had no significant effect on CLi (230 +/- 33 vs. 234 +/- 35 microliters min-1 gKW-1); both urine flow and CNa increased significantly (P < 0.01) (5.4 +/- 0.4 vs. 9.6 +/- 1.3, and 2.4 +/- 0.5 vs. 10.7 +/- 2.4 microliters min-1 gKW-1, respectively). GFR and calculated proximal tubular reabsorption rate (APR) both decreased by approximately 100 microliters min-1 gKW-1 (both P < or = 0.025), suggesting a moderate inhibition of APR after the acute elevation of plasma Li+ concentration. It is concluded that the endogenous CLi may be used as an alternative to the conventional method.