Blunted renal dopaminergic system activity in puromycin aminonucleoside-induced nephrotic syndrome.

BACKGROUND: A primary tubular sodium handling abnormality has been implicated in the edema formation of nephrotic syndrome. Dopamine synthesized by renal proximal tubules behaves as an endogenous natriuretic hormone by activating D(1)-like receptors as a paracrine/autocrine substance. METHODS: We examined the time courses of the urinary excretion of sodium, protein and dopamine in puromycin aminonucleoside (PAN)-treated and control rats. The rats were sacrificed during greatest sodium retention (day 7) as well as during negative sodium balance (day 14) for the evaluation of renal aromatic l-amino acid decarboxylase (AADC) activity, the enzyme responsible for the synthesis of renal dopamine. Also, the influence of volume expansion (VE) and the effects of the D(1)-like agonist fenoldopam (10 microg/kg bw/min) on natriuresis and on proximal tubular Na(+),K(+)-ATPase activity were examined on day 7. RESULTS: The daily urinary excretion of dopamine was decreased in PAN-treated rats, from day 5 and beyond. This was accompanied by a marked decrease in the renal AADC activity, on days 7 and 14. During VE, the fenoldopam-induced decrease in proximal tubular Na(+),K(+)-ATPase activity was more pronounced in PAN-treated rats than in controls. However, the urinary sodium excretion during fenoldopam infusion was markedly increased in control rats but was not altered in PAN-treated animals. CONCLUSION: PAN nephrosis is associated with a blunted renal dopaminergic system activity which may contribute to enhance the proximal tubular Na(+),K(+)-ATPase activity. However, the lack of renal dopamine appears not to be related with the overall renal sodium retention in a state of proteinuria.     

Response of jejunal Na+, K+-ATPase to 5-hydroxytryptamine in young and adult rats: effect of fasting and refeeding

The present study is aimed to evaluate the effects of 5-hydroxytryptamine (5-HT) upon jejunal Na+,K+-ATPase in young (20-day-old) and adult (60-day-old) rats, and determine the effect of food intake on the response of the sodium pump to the amine. Basal Na+,K+-ATPase activity in jejunal epithelial cells from young rats was twice that in adult animals and responded to 5-HT with stimulation. In adult rats, fasting reduced by 25% basal jejunal Na+, K+-ATPase activity, whereas in young rats, no such change was observed. The sensitivity of jejunal Na+,K+-ATPase to 5-HT in young fasted rats was similar to that observed in fed animals. The effect of refeeding in young rats was a 2-fold increase in jejunal Na+, K+-ATPase activity, this being accompanied by insensitivity to 5-HT. In adult rats, refeeding was accompanied by an increase in jejunal Na+,K+-ATPase activity. It is concluded that the stimulatory effect of 5-HT upon jejunal Na+,K+-ATPase activity is a phenomenon dependent on both age and type of diet. In young rats, it is the food intake that plays an important role in development of insensitivity of Na+,K+-ATPase to stimulation by 5-HT, while in adult animals fasting or fasting followed by refeeding does not play a major role in regulating its sensitivity to the amine.     

Food deprivation increases alpha(2)-adrenoceptor-mediated modulation of jejunal epithelial transport in young and adult rats

This study examined the effect of food deprivation on the jejunal response to alpha(2)-adrenoceptor activation in young (20-d-old) and adult (60-d-old) rats, using short-circuit (I(sc)) measurements in the absence or presence of furosemide (1 mmol/L). The effect of alpha(2)-adrenoceptor stimulation by 5-bromo-N:-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304; 0.3-3000 nmol/L) was a concentration-dependent decrease in I(sc) with similar half-maximal effective concentration (EC(50); 12.3 +/- 1.1 vs. 9.6 +/- 1.1 nmol/L) and maximal effect (E(max); 70.6 +/- 6.9 vs. 80.6 +/- 4.5% of reduction) values in adult food-deprived and fed rats. The effect of UK 14,304 on I(sc) in fed and food-deprived rats was markedly (P: < 0.05) attenuated by furosemide (1 mmol/L). E(max) values for UK 14,304 in 20-d-old food-deprived rats were higher (P: < 0.05) than those observed in fed rats (93.3 +/- 3.3 vs. 67.0 +/- 11.3% of reduction), without differences in EC(50) values. The effect of UK 14,304 on I(sc) in 20-d-old fed rats was completely abolished by furosemide (1 mmol/L). In food-deprived young rats, the effect of UK 14,304 was also markedly (P: < 0.05) antagonized by furosemide, but not completely abolished. Specific [(3)H]-rauwolscine binding in membranes from jejunal epithelial cells revealed the presence of a single class of binding sites, with an apparent K:(D) in the low nmol/L range. In 20-d-old food-deprived rats, specific [(3)H]-rauwolscine binding was markedly increased, and this was reversed by refeeding. Na(+),K(+)-ATPase activity in isolated jejunal epithelial cells from 60-d-old fed rats was twice that in 20-d-old fed rats [117 +/- 14 vs. 52 +/- 5 nmol free inorganic phosphorus/(mg protein.min)]. Food deprivation in adult rats, but not in 20-d-old rats, was accompanied by a significant decrease in Na(+),K(+)-ATPase activity. In both young and adult rats (fed and food-deprived), UK 14,304 did not affect Na(+),K(+)-ATPase activity. In conclusion, food deprivation in 20-d-old rats enhanced the response to alpha(2)-adrenoceptor stimulation. This effect, which depends primarily on the stimulation of a furosemide-sensitive antisecretory mechanism, is suggested to result from increases in the number of jejunal epithelial alpha(2)-adrenoceptors.     

The O-methylated derivative of L-DOPA, 3-O-methyl-L-DOPA, fails to inhibit neuronal and non-neuronal aromatic L-amino acid decarboxylase

The present study examined whether the O-methylated derivative of L-DOPA, 3-O-methyl-L-DOPA (3-OM-L-DOPA), inhibits neuronal (brain) and non-neuronal (liver and kidney) aromatic L-amino acid decarboxylase (AADC) activity. The incubation of brain, liver and kidney homogenates with 3-OM-L-DOPA (5 mM) did not result in the formation of 3-methoxytyramine, the compound expected to result from the decarboxylation of 3-OM-L-DOPA. Incubation of tissue homogenates with L-DOPA resulted in a concentration-dependent formation of dopamine, revealing K(m) values (in mM) of similar magnitude for brain (0.8), liver (1.6) and kidney (1.0). Both benserazide and L-5-hydroxytryptophan (L-5-HTP) were found to produce concentration dependent decreases in AADC activity with K(i) values in the microM range. By contrast, 3-OM-L95% reduction) in liver and kidney AADC activity accompanied by a marked decrease (49% reduction) in brain AADC activity. By contrast, the administration of 30 mg/kg (p.o.) 3-OM-L-DOPA, which generates levels in brain, liver and kidney six-fold those in L-DOPA-treated rats, was found to change neither neuronal nor non-neuronal AADC activity. In conclusion, 3-OM-L-DOPA fails to interact with neuronal and non-neuronal AADC, either as substrate or inhibitor.     

Treg Vaccination in Autoimmune Type 1 Diabetes

Foxp3⁺ regulatory T (Treg) cells are critical contributors to the establishment and maintenance of immunological self-tolerance. Autoimmune type 1 diabetes (T1D) is characterized by the loss of self-tolerance to the insulin-producing β cells in the pancreas and the destruction of β cells, resulting in the development of chronic hyperglycemia at diagnosis. The application of strong-agonistic T-cell receptor ligands provided under subimmunogenic conditions functions as a critical means for the efficient de novo conversion of naive CD4⁺ T cells into Foxp3⁺ Treg cells. The specific induction of Treg cells upon supply of strong-agonistic variants of certain self-antigens could therefore function as a critical instrument in order to achieve safe and specific prevention of autoimmunity such as T1D via the restoration of self-tolerance. Such immunotherapeutic strategies are being developed, and in the case of T1D aim to restrict autoimmunity and β-cell destruction. In this review, we discuss the requirements and opportunities for Treg-based tolerance approaches with the goal of interfering with autoimmune T1D.