Renal D1 Research Articles

Role of nuclear factor kappa B (NF-κB) in oxidative stress-induced defective dopamine D1 receptor signaling in the renal proximal tubules of Sprague-Dawley rats

Dopamine promotes sodium excretion, in part, via activation of D1 receptors in renal proximal tubules (PT) and subsequent inhibition of Na, K-ATPase. Recently, we have reported that oxidative stress causes D1 receptor–G-protein uncoupling via mechanisms involving protein kinase C (PKC) and G-protein-coupled receptor kinase 2 (GRK 2) in the primary cultures of renal PT of Sprague-Dawley (SD) rats. There are reports suggesting that redox-sensitive nuclear transcription factor, NF-κB, is activated in conditions associated with oxidative stress. This study was designed to identify the role of NF-κB in oxidative stress-induced defective renal D1 receptor–G-protein coupling and function. Treatment of the PT with hydrogen peroxide (H 2O 2, 50 μM/20 min) induced the nuclear translocation of NF-κB, increased PKC activity, and triggered the translocation of GRK 2 to the proximal tubular membranes. This was accompanied by hyperphosphorylation of D1 receptors and defective D1 receptor–G-protein coupling. The functional consequence of these changes was decreased D1 receptor activation-mediated inhibition of Na, K-ATPase activity. Interestingly, pretreatment with pyrrolidine dithiocarbamate (PDTC, 25 μM/10 min), an NF-κB inhibitor, blocked the H 2O 2-induced nuclear translocation of NF-κB, increase in PKC activity, and GRK 2 translocation and hyperphosphorylation of D1 receptors in the proximal tubular membranes. Furthermore, PDTC restored D1 receptor G-protein coupling and D1 receptor agonist-mediated inhibition of the Na, K-ATPase activity. Therefore, we suggest that oxidative stress causes nuclear translocation of NF-κB in the renal proximal tubules, which contributes to defective D1 receptor–G-protein coupling and function via mechanisms involving PKC, membranous translocation of GRK 2, and subsequent phosphorylation of dopamine D1 receptors.

Mice with Impaired Extrathyroidal Thyroxine to 3,5,3′-Triiodothyronine Conversion Maintain Normal Serum 3,5,3′-Triiodothyronine Concentrations

For T(3) to mediate its biological effects, the prohormone T(4) must be activated by removal of an outer-ring iodine by the type 1 or 2 deiodinases (D1 and D2) with approximately 60% of the daily T(3) production in rodents being produced extrathyroidally through this pathway. To further define the role of these enzymes in thyroid hormone homeostasis, we backcrossed the targeted disruption of the Dio2 gene into C3H/HeJ (C3H) mice with genetically low D1 expression to create the C3H-D2KO mouse. Remarkably, these mice maintain euthyroid serum T(3) levels with normal growth and no decrease in expression of hepatic T(3)-responsive genes. However, serum T(4) is increased 1.2-fold relative to the already elevated C3H levels, and serum TSH is increased 1.4-fold. Despite these increases, thyroidal (125)I uptake indicates no difference in thyroidal activity between C3H-D2KO and C3H mice. Although C3H-D2KO hepatic and renal D1 activities were well below those observed in wild-type mice (approximately 0.1-fold for both), they were 8-fold and 2-fold higher, respectively, relative to C3H mice. Thyroidal D1 and cerebral cortical type 3 deiodinase activity were unchanged between C3H-D2KO and C3H mice. In conclusion, C3H-D2KO mice have notably elevated serum T(4) levels, and this, in conjunction with residual D1 activity, is likely an important role in the maintenance of euthyroid serum T(3) concentrations.

A New Approach for Treatment of Hypertension: Modifying D1 Dopamine Receptor Function

Essential hypertension is a major factor for myocardial infarction, heart failure and kidney failure. Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and vasodilatation directly or indirectly with other hormones and humoral factors, such as reactive oxygen species and the renin-angiotensin system. Dopamine receptors are classified into five subtypes based on their structure and pharmacology. Among those dopamine receptor subtypes, D(1) receptor is the most important one, during conditions of moderate sodium intake, more than 50% of renal sodium excretion is regulated by D(1)-like receptors. Decreased renal dopamine production and/or impaired D(1) receptor function have been reported in hypertension. Disruption of D(1) receptor results in hypertension. In this paper, we review the mechanisms by which hypertension develops when D(1) receptor function is perturbed. We also discuss possible new approaches developing anti-hypertensive medicine by increasing renal dopamine production, enhancing D(1) receptor function, or modifying its interactions with other blood pressure-regulating systems.

Developmental Control of Iodothyronine Deiodinases by Cortisol in the Ovine Fetus and Placenta Near Term

Preterm infants have low serum T4 and T3 levels, which may partly explain the immaturity of their tissues. Deiodinase enzymes are important in determining the bioavailability of thyroid hormones: deiodinases D1 and D2 convert T4 to T3, whereas deiodinase D3 inactivates T3 and produces rT3 from T4. In human and ovine fetuses, plasma T3 rises near term in association with the prepartum cortisol surge. This study investigated the developmental effects of cortisol and T3 on tissue deiodinases and plasma thyroid hormones in fetal sheep during late gestation. Plasma cortisol and T3 concentrations in utero were manipulated by exogenous hormone infusion and fetal adrenalectomy. Between 130 and 144 d of gestation (term 145+/-2 d), maturational increments in plasma cortisol and T3, and D1 (hepatic, renal, perirenal adipose tissue) and D3 (cerebral), and decrements in renal and placental D3 activities were abolished by fetal adrenalectomy. Between 125 and 130 d, iv cortisol infusion raised hepatic, renal, and perirenal adipose tissue D1 and reduced renal and placental D3 activities. Infusion with T3 alone increased hepatic D1 and decreased renal D3 activities. Therefore, in the sheep fetus, the prepartum cortisol surge induces tissue-specific changes in deiodinase activity that, by promoting production and suppressing clearance of T3, may be responsible for the rise in plasma T3 concentration near term. Some of the maturational effects of cortisol on deiodinase activity may be mediated by T3.

OXIDATIVE STRESS INDUCED RENAL D1 RECEPTOR DYSFUNCTION CAUSES SALT SENSITIVE HYPERTENSION IN SPRAGUE DAWLEY RATS

Renal dopamine, via activation of D1 receptors plays a role in maintaining sodium homeostasis and blood pressure during increased sodium intake. Animal models of Salt sensitive hypertension are associated with defective renal D1 receptor function and increased oxidative stress. The objective of the present study was to determine whether oxidative stress associated with high salt intake will produce hypertension resulting from defective renal D1 receptor function. Male Sprague Dawley rats were divided into four groups and received tap water (vehicle), 30mM L-buthionine sulfoximine (BSO), an oxidant, 1% NaCl (high salt), and BSO plus NaCl in drinking water for ten days. Compared to vehicle, BSO treatment caused elevated levels of renal tubular malondialdehyde, reduced levels of glutathione, increased urinary excretion of 8-isoprostane and decreased D1 receptor numbers and coupling to G proteins. Furthermore, SKF-38393, a D1 receptor agonist, failed to inhibit Na, K-ATPase in proximal tubules and was unable to promote sodium excretion. Animals provided with high salt alone did not show any increase in oxidative stress or defect in D1 receptor function. Concomitant administration of BSO and high salt caused significant increase in renal tubular oxidative stress, abrogation of D1 receptor function and showed marked decrease in basal sodium excretion and significant increase in blood pressure. It appears that during high salt intake oxidative stress induced D1 receptor dysfunction leads to salt retention and hypertension. These phenomena may explain, in part, the mechanism for salt sensitive hypertension associated with oxidative stress. [Supported in part by NIH grant DK-58743]

Defective Renal Dopamine D1 Receptor Function Contributes to Hyperinsulinemia-Mediated Hypertension

Hyperinsulinemia is reported to play a role in hypertension, as abnormalities in blood pressure regulation and sodium handling exist in diabetes mellitus. Kidney dopamine promotes sodium excretion via the activation of renal D1 receptors. Because there is a close relationship between renal D1 receptor function and sodium excretion, it is hypothesized that a defect in this mechanism may contribute to decreased sodium excretion and hypertension during hyperinsulinemia. Renal D1 receptor function was studied in insulin-induced hypertension in male Sprague Dawley rats. Insulin pellets were implanted subcutaneously for controlled insulin release for three weeks; sham rats served as a control. Compared to control rats, insulin pellets increased plasma insulin levels by eight fold and decreased blood glucose by 40%. Insulin also caused a 22 mmHg increase in mean arterial blood pressure compared to control animals. The intravenous infusion of SKF-38393, a D1 receptor agonist, increased sodium excretion in control rats, but SKF-38393 failed to produce natriuresis in hyperinsulinemic animals. Renal proximal tubules from hyperinsulinemic rats had a reduced D1 receptor number, defective receptor-G protein coupling, and blunted SKF-38393 induced Na, K-ATPase inhibition. Insulin seems to reduce D1 receptor expression and coupling to the G-protein, leading to a reduced D1 receptor-mediated Na, K-ATPase inhibition, and a diminished natriuretic response to SKF-38393. These phenomena could account for sodium retention and hypertension associated with hyperinsulinemia.

Protein Phosphatase 2A B56α During Development in the Spontaneously Hypertensive Rat

Mistargeting of the regulatory subunit of protein phosphatase 2A (PP2A), B56α is involved in the hyperphosphorylation and desensitization of the D1 dopamine receptor in renal proximal tubules of spontaneously hypertensive rats (SHRs). However, the renal expression of B56α before hypertension develops is not known. Therefore, we studied the expression of B56α and PP2A activity in the kidney during development in the SHR and its normotensive control, the Wistar‐Kyoto (WKY) rat. PP2A B56α was expressed in proximal and distal tubules with no differences in the pattern of expression in WKY and SHRs at any age. In brush border membranes of renal proximal tubules, PP2A B56α protein was greatest in the immature rats and decreased with development. However, PP2A activity did not change with age. PP2A B56α protein and PP2A activity were similar in WKY and SHRs except at 2 weeks when both PP2A B56α protein and PP2A activity were higher in SHRs than in WKY rats. The PP2A catalytic subunit co‐immunoprecipitated with the D1 receptor in renal proximal tubule cells. It is possible that the increased expression of PP2A B56α and increased basal PP2A activity in the young, especially in the SHRs, may serve as a compensatory mechanism in the increased phosphorylation and decreased renal D1 receptor function, including D1‐receptor mediated stimulation in renal proximal tubules of SHRs.

Dynamics and regulation of intracellular thyroid hormone concentrations in embryonic chicken liver, kidney, brain, and blood

The intracellular thyroid hormone (TH) availability is influenced by different metabolic pathways. We investigated the relationship between tissue and plasma TH levels as well as the correlation with changes of deiodination and sulfation during chicken embryonic development. From day 14 until day 19, T 3 remains unchanged in liver and kidney in spite of increasing plasma T 4 and T 3 levels and a slightly increased T 4 availability in these tissues. During this period, the T 3 breakdown capacity by type III deiodinase (D3) is high in liver but low in kidney. The TH inactivation capacity of type I deiodinase (D1), with production of inactive rT 3 instead of T 3, in kidney seems to be potentiated by the sulfation pathway. A sharp rise in T 3 and T 4 is detected in all tissues examined when the embryo switches to lung respiration. The same day, T 4 content in liver is sharply enhanced and sulfation activity is decreased. So, T 4 availability in liver is increased while a declined D3 activity allows for the accumulation of hepatic T 3. The increase in renal T 3 and T 4 are more closely related to plasma TH profiles and a lack of correlation with the changes in renal D1 and D3 activity suggests that T 4 and T 3 content in this organ is strongly dependent on direct uptake from the blood. Despite much lower T 4 levels, T 3 levels in brain are in the same range as in liver and kidney and intracellular T 3 even exceeds the T 4 levels towards the end of development. The rise in TH content coincides with a drop in D3 activity, low sulfation activity and an increased T 3 production capacity via type II deiodinase (D2). In conclusion, the current study describes the dynamics of intracellular TH concentrations in liver, kidney, and brain during chicken development and investigates their relationship with circulating TH levels and changes of deiodinases and sulfotransferases. The clear differences in intracellular TH profiles among the different tissues demonstrate that circulating levels are not necessarily representative for the local TH changes. Some of the changes in intracellular TH availability can be linked to changes in local deiodination and sulfation capacities, but the importance of these enzyme systems in relation to other factors, such as hormone uptake, differs between liver, kidney, and brain.

Dopamine and the kidney: a role in hypertension?

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension. Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis. GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

Regulation of iodothyronine deiodinases in the Pax8-/- mouse model of congenital hypothyroidism.

Thyroid hormones are essential for a variety of developmental and metabolic processes. Congenital hypothyroidism (CHT) results in severe defects in the development of different tissues, in particular brain. As an animal model for CHT, we studied Pax8(-/-) mice, which are born without a thyroid gland. We determined the expression of iodothyronine deiodinase D1 in liver and kidney, D2 in brain and pituitary, and D3 in brain, as well as serum T(4), T(3), and rT(3) levels in Pax8(-/-) vs. control mice during the first 3 wk of life. In control mice, serum T(4) and T(3) were undetectable on the day of birth (d 0) and increased to maximum levels on d 15. In Pax8(-/-) mice, serum T(4) and T(3) remained below detection limits. Serum rT(3) was high on d 0 in both groups and rapidly decreased in Pax8(-/-), but not in control mice. Hepatic and renal D1 activities and mRNA levels were low on d 0 and increased in control mice roughly parallel to serum T(4) and T(3) levels. In Pax8(-/-) mice, tissue D1 activities and mRNA levels remained low. Cerebral D2 activities were low on d 0 and increased to maximum levels on d 15, which were approximately 10-fold higher in Pax8(-/-) than in control mice. D2 mRNA levels were higher in Pax8(-/-) than in control mice only on d 21. Cerebral D3 activities and mRNA levels were high on d 0 and showed a moderate decrease between d 3 and 15, with values slightly lower in Pax8(-/-) than in control mice. One day after the injection of 200 ng T(4) or 20 ng T(3)/g body weight, tissue deiodinase activities and mRNA levels were at least partially restored toward control levels, with the exception of cerebral D3 activity. In conclusion, these findings show dramatic age and thyroid state-dependent changes in the expression of deiodinases in central and peripheral tissues of mice during the first 3 wk of life.

Rosiglitazone Treatment Restores Renal Dopamine Receptor Function in Obese Zucker Rats

Earlier we have reported a defective dopamine D1-like receptor function, which was accompanied by a decrease in D1 receptor numbers and the inability of dopamine to inhibit Na,K-ATPase and Na,H-exchanger in proximal tubules of hyperinsulinemic obese Zucker rats. The present study was designed to test the hypothesis that the defect in dopamine receptor function is a result of hyperinsulinemia in obese rats. We designed experiments to study D1 receptor function in obese Zucker rats treated with rosiglitazone, as it lowers plasma insulin by improving insulin sensitivity. A group of untreated lean and obese rats served as controls. Rosiglitazone treatment (10 mg/kg orally, 4 weeks) caused significant decreases in plasma insulin, blood glucose, and blood pressure while causing an increase in renal sodium excretion compared with untreated obese rats. In the isolated proximal tubules obtained from untreated lean rats, dopamine caused concentration-dependent inhibition of the Na,K-ATPase activity, but this inhibitory effect was absent in untreated obese rats. In rosiglitazone-treated obese rats, the inhibitory effect of dopamine on Na,K-ATPase was significantly restored. This was accompanied by a complete restoration of D1 receptor numbers in proximal tubular membranes of treated obese rats. In another set of experiments, treatment of primary proximal tubule epithelial cells in culture medium with insulin caused a significant decrease in the D1 receptor abundance, suggesting a direct role of insulin on D1 receptor regulation. We conclude that hyperinsulinemia causes downregulation of D1 receptor function and lowering of plasma insulin levels leads to restoration of renal D1 receptor function.

Desensitization of human renal D1 dopamine receptors by G protein-coupled receptor kinase 4

The D1 dopamine receptor, expressed in several nephron segments, participates in the regulation of water and electrolyte transport. Because the renal D1 receptor is desensitized in genetic hypertension, we sought to determine the mechanism(s) of the desensitization of D1 receptors endogenously expressed in renal proximal tubules. The mechanisms involved in the homologous desensitization of the D1 receptor in human renal proximal tubule cells were studied by measuring the production of cAMP in response to stimulation or inhibition of G protein-coupled receptor kinase (GRK) activity and expression. Protein expression was assessed by immunoblotting. In human renal proximal tubule cells, the D1 agonist, fenoldopam, increased cAMP accumulation (73 +/- 2%). Fenoldopam pre-treatment decreased the responsiveness to subsequent fenoldopam stimulation (t(1/2) approximately equal to 20 min) with complete desensitization at 30 minutes. Recovery occurred gradually (t(1/2) approximately equal to 20 min) with full recovery at 60 minutes. Forskolin pretreatment minimally affected the fenoldopam effect, indicating a minor involvement of protein kinase A in the homologous desensitization process. Because GRKs are involved in the homologous desensitization process, we determined the consequences of inhibition of GRK expression and activity. Heparin, an inhibitor of GRK activity, decreased the expression of GRK2 and GRK4 and attenuated the desensitization of the D1 receptor (85 +/- 1%). Antisense oligonucleotides (GRK4> GRK2) blunted the D1 receptor desensitization. However, the first 20 minutes of homologous desensitization were not affected by either heparin or GRK antisense oligonucleotides. These studies document the critical role of GRK4, relative to GRK2, in the homologous desensitization of D1 receptors in renal proximal tubule cells. However, the early phase of homologous desensitization is regulated by a non-GRK-mediated pathway.

Effects of dexamethasone treatment on iodothyronine deiodinase activities and on metamorphosis-related morphological changes in the axolotl ( Ambystoma mexicanum)

In amphibians, there is a close interaction between the interrenal and the thyroidal axes. Hypothalamic corticotropin-releasing hormone or related peptides stimulate thyroidal activity by increasing thyrotropin synthesis and release, while corticosterone accelerates both spontaneous and thyroid hormone-induced metamorphosis. One of the mechanisms that is thought to contribute to this acceleration is a corticosterone-induced change in peripheral deiodinating activity. The present experiments were designed to investigate further the effects of glucocorticoid treatment on amphibian deiodinase activities and to explore the possible role of these effects in metamorphosis. Neotenic axolotls ( Ambystoma mexicanum) were treated either acutely or chronically with dexamethasone (DEX) and changes in type II and type III iodothyronine deiodinase (D2 and D3) activities were studied in liver, kidney, and brain. In addition, gill length, tail height, and body weight were measured at regular intervals in the chronically treated animals in search of metamorphosis-related changes. A single injection of 50 μ g DEX decreased hepatic D3 activity (6–48 h) while it increased D2 activity in brain (6–48 h) and to a lesser extent in kidney (24 h). These changes were accompanied by an increase in plasma T 3 levels (48 h). Samples taken during chronic treatment with 20 or 100 μ g DEX showed that both hepatic D2 and D3 activities were decreased on day 26, while renal D3 activity was decreased but only in the 20 μ g dose group. All other deiodinase activities were not different from those in control animals. At 25 days, all DEX-treated axolotls showed a clear reduction in gill length, tail height, and body weight, changes typical of metamorphosis. Prolongation of the treatment up to 48 days resulted in complete gill resorption by days 44–60. Although probably several mechanisms contribute to these DEX-induced metamorphic changes, the interaction with thyroid function via a sustained downregulation of hepatic D3 may be one of them.

Ligand-independent desensitization of the renal D1 receptor in essential hypertension

Ligand-independent desensitization of the renal D1 receptor in essential hypertension

Environmental salinity selectively modifies the outer-ring deiodinating activity of liver, kidney and gill in the rainbow trout

We here analyzed the effect of a mild hyperosmotic challenge on the activities of deiodinases type I (D1) and II (D2) in the trout liver, and D1 in kidney and gill, two organs involved in osmoregulation. FW-adapted immature rainbow trout were transferred to 5‰ SW and killed 0.5, 1, 2, 4, 8 12, 24 and 48 h post-transfer (PT). Fish maintained in FW served as controls. Hepatic, renal and branchial D1 and hepatic D2 activities were assessed as well as circulating levels of T 3, T 4 and cortisol. Hyperosmotic challenge elicited significant and sustained decreases in kidney D1 and liver D2 activities at 8 h PT, which returned to control values at 48 h PT. In contrast, liver and gill D1 activities exhibited no significant change throughout the study. Also, significant increases in circulating T 4 at 2–4 and 48 h PT were observed. Circulating T 3 remained unmodified until 24–48 h PT, when it rose sharply. Simultaneously, cortisol showed a trend towards increase during the initial 4 h PT, which attained significance at 48 h PT. The present findings demonstrate that a mild hypertonic challenge is sufficient to elicit responses in the trout thyroidal axis. Hormonal changes in the circulatory compartment are in accordance with those previously described for migratory salmonids. A novel aspect of our findings is the organ-specific differential response exhibited by ORD-enzymes when trout are exposed to a mildly different osmotic environment. Our findings further establish the uniqueness of fish thyroid physiology, and can be of value in further understanding the evolutionary aspects of this ORD family of deiodinases.

Desensitization of renal D1 dopamine receptors by G protein-coupled receptor kinases and endocytosis

Desensitization of renal D1 dopamine receptors by G protein-coupled receptor kinases and endocytosis

Regulation of Thyroid Hormone Metabolism during Fasting and Refeeding in Chicken

Fasting and refeeding have considerable effects on thyroid hormone metabolism. In the present study, 8-day-old meat-type cockerels were subjected to a 2-day starvation period followed by 3 days' refeeding. Blood and tissue samples were collected at the start of the experiment, at 4, 24, and 48 h of starvation, and at 4, 8, 24, 48, and 72 h of refeeding. This study demonstrates that in chicken, fasting decreased plasma T3 and TSH levels and increased plasma T4 concentrations. This was accompanied by increased hepatic type III deiodinase (D3) and decreased renal D3 activity. There were no changes in hepatic or renal type I deiodinase (D1). Refeeding restored normal plasma T3, T4, and TSH levels, while hepatic D3 and renal D3 activities returned to prefasting levels. Again hepatic D1 was not affected, but renal D1 was lower than the ad libitum values during the entire refeeding period. These results confirm that liver D3 is involved in the regulation of plasma T3 during fasting and refeeding in the chicken. Northern blot analysis demonstrated increased hepatic D3 mRNA levels during the first day of starvation that disappeared by the end of the second day; refeeding had no additional effects. These results suggest that in fasted chickens the rapid upregulation of hepatic D3 occurs predominantly at a pretranslational level, whereas the drop in hepatic D3 activity after refeeding is probably regulated at a posttranslational level. In addition, renal D3 may play a role in the regulation of local T3 availability.

Effect of streptozotocin-induced diabetes mellitus on type 1 deiodinase (D1) in inherited D1-deficient mice.

We investigated the effects of streptozotocin (STZ)-induced diabetes on thyroid hormone levels, type 1 deiodinase (D1) activity and messenger RNA (mRNA) levels in inherited D1 deficient C3H mice in a comparative manner with control C57 mice. The apparent maximum velocity (Vmax) D1 values in C3H mice were 3% (liver) and 26% (kidney) of those in C57 mice. In C3H mice, similar serum T3, slightly higher T4, and 2.6-fold higher rT3 levels were observed compared with C57 mice. In STZ-induced diabetes, serum T4 level markedly decreased in both C3H and C57 mice. Serum T3 levels in STZ-C3H mice similarly decreased as in STZ-C57 mice. On the other hand, serum rT3 levels increased to 3.3-fold higher in STZ-C3H than in STZ-C57 mice. The Vmax values were decreased to 12% (STZ-C3H) and to 30% (STZ-C57) in liver, and decreased to 33% (both STZ-C3H and STZ-C57) in kidney. The changes in D1 mRNA levels in diabetes versus control were comparable to those of D1 activities in both strains. In summary, similar mechanism(s) to those which decrease the D1 expression and the serum T3 level in diabetes, function in D1 deficient C3H mice as in C57 mice. It appears that hepatic and renal D1 activity alone can not explain the similar reduction in T3 level in STZ-C3H mice and STZ-C57 mice.

Dopamine in the developing kidney.

The adult kidney has a high rate of dopamine (DA) production, metabolism, and signalling. The non-neuronal DA system in the adult kidney is of utmost importance for the regulation of salt metabolism. DA may also act as a transcription factor and may be of importance for tissue differentiation. In the central nervous system, D1 receptors require the dopamine- and cAMP-regulated phosphoprotein with a molecular weight of 32,000 Dalton (DARPP-32) to mediate their actions. The renal D1 mediates DARPP-32 activation via a cascade involving cAMP and PKA, and protein kinase C (PKC) activation via phospholipase C. Active DARPP-32 has a specific inhibitory effect on protein phosphatase 1 (PP1), leaving, e.g. Na+,K+-ATPase in a phosphorylated, inactive, state. Thus, dopamine acts as a natriuretic hormone in the mature kidney. Here, we discuss the age-dependent distribution and some functional aspects of several parts of the renal dopamine system (dopamine, AADC, COMT, D1 receptor, and DARPP-32) during renal morphogenesis.

Alteration of association of agonist-activated renal D1(A) dopamine receptors with G proteins in proximal tubules of the spontaneously hypertensive rat.

Defective D1A dopamine receptor-G protein coupling has been identified in renal proximal tubules of the spontaneously hypertensive rat (SHR). To determine whether association of D1A dopamine receptors with the alpha subunits of G proteins in kidney of SHR is normal. We analyzed the association of agonist-activated [1251]-labeled D1A dopamine receptors in kidneys of SHR and the normotensive Wistar-Kyoto (WKY) rat through immunoprecipitation, using highly specific antipeptide antibodies directed against alpha subunits of G proteins. We have shown for the first time that the D1A receptors of renal proximal tubules are associated with the adenylyl cyclase inhibitory G proteins G(i)alpha. The association of WKY rat proximal tubule D1A receptors with Gi1alpha and Gi2alpha in the presence of agonist is significantly (P<0.01) greater (2.4-fold and 3.1-fold greater, respectively) than it is without agonist D1A receptors of WKY rat also exhibit (twofold greater) association with G(s)alpha, consistently with the ability of these receptors to mediate stimulation of adenylyl cyclase. The WKY rat D1A receptors do not associate either with G(o)alpha or with G(q)alpha. The D1A receptors of SHR proximal tubule membranes appear to be resistant to activation by agonist and do not associate with G(s)alpha, G(o)alpha and any of the subunits of G(i)alpha. However, the SHR D1A sites exhibit a modestly (1.7-fold) greater association with G(q)alpha, which was not statistically significant. The differences among associations of the D1A receptors of WKY rat and SHR with these Galpha proteins may be important in understanding renal dopaminergic functions in normal and pathophysiologic states.