Endogenous cardiotonic steroids and salt-sensitive hypertension

Reactive Oxygen Species Modulation of Na/K-ATPase Regulates Fibrosis and Renal Proximal Tubular Sodium Handling

The Na/K-ATPase is the primary force regulating renal sodium handling and plays a key role in both ion homeostasis and blood pressure regulation. Recently, cardiotonic steroids (CTS)-mediated Na/K-ATPase signaling has been shown to regulate fibrosis, renal proximal tubule (RPT) sodium reabsorption, and experimental Dahl salt-sensitive hypertension in response to a high-salt diet. Reactive oxygen species (ROS) are an important modulator of nephron ion transport. As there is limited knowledge regarding the role of ROS-mediated fibrosis and RPT sodium reabsorption through the Na/K-ATPase, the focus of this review is to examine the possible role of ROS in the regulation of Na/K-ATPase activity, its signaling, fibrosis, and RPT sodium reabsorption.

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

Erythrocyte Na+-Li+ counter-transport activity and digoxin-like substances in insulin dependent diabetic women with preexisting preeclampsia

AIM OF THE STUDY

To determine whether there is pathogenetic link between red cells sodium-lithium counter-transport activity and digoxin-like immunoreactive substances (DLIS) in plasma of insulin-dependent diabetic (IDDM) and non-diabetic women with preexisting preeclampsia (PE).

SUBJECTS AND METHODS

We studied Na(+)/Li(+) CT activity in red cells and plasma levels of DLIS in 11 IDDM women with preexisting PE (Group 1), 13 IDDM without preexisting PE (Group 2) 23 non-diabetic women with preexisting PE (Group 3) and 12 non-diabetic women with normal pregnancy (Group 4) at least 4 months after delivery.

RESULTS

Na(+)/Li(+) CT activity was higher in Group 1 compared to Group 2 (mean ± SEM 0.316 ± 0.05 vs 0.190 ± 0.02 mmol/LRBC/hr p < 0.05) and in Group 3 compared to Group 4 (0.365 ± 0.004 vs 0.168 ± 0.01 mmol/LRBC/hr, p < 0.01). Plasma levels of DLIS were higher in Group 3 compared to Group 4 (0.727 ± 0.189 vs 0.295 ± 0.066 ng/ml; p<0.05); there was no statistically significant difference between the two diabetic groups. In Groups 1 and 3, Na(+)/Li(+) CT activity was correlated to the plasma levels of DLIS (r = 0.927; p < 0.001 and r = 0.485; p<0.05 respectively).

CONCLUSION

Increased Na(+)/Li(+) CT activity and increased plasma levels of DLIS may contribute to PE in IDDM and non-diabetic women.

Isolation of marinobufotoxin from the supernatant of cultured PC12 cells

1. Digitalis-like factors (DLFs) are believed to be involved in sodium metabolism via inhibition of Na(+) /K(+) -ATPase and may cause hypertension. Yet, the source and regulation of secretion of DLFs remain unknown. Recently, marinobufagenin (MBG) was isolated in mammals and implicated in renal sodium and water metabolism. More recently, we isolated marinobufotoxin (MBT), a suberoyl arginine ester of MBG, in Y-1 cells. We have developed an ELISA to measure MBG-like immunoreactivity (MBG-IR) and have characterized MBG-IR using chromatography. We have also identified a ouabain-like factor in cultured PC12 cells from a phaeochromocytoma cell line. In the present study, we examined whether MBT was produced in the adrenal medulla. 2. PC12 cells were cultured in serum-free medium and culture supernatants were collected over a period of 24 h. The supernatants were analysed by ELISA and HPLC to determine MBG-IR content. The HPLC fraction containing the main peak of MBG-IR was characterized by LC/MS. 3. Compared with samples collected at 0.5 h, the concentration of MBG-IR in culture supernatants increased significantly after 2 h and continued to increase until 24 h. The fraction with the highest ELISA peak for MBG-IR had the same HPLC elution time as authentic MBT. Furthermore, tandem mass spectrometry indicated that each fraction of MBT and MBG had the correct specific daughter ions. 4. The results indicate that MBT and MBG are produced and/or secreted by adrenomedullary cells.

Ouabain stimulates a Na+/K+-ATPase-mediated SFK-activated signalling pathway that regulates tight junction function in the mouse blastocyst

The Na(+)/K(+)-ATPase plays a pivotal role during preimplantation development; it establishes a trans-epithelial ionic gradient that facilitates the formation of the fluid-filled blastocyst cavity, crucial for implantation and successful pregnancy. The Na(+)/K(+)-ATPase is also implicated in regulating tight junctions and cardiotonic steroid (CTS)-induced signal transduction via SRC. We investigated the expression of SRC family kinase (SFK) members, Src and Yes, during preimplantation development and determined whether SFK activity is required for blastocyst formation. Embryos were collected following super-ovulation of CD1 or MF1 female mice. RT-PCR was used to detect SFK mRNAs encoding Src and Yes throughout preimplantation development. SRC and YES protein were localized throughout preimplantation development. Treatment of mouse morulae with the SFK inhibitors PP2 and SU6656 for 18 hours resulted in a reversible blockade of progression to the blastocyst stage. Blastocysts treated with 10(-3) M ouabain for 2 or 10 minutes and immediately immunostained for phosphorylation at SRC tyr418 displayed reduced phosphorylation while in contrast blastocysts treated with 10(-4) M displayed increased tyr418 fluorescence. SFK inhibition increased and SFK activation reduced trophectoderm tight junction permeability in blastocysts. The results demonstrate that SFKs are expressed during preimplantation development and that SFK activity is required for blastocyst formation and is an important mediator of trophectoderm tight junction permeability.

Impairment of Na/K-ATPase signaling in renal proximal tubule contributes to Dahl salt-sensitive hypertension

We have observed that, in renal proximal tubular cells, cardiotonic steroids such as ouabain in vitro signal through Na/K-ATPase, which results in inhibition of transepithelial (22)Na(+) transport by redistributing Na/K-ATPase and NHE3. In the present study, we investigate the role of Na/K-ATPase signaling in renal sodium excretion and blood pressure regulation in vivo. In Sprague-Dawley rats, high salt diet activated c-Src and induced redistribution of Na/K-ATPase and NHE3 in renal proximal tubules. In Dahl salt sensitive (S) and resistant (R) rats given high dietary salt, we found different effects on blood pressure but, more interestingly, different effects on renal salt handling. These differences could be explained by different signaling through the proximal tubular Na/K-ATPase. Specifically, in Dahl R rats, high salt diet significantly stimulated phosphorylation of c-Src and ERK1/2, reduced Na/K-ATPase activity and NHE3 activity, and caused redistribution of Na/K-ATPase and NHE3. In contrast, these adaptations were either much less effective or not seen in the Dahl S rats. We also studied the primary culture of renal proximal tubule isolated from Dahl S and R rats fed a low salt diet. In this system, ouabain induced Na/K-ATPase/c-Src signaling and redistribution of Na/K-ATPase and NHE3 in the Dahl R rats, but not in the Dahl S rats. Our data suggested that impairment of Na/K-ATPase signaling and consequent regulation of Na/K-ATPase and NHE3 in renal proximal tubule may contribute to salt-induced hypertension in the Dahl S rat.