Endogenous bufadienolide mediates pressor response to ethanol withdrawal in rats.

An endogenous natriuretic and vasoconstrictor Na/K-ATPase inhibitor, marinobufagenin (MBG), is implicated in NaCl-induced hypertension and in ethanol addiction. In rats, MBG suppresses voluntary alcohol intake, while immunization against MBG induces alcohol-seeking behavior. Since alcohol withdrawal is associated with elevation of blood pressure (BP) and renal sodium retention, we hypothesized that MBG mediates pressor response to ethanol withdrawal. In male Sprague-Dawley rats, forced ethanol intake (20% v/v, 2.8+/-0.2 g/day for 7 days) did not affect BP and MBG excretion. Ethanol withdrawal was associated with a 21 mm Hg increase in BP, a 10% decrease in hematocrit, and a three-fold increase in renal MBG excretion. In vivo administration of anti-MBG antibody to rats prevented withdrawal-induced BP elevation. Therefore, MBG mediates pressor response to ethanol withdrawal, and may link mechanisms of ethanol dependence and hypertension.

Endogenous sodium pump inhibitors, diabetes mellitus and preeclampsia Preliminary observations and a hypothesis.

Endogenous inhibitors of the Na/K-ATPase (NKA) and diabetes mellitus (DM) are both risk factors for preeclampsia and NaCl sensitive hypertension. Our goal was to test the hypothesis that NaCl supplementation, induces preeclampsia-like symptoms in pregnant rats with DM via stimulation of marinobufagenin (MBG), a natriuretic and vasoconstrictor inhibitor of the NKA. Type 2 DM in female Sprague-Dawley rats was induced by administration of 65mg/kg streptozotocin at day 4 post-partum. In intact rats, pregnancy was associated with a twofold increase in MBG levels and a mild impairment in glucose tolerance. Pregnant rats with DM exhibited fetal macrosomia, greater impairment of glucose tolerance, and higher levels of MBG as compared to that in normal pregnant rats. As compared to intact pregnant rats, NaCl supplementation of diabetic pregnant rats (drinking 1.8% NaCl during days 12-19 of pregnancy) was associated with an increase in systolic blood pressure, decreased fetal and placental weight, fivefold elevation of MBG excretion, and 42% inhibition of NKA in erythrocytes. In nonpregnant rats, in vivo pretreatment with anti-MBG antibody produced an exaggerated response of plasma levels of glucose and insulin in oral glucose tolerance test. These results suggest that MBG is a common factor in the pathogenesis of DM and preeclampsia, and that regulation of glucose tolerance may be one of the physiological functions of endogenous cardiotonic steroids.

Endogenous digitalis-like ligands and Na/K-ATPase inhibition in experimental diabetes mellitus.

Dysregulation of the Na/K-ATPase (NKA) in the kidney, cardiovascular system, and peripheral nervous system is believed to contribute to pathogenesis of diabetes mellitus (DM) and its complications. Recently we demonstrated that, in addition to endogenous ouabain (EO), mammalian tissues contain another NKA inhibitor, a bufadienolide marinobufagenin (MBG). In vitro MBG, a natriuretic and a vasoconstrictor, acts as a selective inhibitor of alpha-1 NKA, the main isoform of the sodium pump in renal tubules and vascular smooth muscle. To determine whether digitalis-like NKA inhibitors are linked to NKA dysregulation in DM, we studied changes in renal excretion and plasma levels of MBG and EO and the activity of erythrocyte NKA in male Wistar rats with type 1 DM and type 2 DM. Rats with type 1 DM were studied four weeks following a single intraperitoneal injection of 65 mg/kg streptozotocin (STZ)(n = 12), and rats with type 2 DM were studied 10 weeks after intraperitoneal injection of STZ during the neonatal period (n = 12). Renal excretion and plasma levels of EO did not change in rats with both types of DM as compared to that in the control groups. Renal excretion (57.5 +/- 9.4 pmol/kg/ 3 hours vs. 12.6 +/- 2.1 pmol/kg/ 3 hours; P less than 0.01) and plasma levels (2.23 +/- 0.82 nmol/L vs. 0.29 +/- 0.07 nmol/L; P less than 0.01) of MBG increased, and NKA activity in erythrocytes was inhibited by 50% in rats with type 1 DM as compared to controls. In rats with type 2 DM, plasma levels (1.48 +/- 0.09 nmol/L vs. 0.46 +/- 0.02 nmol/L; P less than 0.01) and renal excretion (21.3 +/- 3.2 pmol/kg/ 3 hours vs. 13.1 +/- 2.1 pmol/kg/ 3 hours) of MBG also became elevated, but less than in the animals with type 1 DM. Accordingly, activity of NKA in erythrocytes from rats with type 2 DM was inhibited by 35%. In vitro treatment of erythrocytes from rats with type 1 and type 2 DM with anti-MBG antibody reversed the DM induced inhibition of the NKA. These results suggest that digitalis-like factors are involved in the pathogenesis of DM and that MBG, rather than EO, is responsible for DM-induced NKA inhibition.

Marinobufagenin (MBG) suppression of ethanol-seeking behavior is associated with inhibition of brain cortex Na/K-ATPase in mice.

In the present study the hypothesis was tested that sodium pump ligands (SPL) can modulate alcohol-seeking behavior and that this effect is related to changes in Na/K-ATPase activity in the central nervous system. Mice were tested for initiation of ethanol intravenous self-administration (IVSA) following i.p. pretreatment with vehicle or the endogenous SPL, marinobufagenin (MBG). Drug- and experimentally-naive mice acquired IVSA of 2% ethanol during a single 30-min session. MBG was found to dose-dependently attenuate (1.25-2.5 microg/kg) initiation of ethanol IVSA producing a decrease in the ratio and in the difference between operant responses of response-dependent and yoked animals as well as a decrease in percentage of mice demonstrating ethanol-seeking behavior. Attenuation of the reinforcing effect of ethanol resulting from MBG was associated with brain levels of this steroid capable of concurrently inhibiting Na/K-ATPase in the brain cortex. We hypothesize that endogenous digitalis-like factors could modulate the reinforcing effect of ethanol.

Endogenous digitalis-like ligands of the sodium pump: possible involvement in mood control and ethanol addiction.

This review addresses possible involvement of endogenous digitalis-like sodium pump ligands (SPL) in the mood control and ethanol addiction. Endogenous SPL include cardenolide and bufadienolide classes. Multiple SPL and multiple isoforms of the Na/K-ATPase, one of the key membrane enzymes, comprise a complex regulatory system. In the nervous system, pattern of expression of Na/K-ATPase is based on multiple alpha/beta isoform combinations. Clinical studies demonstrate changes in the activity of Na/K-ATPase in patients with bipolar and unipolar mood disorders. The effects of ethanol on the Na/K-ATPase are concentration-dependent and are associated with both inhibition and activation of enzyme activity. Reinforcing effect of ethanol as well as its voluntary consumption may be affected by digitalis glycosides and endogenous SPL.