Endogenous digitalislike circulating substances in spontaneously hypertensive rats

Peritubular membrane potential in kidney proximal tubular cells of spontaneously hypertensive rats

Peritubular membrane potential in kidney proximal tubular cells of spontaneously hypertensive rats (SHR-Okamoto strain adult rats) was measured with conventional 3 mol KCl microelectrodes, in vivo. Peritubular cell membrane potential was not different in SHR (-66.5 ± 0.7 mV) as compared with normotensive control Wistar rats (-67.5 ± 1.2 mV). To test the effects of possible altered sodium membrane transport in SHR on proximal tubule peritubular membrane potential, we allowed SHR and control rats to drink 1% NaCl for two weeks. Again, proximal tubule peritubular membrane potential was not different in SHR on 1% NaCl (-67.0 ± 1.0 mV) as compared with control rats on 1% NaCl (-64.7 ± 1.3 mV). From these results we concluded that peritubular membrane potential in kidney proximal tubular cells of SHR was not different from normotensive Wistar control rats, and if some alteration of sodium transport in kidney proximal tubular cells of SHR could exist, that was not possible to evaluate from the measurements of peritubular membrane potential in kidney proximal tubular cells.

Purification and characterization of endogenous digoxin-like immunoreactive factors in chicken blood

Studies have been performed to determine whether an endogenous material capable of binding to digoxin antibodies is present in the chicken plasma. In the blood of 12 chickens without feed control, endogenous digoxin-like immunoreactive factors (DLIF) binding of digoxin antibodies in enzyme immunoassays amounted to 866 / 302 pg digoxin equivalents/mL of plasma (mean +/- SEM). Immunoreactivity of DLIF increased to 1848***331 pg/mL with a double value of control after boiling and acid pretreating the plasma. The major purification steps employed in this report were gel filtration column chromatography, high performance liquid chromatography (HPLC) and isoelectric focusing (IEF). Using HPLC for the separation, at least 10 chicken DLIFs with different molecular weight (MW) have been found. The MW of the smallest is 300 daltons (Da) while the largest is 100 kDa. The value of the isoelectric point of the most abundant type of DLIF from untreated chicken plasma is 6.3 as determined by IEF. The partially purified DLIF inhibits Na+, K(+)-ATPase from a porcine cerebral cortex as well as three human red blood cell membrane preparations in a dose-response fashion.

Effects of two endogenous Na+,K(+)-ATPase inhibitors, marinobufagenin and ouabain, on isolated rat aorta

Previously, we reported that the venom of Bufo marinus toad contains a Na+,K(+)-ATPase inhibitor with potent vasoconstrictor activity. In the present study, using thin-layer chromatography in Silicagel 60 F254 + 366, we separated a vasoactive substance from a mixture of steroids from Bufo marinus venom. Based on chromatographic mobility of this substance and typical color reaction after its vizualization with SbCl3, we identified it as a previously described steroid, marinobufagenin. Vasoconstrictor and Na+,K+ pump inhibitory properties of marinobufagenin were studied in isolated rat aortic rings and compared with those of ouabain. Ouabain (10-100 mumol.1-1) produced weak vasoconstriction, which was blocked by 2 mumol.1-1 phentolamine. 10 mumol.1-1 ouabain stimulated, and at higher concentrations inhibited, the Na+,K+ pump. 2 mumol.1-1 phentolamine abolished the activating effect of 10 mumol.1-1 ouabain on the Na+,K+ pump, but did not alter the inhibitory action of higher concentrations of ouabain. By contrast, marunibufagenin elicited rapid and strong vasoconstriction and inhibited ouabain-sensitive 86Rb uptake. Antidigoxin antibody antagonized the vasoconstrictor responses to marinobufagenin, but not to ouabain. 2 mumol.1-1 phentolamine did not alter the constrictor effect of marinobufagenin. In solid-phase digoxin immunoassay, marinobufagenin demonstrated higher digoxin-like immunoreactivity than ouabain.

Insulin resistance and endogenous digoxin-like factor in obese hypertensive patients with glucose intolerance

Hypertensive obese subjects with glucose intolerance have hyperinsulinaemia, insulin resistance and intracellular cation imbalance resulting in increased sodium content. The aim of our study was to assess in these patients plasma levels of endogenous digoxin-like factor (EDLF), an inhibitor of the sodium-pump mechanism. We studied 14 hypertensive and 12 normotensive subjects with obesity and glucose intolerance for fasting blood glucose, and plasma insulin, C-peptide and EDLF levels: the two groups were matched for age and BMI and were studied after a 2-week wash-out period from hypotensive drugs. Compared with normotensives, hypertensive subjects had higher plasma insulin levels, a greater immunoreactive insulin/C-peptide ratio, a lower glucose/insulin ratio and higher plasma EDLF levels. Our results confirm that among obese people with glucose intolerance, hypertensives are more hyperinsulinaemic and insulin-resistant than normotensives and indicate that the intracellular cation imbalance in these patients may be attributable, at least in part, to EDLF.

Pathogenesis of hypertension in the sinoaortic-denervated spontaneously hypertensive rat

The present study was performed to examine the relation between the gain of the baroreceptor reflex and the pathogenesis of hypertension in the spontaneously hypertensive rat. Spontaneously hypertensive or Wistar-Kyoto rats underwent either sinoaortic baroreceptor denervation or sham denervation at 28-35 days of age. Four months later these rats were chronically instrumented for measurements of arterial pressure and heart rate. Sixty-minute computerized measurements of arterial pressure showed no difference between spontaneously hypertensive sham (163 +/- 5 mm Hg) and spontaneously hypertensive baroreceptor-denervated (166 +/- 5 mm Hg) rats, or Wistar-Kyoto sham (114 +/- 3 mm Hg) and Wistar-Kyoto baroreceptor-denervated (121 +/- 4 mm Hg) rats. The gain of baroreceptor reflex control of heart rate was assessed by measuring maximal heart rate responses to changes in arterial pressure elicited by bolus injection of phenylephrine and nitroprusside (gain = slope of linear regression equation of change in heart rate versus change in arterial pressure). Baroreceptor reflex gain was significantly higher in Wistar-Kyoto sham rats (-2.10 beats/min/mm Hg) than spontaneously hypertensive sham rats (-0.94 beats/min/mm Hg). Baroreceptor denervation significantly decreased baroreceptor reflex gain in both Wistar-Kyoto (-0.26 mm Hg) and spontaneously hypertensive (-0.22 beats/min/mm Hg) groups. Since baroreceptor denervation did not exacerbate the development of hypertension in adult spontaneously hypertensive rats or lead to hypertension in Wistar-Kyoto rats, we conclude that a primary dysfunction in the baroreceptor reflex alone is not responsible for the development of hypertension in this model.

Apparent upregulation of Na+,K+ pump sites in SHR skeletal muscle with reduced transport capacity

Slow-twitch, oxidative skeletal muscles in SHR exhibit several physiological defects, including a reduced ability to maintain force during high frequency repetitive stimulation (1). Muscle fatigue may be produced by one of a variety of factors acting at different levels of the neuromuscular system. Several lines of evidence, however, suggest that SHR soleus fatigues more rapidly than WKY soleus because SHR muscles allow more K+ to accumulate in the extracellular space during repetitive muscle activity. An increase in extracellular K+ can lead to a failure in the generation or conduction of muscle action potentials. Comparison of the compound action potentials recorded from SHR and WKY muscles during repetitive stimulation provided evidence for a decrease in excitability of SHR soleus. Since the K+ released from muscle fibers during exercise is returned to the fiber principally via the activity of the Na+, K+ pump, the increase in extracellular K+ in SHR muscle may reflect a decrease in pump capacity. Measurements including intracellular K+ and Na+ content at rest, the level of hyperpolarization produced by the addition of epinephrine and insulin to SHR soleus and the post-exercise recovery of resting membrane potentials all appear to indicate that Na+, K+ pump capacity is reduced in SHR soleus muscles. Nonetheless, ouabain binding studies show a significantly greater number of pump sites in SHR muscles. The data suggest that Na+ pump activity is decreased in SHR soleus muscles without an apparent reduction in either the number of pump sites or in pump binding affinity.

Typology of Na+ transport abnormalities in erythrocytes from essential hypertensive patients. A first step towards the diagnosis and specific treatment of different forms of primary hypertension

Over the last 5 years, several authors have measured apparent affinities and maximal translocation rates of the different erythrocyte Na+ transport systems in essential hypertensive patients. These kinetic studies have clearly shown that no unique red cell Na+ transport defect characterizes the whole population of essential hypertensive patients. Conversely, several complex patterns of erythrocyte Na+ transport abnormalities may be present in different subsets of essential hypertensive patients. These kinetic studies are now providing a more profound biochemical insight into the molecular heterogeneity of primary hypertension. In particular, they may permit the diagnosis and specific treatment of different forms of primary hypertension in the next decade.

Modulatory effect of some steroid hormones, their glucuronides and ouabain-like compounds on Cavia cobaya kidney Na+,K(+)-ATPase activity

1. Ouabain-like compounds (approx. mol. wt 700, 2,000 and 4,000 Da) were purified from plasma of essential hypertensive patients. 2. Dose-response experiments performed with (a) steroid hormones, (b) their glucuronides and (c) ouabain-like compounds, emphasize a modulatory effect [activation of the Na,K-ATPase at very low concentrations of ligand, inhibition at higher levels; apparent Ki: (a) between 1 and 0.5 mM; (b) between 1 and 0.5 microM; and (c) between 10 and 1 nM; maximum enhancement of the enzymatic activity: (a) +20%; (b) +45%; and (c) +100%]. 3. Displacement experiments of [3H]ouabain evidence a high competition of the ligands towards the cardioglycoside. The relative I50s are: (a) between 1 and 0.5 mM; (b) between 10 and 1 microM; and (c) between 10 and 0.01 nM.

Body fluid variations and endogenous digitalis-like compounds during chronic NaCl loading in Wistar rats

Circulating digitalis-like compounds have been proposed to be raised in volume expanded hypertension and to participate in Na+ homeostasis. We have investigated the temporal relationships between the activity of these circulating digitalis-like compounds, blood pressure and body fluid volume variations during a chronic NaCl load in the Wistar rat. Characteristics of salt-loaded rats were compared to those of weight-matched controls. At one week, when extracellular fluid volume (ECFV) was elevated, the capacity of plasma extracts to inhibit the Na+K+ATPase activity begun to rise. At two weeks, ECFV remained elevated, and plasma volume, blood pressure and the activity of plasma digitalis-like compounds increased. After 13 weeks, the continuous rise in plasma digitalis-like activity and in blood pressure was accompanied by the return of body fluid volumes towards control values. These changes in plasma digitalis-like activity, body fluid volumes, and systolic blood pressure during a high NaCl diet are compatible with the proposed role of circulating digitalis-like compounds as natriuretic and hypertensive factors.