Pathophysiological role of cation transport and natriuretic factors in hypertension

Increased sodium influx and calcium uptake in erythrocytes in hyperthyroidism: role of abnormal membrane lipid levels

The study was designed to examine the effects of thyroid hormones on red blood cell (RBC) membrane phospholipids and ion transport. We demonstrated that in untreated Graves' disease, an alteration in the phospholipid pattern is present at cellular levels, with a concomitant derangement in membrane permeability defined as (22)Na influx and (45)Ca uptake. Thionamide therapy replaced the normal membrane permeability, presumably as a consequence of restoring the normal phospholipid membrane composition. We conclude that thyroid hormones are able to induce a quick breakdown of a large number of membrane components such as membrane phospholipids.

Role of dietary salt in hypertension

Hypertension is the most common chronic disease in the United States and, untreated, results in disability or death due to stroke, heart failure or kidney failure. Fortunately the results of hypertension can be avoided to a large extent by proper treatment. One treatment which is effective in some cases is the restriction of dietary NaCl intake. This review considers the role of dietary NaCl in the genesis, therapy and prevention of hypertension. Most people can eat as much NaCl as they like; they have good kidneys which, within about 24 hours, excrete the NaCl as fast as it is taken in and nothing happens to blood pressure. A few, especially those with kidney disease, do not excrete it as fast as it is taken in and blood pressure rises. They are "salt sensitive". Once hypertension is established, the proportion who are "NaCl sensitive" is much higher. About 60% of people with hypertension respond to a high NaCl intake with a rise in pressure and to NaCl restriction with a fall in pressure and reduction in the need for antihypertensive medication. These are the same people that respond to diuretics with a fall in blood pressure. Many are black and elderly and have low plasma renin activity (low-renin hypertension) but some have normal or high plasma renin activity (normal or high-renin hypertension). Evidence suggests that very early they have a subtle kidney defect which causes them to excrete NaCl and water more slowly, e.g., even before they become hypertensive, black and elderly subjects excrete intravenously administered NaCl more slowly than white and young subjects. How does NaCl retention raise blood pressure? One possibility is that the NaCl retention causes water retention which releases a digitalis-like substance that increases the contractile activity of heart and blood vessels. Another is that the sodium itself penetrates the vascular smooth muscle cell, causing it to contract. "Salt sensitive" hypertension also responds to increased potassium and calcium intakes, perhaps in part because they increase NaCl urinary excretion.

Effects of bufalin on norepinephrine turnover in canine saphenous vein

Abundant experimental data suggest that an endogenous digitalislike factor is responsible for some essential hypertension. Some forms of hypertension have also been associated with increased levels of catecholamines. We therefore designed experiments to investigate the role of digitalislike factors in the regulation of norepinephrine turnover in the neurovascular junction. We chose bufalin, an amphibian-derived compound that shares many of the physiological properties postulated as characteristic of digitalislike compounds, as a model of the mammalian compound. In vitro experiments in canine saphenous veins showed that, in addition to inhibiting norepinephrine uptake, bufalin increased norepinephrine overflow by an amount larger than could be explained solely by uptake inhibition. The effect of bufalin on norepinephrine overflow is inhibited by tetrodotoxin, which suggests a dependence of this response on Na+ influx through the neuronal membranes. We propose that Na+,K(+)-ATPase inhibition resulting in neuronal depolarization is responsible for the augmented norepinephrine turnover caused by bufalin and that these indirect effects of norepinephrine on the cardiovascular system may play a role in the etiology of hypertension.

Endogenous digitalis-like factor as a stimulator of endothelin secretion from endothelial cells

Effects of human urine-derived endogenous digitalis-like factor (EDLF) and ouabain on endothelin (ET) secretion were examined in cultured endothelial cells. ET was secreted in a linear fashion over 5 hours from bovine pulmonary artery endothelium into serum-free medium. EDLF stimulated ET secretion in a dose-dependent manner. In contrast, ouabain did not affect ET secretion at the concentration of 10(-9)-10(-5) M. These results indicate that human urine-derived EDLF is distinct from plant-derived ouabain and act as a stimulator of ET secretion by endothelial cells.

Digitalislike circulating factor in hypertension: potential messenger between salt balance and intracellular sodium

Sodium chloride has no clearly established local direct action on blood vessels to produce constriction; on the contrary, it has an immediate local indirect action via osmolality, which produces vasodilation. Thus in order to explain salt-induced hypertension, a delayed remote indirect vasoconstrictor action must be postulated. This indirect vasoconstrictor action is apparently the result of volume expansion. Acute volume expansion imparts three physiologic properties to the plasma; these are the ability to inhibit Na,K-ATPase and the Na-K pump, to cause natriuresis, and to sensitize blood vessels to vasoconstrictor agents. Similarly, low-renin, volume-expanded hypertension endows the plasma with the capacity to inhibit the Na,K-ATPase pump, to sensitize blood vessels to vasoconstrictor agents, and to raise blood pressure. These properties apparently result from a circulating digitalislike substance(s), perhaps derived from the hypothalamus and/or adrenals. We here review the considerable effort expended in identifying the agent or agents, and conclude that both steroidal and peptidic structure must be considered. Regardless of its structure, we hypothesize that when sodium excretion does not keep pace with sodium intake, its release leads to increased contractile activity of cardiac and vascular smooth muscle and hence hypertension. Inhibition of the Na-K pump increases the intracellular sodium concentration, particularly when superimposed on genetic- or aldosterone-induced increased sodium permeability, resulting in depolarization and increased calcium influx (vascular smooth muscle) or altered Na(+)-Ca2+ exchange and decreased calcium efflux (heart muscle). The increased intracellular calcium concentration then accounts for the increased contractile activity. Depolarization may also increase the sensitivity of vascular smooth muscle to vasoconstrictor agents such as norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced intracellular pH in lymphocytes from the spontaneously hypertensive rat

This study was designed to determine the cytoplasmic pH (pHi) profile of lymphocytes from a rat model of genetic hypertension that is well suited for study before and after the development of spontaneous hypertension. For this purpose, pHi was measured in thymic lymphocytes obtained from spontaneously hypertensive rats (SHR) and from age-matched Wistar-Kyoto (WKY) control rats using 2',7'-bis carboxyethyl-5,6-carboxyfluorescein (BCECF), a pH-sensitive fluorescence probe. At the age of 16-20 weeks, pHi of lymphocytes suspended in a HCO3-free HEPES-buffered solution, was markedly lower in the SHR than in the WKY rats (7.07 +/- 0.02, n = 16 and 7.22 +/- 0.01, n = 15, respectively, p less than 0.001), whereas systolic blood pressure was higher in SHR than in WKY rats (175 +/- 5.0 and 105 +/- 3.0 mm Hg, respectively, p less than 0.001). In rats less than 5 weeks of age, pHi was also lower in SHR than in WKY rat lymphocytes (7.12 +/- 0.04, n = 11 and 7.23 +/- 0.04, n = 11, respectively, p less than 0.05), although at this age systolic blood pressure was not different between the two groups (87 +/- 4.0 and 85 +/- 3.0 mm Hg, respectively). In lymphocytes suspended in a more physiological HCO3/CO2-buffered solution, pHi was again lower in the adult SHR than in the WKY rat (7.18 +/- 0.02, n = 16 and 7.31 +/- 0.02, n = 16, respectively, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary sodium pump inhibitor raises cytosolic free calcium concentration in rat aorta

We were able to purify two distinct sodium pump inhibitors to homogeneity from human urine based on [3H]ouabain-displacing activity from intact human erythrocytes. The polar and less polar compounds were eluted off the C18 reverse-phase column with 18% and 31% acetonitrile, respectively. The polar compound cross-reacted very weakly with specific antidigoxin antibody and lacked a characteristic ultraviolet absorption peak between 190 and 300 nm. The less polar compound showed a prominent digoxinlike immunoreactivity and had an ultraviolet spectrum similar to that of digoxin. We examined the effects of these compounds on cytosolic free calcium concentration in cultured rat vascular smooth muscle cells (A10 cells) using the fluorescent calcium chelator fura-2. Only the polar ouabain-displacing compound caused a significant increase, from 108 +/- 7 to 162 +/- 8 nM (n = 6, p less than 0.01), in cytosolic free calcium concentration in A10 cells. The rise in cytosolic free calcium concentration induced by the polar ouabain-displacing compound tended to be slower in onset and more sustained than that induced by arginine vasopressin. In contrast, ouabain and bufalin had no appreciable effects on cytosolic free calcium concentration in A10 cells. These results suggest that the polar ouabain-displacing compound we isolated from human urine may possess a vasoactive property and may play an important role in the modulation of vascular tone.

Free cytosolic calcium in renal proximal tubules from the spontaneously hypertensive rat

Free intracellular calcium was measured in renal proximal tubules obtained from spontaneously hypertensive rats (SHR) and from age-matched Wistar-Kyoto rats (WKY) ingesting a normal diet. Experiments were performed on renal proximal tubule suspensions using fura-2 to monitor cytosolic calcium. In 4-week-old rats, when systolic blood pressure was not significantly different between the two groups, renal proximal tubule cytosolic calcium was similar (143 +/- 28 and 144 +/- 15 nM, respectively). By the age of 5 weeks, cytosolic calcium increased significantly in both SHR and WKY (214 +/- 24 and 262 +/- 34 nM, respectively, p less than 0.05). Calcium, however, was not significantly different between the two groups, even though at this age blood pressure was higher in SHR than in WKY. As compared with values in 4-week-old rats, cytosolic calcium was also found increased in tubules from both SHR and WKY aged 10 to 12 weeks (261 +/- 42 and 279 +/- 30 nM, respectively) and 20 to 24 weeks (263 +/- 42 and 308 +/- 28 nM, respectively). However, no significant differences in cytosolic calcium were found between SHR and WKY even though at these ages systolic blood pressure increased markedly in the SHR. Moreover, regression analysis failed to reveal a correlation between cytosolic calcium and blood pressure when data from either group of rats of all ages studied were pooled. Exposure to ouabain (10(-3) M) to inhibit Na+,K+-adenosine triphosphatase and increase intracellular sodium had no significant effect on cytosolic calcium in tubules from either SHR or WKY (260 +/- 69 and 250 +/- 45 nM, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)