Calcium antagonist nifedipine normalizes high blood pressure and prevents mortality in salt-loaded ds substrain of Dahl rats

Impaired renal vasodilation and urinary cGMP excretion in Dahl salt-sensitive rats

We previously have shown that Dahl salt-sensitive rats increase renal vascular resistance in response to excessive salt feeding before total peripheral resistance increases and hypertension occurs. Failure of renal vasculature to dilate, as normally occurs in Dahl salt-resistant rats fed a high salt diet, may play a role in the development of hypertension in Dahl salt-sensitive rats. We also showed that renal vasculature in salt-sensitive rats is hyperreactive to vasoconstrictors and hyporeactive to vasodilators. Atrial natriuretic peptide, by stimulating cell-bound receptors, and nitroprusside, by generating nitric oxide, cause renal vasodilation by generating cGMP. Studies were undertaken to determine whether defective renal vasodilation in Dahl salt-sensitive rats is due to impaired production of cGMP. We examined the influence of nitroprusside infusion and salt intake on renal vascular resistance and cGMP excretion in salt-sensitive rats. Results demonstrate that salt feeding and nitroprusside infusion increase cGMP excretion and decrease renal vascular resistance in salt-resistant rats (P < .01), and, although this relationship was less clear in salt-sensitive rats, there was a reciprocal relationship between renal vascular resistance and cGMP excretion in all animals studied. Salt feeding and nitroprusside infusion caused less of an increase in cGMP excretion in salt-sensitive than in salt-resistant rats (P < .01). In conclusion, these studies support the concept that impairment in cGMP generation may play a primary role in the inability of the kidneys of Dahl salt-sensitive rats to vasodilate in response to increased salt intake. Such an impairment could contribute to salt retention and the development of hypertension.

Role of calcium antagonists in progression of arteriosclerosis. Evidence from animal experiments and clinical experience. Part I. Preventive effects of calcium antagonists in animal experiments

The quantitative predominance of free and total cholesterol over the amount of mural calcium is a most significant criterion of healthy human coronary arteries during the whole life span (0-90 years). However, this normal ratio increasingly changes as soon as arteriosclerotic alterations of the coronary walls set in. Accordingly, the mural calcium content steadily rises from fatty streaks over severe arteriosclerosis and, lastly, seems to reach a climax in plaques which caused lethal coronary infarction. Furthermore, the severe arteriosclerosis of human art. dorsalis pedis with gangrene (and amputation) is characterized by a tremendous calcium incorporation and absence of any mural cholesterol changes. Only in rare cases of human basilary plaques was a dangerous cholesterol incorporation in brain arterial wall found without significant elevation of serum cholesterol levels. The presented data indicate the existence of two different types of arteriosclerosis in one and the same patient and two basically different types of experimental coronary plaques according to their chemical composition, microscopic aspect and responsiveness to calcium antagonists: 1) the calcium type, developing in vitamin-D3-treated rats, and 2) the cholesterol type, represented by fatty coronary atheromata of cholesterol-fed rabbits. Coronary atheromata of cholesterol-fed New Zealand rabbits may be suitable models for coronary heart disease in rare cases of human familiar hypercholesterolemia. The formation of conventional human coronary artery plaques, however, essentially requires a progressive uptake of calcium, thereby representing a calcium dominated type of arteriosclerosis. Calcium antagonists specifically inhibit progredient mural calcium uptake in all experimental models of arteriosclerosis tested so far. However, neither in atheromatous arteries nor in afflicted organs (myocardium, liver, kidneys) of cholesterol-fed rabbits were we able to find any significant prevention of cholesterol accumulation by calcium antagonist.

Deuterium oxide prevents hypertension and elevated cytosolic calcium in hypertensive rats

Increased calcium uptake in vascular tissue, leading to elevated cytosolic free calcium, has been implicated in the pathophysiology of hypertension. This study examined the dose-dependent effect of deuterium oxide (5%, 10%, or 20% in drinking water) on systolic blood pressure, aortic calcium uptake, and platelet cytosolic free calcium in spontaneously hypertensive rats. Starting at age 8 weeks, spontaneously hypertensive rats were divided into four groups of six animals each. The drinking water of groups 1, 2, 3, and 4 was replaced by 100% water and 5%, 10%, and 20% deuterium oxide in water, respectively, for another 7 weeks. Ten Wistar-Kyoto rats, age 8 weeks, were given 100% water for the next 7 weeks. The usual increase in systolic blood pressure and the associated increase in aortic calcium uptake and platelet cytosolic free calcium in spontaneously hypertensive rats at age 15 weeks was lowered in a dose-dependent manner by deuterium oxide. Deuterium oxide also prevented renal vascular changes in spontaneously hypertensive rats. A minimum dose of 10% deuterium oxide was needed to completely prevent the development of hypertension, elevated aortic calcium uptake, platelet cytosolic free calcium, and renal vascular changes in spontaneously hypertensive rats.

Comparison of converting enzyme inhibitor and calcium channel blocker in hypertensive glomerular injury

The protective effect of converting enzyme inhibitors in experimental hypertensive glomerular injury is associated with decreased systemic arterial and glomerular capillary pressure. Although calcium channel blockers effectively lower systemic blood pressure, their effect on glomerular capillary pressure and on hypertensive glomerular injury is uncertain. We compared equihypotensive treatment with the calcium antagonist TA 3090 or the converting enzyme inhibitor captopril in post-salt hypertensive Dahl salt-sensitive (DS) rats for up to 5 weeks after five sixths nephrectomy. Before the nephrectomy, all rats demonstrated hypertension (mean 177 mm Hg), proteinuria (mean 175 mg/day), and mild glomerulosclerosis (mean injury score 35). Rats treated with captopril or TA 3090 demonstrated a significant and equivalent decrease in systolic blood pressure compared with untreated rats at 2, 3, and 5 weeks after five sixths nephrectomy; however, only captopril reduced proteinuria. Final proteinuria was actually increased in rats treated with TA 3090 compared with untreated rats. Glomerular injury score was significantly decreased in captopril-treated compared with untreated rats at 2 weeks (33 +/- 9 versus 117 +/- 10, p less than 0.05) and 5 weeks (46 +/- 9 versus 94 +/- 24, p less than 0.05), whereas treatment with TA 3090 delayed but did not prevent progressive glomerular injury (2-week score 35 +/- 7, p less than 0.05 versus untreated; 5-week score 109 +/- 19, p = NS versus untreated). Thus, in hypertensive DS rats after subtotal nephrectomy, treatment with a converting enzyme inhibitor reduced systemic blood pressure, proteinuria, and glomerulosclerosis. However, equihypotensive treatment with a calcium channel blocker did not reduce proteinuria and delayed but did not prevent glomerulosclerosis. Thus, in the rat similar reductions in systemic blood pressure with these two classes of agents have disparate effects on the progression of chronic renal failure.

Deuterium oxide normalizes blood pressure and vascular calcium uptake in Dahl salt-sensitive hypertensive rats

This study examined the effect of 25% deuterium oxide in drinking water on systolic blood pressure, uptakes of calcium, and rubidium 86 by aortas of Dahl salt-sensitive rats on 0.4% (low) and 8% (high) sodium chloride (salt) diet. Twenty-four rats were divided into four groups. Groups I and II were on the low salt diet and groups III and IV on the high salt diet from 6 weeks of age. Additionally, at 10 weeks of age groups I and III were placed on 100% water and groups II and IV on 25% deuterium oxide. At 14 weeks, systolic blood pressure, uptakes of calcium, and rubidium 86 by aortas were significantly higher (p less than 0.01) in rats on the high salt diet as compared with those on the low salt diet. Deuterium oxide intake normalized systolic blood pressure and aortic calcium uptake but not aortic rubidium 86 uptake in hypertensive rats on the high salt diet. Deuterium oxide had no effect on blood pressure or aortic calcium uptake in rats on the low salt diet. The parallel increase in systolic blood pressure and vascular calcium uptake suggests that increased calcium uptake mechanisms are associated with hypertension in salt-sensitive Dahl rats. Furthermore, deuterium oxide appears to normalize elevated blood pressure in salt-sensitive hypertensive rats by normalizing elevated vascular (aortic) calcium uptake.

Amlodipine, a new 1,4-dihydropyridine calcium antagonist with a particularly strong antihypertensive profile

The effects of a new 1,4-dihydropyridine derivative amlodipine have been compared with results from our previous work. Application of amlodipine at a concentration of 1.6 X 10(-6) M to isolated guinea-pig papillary muscle for 120 minutes produced a 50% reduction in tension development compared with a concentration of 3.7 X 10(-7) M nifedipine needed to produce the same result under identical conditions. This suggests that amlodipine has even weaker negative inotropic effects than nifedipine. In isolated porcine coronary strips, the K+-induced contractions were approximately 10,000 times more sensitive to the relaxing effects of nisoldipine, nitrendipine and nicardipine than to those of papaverine, whereas nifedipine and amlodipine were 3,000 times more potent than papaverine. However, in comparison with these in vitro actions, the efficacy of amlodipine appears to be greater in vivo: Simultaneous subcutaneous injection of nifedipine (20 mg/kg) and of equimolar doses of nisoldipine and felodipine attenuated the myocardial calcium uptake by rat hearts in situ (stimulated with a single subcutaneous dose of 30 mg/kg isoproterenol) with the same efficacy, whereas the actions of nitrendipine and nimodipine were considerably weaker. In contrast, amlodipine antagonized isoproterenol-stimulated myocardial calcium accumulation more effectively. Furthermore, amlodipine exhibited a high antihypertensive potency combined with rapid onset and long duration of action: Amlodipine (10 mg/kg orally [p.o.]) reduced the blood pressure of spontaneously hypertensive rats almost to the same extent as nifedipine, nitrendipine, verapamil and felodipine administered at the much higher doses of 100 mg/kg p.o. Amlodipine (20 mg/kg/day p.o.) maintained normal blood pressure during the whole life span of Dahl-S rats (5 months), but this dose is considerably lower than that reported for other 1,4-dihydropyridines. The survival of NaCl-loaded Dahl-S rats increased from 20 to 100% after administration of amlodipine (20 mg/kg/day p.o.) over 10 weeks: The effective dose of other calcium antagonists is approximately 5 times higher, but well tolerated as, e.g., demonstrated in long-term studies on Dahl-S rats with nitrendipine over 12 months. Increases in systemic arteriolar tone can be visualized in the ocular fundus of spontaneously hypertensive rats. After amlodipine (10 mg/kg p.o.) arteriolar spasm declines. Prophylaxis with 2 doses of 20 mg/kg amlodipine daily in NaCl-loaded Dahl-S rats abolished the macroscopic and histologic changes that are normally seen in branches of the mesenteric artery. With use of electron microscopy, calcium accumulation in the lamina elastica interna was demonstrated by the potassium-pyr-oantimonate technique.(ABSTRACT TRUNCATED AT 400 WORDS)

Future directions in the use of calcium antagonists

There are several reasons to expect that the use of calcium antagonists to treat cardiovascular disease will continue to spread. The scope of indications for existing calcium antagonists is expanding; new calcium antagonists with more selective organ affinity are being developed and these drugs may be given over the long term for prophylaxis against hypertension and for vasoprotection. In all probability, the long-term prophylactic use of calcium antagonists offers the most promise. The long-term effects of calcium antagonists for treating hypertension as well as for preventing vascular damage due to calcinosis and sclerosis will be discussed.

Antihypertensive and arterial anticalcinotic effects of calcium antagonists

In vascular smooth muscle (as in myocardial fibers), a transmembrane supply of calcium ions is required for active tension development. In consequence, calcium antagonists possess a wide scope of action against practically all types of vasoconstrictor or spastic responses of arterial smooth muscle cells. Calcium antagonists are the drugs of choice for the treatment of coronary, pulmonary, cerebral or mesenteric artery spasms. Other clinically important targets of calcium antagonists are the systemic resistance vessels that rapidly dilate, which explains why calcium antagonists are increasingly used for the treatment of acute hypertensive crises as well as for antihypertensive long-term therapy. In physiologic experiments, calcium antagonists normalize the blood pressure of spontaneously hypertensive rats, neutralize various vasoconstrictor agents (if they act via promotion of transmembrane calcium influx), and greatly reduce the sensitivity of the systemic arteries and arterioles to mechanical stimuli, which can produce additional vasoconstriction, if a rise in intraluminal pressure stretches the vascular wall (Bayliss effect). Calcium antagonists also prevent noxious arterial calcium overload in animals. In the human arterial walls, at an advanced age, pathogenic degrees of calcium accumulation are reached and probably play an important role in both the development of hypertension and of arteriosclerotic lesions. Hypertensive rats exhibit progressive arterial calcium overload that responds well to the calcium antagonists nifedipine, nimodipine, nisoldipine and nitrendipine, as well as to verapamil.

Experimental basis of the long-term therapy of arterial hypertension with calcium antagonists

In vascular smooth muscle (as in myocardial fibers) a transmembrane supply of Ca++ ions is required for active tension development. In consequence, Ca++ antagonists, which restrict transmembrane Ca++ delivery, possess a wide scope of action against practically all types of vasoconstrictor or spastic reactions on coronary, pulmonary, cerebral, renal or mesenteric arteries. Another important target of Ca++ antagonists is the resistance vessels of systemic circulation in both animals and humans. Thus, Ca++ antagonists are successfully used for the treatment of acute hypertensive crises as well as for long-term antihypertensive therapy. Spontaneously hypertensive rats (SHRs) also respond to suitable Ca++ antagonists with a dramatic fall in blood pressure. The acute spasmolytic effects of Ca++ antagonists can be directly visualized by ophthalmoscopic examination of the arterioles in the ocular fundus. In untreated SHRs these arterioles are always heavily constricted, and aneurysm-like luminal protuberances develop. In contrast, after 1 adequate Ca++ antagonist dose, blood pressure instantaneously falls to its normal level while the retinal arterioles dilate. A further important effect of Ca++ antagonists that we have demonstrated in experiments on SHRs is the prevention of progressive arterial Ca++ overload, which otherwise produces severe calcinotic or arteriosclerotic damage of the arterial walls. However, with the help of Ca++ antagonists, arterial integrity can be totally preserved. Interestingly, this anticalcinotic arterial protection by Ca++ antagonists may also manifest itself independent of any blood pressure change. This is true of arterial calcinosis in normotensive rats because of advanced age, alloxan diabetes or intoxication with overdoses of vitamin D3, dihydrotachysterol or nicotine.

[Calcium antagonists in the therapy of hypertension]

Calcium antagonists (nifedipine, verapamil, diltiazem) are potent vascular smooth muscle relaxants. Experimental and clinical investigations provide growing evidence that they are effective in acute and (sub)chronic therapy of arterial hypertension by lowering peripheral vascular resistance and improvement of altered hemodynamics--independent from pathogenesis of hypertension. Due to its prompt and profound hypotensive action, sublingual or oral nifedipine has been used successfully in hypertensive crises. The hypotensive effect usually correlated closely with the severity of hypertension and is nearly absent in normotensive controls. Since the blood pressure drop may occasionally results in absolute or relative hypotension, the initial dose should be as low as possible. The activation of the adrenergic and renin angiotension systems seen after nifedipine administration is less pronounced after chronic administration of the drug and is nearly absent after verapamil and diltiazem. Plasma aldosterone concentrations remain constant or are slightly decreased. In contrast to classic vasodilators, the long-term administration of calcium antagonists usually does not result in tachycardia (nifedipine), but slight sinus bradycardia (verapamil, diltiazem). Peripheral edema may occasionally occur after nifedipine. A tolerance has been observed during long-term treatment of hypertension. Combining these drugs (verapamil, diltiazem) with betablockers is not recommended due to the negative inotropic and bathmotropic effects. Simultaneous administration of nifedipine and beta-blockers enhances the hypotensive action, but favours the development of peripheral edema and in rare cases (especially in severe coronary heart disease) results in a dramatic drop in blood pressure and/or congestive heart failure. Further clinical evaluation and long-term trials of calcium antagonists as antihypertensive agents will be needed before definite conclusions can be drawn.

Prevention of malignant hypertension in salt loaded "S" Dahl rats with the calcium antagonist nifedipine

Female "S" Dahl rats, fed an 8% NaCl diet from the 6th week of age developed malignant hypertension and heart hypertrophy in the course of 6 weeks on the salt regimen. Simultaneous treatment with nifedipine (300 ppm in diet additional to 8% NaCl) prevented the increase in blood pressure, reduced heart hypertrophy and mortality and improved the impaired renal function in "S" rats. The decrease in hematocrit in salt loaded "S" rats was prevented by nifedipine. The prophylactic effect of the calcium antagonist nifedipine is only partly due to the prevention of increase in systemic vascular resistance. In addition an improvement of failed intrarenal hemodynamics by nifedipine can be postulated. This enables the kidney of "S" rats to excrete the salt load without increasing blood pressure and plasma volume.