G. Fleckenstein-Grün

The role of calcium in the pathogenesis of experimental arteriosclerosis

Abstract It is well established that high plasma cholesterol is associated with arteriosclerosis. However, increases in arterial cholesterol are paralleled by increases in arterial Ca 2+ content, a phenomenon which is generally considered to be of secondary importance. Moreover, various animal models of arteriosclerosis have demonstrated that Ca 2+ antagonists given prophylactically prevent the build-up of atherosclerotic plaques . Albrecht Fleckenstein and colleagues predict that in future a more comprehensive theory of arteriosclerosis should take into account disorders of both lipid and calcium metabolism.

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.

Calcium Overload — An Important Cellular Mechanism in Hypertension and Arteriosclerosis

SummaryArterial hypertension and arteriosclerosis are dramatic consequences of vascular calcium overload. Acute intracellular calcium overload of vascular smooth muscle cells produces hypercontractility. Hypertension develops if a general increase in systemic arteriolar tone leads to a rise in peripheral flow resistance. Moreover, progressive elevation of calcium destroys the structural integrity of the arterial and arteriolar walls. Thus, in various animal models, calcium overload initiates lesions of an arteriosclerotic character. Interestingly, conventional human coronary plaques also represent a calcium-dominated type of arteriosclerosis.With the advent of specific calcium antagonists, the pathogenic effects of calcium overload and its deleterious consequences have become, for the first time, accessible to therapeutic intervention. Accordingly, adequate treatment with calcium antagonists prevents calcium overload and can thereby protect arteries and arterioles from functional disturbances and structural damage. In spontaneously hypertensive rats, specific calcium antagonists of the verapamil, nifedipine and diltiazem type normalise blood pressure (BP) by reducing transmembrane calcium influx into vascular smooth muscle cells. However, in addition to controlling BP, these drugs also act as tissue protective agents. The long term effects of calcium antagonists such as verapamil in experimental hypertension include the prevention of severe arteriosclerosis, myocardial hypertrophy, and malignant nephrosclerosis. In humans, the antihypertensive efficacy of verapamil is well documented. Further clinical studies have yet to evaluate the antiarteriosclerotic and tissue protective potential of verapamil in humans.

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.

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.

Excessive mural calcium overload — A predominant causal factor in the development of stenosing coronary plaques in humans

SummaryHealthy human coronary artery walls contain, over their entire lifetime, more free and total cholesterol than calcium. However, as soon as arteriosclerotic alterations set in, the calcium content increases. Thus in coronary fatty streaks (arteriosclerotic plaques of WHO stage I), calcium was increased 13 times, in stage II plaques 25 times, and in fully developed stage III plaques 80 times above normal on average. The most dramatic calcium incrustation was found in coronary stage III plaques that had produced massive fatal coronary infarction. Here, the proportion of calcium salts (particularly hydroxyapatite) may amount to almost 50% of dry weight. Thus the most excessive accumulation of calcium seems to be correlated with the highest fatality. In contrast, there is no correlation between mural coronary free or total cholesterol content, and plaque severity. Accordingly, stenosing coronary stage III plaques contain less cholesterol than do fatty streaks. Moreover, in coronary stage III plaques the proportion of free cholesterol was 1.37%, and of total cholesterol only 2.34% of the whole mass, certainly not enough for directly causing coronary occlusion. Thus the calcium-rich plaques of human coronary arteries considerably differ from the well-known cholesterol-rich plaques (stage I and II) of human aortae. Our findings justify a new prophylactic approach with suitable calcium antagonists, to interfere with deleterious calcium uptake in coronary plaque development.

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)

Calcium antagonists: mechanisms and therapeutic uses

Abstract Two classes of Ca-antagonists with differing potencies and specificities have been identified. These drugs can effect the myocardium, cardiac pacemakers and vascular smooth muscle differentially and therefore have a range of therapeutic applications.

Calcium — a Neglected Key Factor in Arteriosclerosis. The Pathogenetic Role of Arterial Calcium Overload and Its Prevention by Calcium Antagonists

Using specific calcium antagonists as experimental tools, both the physiological messenger and current carrying function of calcium ions as well as their pathogenetic potencies could be elucidated. Notably, excess intracellular calcium signalling and intra- and extracellular calcium overload turned out to be pathogenetic principles of general importance. In this context, progressive calcium overload of arteriosclerotic vascular walls and the antiarteriosclerotic effects of calcium antagonists, deserve particular interest. In fact, with the help of calcium antagonists, arterial calcium overload as decisive component of various types of experimental arteriosclerosis became accessible to a direct therapeutic intervention. According to their responsiveness to calcium antagonists, two pathophysiologically different types of experimental coronary plaques could be characterized: (1) The calcium type, i.e. coronary calcinosis of vitamin D3-intoxicated rats highly sensitive to calcium antagonist treatment, (2) the cholesterol type, represented by coronary atheromata of cholesterol-intoxicated rabbits; this primary coronary cholesterol accumulation could not be inhibited by calcium antagonists. The formation of conventional human coronary artery plaques is characterized from the very early lesion onwards by a progressive local uptake of calcium, finally leading to lethal consequences. Conversely, the analysis of the mural cholesterol does not allow to discriminate arteriosclerotic from normal coronary artery segments. Thereby, conventional human coronary plaques typically represent a calcium-dominated type of human arteriosclerosis and differ widely from plaques produced in cholesterol-fed rabbits. The results indicate the decisive pathophysiological role of calcium and calcium overload in both calcium-dominated types of experimental arteriosclerosis and conventional human coronary artery plaques. Moreover, the antiarteriosclerotic effects of calcium antagonists are demonstrated to be based--in various types of experimental arteriosclerosis--on the inhibition of intra- and extracellular calcium overload of arterial walls evoked by various risk factors (vitamin D3 intoxication, hypertension, nicotine, diabetes).

Contribution of store-operated Ca2+ entry to pHo-dependent changes in vascular tone of porcine coronary smooth muscle.

Vascular smooth muscle contracts on increases of extracellular pH (pH(o)) and relaxes on pH(o) decreases possibly resulting from changes in transsarcolemmal Ca(2+) influx. Therefore, we studied store-operated Ca(2+) entry (SOCE; i.e. capacitative Ca(2+) entry (CCE)) during acidification (pH(o)=6.5) and alkalinization (pH(o)=8.0) in isolated porcine coronary smooth muscle cells (SMCs) by monitoring cytoplasmic Ca(2+) ([Ca(2+)](i)) and divalent cation entry (Mn(2+) quench) with fura-2/AM-fluorometry. Additionally, we evaluated the contribution of SOCE to pH(o)-dependent changes in isometric tension of porcine coronary smooth muscle strips. SOCE elicited in SMCs by the SERCA inhibitor BHQ was strongly modulated by pH(o) showing a decrease upon acidification and vice versa an increase upon alkalinization. BHQ-mediated tension of smooth muscle strips also revealed strong pH(o) dependence. In contrast, L-VOC-dependent tension ([K(+)](o)=20 and 40 mmol l(-1)) was remarkably less affected by pH(o) changes. Moreover, refilling of depleted Ca(2+) stores after repeated M(3)-cholinergic receptor stimulation could be almost completely inhibited by SKF 96365 and was markedly reduced by acidification and considerably enhanced by alkalinization pointing to a major role of SOCE in refilling. We conclude that vascular tone particularly responds to alterations in pH(o) whenever SOCE substantially contributes to the amount of activator Ca(2+) for contraction.