No abstract

In the mid 1960s, experimental work on molecules under screening as coronary dilators allowed the discovery of the mechanism of calcium entry blockade by drugs later named calcium channel blockers. This paper summarizes scientific research on these small molecules interacting directly with L-type voltage-operated calcium channels. It also reports on experimental approaches translated into understanding of their therapeutic actions. The importance of calcium in muscle contraction was discovered by Sidney Ringer who reported this fact in 1883. Interest in the intracellular role of calcium arose 60 years later out of Kamada (Japan) and Heibrunn (USA) experiments in the early 1940s. Studies on pharmacology of calcium function were initiated in the mid 1960s and their therapeutic applications globally occurred in the the 1980s. The first part of this report deals with basic pharmacology in the cardiovascular system particularly in isolated arteries. In the section entitled from calcium antagonists to calcium channel blockers, it is recalled that drugs of a series of diphenylpiperazines screened in vivo on coronary bed precontracted by angiotensin were initially named calcium antagonists on the basis of their effect in depolarized arteries contracted by calcium. Studies on arteries contracted by catecholamines showed that the vasorelaxation resulted from blockade of calcium entry. Radiochemical and electrophysiological studies performed with dihydropyridines allowed their cellular targets to be identified with L-type voltage-operated calcium channels. The modulated receptor theory helped the understanding of their variation in affinity dependent on arterial cell membrane potential and promoted the terminology calcium channel blocker (CCB) of which the various chemical families are introduced in the paper. In the section entitled tissue selectivity of CCBs, it is shown that characteristics of the drug, properties of the tissue, and of the stimuli are important factors of their action. The high sensitivity of hypertensive animals is explained by the partial depolarization of their arteries. It is noted that they are arteriolar dilators and that they cannot be simply considered as vasodilators. The second part of this report provides key information about clinical usefulness of CCBs. A section is devoted to the controversy on their safety closed by the Allhat trial (2002). Sections are dedicated to their effect in cardiac ischemia, in cardiac arrhythmias, in atherosclerosis, in hypertension, and its complications. CCBs appear as the most commonly used for the treatment of cardiovascular diseases. As far as hypertension is concerned, globally the prevalence in adults aged 25 years and over was around 40% in 2008. Usefulness of CCBs is discussed on the basis of large clinical trials. At therapeutic dosage, they reduce the elevated blood pressure of hypertensive patients but don't change blood pressure of normotensive subjects, as was observed in animals. Those active on both L- and T-type channels are efficient in nephropat...

With the endothelium present, the maximum response of rat isolated aorta to clonidine was much lower than that to noradrenaline. Removal of endothelium enhanced the response to both adrenoceptor agonists and the clonidine-induced maximum contraction became almost equal to that produced by noradrenaline, although it was much more sensitive to inhibition by flunarizine and nifedipine. These results indicate that clonidine and noradrenaline activate receptors present in the endothelial cells and that these receptors are highly sensitive to clonidine.

1. Mesenteric arteries immersed in a depolarizing solution contract in the presence of calcium. These contractions are proportional to the calcium concentration and are reversible. 2. Mesenteric arteries immersed in a calcium-free depolarizing solution contract in the presence of adrenaline. Under the experimental conditions reported here, this response develops only albout one-third of the contractile tension developed in polarizing solution (modified Krebs bicarbonate). 3. Cinnarizine and chlorpromazine inhilbit the contractile response to calcium and induce relaxation of depolarized muscle previously contracted by calcium; cinnarizine was 4 times more potent than chlorpromazine in such activity. 4. Chlorpromazine inhibits the response to adrenaline in lboth polarizing and calcium-free depolarizing solutions, whereas cinnarizine inhibits the response in polarizing solution but not that in calcium-free depolarizing solution. 5. The significance of these results is discussed.TIhe contractile response of vascular smooth muscle to depolarization by potassium is reduced by phenoxybenzamine, dibenamine and chlorpromazine (Bevan,

SUMMARY1. The Na, K, Ca and Mg content and the 45Ca uptake and loss were determined in rat aortae incubated in physiological solution or in solution containing LaCl3 instead of CaCl2.2. Aortae washed in La-solution contained less Ca and Na than controls in physiological solution, the K content was not modified and the Mg content was slightly decreased. 4. Noradrenaline increased the rate of uptake of 45Ca into the Ca fraction resistant to displacement by La. This increase was dose dependent, a response of 50 % of the maximum being produced by 2 x 10-8 noradrenaline as for the contraction. In the presence of phentolamine, the doseeffect curves for the action of noradrenaline on 45Ca uptake were displaced in a manner characteristic of competitive antagonism. The pA2 for phentolamine was 7-8.5. In physiological solution, the rate of loss of 45Ca, from the Ca fraction resistant to displacement by La, was increased by noradrenaline the ED50 was 2 x 104M, and the effect was abolished by phentolamine.6. In view of the similarity of phentolamine pA2 estimated by measuring noradrenaline sensitive 45Ca uptake or noradrenaline evoked contraction, it is likely that the activation of ac-adrenergic receptors is responsible for both effects.

The influence of endothelium on the response of rat isolated aorta to alpha-adrenergic agonists has been studied by comparing the response of intact and denuded preparations before and after treatment with calcium entry blockers flunarizine and nifedipine. Endothelium removal enhanced the response of the preparations, especially to alpha 2-agonists that had a weak effect in intact preparations. In the absence of endothelium, about 80% of the maximum response to clonidine was blocked by calcium entry blockers, whereas only 25% of the maximum response to noradrenaline was sensitive to them. In contrast about 40% of maximum noradrenaline-evoked contractions was sensitive to calcium entry blockers in the presence of endothelium. This may be attributed to an increase in the intracellular exchangeable calcium fraction, likely to be due to a slight depolarization of smooth muscle membrane of denuded preparations, which are highly sensitive to the calcium agonist BAY K 8644. The results indicate that a factor liberated by endothelial cells controls both calcium entry and calcium release evoked by alpha-adrenoceptor agonists in vascular smooth muscle.

Drugs currently known as calcium channel blockers (CCB) were initially called calcium antagonists because of their ability to inhibit calcium-evoked contractions in depolarized smooth muscles. Blocking the entry of calcium reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological property has been the basis for the use of CCBs in the management of hypertension and coronary heart disease. A major question is whether drugs reducing blood pressure have other effects that help prevent the main complications of hypertension, such as atherosclerosis, stroke, peripheral arterial disease, heart failure and end-state renal disease. Experimental studies that focus on this question are reviewed in the present paper.

To better understand excitation-contraction coupling in cardiac muscle, we investigated the main Ca2+ channels involved in that process in adult and neonatal rat ventricle. Voltage-dependent (L-type) Ca2+ channels and sarcoplasmic reticulum Ca2+ release channels were labeled by means of [3H] (+)-PN200-110 and [3H]ryanodine, respectively. The number of [3H]ryanodine binding sites (per gram tissue) increased more than that of [3H] (+)-PN200-110 binding sites over the postnatal period (2.1-fold versus 1.35-fold, respectively). After equilibration of microsomal fractions in density gradient, ryanodine receptors were characterized by a heavy distribution pattern that did not change appreciably between days 1 and 30 after birth. In neonatal tissue, 1,4-dihydropyridine receptors were found mainly in low-density subfractions, together with other sarcolemmal constituents, whereas in adult tissue, they were recovered predominantly in high-density subfractions, together with ryanodine receptors. Thus, after birth, and in parallel with the development of T tubules, there was a progressive concentration of L-type Ca2+ channels in junctional structures of high equilibrium density, where they were situated close to the Ca2+ release channels of the sarcoplasmic reticulum. In adult ventricle, L-type channels were, on an average, threefold more abundant in T tubules than in external sarcolemma. In parallel mechanical studies, we found that the inhibitory action of ryanodine on systolic contraction was much more pronounced in adult than in neonatal right ventricle, and that, conversely, neonatal tissue was more sensitive that adult tissue to inhibitors of L-type channels. We conclude that, in view of the presumed mechanism of Ca2+ release from the sarcoplasmic reticulum, that is, Ca(2+)-induced Ca2+ release, the predominant localization in adult rat ventricle of the major Ca2+ entry pathway in the vicinity of the Ca2+ release pathway is of great functional significance. Furthermore, owing to the relative stoichiometry of Ca2+ entry and Ca2+ release channels in junctional structures (about 1:9), a physical link between these channels is not likely to be involved in the modulation of Ca2+ release from the sarcoplasmic reticulum in cardiac muscle.