The effect of verapamil, diltiazem and nifedipine on baroreceptor reflexes.

Heart rate in nifedipine treated patients tends to be higher than in those treated with verapamil or diltiazem. This may be due, among other causes, to a differential action on cardiac baroreflexes. As conclusions of previous studies were controversial, we decided to study the effect of verapamil, diltiazem and nifedipine on baroreflex sensitivity. After 3-5 week treatment, heart rate response to various doses of i.v. phenylephrine and sodium nitroprusside was recorded in conscious rats. The slope of the regression line relating change of heart rate to change in blood pressure, was depressed in verapamil treated rats compared to that in control, diltiazem or nifedipine treated rats (p less than 0.0001). The role of the autonomic nervous system in modulation of baroreflexes was investigated by pretreatment of the animals with propranolol or atropine. The results were compatible with the hypothesis that verapamil depresses adrenergic nerve activity and this may contribute to decreased, vasodilation-induced baroreceptor reflexes.

Effects of calcium antagonists on sympathetic nerve activity.

The effect of oral treatment with calcium antagonistic drugs on the activity of sympathetic nerves was investigated by studying the uptake and release of 3H-norepinephrine and the residual 3H-content in arteries of spontaneously hypertensive rats. After three to four weeks administration of verapamil, diltiazem or nifedipine, each at two dose levels, the uptake of 3H-norepinephrine was significantly lower than in control animals. Transmural stimulation induced release of 3H-norepinephrine had a tendency to be lower in treated than in untreated rats. Residual 3H in the vessels incubated with 3H-norepinephrine was significantly lower in all but one group of animals than in controls. In previous studies we found that calcium entry blockers in vitro can release norepinephrine from tissues with sympathetic innervation. The results of present experiments suggest that oral administration of verapamil, diltiazem and nifedipine can release norepinephrine and can lead, after a few weeks, to partial depletion of norepinephrine in the vessels.

The effect of nifedipine on norepinephrine release.

Recently, we reported that verapamil and diltiazem caused a marked release of 3H from 3H-norepinephrine labelled tissues with rich sympathetic innervation. In the present study, we investigated the effect of nifedipine in rat tail artery and guinea-pig vas deferens. In vitro superfusion of the arteries with nifedipine (10(-6) - 10(-4) M) caused a significant release of 3H both in spontaneously hypertensive and in normotensive rats, particularly after the highest concentration of the drug. Release of 3H induced by transmural stimulation (1 Hz, 2msec, 10 V) was also significantly greater in vessels superfused with nifedipine than in controls. Column chromatographic analysis revealed that nifedipine did not alter the proportion of norepinephrine and its metabolites in spontaneous or electrical stimulation induced 3H-overflow suggesting that it releases norepinephrine from sympathetic nerves intracellularly. Nifedipine (10(-9) - 10(-4) M) superfusion caused release of 3H-norepinephrine also in the vas deferens of guinea-pig. The effect of nifedipine observed in the present study was similar but less pronounced than that described by us for verapamil and diltiazem. The results indicate that calcium entry blockers act on sympathetic nerves and that this effect is likely explained by an action other than inhibition of Ca2+ influx into the cells.

The effect of diltiazem on noradrenaline release.

The effects of diltiazem in rat tail arteries and guinea-pig vasa deferentia have been investigated. Superfusion of the rat tail artery with diltiazem (10(-6) - 10(-4) M) resulted in a dose-related increase in 3H-overflow (P less than 0.001) both in Wistar Kyoto (WKY) and in spontaneously hypertensive (SHR) rats. Release of 3H by transmural stimulation (1 Hz, 2 ms, 10 V) was also much greater in vessels perfused with diltiazem; this effect was dose-dependent. Diltiazem did not significantly alter the proportion of noradrenaline and its metabolites in 3H-overflow, as analysed by column chromatography. In the vasa deferentia of guinea-pigs, diltiazem (10(-9) - 10(-5) M) increased spontaneous 3H-release. The results indicate that diltiazem acts on sympathetic nerves and causes the release of noradrenaline.

Effects of verapamil on [3H]norepinephrine release.

If calcium entry blocking drugs affect norepinephrine release, this may alter their cardiovascular action by modifying the activity of baroreceptor reflexes. We investigated the effect of verapamil on 3H-release in [3H]norepinephrine-incubated rat arteries and guinea pig vas deferens. Superfusion of tail artery with verapamil (10(-6) - 10(-4) M) increased the 3H-overflow induced by transmural stimulation (1 Hz, 2 ms, 10 V) both in Wistar Kyoto and in spontaneously hypertensive rats. The effect was present also in vessels pretreated with cocaine to inhibit neuronal uptake or with yohimbine to block alpha 2-adrenoceptors. The greatest increase in 3H-overflow--around 400% in the various groups--was observed after perfusion with 10(-4) M verapamil. In other experiments, we found that verapamil also enhanced 3H-overflow from vessels not stimulated transmurally; this effect again was dose-related (p less than 0.001). In the vas deferens of the guinea pig, 10(-5) M verapamil increased spontaneous and electrical stimulation-induced 3H-release (p less than 0.001), whereas superfusion of the tissue with 10(-9) and 10(-7) M verapamil was ineffective. The results indicate that verapamil can act on sympathetic nerves to release norepinephrine.

Vasodilators.

This article reviews the types, mechanisms of action and therapeutic applications of currently used vasodilators. Vasodilators have little value if there is vascular obstruction but are effective in the treatment of systemic hypertension and of myocardial ischemia and heart failure. Since the various groups of vasodilators have widely different actions on the coronary arteries, the peripheral arterial circulation and venous tone, an understanding of the specific modes of action and of the systemic effects of these drugs is essential if they are to be used appropriately and their side effects minimized.

Calcium antagonists. Pharmacodynamic effects and mechanism of action.

Calcium antagonistic drugs (also called slow channel or calcium channel inhibitors or calcium entry blockers) represent a major development in cardiovascular pharmacology. Their main site of action is at the slow channels where they inhibit Ca2+ influx into the cells. This characteristic distinguishes them from other drugs such as sodium nitroprusside, papaverine, hydralazine and diazoxide which interfere with the availability of calcium ions for their physiological functions by acting at sites other than the 'calcium channels'. There is considerable evidence, however, that calcium antagonistic drugs act at an intracellular site(s) as well as the 'calcium channels'. At present, verapamil, nifedipine and diltiazem are the most important representatives of this new class of drugs. Their chemical structures are quite different but their pharmacological characteristics are similar. The action of these drugs is primarily confined to the cardiovascular system. In the heart they depress cardiac contractions and heart rate and protect the ischaemic myocardium from calcium injury. Furthermore, verapamil and diltiazem (but not nifedipine) prolong AV conduction and refractoriness, which is important for their use as antiarrhythmic agents. All 3 drugs are powerful dilators of the coronary and peripheral arteries. These in vitro effects can be substantially altered by activation of baroreceptor reflexes in vivo, as is expected with vasodilators that cause little or no inhibition of noradrenaline release from sympathetic nerve endings. The combination of coronary dilatation with decreased oxygen demand of the myocardium and with decreased preload explains their value in the treatment of vasospastic and effort angina.

Norepinephrine uptake in arteries of spontaneously hypertensive rats.

The aim of our study was to investigate the activity of sympathetic nerves in arteries as a possible factor in the development of hypertension. In this paper, we report our results on the uptake of norepinephrine by the arteries of spontaneously hypertensive rats (SHR). Tail arteries of 7--9 week-old SHR and of normotensive controls (WKY) were incubated with [3H]norepinephrine for various periods of time. The 3H content in vessels of SHR and WKY was identical after 5 min but significantly higher in SHR after 15, 30, 60 and 90 min incubation. The rate of time-related uptake was greater in SHR as revealed by analysis of variance. The uptake of [3H]norepinephrine after 60 min was substantially less in vessels treated with cocaine in inhibit neuronal uptake or with desoxycorticosterone to inhibit extraneuronal uptake. After MAO activity was blocked with pargyline. 3H content remained higher in arteries of SHR than in those of WKY but after catechol-O-methyltransferase (COMT) was inhibited by U-0521, the difference was not significant. Our results demonstrate an alteration in the function of the sympathetic nerves in arteries as indicated by enhanced uptake of norepinephrine in the tail arteries of young SHR prior to the full development of hypertension.

Norepinephrine release in arteries of spontaneously hypertensive rats.

The role of the sympathetic nervous system in arterial hypertension cannot be properly evaluated until we know about its activity in the vessels themselves. In this study we investigated the effect of transmural stimulation on the tail artery - labelled in vitro with 3H-norepinephrine - of 7-9 week old spontaneously hypertensive rats (SHR) and Wistar Kyoto controls (WKR). Electrical stimulation using two frequencies (2 and 10 Hz) resulted in significantly more 3H overflow in vessels from SHR than from WKR. With 10 Hz stimulation the fractional release was also greater. Column chromatographic analysis of 3H overflow revealed that transmural stimulation in arteries of SHR enhanced mainly the release of norepinephrine and not of its metabolites. Significantly, an increased release of 3H-norepinephrine on stimulation was observed in SHR before the full development of hypertension suggesting that it might be a cause rather than a consequence of high blood pressure.

Calcium antagonistic drugs. Mechanism of action.

Calcium antagonists represent a new class of drugs, which were suggested to act by a selective inhibition of Ca2+ influx through cell membranes. We studied the mechanism of action of three calcium antagonists, diltiazem, nifedipine, and verapamil, by investigating the effect on 45Ca uptake and efflux in rat heart and aorta and in rabbit vessels. The uptake of La3+-resistant 45Ca was not decreased by nifedipine or verapamil either in the heart or in the vessels and was increased by diltiazem in rabbit vessels. The efflux of 45Ca from the mesenteric vein of rabbit, originating presumably from intracellular and membrane-boudn fractions, was enhanced by nifedipine. These effects were observed with drug concentrations inhibiting contractions in isolated atria and the spontaneous and norepinephrine-, potassium-, or barium-induced contractions in the portal vein of rats. Thus, our results suggest that calcium antagonistic drugs act by other mechanisms than the inhibition of transmembranous Ca flux, probably on the release and binding of Ca2+ in intracellular pools. The relatively greater inhibition of norepinephrine- than K+-induced contractions in vessels by the calcium antagonistic drugs and the abolition of the inotropic effect of norepinephrine in rat atrium exposed to 0-Ca Krebs solution for a short period are other effects suggesting an intracellular action for these drugs.

Effect of antihypertensives on calcium kinetics in the aorta of spontaneously hypertensive rats.

This study was undertaken to determine whether alterations in 45Ca kinetics, found previously in the aorta of spontaneously hypertensive rats (SHR), are the cause or the consequence of high blood pressure. SHR were treated for 3-6 weeks with hydralazine, propranolol or timolol before the uptake and efflux of 45Ca was measured in the aorta and compared with those in untreated animals. Hydralazine prevented the development of hypertension in the rats but did not alter the uptake or efflux of 45Ca. It is concluded that calcium handling in SHR is abnormal also in the absence of high arterial pressure.

The effect of magnesium on the response of smooth muscle to 5-hydroxytryptamine.

1 The responses to 5-hydroxytryptamine (5-HT) of rabbit isolated mesenteric artery and vein and longitudinal smooth muscle of guinea-pig ileum were examined in Krebs solution containing 0, 1.2 or 2.4 mM Mg2+. 2 When the concentration of Mg2+ was raised the spontaneous contractile activity of rabbit mesenteric vein was depressed. The responses to 5-HT in rabbit mesenteric artery and vein and guinea-pig ileum were greater in the absence of Mg2+. The initial fast component of 5-HT-induced contractions in rabbit mesenteric vein was reduced more consistently than the subsequent slow component by increasing the Mg2+ concentration. 3 Exposure of mesenteric vein to Ca-free solution containing ethyleneglycoltetra-acetic acid (EGTA) promptly abolished 5-HT contraction in normal-Mg but not in low-Mg Krebs solution. 4 In mesenteric veins, no difference was observed in either the 'lanthanum-resistant' uptake of 45Ca or total tissue Ca, measured by atomic absorption spectrophotometry, after 60 min exposure to either low-Mg or normal-Mg Krebs solution. On the other hand, after 5 min exposure, the 'lanthanum-resistant' uptake of 45Ca was greater in the absence of Mg2+ than in the presence of higher Mg2+ concentrations. 5 It is suggested that Mg2+ depressed the 5-HT response at least partly by reducing the availability of Ca2+ from a rapidly equilibrating intracellular pool.

The effect of sodium nitroprusside on the uptake and efflux of 45Ca from rabbit and rat vessels.

In this study we investigated if sodium nitroprusside (NaNP), a vasodilator with direct action on smooth muscle, would have any effect on calcium kinetics in blood vessels. The effect of NaNP, in 0.1-10 micrometer concentration, was studied in vitro on the uptake of 45Ca with the lanthanum method and on the efflux of 45Ca. The drug failed to alter the uptake of 'La-resistant' 45Ca in rabbit mesenteric artery and vein. This was true even if Ca2+ uptake was increased in the vessels by 60 mM KCl. The efflux of 45Ca on the other hand was enhanced from rabbit mesenteric vein and from rat aorta exposed to NaNP, in a dose-related way. The present study, like our previous one on other vasodilators, indicates that NaNP has en effect on vascular calcium kinetics but this is the first time that a relationship could be established between the dose of the drug and the effect on 45Ca efflux.

Calcium kinetics in the aorta of spontaneously hypertensive rats.

In the search for abnormalities in hypertensive vessels Ca kinetics was studied in the aorta of spontaneously hypertensive rats (SHR). The uptake of 45Ca was significantly less in hypertensive animals but the uptake of 'Lanthanum resistant' 45Ca was not. The efflux of 45Ca from aorta of SHR was different (P less than 0.01) from that of control rats because of faster initial efflux. The half-life of the first 2 components of efflux curves was significantly shorter in SHR. Results are interpreted as consequences of decreased Ca binding by smooth muscle membrane in hypertensive vessels.

Isoxsuprine as an oral vasodilator.

The effect of isoxsuprine, administered orally as a single 20-mg tablet, was compared with that of a placebo in a double-blind crossover study in 12 volunteers with and in 12 without peripheral vascular disease. Isoxsuprine failed to increase blood flow in the calf, foot or hand, and did not alter blood pressure or heart rate significantly. Claudication time was not prolonged after taking the drug. This study does not support the value of oral administration of isoxsuprine as a peripheral vasodilator.