Characterization of two different calcium entry pathways in small mesenteric arteries from rat

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

Calcium is a highly positively charged ionic species. It regulates all cell types' functions and is an important second messenger that controls and triggers several mechanisms, including membrane stabilization, permeability, contraction, secretion, mitosis, intercellular communications, and in the activation of kinases and gene expression. Therefore, controlling calcium transport and its intracellular homeostasis in physiology leads to the healthy functioning of the biological system. However, abnormal extracellular and intracellular calcium homeostasis leads to cardiovascular, skeletal, immune, secretory diseases, and cancer. Therefore, the pharmacological control of calcium influx directly via calcium channels and exchangers and its outflow via calcium pumps and uptake by the ER/SR are crucial in treating calcium transport remodeling in pathology. Here, we mainly focused on selective calcium transporters and blockers in the cardiovascular system.

Historical Overview of Store-Operated Ca(2+) Entry

Calcium influx is an essential mechanism for the activation of cellular functions both in excitable and non-excitable cells. In non-excitable cells, activation of phospholipase C by occupation of G protein-coupled receptors leads to the generation of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which, in turn, initiate two Ca(2+) entry pathways: Ca(2+) release from intracellular Ca(2+) stores, signaled by IP3, leads to the activation of store-operated Ca(2+) entry (SOCE); on the other hand, DAG activates a distinct second messenger-operated pathway. SOCE is regulated by the filling state of the intracellular calcium stores. The search for the molecular components of SOCE has identified the stromal interaction molecule 1 (STIM1) as the Ca(2+) sensor in the endoplasmic reticulum and Orai1 as a store-operated channel (SOC) subunit. Furthermore, a number of reports have revealed that several members of the TRPC family of channels also take part of the SOC macromolecular complex. This introductory chapter summarizes the early pieces of evidence that led to the concept of SOCE and the components of the store-operated signaling pathway.

Chronic exposure to cyclosporine affects endothelial and smooth muscle reactivity in the rat aorta

Chronic exposure to cyclosporine affects vascular reactivity. Experiments were designed to characterize the endothelium-dependent and endothelium-independent vascular reactivity of rats exposed to oral cyclosporin A (CyA). Two subsets of rats (n = 6) were treated with CyA (20 mg/kg/day) and olive oil (cyclosporine vehicle), respectively, for a period of 8 weeks. Aortic rings (4-5 mm) were suspended for isometric force measurement in organ chambers containing Krebs Ringer solution (37 degrees C, 95% O2, 5% CO2). The maximal endothelium-dependent relaxation to cumulative doses of acetylcholine was significantly decreased in the CyA-treated aortic rings compared to olive oil-treated ones (data expressed as percent of initial contraction; CyA, 50% +/- 3% versus olive oil, 37% +/- 7%; p < 0.05). However, endothelium-dependent relaxations to histamine and adenosine diphosphate and endothelium-independent relaxation to sodium nitroprusside were not affected in both groups. An endothelium-dependent contraction to serotonin and aggregating platelets were observed in the CyA group, but not in the control group. The endothelium-independent contraction to norepinephrine was enhanced in the CyA group (CyA ED50, log -7.66 +/- 0.18 mol/L versus olive oil ED50, log -7.01 +/- 0.11 mol/L; p < 0.01). These experiments suggest that chronic exposure to cyclosporine A could contribute to augmenting vascular tone by (1) decreased release of endothelial relaxing factor mediated by muscarinic receptors, (2) increased production of endothelium-related constricting factor mediated by serotoninergic receptors, and (3) greater vascular smooth muscle sensitivity to circulating catecholamine.

Influence of the endothelium on Ca2+ dependency of angiotensin II-induced contractions of rat aortic rings

Endothelium-dependent relaxation has been demonstrated to be involved in the regulation of vascular tone and extracellular Ca2+ was found to play a prominent role in this process. Since the dependency on extracellular Ca2+ appeared to differ considerably within the arterial tree, possibly as a consequence of vessel-related endothelium-dependent mechanisms, we investigated the effects of different compounds affecting Ca2+ entry (nifedipine, CoCl2) on angiotensin II-induced contractions of rat aortic rings with and without endothelium as well as the responses in a Ca(2+)-"free" solution. For this purpose, rat aortic rings were either undone from their endothelial layer by gentle mechanical rubbing or care was taken to keep the intima intact in case rings where endothelium were required. The presence of an intact endothelium was confirmed by acetylcholine-induced relaxation. A stronger responsiveness towards angiotensin I, both after a complete concentration-response curve and after a single maximal concentration of angiotensin II was observed in arterial segments without endothelium. The maximal contraction to a single concentration of angiotensin II (0.1 microM) in the rings without endothelium amounted to 75.8 +/- 3.8% of the preceding response to a supramaximal concentration of noradrenaline (= Emax). In rings without the endothelial layer, the contraction was 34.8 +/- 3.7% of Emax. This indicates an endothelium-induced relaxation in aortic rings with endothelium. After incubation with the Ca2+ entry blocker nifedipine (1 microM) both rings with and without endothelium were inhibited to the same extent, contractions amounted to 30.7 +/- 1.8% and 19.6 +/- 1.3% of Emax, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood pH and the actions of nifedipine on cardiac excitation-contraction coupling

Hearts from male Sprague-Dawley rats were perfused, by the Lagendorff method, with calcium-free Krebs solution (containing either adrenaline or a high level of potassium) at pH 7.48 (control), 7.26 (acidosis) or 7.69 (alkalosis). When the hearts stopped contracting, a dose of nifedipine or its vehicle was given before measuring the force of contraction, coronary perfusion pressure and heart rate in response to graded doses of calcium. The calcium-antagonising efficacy of nifedipine was reduced during acidosis in both adrenaline- and potassium-stimulated hearts, but the reduction was greater in the adrenaline-stimulated hearts. Alkalosis led to a small increase in the efficacy of nifedipine on adrenaline- and potassium-stimulated contractions.

(+)-S-12967 and (-)-S-12968: 1,4-dihydropyridine stereoisomers with calcium channel agonistic and antagonistic properties in rat resistance arteries

1. The actions of (+)-S-12967 and (-)-S-12968 two isomers of a new 1,4-dihydropyridine (DHP) derivative, were studied on 125 mM K(+)-, Ca(2+)- and noradrenaline-induced contractions in rat isolated mesenteric resistance arteries and compared to those of nifedipine. 2. The action of (+)-S-12967 and (-)-S-12968 was slow in onset in contrast to nifedipine. Both isomers had a dual contractile and relaxant action in arteries contracted with 125 mM K+; however, the (-)-isomer was about 300 times more potent than the (+)-isomer. The response to 125 mM K+, being depressed by 70%, recovered within 20 to 30 min for all DHP derivatives. All vessels were treated with 1 x 10(-6) M phenoxybenzamine thus excluding the possibility that the contraction is mediated by activation of amine-receptors. 3. Both (+)-S-12967 and (-)-S-12968 at low concentrations potentiated responses induced by Ca2+ in arteries activated by 125 mM K+ and inhibited the responses at higher concentrations. (+)-S-12967 and (-)-S-12968 had no contractile action in arteries kept in normal buffer. Nifedipine had only an inhibitory action on vessel responses to 125 mM K+ and Ca2+. 4. Both isomers and nifedipine depressed the maximal vessel response to noradrenaline by about 20% and 44%, respectively. 5. The results confirm that DHP calcium antagonists selectively inhibit vascular smooth muscle responses induced by high potassium and that the potency of 1,4-DHP isomers may vary considerably. Furthermore, since the agonistic/antagonistic properties on the calcium channel were shared by both stereoisomers of the 1,4-DHP molecule and apparently dependent on their concentration and the vascular smooth muscle membrane potential, it suggests that the agonistic action of 1,4-DHPs may be ascribed to functional characteristics of their binding site regulating the Ca2l -channel.

Interactions between analgesics and calcium channel blockers

1. The findings, derived from different experimental models, examined in this review, provide evidence that the calcium channel blockers and related drugs possess analgesic effects. 2. The antinociceptive action that some analgesic drugs exhibit may be related to calcium channel blockade. 3. Evidence from a variety of biochemical and pharmacological experimental approaches, support the existence of an interelation between the calcium modulators and the opioid drugs. 4. This idea agrees with the novel neuropharmacological hypothesis that a common very high affinity binding site for multiple neurotransmitters could exist, as has been proposed by Pasternak and Wood (1986). 5. This hypothesis could be extended to the neuromodulators or other neuromediators.

Effects of ryanodine on tension development in rat aorta and mesenteric resistance vessels

1. The effect of ryanodine on contractile responses dependent either on intracellular Ca2+ release or on extracellular Ca2+ influx were studied in aorta and mesenteric resistance vessels of the rat. 2. In aorta, in the presence of extracellular Ca2+, pretreatment with ryanodine (10(-5)M) did not modify contractile responses to noradrenaline (NA) (10(-6)M) whereas in the absence of Ca2+, pretreatment with ryanodine reduced to about 25% the contractile response to NA (10(-6)M) and totally abolished the transient contraction elicited by caffeine (5 x 10(-2)M). 3. In mesenteric resistance vessels, ryanodine (10(-5)M) had no effects on NA (10(-5)M)-induced tension in the presence of extracellular Ca2+ but totally abolished contractile responses to caffeine (10(-2)M) in the absence of Ca2+. 4. In K+ -depolarized mesenteric resistance vessels, pretreatment with ryanodine (10(-5)M) significantly enhanced contractile responses to Ca2+ concentrations higher than 10(-4)M and 10(-3)M for arteries depolarized with 30 mM and 40 mM K+ respectively. Concentrations of either diltiazem (6 x 10(-7)M) or nifedipine (10(-8)M) that abolished contractile responses to Ca2+ in depolarized arteries (K+, 40 mM) did not totally inhibit the enhancement of Ca2+ -induced contractions obtained in the presence of ryanodine. 5. Ryanodine did not modify the Ca2+ concentration-effect relationships in mesenteric resistance vessels exposed to NA or arginine vasopressin. 6. These data are consistent with the hypothesis that ryanodine induces a release of Ca2+ from intracellular stores, resulting in a subsequent reduction of the amplitude of contractions dependent upon intracellular Ca2+ liberation. Furthermore, the ability of sarcoplasmic reticulum to buffer rises in cytoplasmic Ca2+ may be reduced in the presence of ryanodine, thereby accounting for the potentiation of contractile responses to Ca2+ in K+-depolarized mesenteric resistance vessels.

Vasodilator drug effects on internal mammary artery and saphenous vein grafts

The internal mammary artery is a dynamic conduit used for myocardial revascularization in which potential exists for spasm as well as for vasodilation. This study investigated vasodilator drug effects on the mammary artery using nitroprusside and nitroglycerin in vitro to measure the inhibition of contraction of human internal mammary artery and in vivo to examine blood flow through a canine mammary artery. In the in vitro study, ring segments of human internal mammary arteries were suspended on strain gauges in muscle baths containing 37 degrees C Krebs solution for measurement of isometric tension in vitro. Arterial contraction was stimulated with 70 mM potassium and 10 microM norepinephrine, and inhibition of contraction by vasodilators was measured. Nitroprusside was more potent and effective than was nitroglycerin in inhibiting potassium and norepinephrine contraction. The in vivo study utilized a canine (n = 8) right heart bypass preparation that allowed precise control of cardiac output, blood pressure and heart rate, which were maintained constant. The internal mammary artery graft and the saphenous vein graft perfused the same coronary artery bed. Electromagnetic flow probes measured graft flow (with the other graft occluded) before and after 15 min of drug infusion (1 microgram/kg per min). Nitroglycerin significantly increased mammary artery flow 36 +/- 13%, whereas nitroprusside significantly decreased it 12 +/- 2%. Saphenous vein grafts responded differently; graft blood flow decreased with nitroglycerin and increased with nitroprusside. Thus, although nitroprusside was more effective than nitroglycerin in inhibiting mammary artery contraction in vitro, it decreased internal mammary artery graft flow measured in vivo. Nitroglycerin had the opposite effect, increasing mammary graft flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Source of calcium for contractions induced by depolarization and muscarinic receptor stimulation in rabbit urinary bladder

Omission of calcium or the inclusion of lanthanum in the bathing medium resulted in an almost complete inhibition of contractile responses induced by either K+ depolarization or carbachol in strips of rabbit urinary bladder. D-600 inhibited K+-induced contractions significantly more than carbachol-induced responses. The influx of 45Ca into cells was stimulated both by K+ depolarization and carbachol. Over a 2-min period the increase in 45Ca influx induced by high K+ and carbachol was 98 and 65%, respectively. Both lanthanum and D-600 blocked 45Ca influx stimulated by either K+ depolarization or carbachol. The inhibition of 45Ca influx by these calcium-channel blocking agents, particularly by D-600, was dependent on the length of exposure. Application of carbachol during 45Ca efflux in pre-loaded muscle strips had no effect on the rate of 45Ca efflux. These results indicate that the contractile responses of the urinary bladder to depolarization and to carbachol are highly dependent on an extracellular source of calcium.

Calcium dependency of massive postmortem bronchoconstriction in guinea pig lungs

Calcium antagonists were employed to examine whether or not guinea pig's postmortem bronchoconstriction is calcium dependent. First, a group of 17 young (3-4-week-old) animals were divided in three subgroups: control, nifedipine, and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8, an intracellular calcium antagonist). A second group of 12 mature (5-7-week-old) animals were divided into two subgroups: control and nifedipine. Baseline pressure-volume (P-V) curves were performed prior to cannulation of the pulmonary artery in the anesthetized, opened chested animal. The lungs of the animal were then perfused and P-V curves were obtained again at intervals for up to 60 min. Nifedipine (10(-6) M) or TMB-8 (5 X 10(-4) M) was added to the perfusate in experimental groups. A temporal decrease in inflation volume (IV, the lung volume between transpulmonary pressure of 0 and 30 cm H2O during inflation) was used as an indicator of bronchoconstriction. In control subgroups, the decrease in IV was significantly greater (P less than 0.05) in the young (29.5 +/- 12.9% baseline at 20 min) than in the mature animals (75.0 +/- 7.0% baseline at 20 min). Nifedipine significantly alleviated (P less than 0.05) the decrease in IV in young animals at 15-30 min (58.6 +/- 8.4% baseline at 20 min) while TMB-8 (90.6 +/- 2.1% baseline at 20 min) prevented the decrease even further. Nifedipine did not have a significant effect on IV in mature animals. These data suggest that the postmortem bronchospasm is much more severe in young animals and that this severe constriction is calcium dependent.

Classification of calcium antagonists

Drugs of several chemical families have been identified as calcium antagonists. This article examines some pharmacologic properties of these drugs to clarify their terminology and their classification and to provide a rationale for their clinical use. Studies with nifedipine show quantitatively that the therapeutic effect in angina is related to the interaction of this drug with membrane calcium channels in human coronary arteries. This gives support to a classification based on studies at the molecular, tissue and organ levels. Among calcium antagonists, calcium entry blockers are defined as agents able to block calcium inward fluxes evoked by various stimuli. They may be subdivided in 2 groups. Group I is the group of selective calcium entry blockers. Group IA consists of those agents selective for slow calcium channels in myocardium (slow channel blockers); the leading agents are verapamil, nifedipine and diltiazem. Group IB contains agents without action on slow calcium channels in myocardium but with selective action on arteries; the leading agents are cinnarizine and flunarizine. Group II is the group of nonselective calcium entry blockers. Group IIA contains agents acting at similar concentration on calcium and on fast sodium channels. Group IIB consists of agents interacting with calcium channels while having another primary site of action. Other agents modulate calcium movements by an action on sodium-calcium exchange and by an action within the cell. Their identification requires the use of cell biology. The actual clinical uses of these drugs are consistent with this pharmacologic classification.

Actions of some prostaglandins and leukotrienes on rat cerebral and mesenteric arteries

The effects of some prostaglandins (PG's) and leukotrienes (LT's) on rat middle cerebral, basilar and mesenteric arteries were evaluated in vitro. The order of potency of some prostanoids with respect to their contractile effects in basilar arteries was: U44069 greater than PGF2 alpha greater than PGI2 approximately equal to PGE2 greater than 6-keto-PGE1 greater than 6-keto-PGF1 alpha, whereas 6,15-diketo-PGF1 alpha was inactive. Middle cerebral and basilar arteries were 3-5 times more sensitive than mesenteric arteries to PGF2 alpha. LTD4 and LTC4 were inactive in all three vessel types. PGI2 produced a concentration-related relaxation of similar potency in all three arteries contracted by PGF2 alpha. Arteries preactivated by other agents (K+, noradrenaline, 5-hydroxytryptamine) either failed to relax or inconsistently relaxed after PGI2 application. Among the PGI2 metabolites (6-keto-PGF1 alpha, 6,15-diketo-PGF1 alpha, 6-keto-PGE1), only 6-keto-PGE1 elicited relaxation in the PGF2 alpha-contracted basilar artery. However, the drug potency was significantly smaller than that of PGI2. Nifedipine inhibited the PGF2 alpha-induced contraction by 68% in middle cerebral arteries and by 80% in mesenteric arteries. Exposure to Ca2+-free medium for a time period which almost completely abolished the contractile response to K+ (less than 5% left), reduced the PGF2 alpha-induced contraction by 54, 61 and 85% in middle cerebral, basilar and mesenteric arteries, respectively. The PGF2 alpha-induced contraction of cerebral arteries in Ca2+-free medium was usually composed of a rapidly developing first phase, which levelled off after 1-2 min, and a second slowly developing tonic phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of adrenoceptor mechanisms in isolated guinea-pig uterine arteries

The adrenoceptors of the guinea-pig uterine artery were characterized pharmacologically. Circular segments of the artery, approximately 2 mm long, and with an external diameter of 250 micron, were mounted in miniaturized tissue baths. Noradrenaline, methoxamine and phenylephrine (concentrations ranging from 10 nM to 1 mM), in the presence of propranolol (0.1 microM) and cocaine (1 microM), induced concentration-dependent contractions of the arterial segments. Clonidine (10 nM to 0.1 mM) was less effective in producing contraction of the vessel. Prazosin (10 nM to 1 microM) antagonized noradrenaline-induced contractions; its pA2 was 7.68. Rauwolscine (10 nM to 1 microM) had no effect on noradrenaline-induced contractions. Isoprenaline (10 nM to 0.1 mM) in the presence of prazosin (1 microM) and cocaine (1 microM) had no relaxant effect on arteries contracted submaximally by prostaglandin F2 alpha (5 microM). Cocaine or normetanephrine treatment did not influence the noradrenaline-induced contractions. It is concluded that in guinea-pig uterine arteries, amine-induced contractions are mediated predominantly by alpha 1-adrenoceptors and that in this arterial preparation, relaxant beta-adrenoceptor effects and neuronal or extraneuronal uptake are of minor if any importance.

Effects of calcium entry blockers on Ca2+-induced contraction of depolarized and noradrenaline-exposed rat resistance vessels

The effects of four calcium entry blockers (CEBs), diltiazem (D), verapamil (V), nifedipine (NF) and nicardipine (NC), were investigated on Ca2+ concentration-effect curves of rat depolarized (K+, 40 mM) or noradrenaline (NA, 3 microM)-exposed mesenteric resistance vessels. Under control conditions, NA-exposed vessels were more sensitive to Ca2+ (pD2: 4.12 +/- 0.11) than depolarized vessels (pD2: 3.16 +/- 0.02, P less than 0.01) whereas the maximal active wall tensions were not significantly different (2.86 +/- 0.11 mN/mm and 2.11 +/- 0.34 mN/mm respectively). In depolarized vessels, D, V, NF and NC induced a concentration-dependent shift to the right and a depression of the maximal effect of the Ca2+ curves, which suggested a non-competitive antagonism. The IC50 (concentration of CEB producing a 50% inhibition of the maximal contractile response from control curve) values were: D: 3 X 10(-7), V: 1.3 X 10(-7), NF: 4.5 X 10(-9), NC: 3 X 10(-9) M. In NA-exposed vessels, the CEBs produced a concentration-dependent shift to the right of the Ca2+ curves before depressing their maximal effect. This suggested that the antagonism was different from that observed in depolarized arteries. In this case, the IC50 values were: D: 4.5 X 10(-7), V: 2 X 10(-7), NF: 9 X 10(-9), NC: 7 X 10(-9) M. Although the gating mechanisms activated in this study were differently affected by CEBs, since there were marked qualitative differences in their antagonistic effects on Ca2+ concentration-effect curves, depolarization and NA promoted the entry of Ca2+ into smooth muscle cells of rat resistance vessels by mechanisms with the same sensitivity to CEBs as expressed by IC50 values.

Contractile effects of various vasoactive agents in small rat portal veins and hepatic arteries and the influence of sympathetic denervation on the noradrenaline response

Contractile responses were studied in isolated tubal segments of branches of the rat portal vein (luminal diameter approximately 300 microns) and hepatic artery (luminal diameter approximately 200 microns). Portal veins were approximately three times more sensitive to noradrenaline (NA) than hepatic arteries. 5-hydroxytryptamine contracted hepatic arteries concentration-dependently, whereas it produced only weak and inconsistent contractions in portal veins. Vasopressin effectively contracted hepatic arteries, whereas it had no effect on portal veins. Both vessel types responded to prostaglandin F2 alpha with contractions, although the drug potency was relatively low (EC50 greater than 10(-5) mol l-1). Histamine and carbachol failed to induce (hepatic arteries) or caused only weak (portal veins) contractions. Microsurgical hepatic hilar denervation reduced the catecholamine content of the parenchyma to less than or equal to 25% of controls. In both portal veins and hepatic arteries, the denervation procedure increased the NA sensitivity by factors of 3.1 and 2.0, respectively. In non-denervated livers, cocaine produced a similar increase of the NA sensitivity, whereas the drug had no significant effect in vessels from denervated animals. Thus, there was a marked difference between rat portal veins and hepatic arteries in their responsiveness to several contractile agents. Furthermore, the results of the present study indicate that the adrenergic nerves in both vessel types can be adequately removed by the microsurgical denervation procedure used.