Influence of extracellular calcium and nifedipine on alpha 1- and alpha 2-adrenoceptor-mediated contractile responses in isolated rat and cat cerebral and mesenteric arteries

Possible role of Ca2+ channels in the vasodilating effect of 5beta-dihydrotestosterone in rat aorta

It has previously been shown that the androgen, 5beta-dihydrotestosterone (17beta-hydroxy-5beta-androstan-3-one, 5beta-DHT), is able to produce an endothelium-independent vasodilating effect in rat aorta. The present study analyzed the mechanisms underlying the above vasodilator effect of 5beta-dihydrotestosterone, with particular emphasis on verifying a possible interaction with GABA(A) receptors, beta-adrenoceptors and Ca2+ channels. Rat aortic rings without endothelium were isometrically recorded. 5Beta-dihydrotestosterone produced a concentration-dependent relaxation on the contractions induced by noradrenaline (NA; 0.3 microM) or K+ (KCl; 60 mM), with the latter being more sensitive to 5beta-dihydrotestosterone-induced relaxation than the former; the concentration-response curves showed that 5beta-dihydrotestosterone is significantly more potent than 17beta-estradiol(1,3,5(10)-estratrien-3,17beta-diol) to induce vasodilatation. The vasodilating effect of 5beta-dihydrotestosterone on noradrenaline-induced contraction was resistant to blockade by the GABA(A) receptor antagonists, picrotoxin or bicuculline, and the beta-adrenoceptor antagonist, propranolol, a finding that excludes an interaction of the steroid with GABA(A) receptors and beta-adrenoceptors. Interestingly, the contractions evoked by calcium in depolarized tissues were substantially inhibited by 5beta-dihydrotestosterone, implying that this steroid could be an endogenous calcium channel blocker; consistent with this finding, 5beta-dihydrotestosterone was able to relax tissues precontracted with the calcium channel opener, Bay K 8644. Moreover, although the rings precontracted with noradrenaline and potassium were almost equipotently relaxed by 5beta-dihydrotestosterone. Nifedipine was more potent than 5beta-dihydrotestosterone to block the potassium-induced contraction, but the steroid was more effective than nifedipine to prevent noradrenaline-induced contraction. The above results suggest that 5beta-dihydrotestosterone causes relaxation of rat aorta by acting directly on the membrane of smooth muscle cells; this non-genomic action may be explained in terms of a blockade of voltage- and receptor-dependent calcium channels, a mechanism that restricts the availability of extracellular calcium in the contractile machinery.

Variation in mRNA expression of alpha-adrenergic, neurokinin and muscarinic receptors amongst four arteries of the rat

Different mechanisms mediate constriction and dilation in different vascular beds. We have used reverse transcription-polymerase chain reaction to investigate whether specific patterns of receptor gene expression may underlie these variable responses. Total RNA, from the basilar, pulmonary, mesenteric and tail arteries of anaesthetised adult Wistar rats, was reverse transcribed and amplified using primers specific for the molecular subtypes of the alpha 1(A, B, D)- and alpha 2(A, B, C)-adrenergic, neurokinin (NK1-NK3) and muscarinic (m1-m5), receptors. Results showed that the pattern of gene expression was variable with no two arteries having the same receptor profile. Messenger RNA for the alpha 1A, alpha 1B, alpha 2B, NK1, NK3, m3 and m5 receptor subtypes were detected in all vessels studied while the remaining subtypes showed a variable expression amongst the arteries. This is the first description of mRNA for the m5 muscarinic receptor in peripheral tissue. The NK3 receptor was the major neurokinin receptor expressed in all vessels except the pulmonary artery, in which the NK1 receptor was also strongly expressed. We conclude that each artery expressed a specific receptor array which may permit some unique neural and hormonal controls.

Modification of adrenergic reactivity in rat tail artery by dietary lipids and calcium channel antagonists

1. The antiatherosclerotic activity of dihydropyridines (DHP), potent calcium antagonists, was studied with respect to prevention of hypercontractility of perfused rat tail arteries. 2. Used for 1 month, atherogenic diet increased pressor responses to norepinephrine (NE) in Ca(2+)-free physiological salt solution (PSS), and PSS containing Ca(2+). 3. When nifedipine (NIF) or nitrendipine (NIT) was administered simultaneously with an atherogenic diet, the contractile activity of NE in Ca(2+)-free PSS was attenuated. Moreover, vasoconstrictor responses to NE in PSS containing Ca2+ were inhibited after 1-month treatment with NIT and nimodipine (NIM). 4. NIF, NIT and NIM prevented atherosclerosis-induced vascular hyperreactivity to alpha-adrenoceptor agonists in rat tail artery.

Calcium channels in cerebral arteries

Electrophysiological evidence shows the existence of voltage-operated Ca2+ channels of the L- and, in some cases, T- and B-, type in the smooth muscle cells of major cerebral arteries and arterioles. Current intensity through L-type Ca2+ channels is higher in cerebral than in peripheral arteries, which points to a greater dependence on extracellular Ca2+ of contractile responses in cerebral arteries. The increase in cytosolic Ca2+ concentration is the key event leading both to maintenance of basal cerebrovascular tone and to contraction of cerebral arteries in response to depolarization and agonist-receptor interaction. Such an increase results from increased transmembrane influx of Ca2+ through L-type Ca2+ channels, as well as from the release of Ca2+ from intracellular Ca2+ stores. Ca2+ entry modulators (dihydropyridines, phenylalkylamines, benzothiazepines, and diphenylpiperazines) bind to allosterically coupled sites in the Ca2+ channel, thus inhibiting (Ca2+ entry blockers) or stimulating (Ca2+ entry activators) Ca2+ influx and, therefore, contractile responses of the cerebral arteries. In vivo, Ca2+ entry blockers increase pial vascular caliber and cerebral blood flow by their direct action on the cerebroarterial wall. However, such an action also takes place on several peripheral vascular beds, which leads to hypotension. Therefore, the brain cannot be considered a "privileged" organ when the vasodilatatory action of Ca2+ entry blockers is considered. Since increased cytosolic Ca2+ concentration (and, therefore, activation of Ca2+ channels) plays a crucial role in the pathogenesis of ischemic brain damage (e.g., acute stroke and subarachnoid hemorrhage), Ca2+ entry blockers could be useful cytoprotective drugs. However, with the exception of nimodipine in the management of subarachnoid hemorrhage, clinical trials have yielded results that are not so promising as one could expect from those obtained in experimental research.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Contraction of the sheep middle cerebral, pulmonary and coronary arteries initiated by release of intracellular calcium

The aim of the study was to compare contraction initiated by intracellular Ca2+ release in the middle cerebral, coronary and pulmonary arteries of the sheep. With all three arteries from the sheep, incubation in Ca(2+)-free physiological salt solution (PSS) reduced agonist-induced contraction much more than occurred with the rabbit aorta. The intracellular Ca2+ store appeared to be of limited capacity, since contraction was transient in Ca(2+)-free conditions with most agonists. In the middle cerebral artery, contraction in Ca(2+)-free conditions was much reduced if a previous contraction had been obtained (for 5-hydroxytryptamine, 5-HT, from 11 +/- 4 to 1 +/- 0.5% of control contraction in 2.5 mM Ca2+), suggesting that the previous contraction had partly discharged the intracellular Ca2+ store. Contraction was less affected in the pulmonary artery and almost unaffected in the coronary artery (for 5-HT, from 15 +/- 1 to 11 +/- 1%) by a previous contraction in Ca(2+)-free conditions. Rings prepared from small branches of the pulmonary and coronary arteries were affected by Ca2+ deprivation in a similar manner to large diameter pulmonary and coronary artery rings. In Ca(2+)-free PSS, contraction induced by prostaglandin E2 was almost eliminated (3 +/- 1% of control contraction in 2.5 mM Ca2+), contractions induced by 5-HT and noradrenaline were reduced, and contraction induced by the thromboxane mimetic U46619 was least affected (up to 73 +/- 8%). Increasing agonist concentration from EC50 to the maximally effective concentration raised the percentage contraction remaining in the middle cerebral artery (for noradrenaline from 7 +/- 2% to 12 +/- 3%) but not in the pulmonary artery (for noradrenaline from 22 +/- 2% to 24 +/- 6%). The present study has revealed notable differences, in coupling to intracellular Ca2+ release between the three vascular territories studied.

In vitro effect of nifedipine on KCl and 5-hydroxytryptamine-induced contractions of the sheep coronary, cerebral and pulmonary arteries

The aim of the present study was to compare the sensitivity to nifedipine of contraction, obtained with 5-hydroxytryptamine (5-HT) and high K+, of arteries from three vascular territories (coronary, cerebral and pulmonary), selecting arteries of equivalent diameter (0.4-0.8 mm). In the coronary and middle cerebral arteries, contraction produced by KCl was abolished by nifedipine 10(-7) M. However, in the pulmonary artery 39 +/- 2% (n = 5) of the KCl contraction remained in the presence of nifedipine 10(-6) M and a similar sized contraction remained in the presence of Ni2+ 10 mM, La3+ 1 mM or Ca(2+)-free conditions. Nifedipine caused less inhibition of 5-HT than of KCl in all three arteries. In the coronary and middle cerebral arteries 5-HT was inhibited to 55 +/- 9% and to 55 +/- 4% and in the pulmonary artery to 78 +/- 3% (n = 6) by nifedipine 10(-6) M. In conclusion, a comparison of coronary, cerebral and pulmonary arteries, of comparable size and from the same species, has shown that there is diversity in the sensitivity of 5-HT-induced contraction to inhibition by nifedipine. Moreover, the KCl-induced contraction in the sheep pulmonary artery is mediated, in part, by a mechanism independent of Ca2+ influx.

Intracellular Ca2+ release in cerebral arteries

Vasoconstricting agonists elevate the intracellular Ca2+ concentration and induce tension development in vascular smooth muscle cells by inducing both Ca2+ influx from the extracellular space and Ca2+ release from cellular stores. The relative importance of Ca2+ release has been found to vary between different sites in the vasculature. This review examines the role of Ca2+ release in the activation of cerebral arteries produced by several vasoconstricting stimuli. Although the activation of cerebral arteries by agonists such as 5-hydroxytryptamine and noradrenaline has typically been found to have little dependence on Ca2+ release, other vasoconstrictors such as thromboxane A2, which may be released from the endothelium by other agonists, appear to induce a substantial intracellular Ca2+ release in cerebral arteries. The limited efficacy of Ca2+ influx blockers in the treatment of delayed cerebrovascular constriction occurring as a result of subarachnoid haemorrhage suggests that intracellular mechanisms such as Ca2+ release and/or the activation of protein kinase C may be important determinants of vasoconstriction under pathological conditions.

Isolated bovine cerebral arteries from rostral and caudal regions: distinct responses to adrenoceptor agonists

Responsiveness to norepinephrine and related agents was compared in isolated bovine anterior cerebral (ACA). middle cerebral (MCA), intracranial internal carotid (ICA), posterior communicating (PCOM), posterior cerebral (PCA) and basilar arteries (BA). Norepinephrine contracted the strips from ACA, MCA and ICA, but relaxed those from PCOM, PCA and BA. In the presence of propranolol, the amine-induced contractions tended to be potentiated in ACA, MCA and ICA, and the relaxations in PCOM, PCA and BA were reversed to contractions. The maximum contractions induced by norepinephrine in ICA and ACA treated with propranolol were significantly greater than those in PCA and BA, but the EC50 values did not differ among arteries. In ACA and MCA, the contractions induced by phenylephrine were greater than those induced by clonidine. The contractions induced by norepinephrine and phenylephrine were attenuated by prazosin but not influenced by yohimbine in ACA and MCA treated with propranolol. These findings indicate that the responses to norepinephrine evidently differ in bovine cerebral arteries of rostral (ACA, MCA and ICA) and caudal regions (PCOM, PCA and BA), possibly due to different functioning of alpha/beta receptors. The amine-induced contraction predominantly seen in the rostral arteries appears to be associated with activation of the alpha 1 adrenoceptor subtype.

Differential effects of calcium antagonists and Bay K 8644 on contractile responses to exogenous noradrenaline and adrenergic nerve stimulation in the rabbit ear artery

1. The effects of three calcium antagonists (nifedipine, verapamil, diltiazem) and the calcium agonist Bay K 8644 were compared on contractile responses of similar amplitude elicited by noradrenaline (NA) and electrical nerve stimulation (ENS) in the rabbit isolated ear artery. 2. Contractions induced by both NA (3 x 10(-7) M) and ENS (10 Hz, 10s) were almost exclusively mediated by alpha 1-adrenoceptors, since 10(-7) M prazosin abolished (NA) or almost abolished (ENS) the responses, and prazosin was more than three orders of magnitude more potent than rauwolscine on both types of response. 3. ENS-induced contractions were considerably less inhibited by nifedipine, verapamil and diltiazem than were those elicited by NA. Bay K 8644 enhanced responses to NA more than those to ENS. 4. The inhibitory effect of nifedipine and Ca2+ deprivation on NA-induced contractions decreased with increasing NA concentration. Reduction of the NA response by prazosin or phenoxybenzamine increased the nifedipine inhibition. 5. Reduction of the ENS-induced contractions by prazosin or phenoxybenzamine, or by use of a lower stimulation frequency did not increase the inhibitory effect of nifedipine. 6. In conclusion, the differential effects of the calcium antagonists on NA- and ENS-induced contractions were not related to differences in alpha-adrenoceptor subtype (alpha 1/alpha 2), receptor reserve or response amplitude, but may rather reflect temporal and spatial differences in alpha-adrenoceptor activation between the responses.

Effects of Ca2+ entry blockers on CaCl2-, KCl- and noradrenaline-induced contractions of goat cerebral arteries

The effects of three Ca2+ entry blockers, nicardipine, nimodipine and verapamil, on CaCl2-, KCl- and noradrenaline-induced contractions were examined in isolated goat middle cerebral artery. The relationship between the effects of Ca2+ entry blockers and the extracellular Ca2+ dependence of the contractions was also examined. In 'nominally' Ca2(+)-free medium, addition of CaCl2 induced concentration-dependent contractions of previously depolarized arteries. Withdrawal of Ca2+ from the extracellular medium caused strong inhibition of the KCl- and noradrenaline-induced arterial contractions. Addition of EGTA to the Ca2(+)-free medium almost abolished the noradrenaline-response but did not increase the inhibition of the KCl-induced contractions. The Ca2+ entry blockers induced concentration-dependent relaxation of the precontracted arteries (100 mM KC1) with the following order of potency: nimodipine greater than nicardipine greater than verapamil. The CaCl2-, KCl- and noradrenaline-induced contractions were depressed in a concentration-related manner by nicardipine, nimodipine and verapamil. Dihydropyridines showed a greater inhibitory effect than verapamil. These results show that Ca2+ entry blockers are able to inhibit the contractile responses of goat cerebral arteries to KCl and noradrenaline, an effect which may be explained by the strong dependence of both responses on extracellular Ca2+.

Pharmacological characterization of postjunctional alpha-adrenoceptors in cerebral arteries from the sheep

1. The responsiveness to noradrenaline was characterized in cerebral arteries from the sheep, since this species was large enough to permit a comparison of arteries from different parts of the cerebral vasculature. 2. Noradrenaline caused contraction of the basilar artery, middle cerebral artery and small pial arteries by stimulation of alpha 1-adrenoceptors. 3. The maximum contraction to noradrenaline was small in the basilar artery (28% of the 5-hydroxytryptamine (5-HT) maximum) but larger in the middle cerebral artery (78% of the 5-HT maximum) and pial artery (92% of the 5-HT maximum) of the sheep. 4. Cocaine (10 microM) potentiated noradrenaline-induced contractions in the sheep middle cerebral artery but not in the sheep basilar artery. 5. The noradrenaline contraction, relative to the 5-HT contraction, was not affected by removal of the endothelium in either the sheep basilar or middle cerebral artery. 6. The results showed a variation within the sheep cerebral vasculature in the response to noradrenaline which cannot be explained by regional differences in alpha-adrenoceptor subtypes, noradrenaline uptake mechanisms or endothelial function.

Influence of Bay K 8644 on vascular responses mediated by alpha 1- and alpha 2-adrenoceptors

1. The calcium channel activator Bay K 8644 increased the potency of noradrenaline in cat middle cerebral (alpha 2-adrenoceptors) and mesenteric (atypical or mixed alpha 1- and alpha 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries (alpha 1-adrenoceptors). 2. In cat arteries, exposure to 15 mM K+ solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K 8644. 3. Bay K 8644 completely reversed the relaxation produced by nifedipine in K+-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K 8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the alpha-adrenoceptor subtype mediating the response.

Effects of neuropeptide Y, calcitonin gene-related peptide, substance P, and capsaicin on cerebral arteries in man and animals

The smooth-muscle tone of pial, middle, and anterior cerebral arteries from humans, cats, and pigs, respectively, was studied in vitro with respect to the effects of capsaicin and various peptides which are present in local perivascular nerves. Neuropeptide Y (NPY) caused concentration-dependent, potent contractions of the cerebral vessels both in the presence and in the absence of endothelium. In contrast to the response to noradrenaline (NA) and K+, the NPY effect was not altered by changes in the extracellular Ca++ concentration. The relaxant action of the calcium antagonist nifedipine on NPY-evoked contraction of cerebral arteries was not inhibited by a Ca++-deficient medium or by a high-Ca++ medium. Calcitonin gene-related peptide (CGRP), substance P (SP), and capsaicin caused relaxation of precontracted cerebral arteries with an intact endothelium. Calcitonin gene-related peptide was the most potent dilatory agent, and removal of the endothelium did not change the CGRP response. In contrast, the ability of SP to cause relaxation was abolished after removal of the endothelium. Capsaicin, which activates sensory nerves, induced long-lasting relaxation in both the presence and absence of endothelium. In conclusion, in contrast to earlier reported data, the contractile effect of NPY seems to be largely independent of extracellular Ca++, while NA- and K+-induced contractions are dependent on extracellular Ca++. The present results suggest that the relaxant effect of nifedipine on cerebral blood vessels may involve actions other than inhibition of Ca++ influx. The relaxant effect of capsaicin is likely to be induced by release of CGRP rather than SP. The potent effects of these peptides on human pial arteries suggest that neuropeptides may be involved in the control of cerebral blood flow in man.

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.

The influence of magnesium on the release of calcium from intracellular depots in vascular smooth muscle cells

Vasoconstrictor responses to prostaglandin F2 alpha and noradrenaline were investigated in ring segments of feline femoral, coronary and cerebral arteries incubated in calcium-free solutions containing different concentrations of magnesium (1.2, 4.4 and 13.2 mM). Contractions produced by prostaglandin F2 alpha and noradrenaline were depressed when calcium was omitted from the incubation solution. The presence of raised concentrations of magnesium (4.4 or 13.2 mM) in the tissue bath further depressed the prostaglandin F2 alpha and noradrenaline contractions in calcium-free medium. In a separate set of experiments the vessel wall contents of cAMP and cGMP were measured before and after the additions of 4.4 or 13.2 mM magnesium; stable relaxations by magnesium were noted but there was no parallel change in the vessel content of cAMP or cGMP. The results indicate that magnesium may interfere with the release of calcium from intracellular depots, and that neither adenylate cyclase, nor guanylate cyclase are involved in the dilator activity of magnesium in isolated arteries.

Compared effects of calcium entry blockers on calcium-induced tension in rat isolated cerebral and peripheral resistance vessels

The effects of the calcium entry blockers verapamil (V), diltiazem (D), nifedipine (NF) and nicardipine (NC) have been studied on calcium concentration-effect curves elicited in depolarized (K+, 40 mmol/l) and in serotonin-exposed (6 mumol/l) rat middle cerebral arteries (RMCA) in order to compare the relative potencies of the blockers against these two calcium channel activating mechanisms. In control conditions, Ca2+ sensitivity expressed as pD2 and maximal active wall tension (AWT) were not significantly different in depolarized and in 5-HT-exposed vessels: pD2: 3.39 +/- 0.08 vs 3.50 +/- 0.06 and AWT: 0.93 +/- 0.15 mN.mm-1 vs 0.90 +/- 0.16 mN.mm-1 respectively. V, D, NF and NC displaced Ca2+ control curves to the right and depressed the maximum contractile response in the two experimental conditions, which suggests a noncompetitive type of antagonism. All the blockers were more potent inhibitors of Ca2+-induced contractions in depolarized than in serotonin-exposed middle cerebral arteries. The IC50 values (concentration of blockers producing a 50% inhibition of maximal control contractile response) were (nmol/l): V = 20, D = 120, NF = 0.4, NC = 1 and V = 400, D = 10,000, NF = 20, NC = 7 in depolarized and serotonin-exposed arteries respectively. From these IC50 values, the relative order of potency of the CEB's was not the same in the two experimental conditions suggesting that while serotonin and K+ both promote the entry of Ca2+ into vascular smooth muscle cells of RMCA, they either activate a different gating mechanism associated with a single common channel or perhaps distinct channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of some calcium-channel blockers on human peripheral arteries and veins

We investigated the effects of five different calcium-channel blockers (CCBs), verapamil, nifedipine, diltiazem, flunarizine and lidoflazine, on contractions evoked in vitro by noradrenaline (NA) in small human arteries and veins from the epigastric region. Vessels were obtained from patients without obvious vascular diseases undergoing surgery because of inguinal hernias. The human superficial epigastric artery has previously been shown to contain mainly alpha 1-adrenoceptors, whereas in the vein alpha 2-adrenoceptors predominate. In experiments where NA (10(-5) M) was added non-cumulatively, it was found that nifedipine was the most potent relaxant agent in both arteries and veins, but that this drug showed no preference for any type of vessel. In contrast verapamil (10(-6) M) and (10(-5) M) diltiazem, flunarizine and lidoflazine inhibited the NA-induced contractions to a significantly greater extent in the arteries than in the veins. Comparison between diltiazem and nifedipine on contractions induced by cumulative addition to NA showed that both drugs had significantly more depressive effects on arteries than on veins if the vessels were contracted by relatively high concentrations of NA (10(-6) and 10(-5) M). The results thus confirm the clinical finding that CCBs have more pronounced effects on the arterial than on the venous side of the circulation. They do not support the view that CCBs are more effective inhibitors of alpha 2- than alpha 1-adrenoceptor mediated contraction in isolated human blood vessels.

Effects of calcium antagonists on isolated bovine cerebral arteries: inhibition of constriction and calcium-45 uptake induced by potassium or serotonin

The purpose of this study was to determine the mechanisms by which organic calcium channel blockers inhibit cerebral vasoconstriction. Isolated bovine middle cerebral arteries were cut into rings to measure contractility or into strips to measure radioactive calcium (45Ca) influx and efflux. Calcium channel blockers (10(-5) M verapamil or 3.3 X 10(-7) M nifedipine) and calcium-deficient solutions all produced near-maximal inhibition of both potassium- and serotonin-induced constriction. In calcium-deficient solutions containing potassium or serotonin, verapamil and nifedipine each blocked subsequent calcium-induced constriction in a competitive manner. Potassium and serotonin significantly increased 45Ca uptake into cerebral artery strips during 5 minutes of 45Ca loading; for potassium 45Ca uptake increased from 62 to 188 nmol/g, and for serotonin from 65 to 102 nmol/g. Verapamil or nifedipine had no effect on basal 45Ca uptake but significantly blocked the increase in 45Ca uptake induced by potassium or serotonin. Potassium, and to a lesser extent serotonin, each induced a brief increase in the rate of 45Ca efflux into calcium-deficient solutions. Verapamil or nifedipine had no effect on basal or potassium-stimulated 45Ca efflux. The results demonstrate that verapamil and nifedipine block 45Ca uptake through both potential-operated (potassium) and receptor-operated (serotonin) channels in bovine middle cerebral arteries.

Effects of extracellular calcium and of the calcium entry blockers flunarizine and nimodipine on contractile responses in human temporal arteries

Contraction induced by 124 mM potassium followed the depolarization of smooth-muscle cells and activation of potential-operated calcium channels in human temporal arteries. The contraction elicited consisted of two phases, one rapid and one slowly developing stable phase; both were affected by the two calcium entry blockers flunarizine and nimodipine but at significantly different concentrations. In calcium-free medium 124 mM potassium resulted in a weak contraction. Addition of calcium caused a concentration-dependent contraction that was attenuated by the calcium entry blockers at concentrations comparable to those inhibiting the second phase. The results suggested that in human temporal arteries flunarizine and nimodipine act as calcium entry blockers; there was good correlation with the therapeutic plasma concentration for nimodipine but not for flunarizine.

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.

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

The effects of Ca2+ removal, nifedipine, and La3+ on contractions induced by 124 mM K+ and 10 microM noradrenaline (NA) were investigated in small mesenteric arteries from rat. Ring segments of the arteries were suspended between two steel wires in a 2.5 ml muscle bath, and the mechanical activity recorded "isometrically". The tonic components of the contractile responses to both K+ and NA were critically dependent on the presence of Ca2+ in the bath solution. Nifedipine effectively relaxed K+-contracted arteries, whereas those activated by NA were considerably less affected by the drug. Application of NA to arteries depolarized by K+ in the presence of nifedipine induced a sustained tonic contraction, which was only approximately 20% smaller than that elicited by NA in "standard" Krebs solution, implicating pharmacomechanical coupling. Unlike nifedipine, La3+ inhibited K+- and NA-induced contractions to approximately the same extent. Re-application of Ca2+ to "Ca2+-depleted" preparations exposed to K+ and/or NA induced concentration-dependent contractions. The experimental results suggested that the effects of K+ and NA on the membrane permeability to Ca2+ were additive. The Ca2+-induced contractions were more inhibited by nifedipine in K+-depolarized than in NA-exposed arteries. It is concluded that K+ and NA utilize partly different Ca2+ entry pathways to increase the myoplasmic Ca2+ concentration in rat mesenteric arteries. Whereas K+ seems to promote the influx Ca2+ by activation of CA2+ channels sensitive to the membrane potential, the nature of the receptor-operated Ca2+ entry pathway remains to be established.