Effects of nifedipine on potassium-induced contraction and noradrenaline release in cerebral and extracranial arteries from rabbit

Contrast Medium-Induced Vasoconstrictions

Angiographic contrast media (CM) may cause both vasodilatation and vasoconstriction. This study evaluates a contrast medium-induced vasoconstriction that occurs when isolated arteries are exposed directly to a CM. Segments of rabbit coronary arteries were mounted in tissue baths containing buffer solution. During the experiments the buffer solution was exchanged with iohexol iso-osmolar with plasma, which caused a temporary vasoconstriction of the vessel segments. The constriction did not depend on the degree of oxygenation of iohexol. The endothelium was not involved in the vasoconstriction. Prazosin slightly decreased the vasoconstriction and a small part of the constriction might thus depend on liberation of norepinephrine by iohexol. The constriction was totally inhibited by the calcium antagonist nifedipine, while it was augmented by addition of low concentrations of KCl to ihoexol. It is concluded that the otherwise safe CM iohexol causes vasoconstriction in vitro by depolarizing the smooth muscle cells and the nerve terminals in the vessel wall.

Non-L-type voltage-dependent calcium channels control vascular tone of the rat basilar artery

1. Constriction of cerebral arteries is considered to depend on L-type voltage-dependent calcium channels (VDCCs); however, many previous studies have used antagonists with potential non-selective actions. Our aim was to determine the expression and function of VDCCs in the rat basilar artery. 2. The relative expression of VDCC subtypes was assessed using quantitative polymerase chain reaction and immunohistochemistry. Data were correlated with physiological studies of vascular function. Domains I-II of the T channel subtypes expressed in the rat basilar artery were cloned and sequenced. 3. Blockade of L-type channels with nifedipine had no effect on vascular tone. In contrast, in the presence of nifedipine, hyperpolarization of short arterial segments produced relaxation, whereas depolarization of quiescent segments evoked constriction. 4. The mRNA and protein for L- and T-type VDCCs were strongly expressed in the main basilar artery and side branches, with Ca(V)3.1 and Ca(V)1.2 the predominant subtypes. 5. T-Type VDCC blockers (i.e. 1 micromol/L mibefradil, 10 micromol/L pimozide and 100 micromol/L flunarizine) decreased intracellular calcium in smooth muscle cells, relaxed and hyperpolarized arteries, whereas nickel chloride (100 micromol/L) had no effect. In contrast with nifedipine, 10 micromol/L nimodipine produced hyperpolarization and relaxation. 6. When arteries were relaxed with 10 micromol/L U73122 (a phospholipase C inhibitor) in the presence of nifedipine, 40 mmol/L KCl evoked depolarization and constriction, which was significantly reduced by 1 micromol/L mibefradil. 7. Sequencing of domains I-II revealed splice variants of Ca(V)3.1, which may impact on channel activity. 8. We conclude that vascular tone of the rat basilar artery results from calcium influx through nifedipine-insensitive VDCCs with pharmacology consistent with Ca(V)3.1 T-type channels.

Inhibitory effect of cilnidipine on vascular sympathetic neurotransmission and subsequent vasoconstriction in spontaneously hypertensive rats

We reported previously that cilnidipine inhibited increases in blood pressure and plasma norepinephrine (NE) level in response to cold stress in spontaneously hypertensive rats (SHRs). In the present study, we investigated the effect of cilnidipine on sympathetic neurotransmission and subsequent vasoconstriction in SHRs. In pithed SHRs, electrical sympathetic nerve stimulation (ESNS) elevated blood pressure, and this pressor response was abolished by guanethidine. Cilnidipine at 10 micrograms/kg, i.v. and phentolamine at 1 mg/kg, i.v. suppressed the pressor response to ESNS by 28 +/- 6% and 67 +/- 3%, respectively. Neither nifedipine nor nicardipine inhibited it. The pressor response to exogenous NE was not influenced by cilnidipine. alpha, beta-Methylene ATP inhibited the pressor response to ESNS in the presence or absence of phentolamine. Cilnidipine also attenuated the phentolamine-resistant pressor response to ESNS. In SHR mesenteric vasculatures preloaded with [3H]-NE, cilnidipine (10(-7) M) as well as omega-conotoxin significantly inhibited the 3H overflow evoked by periarterial nerve stimulation. In radioligand binding experiments, cilnidipine inhibited [125I]-omega-conotoxin binding to rat synaptosomes, but it did not inhibit [3H]-prazosin binding to rat cortex membranes. These results suggest that cilnidipine may reduce electrically stimulated NE release from the sympathetic nerve endings of SHR vasculatures probably through its N-type Ca channel blocking action and that cilnidipine may also inhibit the vasoconstriction induced by ATP released concomitantly during nerve stimulation.

A new synthetic flavonoid protects endothelium-derived relaxing factor-induced relaxation in rabbit arteries in vitro: evidence for superoxide scavenging

A new synthetic flavone derivative, 6,7-dimethoxy-8-methyl-3',4',5-trihydroxyflavone, was studied for its capacity to protect the acetylcholine-induced relaxation of rabbit ear and cerebral arteries from inhibition by superoxide anion. This property was evaluated via two types of in vitro experiments, using rabbit ear or basilar arteries mounted in organ baths equipped for isometric tension measurement. When a high level of superoxide anion was generated by adding 3 x 10(-4) M pyrogallol to the bath, the relaxation to acetylcholine was substantially inhibited. This inhibition was significantly reversed by both superoxide dismutase (25 and/or 50 U/mL) and the flavonoid (3 x 10(-6) M and/or 10(-5) M) in both types of arteries. In the presence of the basal level of superoxide anion, the responses to acetylcholine were significantly potentiated by the flavonoid (10(-5) M) in the ear but not the basilar artery. Thus this flavonoid protects endothelium-dependent relaxation from high levels of superoxide anion possibly by scavenging superoxide anion and may have a certain therapeutic value as an agent capable of promoting natural vasodilatation.

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.

Inhibitory effects of calcium antagonists on alpha-adrenoceptor-mediated contraction in the human internal mammary artery

1. The internal mammary artery has become a preferred coronary bypass graft. Sympathomimetic amines are spasmogens for vasospasm and calcium antagonists are frequently administered drugs perioperatively. The effect of calcium antagonists on alpha-adrenoceptor-mediated contraction depends on the subtype of alpha-adrenoceptor and the type of origin of vascular smooth muscle. This study was designed to investigate the effect of calcium antagonists on alpha-adrenoceptor-mediated contraction in the IMA. 2. Human IMA segments taken from 22 patients undergoing IMA--coronary artery bypass grafting were mounted in an organ bath under the physiological pressure determined from their own length-tension curves. 3. Three ring segments were allocated into three groups. One served as a control and the others were treated with clinically related concentrations of nifedipine (20 or 200 nM) for 25 min before concentration-contraction curves to alpha 1-adrenoceptor agonist methoxamine (MO) or full alpha-adrenoceptor agonist noradrenaline (NA) were established. 4. In separate experiments, the concentration-relaxation curves to nifedipine were established in the IMA rings precontracted with MO (30 microM) or NA (10 microM). Glyceryl trinitrate (GTN, 3 microM) was added to further relax the vessels. 5. Pretreatment with nifedipine (200 nM) only slightly inhibited the MO- (1.74 +/- 0.32 vs 2.88 +/- 0.56 g) or NA- (2.43 +/- 0.66 vs 3.60 +/- 0.82 g) induced contraction without statistical significance (P > 0.05). 6. On the other hand, nifedipine only caused 34.49% relaxation in the MO-precontracted and 24.39% relaxation in the NA-precontracted IMAs. In contrast, GTN caused 76.16% (against MO, P < 0.05) or 92.22% (against NA, P < 0.0001) relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Early changes in rabbit cerebral artery reactivity after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage frequently leads to long-term cerebral artery narrowing called vasospasm. Very early changes in cerebral arteries have not been studied extensively and may be critical for the later pathological developments. We therefore determined what changes in the reactivity of cerebral arteries could be observed after 10 minutes' or 24 hours' contact with subarachnoid blood.

METHODS

Ten minutes or 24 hours after the injection of blood or physiological solution (sham hemorrhage) into the cisterna magna of anesthetized rabbits or no injection (control rabbits), segments of the middle cerebral, basilar, and vertebral arteries were removed for conventional in vitro tension measurements. Concentration-response curves to four endogenous constrictors likely to be released after hemorrhage were obtained, and the maximum relaxation to acetylcholine was determined.

RESULTS

There were no significant differences between the sham hemorrhage and control groups. Compared with control rabbits, treated animals showed increased reactivity to uridine triphosphate in the basilar and vertebral arteries at 10 minutes but not at 24 hours, whereas reactivity was increased in the middle cerebral artery only at 24 hours. Reactivity to serotonin was greatly increased in all arteries at both latencies (up to 2.7 times). Reactivity to noradrenaline was unchanged in the basilar and vertebral arteries at 10 minutes; reactivity in both the basilar and middle cerebral arteries was increased at 24 hours, which is compatible with denervation supersensitivity. There were only minor changes in the reactivity to histamine, and only at 10 minutes. Relaxation to acetylcholine was increased for the middle cerebral artery at 10 minutes but otherwise was not significantly changed.

CONCLUSIONS

Reactivity to uridine triphosphate, serotonin, and noradrenaline greatly increases by 10 minutes to 24 hours after subarachnoid hemorrhage, and this increase is not owing to the mechanical effects of intracranial hypertension, nor is it related to impaired endothelium-dependent relaxation. It is suggested that these and other spasmogens cause excessive muscular calcium loading with a very rapid onset after subarachnoid hemorrhage.

Captopril attenuates reflex adrenergic response in essential hypertension

An attenuation of adrenergic activity during the inhibition of endogenous angiotensin II formation was evaluated by determining plasma norepinephrine concentration after a single oral administration of captopril compared to that after nifedipine in essential hypertension. Captopril produced a fall in mean arterial pressure (-24 +/- 2 mmHg, p less than 0.01) which magnitude was the same as that gained by nifedipine (-22 +/- 3 mmHg, p less than 0.01). Reflex tachycardia due to hypotension was produced (+13 +/- 1 beats/min, p less than 0.01) after nifedipine but not after captopril (-1 +/- 2 beats/min, p greater than 0.05). Although the enhancement of plasma renin activity induced by captopril (+1.54 +/- 0.56 ng/ml/hr, p less than 0.05) was similar (p greater than 0.05) to that by nifedipine (+1.44 +/- 0.47 ng/ml/hr, p less than 0.05), plasma norepinephrine concentration increased less (p less than 0.01) after captopril (+100 +/- 23 ng/ml, p less than 0.05) than after nifedipine (+283 +/- 51 ng/ml, p less than 0.05). Thus, the diminished adrenergic activity is a likely candidate for the abolished reflex tachycardia after the inhibition of angiotensin I converting enzyme activity by captopril in essential hypertension.

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.

Involvement of dihydropyridine-sensitive Ca2+ channels in adenosine-evoked inhibition of acetylcholine release from guinea pig ileal preparation

The effects of adenosine and nifedipine on endogenous acetylcholine (ACh) release evoked by electrical stimulation from guinea pig ileal longitudinal muscle preparations exposed to physostigmine were evaluated using an HPLC with electrochemical detection (ECD) system. Resting ACh release, which was sensitive to tetrodotoxin (0.3 microM), was enhanced by Bay K 8644 (0.5 microM; a Ca2+ antagonist) or 4-aminopyridine (30 microM; a K+ channel blocker) but not by theophylline (100 microM; a P1 purinoceptor antagonist) or atropine (0.3 microM). The enhancement of the resting ACh release by Bay K 8644 was virtually unaffected by atropine. Electrically evoked ACh release was enhanced by around two- to fourfold in the presence of theophylline, atropine, Bay K 8644, 4-aminopyridine, or atropine. On the other hand, the evoked ACh release was reduced by adenosine (10-30 microM), nifedipine (0.1-0.3 microM; a dihydropyridine Ca2+ channel antagonist), or bethanechol (1-3 microM) in a concentration-related fashion. The reduction induced by adenosine or nifedipine was almost abolished by either theophylline or Bay K 8644, whereas that induced by bethanechol was virtually unaffected by these drugs. The inhibition by adenosine of ACh release was not influenced in the presence of 4-aminopyridine or atropine. However, this inhibition by adenosine was considerably enhanced by halving the Ca2+ concentration in the Krebs solution and was diminished by doubling the Ca2+ concentration. These findings suggest that adenosine produces a cholinergic neuromodulation presumably via modifying dihydropyridine-sensitive Ca2+ channel activities in the cholinergic neurons, and thus L-type Ca2+ channels may exist on the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the Ca2+ agonists, Bay K 8644 and CGP 28392, on isolated cat cerebral and peripheral arteries

Ca2+-agonists, Bay K 8644 and CGP 28392, produced concentration-dependent contractions in cat middle cerebral arteries. Only Bay K 8644 induced contractile responses in femoral arteries when they were partly depolarized with 15 mM K+. Nifedipine (0.3 microM) caused a reduction of the contractions elicited by low concentrations of both Ca2+ agonists. In femoral arteries, Bay K 8644 (0.1 microM) increased the contractions elicited by K+ concentrations from 25 to 120 mM, whereas CGP 28392 (0.1 microM) only potentiated the response caused by 25 mM K+. The increase of K+-responses induced by Bay K 8644 was blocked by nifedipine (0.1 microM), whereas that produced by CGP 28392 was not only antagonized but the remaining contractions were less than those obtained in the control situation. In both kinds of arteries preincubated with [3H]noradrenaline, nifedipine (50 microM), CGP 28392 (1 microM and 10 microM) and Bay K 8644 (1 microM) neither modified the spontaneous tritium release nor the stimulated tritium secretion elicited by K+ (25, 50 and 75 mM) or electrical stimulation. These data suggest that CGP 28392 and, mainly, Bay K 8644 facilitate the Ca2+ entry into smooth muscle cells, through voltage-dependent Ca2+ channels (that appear to be preactivated in cerebral arteries), whereas those present in the perivascular adrenergic nerve terminals are unaffected. This fact indicates that the channels existent in both vascular structures seem to have different properties.

Cerebrovascular and cerebral effects of nimodipine--an update

A survey is given on the vascular and neuronal effects of calcium antagonists under physiological and pathological conditions. Special emphasis is put on vasospasm caused by subarachnoid haemorrhage and on postischaemic cerebral hypoperfusion following different forms of cerebral ischaemia, and on the attempts to influence these phenomena pharmacologically. Regarding its neuronal effects it seems likely that nimodipine potently blocks calcium entry during pathological conditions like cerebral ischaemia and spreading cortical depression. Positive effects also have been seen during epileptic seizures and withdrawal syndromes, whereas calcium entry under normal physiological conditions does not appear to be affected. Possible clinical consequences are discussed.

Neuronal versus endothelial origin of vasoactive acetylcholine in pial vessels

Functional pial vessels denuded in situ of the endothelial cell layer exhibit a markedly decreased choline uptake capacity (-53%) but integrally preserved choline acetyltransferase (ChAT) activity and acetylcholine (ACh) release mechanisms. These studies demonstrate that endothelial cells possess a specific choline uptake system. However, the unimpaired ChAT activity in denuded pial vessels implies that the endothelial pool of choline is not significantly metabolized into ACh. In spite of possible differences in the mechanisms that govern release processes in endothelial and neuronal elements, taken together the findings of the present study suggest that the ACh released following depolarization of pial blood vessels originates predominantly from cholinergic perivascular nerve terminals.

Beagle pup model of perinatal asphyxia: nimodipine studies

Calcium antagonists may be of significant benefit in the pharmacotherapy of cerebral ischemia, possibly by improving postischemic cerebral blood flow (CBF). This study evaluated the effects of the calcium antagonist nimodipine on CBF in a newborn beagle pup model of perinatal asphyxia lasting 5 minutes. Immediately after the asphyxial episode, nimodipine (2 micrograms/kg/min) or saline was infused for 10 minutes, following which [14C]iodoantipyrine determinations of CBF were performed. In noninsulted pups, nimodipine caused both significant decreases in CBF to cortical and deep gray structures as well as a decrease in mean arterial blood pressure (MABP) (p less than 0.05). In insulted pups, nimodipine similarly decreased MABP (p less than 0.001) and CBF to cortical and deep gray matter regions. Nimodipine appeared to have no effect on arterial blood gases and EEG tracings in either insulted or noninsulted pups. Although nimodipine may be shown to improve neurologic outcome in asphyxiated newborn infants, the limits of this study do not show the mechanism to be that of improving CBF.

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.

Evidence that intraluminal pressure affects high potassium- and serotonin-induced contractions differently in the bovine middle cerebral artery: an in vitro study

The effects of changing intraluminal pressure on contractions induced by 70 mM potassium (K+) and 10(-7), 10(-6), and 10(-5) M serotonin (5-HT) were studied in vitro in bovine middle cerebral arteries. Changes in vessel outside diameter in whole-mounted cylindrical sections of artery were detected with a photoelectric infrared device. High K+-or 5-HT (10(-5)M)-induced contractions peaked at 25 mm Hg and were significantly correlated with increasing intraluminal pressure between 25 and 175 mm Hg. Contractions induced with lower concentrations of 5-HT (10(-6), 10(-7) M), norepinephrine, and histamine peaked at 75 mm Hg but were not significantly correlated with rising pressure. Phentolamine (2 X 10(-6) M) added to the extraluminal bath had negligible influence on pressure's ability to affect K+- and 5-HT-induced contractions differently. Reducing bath temperature to 27 degrees C reduced the K+ response at each pressure, but similar temperature changes had little affect on the 5-HT-induced contractions. The K+ response became less sensitive to increasing pressure at low temperatures. Nifedipine (10(-7) M) almost totally eliminated K+-induced contractions, while significantly reducing the responses to all concentrations of 5-HT. The 5-HT responses appeared more sensitive to increasing intraluminal pressure in the presence of nifedipine. Maximum Ca++-induced contractions in the presence of 10(-5) M 5-HT and high K+ occurred at 25 mm Hg, while Ca++-induced contractions and Ca++-induced contractions in the presence of 10(-7) 5-HT or K+ plus 5-HT were maximum at 75 mm Hg.(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.

Effect of nimodipine on intracellular brain pH, cortical blood flow, and EEG in experimental focal cerebral ischemia

Intracellular brain pH, cortical blood flow, and electroencephalograms (EEG's) were recorded in severely and moderately ischemic regions in 10 control and 10 nimodipine-treated rabbits prior to and following major branch occlusion of the middle cerebral artery (MCA). Preocclusion cortical blood flow was 51 ml/100 gm/min and intracellular brain pH was 7.01 in both the control and the treated animals. After MCA occlusion, the severely ischemic regions in the control group showed initial and 4-hour postocclusion flows of 12.7 and 5.2 ml/100 gm/min with a brain pH of 6.64 and 6.08, respectively. In animals given nimodipine after MCA occlusion, blood flow increased from 10.5 to 18.8 ml/100 gm/min, with an associated elevation in intracellular brain pH from 6.57 to 6.91. Comparable findings were observed in areas of moderate ischemia. Improvements in cortical blood flow, intracellular brain pH, and EEG attenuations produced by nimodipine were all statistically significant. Inspection of the cortex revealed reversal of cortical pallor and small-vessel spasm following treatment with nimodipine. It is hypothesized that nimodipine exerts its effects through reversal of ischemia-induced secondary vasoconstriction, and that this drug may be an important adjunctive treatment for patients with focal cerebral ischemia.

Nimodipine has an inhibitory action on neurotransmitter release from human cerebral arteries

The effect of the dihydropyridine nimodipine was studied on the resting and K+-evoked release of [3H]acetylcholine (ACh) and [3H]5-hydroxytryptamine (5HT) from postmortem human cerebral arteries. Nimodipine, at a concentration of 30 microM, significantly reduced the K+-evoked release of [3H]ACh from anterior and middle cerebral arteries by 36 and 70%, respectively, and the K+-evoked release of [3H]5HT from basilar and middle cerebral arteries by 55 and 66%, respectively. The mode of action of nimodipine is interpreted in terms of a specific effect on the depolarisation-induced calcium current occurring in neuronal elements present in these preparations but absent from brain.

Autoradiographic localization of calcium channel antagonist receptors in rat brain with [3H]nitrendipine

In vitro autoradiographic techniques have been used to localize [3H]nitrendipine binding sites in the rat brain. The superficial cerebral cortex, the ventral, lateral and posterior nuclei of the thalamus, the molecular layer of the dentate gyrus, the substantia nigra and the external plexiform layer of the olfactory bulb, all contain high densities of silver grains. The level of binding sites are greatly reduced in areas low in synaptic connections. The corpus callosum, the fimbria, the alveus hippocampi and the dorsal commissure of the fornix all lack specific silver grains, as does the lateral olfactory tract of the olfactory bulb. Specific silver grains are not found in the habenula or the hypothalamus. Grains are not associated with blood vessel profiles. The discrete localizations of [3H]nitrendipine binding sites suggest a specific synaptic role.

Effect of tetraethylammonium and verapamil on noradrenaline release induced by field electrical stimulation and potassium from cat cerebral and femoral arteries

Tritium release evoked by field electrical stimulation (FES) or high potassium (K+) from cat cerebral and femoral arteries prelabelled with 3H-noradrenaline was investigated. The release induced by FES and K+ was reduced by Ca2+ suppression and tetrodotoxin (TTX) but not by verapamil in both vessels. Tetraethylammonium (TEA) increased tritium release evoked by FES and K+, when TTX plus TEA were added together radioactivity secretion induced by FES was practically abolished. These results indicate that FES or K+ induce exocytotic noradrenaline release mainly by propagated action potentials and by similar mechanisms in cerebral and femoral arteries.

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.

Differential effects of D600, nifedipine and dantrolene sodium on excitation-secretion coupling and presynaptic beta-adrenoceptor responses in rat atria

The stimulation-evoked release of tritium was measured from rat atria labelled with [3H]-noradrenaline. The calcium dependence of evoked release and the facilitation of this release via activation of presynaptic beta-adrenoceptors were examined using D600 (methoxyverapamil), nifedipine and dantrolene sodium. Both D600 and nifedipine at dose levels of 20 and 100 microM inhibited evoked release. Dantrolene (20, 100 microM) reduced release by 25%, the effect being maximal at 20 microM. In the presence of 20 nM isoprenaline, a facilitation of evoked release occurred, which was blocked by 0.1 microM (-)-propranolol. The facilitatory action of isoprenaline was abolished by omission of calcium from the buffer, or by D600 or nifedipine, (100 microM). In contrast, the response to isoprenaline was not modified by dantrolene (20, 100 microM). It is concluded that the evoked release of noradrenaline (NA) utilizes Ca from both intra- and extracellular sources and that isoprenaline increases NA secretion by promoting the depolarization-induced influx of Ca.

The effects of dihydropyridines on neurotransmitter release from cultured neuronal cells

Depolarizing stimuli increase the release of transmitter substances from cultured PC12 pheochromocytoma cells and reaggregate cultures of mouse mesencephalic dopamine neurones. We measured the stimulated release of (3H) norepinephrine and (3H) dopamine from these systems respectively. In the cultured mouse dopaminergic neurones, several organic calcium channel blockers including nitrendipine, D-600, verapamil and diltiazem were unable to inhibit potassium-evoked transmitter release. However, release was blocked by 3 mM cobalt. The novel dihydropyridine calcium channel agonist BAY K8644 also had no effect on basal or evoked dopamine release. In contrast, BAY K8644 greatly stimulated the potassium-evoked release of (3H) norepinephrine from PC12 cells. The BAY K8644 enhanced release could be blocked by the dihydropyridine antagonist nitrendipine. These results indicate that while stimulus-secretion coupling in the PC12 cell line involves dihydropyridine sensitive calcium channels, this is not the case in primary cultured neurones.

On the postjunctional alpha-adrenoreceptors in rat cerebral and mesenteric arteries

The contractile response to exogenously applied noradrenaline (NA) was examined in vitro in tubal segments (0.2-0.1 mm in diameter) of rat middle cerebral (MCA), basilar (BA) and mesenteric (MA) arteries. In the MCA, the maximum contractile response to NA (10(-4)M) was considerably smaller than that induced by K+ (124 mM) or 5-hydroxytryptamine (10(-5)M), whereas the inverse relationship was found in the MA. NA usually failed to elicit contraction in the BA even in the presence of propranolol and cocaine. In the MCA, propranolol (3 X 10(-7)M) enhanced the maximum contractile response to NA by approximately 100% without affecting the potency of the agonist. In the MA, propranolol had no effect on the concentration-response relationship for NA. Cocaine (10(-5)M) or 6-hydroxydopamine pretreatment increased the NA sensitivity of the MA by a factor of three, whereas these procedures failed to influence the NA sensitivity of the MCA. A marked stereoselectivity was found in the MCA, as (-)-NA was more than 100 times more potent than (+)-NA as a contractile agent. The order of potency of a series of alpha-adrenoreceptor agonists was (-)-adrenaline greater than oxymetazoline greater than (+/-)-NA approximately (-)-phenylephrine greater than methoxamine in the MCA and (+/-)-NA greater than (-)-phenylephrine in the MA. Clonidine failed to elicit contraction in concentrations lower than 3 X 10(-4)M in both types of artery. Prazosin was between three and four orders of magnitude more potent than rauwolscine in inhibiting NA-induced contractions in the MCA and MA. The pA2 values for, respectively, prazosin and rauwolscine were 9.3 and 5.4 in the MCA and 9.7 and 6.8 in the MA. The slope of the Schild plot deviated significantly from unity only for rauwolscine in the MA (0.64). It is concluded that the contractile response to exogenous NA in the MCA and MA is mediated mainly by stimulation of alpha 1-adrenoreceptors, although a small contribution of postjunctional alpha 2-adrenoreceptors in the MA cannot be excluded. In contrast to the MCA, the BA appears to lack contraction-mediating alpha-adrenoreceptors, indicating regional differences in the alpha-adrenoreceptor distribution in the rat cerebrovascular bed.

Interference of calcium entry blockade in vivo with pressor responses to alpha-adrenergic stimulation: effects of two unrelated blockers on responses to both exogenous and endogenously released norepinephrine

To the extent that calcium availability is the final common mediator of vasoconstrictor responses, calcium entry blockade might interfere with physiologic responses to adrenergic stimulation. To test this hypothesis, we studied the effects of calcium entry blockade on pressor responses to norepinephrine in pithed, normal Sprague-Dawley rats in two different ways: (1) by evaluating the effects on pressor responsiveness to exogenous norepinephrine during differential blockade of alpha 1-(prazosin, 0.3 mg/kg) and of alpha 2-receptors (yohimbine, 0.3 mg/kg) and (2) by comparing the effects of calcium entry blockade with those of prazosin and those of rauwolscine (a specific alpha 2-antagonist) on pressor responses to infusions of both endogenously released norepinephrine (electrical stimulation of the pithing rod) and exogenous norepinephrine. In the presence of alpha 1-blockade, both nitrendipine (0.01 mg/kg) and verapamil (0.6 mg/kg) shifted the norepinephrine pressor dose-response curve to the right but were ineffective in alpha 2-blocked animals. Furthermore, nitrendipine (range 0.01 to 0.3 mg/kg) proved to be more effective (p less than .001) against exogenous norepinephrine than against electrical stimulation of the spinal cord, a behavior opposite that of selective alpha 1-blockade (prazosin) and directionally comparable to that of selective alpha 2-antagonism (rauwolscine). These data indicate that calcium entry blockade in vivo preferentially antagonizes the alpha 2-pressor component of exogenous norepinephrine. In addition, both calcium entry blockers were consistently more active (p less than .01) than rauwolscine (0.01 to 1 mg/kg) in antagonizing the pressor response to neural stimulation, suggesting that mechanisms different from "classical" alpha 2-antagonism may also contribute to the overall effect of calcium entry blockade on the adrenergic control of peripheral vascular tone.

Differential effects of nifedipine, verapamil, and diltiazem on noradrenaline-induced contractions, adrenergic transmitter release, and alpha-adrenoceptor binding in the female rabbit urethra

In order to study differences in action between "Ca2+-entry blockers" on smooth muscle and peripheral nerves, the effects of nifedipine, verapamil, and diltiazem on noradrenaline (NA)-induced contractions and electrically evoked release of 3H-NA were investigated in the female rabbit urethra. In addition, possible influences of Ca2+-entry blockers on alpha-adrenoceptors were studied with radioligand binding technique. Exposure to Ca2+-free medium completely abolished the contractile response to 1 microM NA in the rabbit urethra, indicating that the contraction was entirely dependent on influx of extracellular Ca2+. The Ca2+-entry blockers inhibited the NA-induced contractions in the following order of potency: nifedipine greater than verapamil approximately equal to diltiazem. In contrast to nifedipine and diltiazem, which produced a maximum inhibition of between 50 and 60%, verapamil was able to abolish the contractile responses to NA. The electrically evoked efflux of 3H-NA was decreased by diltiazem and increased by verapamil, whereas nifedipine failed to alter the 3H-NA efflux. Only verapamil was effective in inhibiting specific 3H-DHE binding to a crude membrane preparation of the rabbit bladder base and urethra, and the inhibition appeared to be of the competitive type. It is suggested that the effects of verapamil on electrically evoked efflux of 3H-NA and on NA-induced contractions can be partly explained by blockade of pre- and post-junctional alpha-adrenoceptors. The failure of nifedipine and diltiazem to abolish the NA-induced contraction might indicate the existence of different Ca2+-entry pathways in urethral smooth muscle.

Calcium antagonists, cerebral ischemia and vasospasm

The past fifteen years has seen the classification of diverse substances into a group known as calcium antagonists (CAs). They have a common ability to reduce the transmembrane transport of extracellular calcium ions (CAe2+). This flow of calcium into vascular smooth muscle is ultimately associated with the development of tension and vasoconstriction. Some CAs appear to have a predilection for cerebral as opposed to systemic arteries and so may function as specific cerebral arterial vasodilators. It has been proposed that they might be useful in certain types of cerebral ischemia such as that due to arterial occlusion or prolonged vasoconstriction. Animal experiments and initial clinical trials give grounds for cautious optimism that CAs may become as useful in neurology as they have recently become in cardiology.

Effects of Ca antagonists on the norepinephrine release and contractile responses of isolated canine saphenous veins to transmural nerve stimulation

Effects of verapamil, diltiazem and nicardipine on tritium overflow and contraction evoked by transmural nerve stimulation (TNS) were evaluated using canine saphenous vein strips preincubated with [3H]norepinephrine. External Ca2+ was required for both tritium overflow and contraction evoked by TNS. All the Ca antagonists tested significantly increased the spontaneous overflow of tritium in a concentration-dependent manner with no changes in basal tension. Verapamil in concentrations lower than 10(-5) M significantly enhanced the TNS-evoked tritium overflow, but reduced it at 3 X 10(-5) M, while this drug at 3 X 10(-6)-3 X 10(-5) M concentration-dependently inhibited the TNS-evoked contraction. Verapamil, 3 X 10(-5) M, inhibited the TNS-evoked contraction more strongly than the evoked tritium overflow. On the other hand, diltiazem and nicardipine in concentrations higher than 10(-5) M significantly inhibited both tritium overflow and contraction evoked by TNS. There was no significant difference between inhibitions of the TNS-evoked tritium overflow and contraction by either diltiazem or nicardipine. Neither increase in the spontaneous tritium overflow nor inhibitions of the TNS-evoked tritium overflow and contraction by nicardipine appeared to be related to its phosphodiesterase inhibiting activity. These results suggest that diltiazem and nicardipine may inhibit the TNS-evoked contraction mainly by inhibiting Ca2+-dependent transmitter release from the adrenergic nerve endings, whereas verapamil may inhibit it by restricting the availability of Ca2+ at the postsynaptic sites and in the highest concentration used, by additional inhibition of transmitter release.

Effects of verapamil, diltiazem and ryosidine on the release of dopamine and acetylcholine in rabbit caudate nucleus slices

Slices of the rabbit caudate nucleus were preincubated with 3H-dopamine or 3H-choline and then superfused with label-free medium. Release of 3H-dopamine and 3H-acetylcholine was elicited by either electrical stimulation at 8 (in one series 2) Hz, or an increase in the K+ concentration by 50 mmol/l, or addition of L-glutamate 1 mmol/l. Verapamil 1 mumol/l, diltiazem 1 and 10 mumol/l, and ryosidine 1 mumol/l failed to the reduce the electrically-, K+- and glutamate-evoked overflow of tritium. Verapamil 1 mumol/l and diltiazem 10 mumol/l also failed to reduce the electrically-evoked overflow (2 Hz) when dopamine receptors, neuronal dopamine uptake, and neuronal choline uptake were blocked by domperidone, nomifensine and hemicholinium, respectively. Inhibition of the evoked overflow of tritium was only obtained when concentrations were increased to verapamil 10 mumol/l, diltiazem 100 mumol/l and ryosidine 10 mumol/l. The inhibition was generally small. It was more evident for slices preincubated with 3H-choline than for those preincubated with 3H-dopamine, because in the latter verapamil, diltiazem and (much less) ryosidine accelerated the basal efflux of tritium. The inhibition of the K+-evoked overflow of tritium was probably due to blockade of Ca2+ channels because this overflow was Ca2+-dependent but tetrodotoxin-resistant. In contrast, the inhibition of the electrically- and glutamate-evoked overflow possibly involved blockade of Na+ channels as well. The results indicate that three calcium antagonists from different chemical classes are very weak inhibitors of Ca2+ entry into, and hence transmitter release from, the terminal axons of central dopaminergic and cholinergic neurones. The function of the high affinity calcium antagonist binding sites that have been identified in brain remains unknown.

Effects of Ca-antagonists on neuromuscular transmission in the rabbit ear artery

1. The effects of three Ca-antagonists: diltiazem, nicardipine and flunarizine have been studied on excitatory junction potentials (e.j.p.s), force development and efflux of transmitter during stimulation of perivascular nerves in the rabbit ear artery. 2. Stimulation of these perivascular nerves produces excitatory junction potentials and repetitive stimulation causes facilitation. Increasing the frequency or the number of stimuli initiates an action potential and a large contraction. Both phenomena are completely suppressed by 3 X 10(-7) M TTX. 3. Ca-antagonists at 10(-5) M do not affect the resting membrane potential, but flunarizine and nicardipine at concentrations exceeding 10(-5) M reduce the amplitude of e.j.p.s and of the action potentials and also the concomitant contraction induced by nerve stimulation. Diltiazem at concentrations below 3 X 10(-5) M has no effect on e.j.p.s and action potentials while at 10(-4) M, it largely suppresses e.j.p.s, spikes and contraction. This inhibitory effect of the Ca-antagonists increases with prolonged exposure. 4. All these Ca-antagonists induce an increased release of 3H-DOPEG from the nerve terminals, but in our experiments in vitro they do not reduce the efflux of 3H-noradrenaline induced by nerve stimulation. 5. The results indicate that Ca-antagonists might affect the excitation-contraction coupling in vivo by inhibiting the Ca influx activated by endogenous noradrenaline. They do not exert an acute effect on the noradrenaline release induced by stimulation. The increased DOPEG release by Ca-antagonists remains unexplained.