Vasoconstrictor effect of the Ca2+ agonist Bay K 8644 on human cerebral arteries

The effects of Bay K 8644 on the reactivity and 45Ca2+ uptake in segments from human cerebral arteries were studied. Bay K 8644 induced concentration-dependent contractions up to 10(-6) M; 10(-5) M produced a reduction of the maximal response. The Ca2+ agonist elicited these contractions by itself, and no previous depolarization was needed. The response to Bay K 8644 was antagonized competitively by nifedipine (5 x 10(-8) and 10(-7) M, pA2 value of 8.17) and non-competitively by verapamil (10(-6), 5 x 10(-6) and 10(-5) M). The contraction induced by 10(-7) M Bay K 8644 was inhibited by a Ca2+-free medium containing 1 mM EGTA. The subsequent cumulative Ca2+ addition, caused concentration-dependent contractions up to 2.5 mM Ca2+, which were reduced by nifedipine (10(-8) and 10(-7) M) or verapamil (5 x 10(-6) and 10(-5) M). When the EGTA concentration in the Ca2+-free solution was reduced to 0.1 mM, contractions induced by Ca2+ up to 5 mM, including 0 Ca2+, were increased with respect to those obtained in the presence of 1 mM EGTA. Basal 45Ca2+ uptake was not modified with Bay K 8644 (10(-6) M) or nifedipine (10(-6) M). K+ (25 and 50 mM) produced an increase on 45Ca2+ uptake, which was potentiated by Bay K 8644 (10(-6) M) and antagonized by nifedipine (10(-6) M); this latter agent reduced the potentiation elicited by the Ca2+ agonist.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasoconstrictor effect of the Ca agonist Bay K 8644 on human cerebral arteries

The effects of Bay K 8644 on the reactivity and ⁴⁵Ca²⁺ uptake in segments from human cerebral arteries were studied. Bay K 8644 induced concentration-dependent contractions up to 10⁻⁶ M; 10⁻⁵ M produced a reduction of the maximal response. The Ca²⁺ agonist elicited these contractions by itself, and no previous depolarization was needed. The response to Bay K 8644 was antagonized competitively by nifedipine (5 × 10⁻⁸ and 10⁻⁷ M, πA₂ value of 8.17) and non-competitively by verapamil (10⁻⁶, 5 × 10⁻⁶ and 10⁻⁵ M). The contraction induced by 10⁻⁷ M Bay K 8644 was inhibited by a Ca²⁺-free medium containing 1 mM EGTA. The subsequent cumulative Ca²⁺ addition, caused concentration-dependent contractions up to 2.5 mM Ca²⁺, which were reduced by nifedipine (10⁻⁸ and 10⁻⁷ M) or verapamil (5 × 10⁻⁶ and 10⁻⁵ M). When the EGTA concentration in the Ca²⁺-free solution was reduced to 0.1 mM, contractions induced by Ca²⁺ up to 5 mM, including 0 Ca²⁺, were increased with respect to those obtained in the presence of 1 mM EGTA. Basal ⁴⁵Ca²⁺ uptake was not modified with Bay K 8644 (10⁻⁶ M) or nifedipine (10⁻⁶ M). K⁺ (25 and 50 mM) produced an increase on ⁴⁵Ca²⁺ uptake, which was potentiated by Bay K 8644 (10⁻⁶ M) and antagonized by nifedipine (10⁻⁶ M); this latter agent reduced the potentiation elicited by the Ca²⁺ agonist. These results suggest that contractions caused by Bay K 8644 in human cerebral arteries are produced by the opening of the voltage-dependent Ca²⁺ channels present in vascular smooth muscle cells, which appear to be preactivated in a basal situation, facilitating Ca²⁺ influx. Nevertheless, Bay K 8644 needed a previous cell depolarization to produce an increase in ⁴⁵Ca²⁺ uptake.     

Effects of changes in chemoreceptor activity on extracellular K+ and Ca2+ activities in the cat carotid body

In anaesthetized, paralysed and artificially ventilated cats triple-barrelled ion-selective microelectrodes (ISMs) were inserted into the right carotid body in order to measure extracellular activities of K+ ([K+]o) and Ca2+ ([Ca2+]o) simultaneously. In 3 experiments a method involving iron deposition located the tips of the ISMs in the cellular islands of the organ. A thin cannula inserted into the right carotid artery (i.c.) via the lingual artery was used to infuse Ringer-Locke solutions (0.1-0.5 ml/min) containing either sodium cyanide (NaCN), acetylcholine (ACh) or dopamine (DA). Analysis of the effects of administration of NaCN (20-100 micrograms/min i.c.) showed that during this procedure [K+]o increased and [Ca2+]o decreased by mean values (+/- S.D.) of 0.99 +/- 0.82 and 0.22 +/- 0.06 mM respectively. During administration of ACh (20-50 micrograms/min i.c.) [K+]o increased and [Ca2+]o decreased respectively by mean values (+/- S.D.) of 3.18 +/- 3.0 and 0.31 +/- 0.14 mM. Decreases in [K+]o and [Ca2+]o by mean values (+/- S.D.) of 1.53 +/- 1.64 and 0.34 +/- 0.33 mM respectively were associated with administration of DA (20-50 micrograms/min i.c.). The predominant influences exerted by NaCN and ACh on chemoreceptor activity were excitatory whereas administration of DA caused either inhibition, excitation or a combination of these two effects. Stimulation of the sympathetic supply to the carotid body was associated with either increases, decreases or no reaction of chemosensory activity, [K+]o and [Ca2+]o. The changes in [K+]o associated with the various procedures may reflect the state of polarization within the chemoreceptor complex. Decreases in [Ca2+]o usually accompanied the performance of all procedures and may have resulted from an increased influx of this ion from the interstitial fluids into the cells for the purpose of provoking neurotransmitter release. However, the time course of the changes in [K+]o and [Ca2+]o were considerably slower in onset and recovery than the associated alterations in chemoreceptor activity. Therefore, it is unlikely that these ion changes are directly related to chemoreception but rather represent recovery processes after chemoreceptor modulation. It should be noted that the response times of the ISMs used in this study were too slow to register any rapid changes in [K+]o or [Ca2+]o associated with altered chemoreceptor activity.     

Permeabilisation of the sarcolemma in mouse diaphragm exposed to Bay K 8644 in vitro: time course,dependence on Ca2+ and effects of enzyme inhibitors

Summary Treatment of partially depolarised mouse diaphragm muscle in vitro with the Ca²⁺-channel agonist Bay K 8644 (1 M) induces permeabilisation of the sarcolemma (visualised by penetration of procion yellow). Procion yellow staining was widespread (74% of fibres) after 2 h of treatment, but was negligible after 60 min, a time at which myofibre breakdown is well advanced and elevation of [Ca²⁺]_i is minimal (Howl and Publicover 1989). Permeabilisation was inhibited in Ca²⁺-free saline, and was much less pronounced in polarised fibres. Inhibitors of free radical generation (particularly OH) afforded considerable protection to the muscle membrane against Bay K 8644-induced membrane permeabilisation. Inhibition of phospholipase A₂ and lipoxygenase were also effective, but inhibition of xanthine oxidase (by allopurinol) had little effect. It is concluded that the initial effect of Bay K 8644 treatment is to increase Ca²⁺ influx through Ca²⁺ channels at the sarcolemma, and that this action subsequently induces membrane permeabilisation. Membrane damage probably occurs due to free radical generation and activation of phospholipase A₂, both resulting from elevation of [Ca⁺]_i.     

Effect of pentobarbital on the contraction and calcium movements in cat cerebral and peripheral arteries

Pentobarbital (PB) induced concentration-dependent vasodilation in cylindrical segments of cat cerebral and femoral arteries precontracted with 75 mM K+ or 10(-5) M serotonin (5-HT). PB (10(-4) or 10(-3) M, 10 min preincubations) caused concentration-dependent inhibition of the contraction produced by both agents. The exposure of the vessels to a Ca2+-free medium annulled the response elicited by K+ in both kinds of arteries. In this medium, the contraction induced by 5-HT in cerebral arteries was abolished, whereas in femoral ones it was significantly reduced. This residual response was abolished by PB. The intracellular 45Ca2+ uptake (La3+ method) elicited by K+ in both kinds of arteries and by 5-HT in cerebral ones was reduced in a concentration-dependent manner by PB (10(-4)-10(-3) M). The spontaneous 45Ca2+ efflux from these arteries and that induced by La3+ (10 mM) were unaffected by PB (10(-3) M). These results indicate that the vasodepressor effects induced by PB are essentially due to an interference of the barbiturate with the Ca2+ entry into the vascular smooth muscle cell in both kinds of arteries and with the intracellular Ca2+ movements in femoral ones. It does not seem to exist a clear selectivity of PB for brain or peripheral arteries for antagonizing the responses caused by K+ or 5-HT.     

Effects of zonisamide on K+ and Ca2+ evoked release of monoamine as well as K+ evoked intracellular Ca2+ mobilization in rat hippocampus.

To clarify the effects of zonisamide (ZNS) on neurotransmission and intracellular Ca2+ mobilization, both Ca2+ and K+ evoked hippocampal releases of dopamine (DA) and serotonin (5-HT) were determined by in vivo microdialysis, and K+ (25 and 50 mM) evoked elevation of intracellular Ca2+ level was determined by fluorescence microscopy in vitro. Therapeutic concentrations of ZNS had different effects on Ca2+ and K+ evoked release of monoamine. ZNS stimulated Ca2+ evoked monoamine release, while ZNS inhibited K+ evoked monoamine release. ZNS inhibited K+ evoked elevation of hippocampal intracellular Ca2+ levels in a concentration dependent manner. These results suggest that ZNS inhibits the depolarization induced by neuronal excitation, whereas ZNS might enhance the N-type Ca2+ channel activity or N-type Ca2+ channel related exocytosis mechanisms.     

For K+ channels, Na+ is the new Ca2+

Although K+ channels activated by Ca2+ have long been known to shape neuronal excitability, evidence is accumulating that K+ channels sensitive to intracellular Na+, termed K(Na) channels, have an equally significant role. K(Na) channels contribute to adaptation of firing rate and to slow afterhyperpolarizations that follow repetitive firing. In certain neurons, they also appear to be activated by Na+ influx accompanying a single spike. Two genes encoding these channels, Slick and Slack, are expressed throughout the brain. The spatial localization of K(Na) channels along axons, dendrites and somata appears to be highly cell-type specific. Their molecular properties also suggest that these channels contribute to the response of neurons to hypoxia.     

Does calcium influx regulate melatonin production through the circadian pacemaker in chick pineal cells? Effects of nitrendipine, Bay K 8644, Co, Mn, and low external Ca

We have recently described a system, using dispersed chick pineal cells in static culture, which displays a persistent, photosensitive, circadian rhythm of melatonin production and release. Here, we describe the effects of nitrendipine (NTR) (a dihydropyridine ‘antagonist’ of L-type calcium channels), Bay K 8644 (BK) (a dihydropyridine calcium channel ‘agonis’), cobalt and manganese ions (both inorganic calcium channel blockers), and low external calcium concentrations, on the melatonin rhythm. NTR inhibited and BK stimulated melatonin output; they were potent and effective. Co²⁺, Mn²⁺, and low external Ca²⁺ markedly inhibited melatonin output. These results support a role for calcium influx through voltage-dependent calcium channels (L-type) in the regulation of the melatonin production. Four or 8 h pulses of white light or darkness, in otherwise constant red light, cause, in addition to acute effects, phase-dependent phase shifts of the melatonin rhythm in subsequent cycles. Such phase shifts indicate an effect on (proximal to) the pacemaker generating the rhythm. Four or 8 h pulses of NTR, BK, Co²⁺, however, did not appreciably alter the phase of subsequent melatonin cycles. Neither did BK interfere with phase shifts induced by light pulses. Mn²⁺ pulses did induce phase-dependent phase shifts, but, unlike those evoked by light or dark pulses, these were all delays. Such effects of Mn²⁺ in other systems have been attributed to, and are characteristic of, ‘metabolic inhibitors’. On balance, the results fail to support a prominent role for calcium influx in regulating the pacemaker underlying the circadian rhythm in chick pineal cells. Rather, calcium influx appears to regulate melatonin production primarily by acting on the melatonin-synthesizing apparatus, distal to the pacemaker.     

Membrane-bound enzymes of erythrocytes in human muscular dystrophy: (Na+ + K+-ATPase, Ca2+-ATPase, K+- and Ca2+-p-nitrophenylphosphatase.

Four enzyme activities were studied in erythrocyte membranes from patients with Duchenne and congenital myotonic muscular dystrophy. (Na⁺ + K⁺)-stimulated, Mg²⁺-dependent adenosinetriphosphatase, measured in two different media, showed normal activity and ouabain inhibition, as did K⁺-stimulated p-nitrophenylphosphatase. The specific activity of Ca²⁺-stimulated p-nitrophenylphosphatase was twice normal in Duchenne membranes. Ca²⁺-stimulated, Mg²⁺-dependent adenosine-triphosphatase was augmented in membranes from both Duchenne and congenital myotonic muscular dystrophic patients. The cause of the increased activities may be the necessity for compensating an alteration in the calcium metabolism in the dystrophic erythrocytes.Several kinetic parameters of the two Ca²⁺-stimulated enzyme activities were studied in Duchenne and control membranes. Most were not changed, with the exception of the Na⁺-stimulation of Ca²⁺-ATPase. In Duchenne membranes two affinity sites were present with half maximal activating concentrations of 58 ± 4 and 4 ± 1 mM Na⁺. In control membranes only one affinity site was found with K_a = 26 ± 9 mM Na⁺.     

有氧运动对大鼠骨骼肌线粒体Na+，K+-ATP酶和Ca2+-ATP酶及线粒体肿胀的影响

背景：研究表明有氧运动可提高线粒体功能,但在不同时期的作用特点还不明确。 目的：观察不同周期有氧运动对大鼠骨骼肌线粒体Na+,K+-ATP酶和Ca2+-ATP酶以及线粒体肿胀的影响。 方法：将SD大鼠随机分为正常对照组,有氧运动2,4和6周组。正常对照组不进行有氧运动,其余3组则参照BedfordTG标准,采用跑台运动方式,建立有氧运动模型进行相应的运动周期锻炼。测定各组大鼠骨骼肌线粒体Na+,K+-ATP酶和Ca2+-ATP酶的活性以及线粒体肿胀程度。 结果与结论：有氧运动2周组各指标与对照组比较无差异。有氧运动4和6周组线粒体Na+,K+-ATP酶、Ca2+-ATP酶活性均增高(P < 0.05),线粒体肿胀程度降低(P < 0.05)。实验结果表明,有氧运动可保护线粒体Na+,K+-ATP酶、Ca2+-ATP酶的活性,提高线粒体功能,但需要一定的时间积累。     

Cholinergic, nitric oxidergic innervation in cerebral arteries of the cat

Using immunoperoxidase labeling (IPL) and immunofluorescence labeling (IFL) methods, and each followed by NADPH diaphorase (NADPHd) histochemical staining in the same specimen, colocalization of choline acetyltransferase (ChAT) and NADPHd, indicative of nitric oxide synthase (NOS), in cerebral pial arteries and the sphenopalatine ganglia (SPG) of the cat was examined. In addition, retrograde axonal tracing using true blue was performed to determine if cerebral perivascular nerves containing ChAT and NADPHd originate in the SPG. Consistent results were obtained from IPL and IFL methods, indicating that the middle cerebral artery (MCA) and the circle of Willis received dense ChAT-immunoreactive (I) and NADPHd bundles and fine fibers. Almost all ChAT-I fibers and NADPHd fibers were found to be coincident in the arteries examined. A few fine fibers exhibited only NADPHd staining. In the SPG, approximately half of the ganglionic cells were both ChAT-I and NADPHd positive, while the remaining cells were positively only for NADPHd staining. One week after application of true blue on the middle cerebral arteries (MCA), the fluorescent true blue was found in the ganglionic cells of the SPG. Some of the true blue-positive cells contained both ChAT-immunoreactivity and NADPHd staining. These results provide morphological evidence indicating that all ChAT-I fibers in the MCA and the circle of Willis contain NOS, and that these fibers originate in the SPG, although not all NOS-I ganglionic cells in the SPG send fibers to pial vessels. These results also support the hypothesis that acetylcholine (ACh) and nitric oxide (NO) are synthesized and co-released in the same neurons in cerebral perivascular nerves. Based on the reported findings that NO mediates a major component of neurogenic vasodilation, and that ACh acts as a modulator, the present results demonstrate the presence of a cholinergic, nitric oxidergic innervation in cerebral arteries of the cat.     

Role of Ca2+-dependent K+ channels in erythrocyte lysate-induced contraction of rabbit cerebral artery

Abstract

K⁺ channel openers may be useful in the treatment of cerebral vasospasm following subarachnoid hemorrhage. However, the role of Ca²⁺ -dependent K⁺ channel (KCa) openers in cerebral vasospasm remain unclear. This study was undertaken to examine the role ofKCa in hemolysate-induced contraction of rabbit cerebral and peripheral arteries: 7. Iberiotoxin (IBX), a selective KCa channel blocker, produced more pronounced contraction in basilar than in those of carotid or femoral arteries, indicating KCa channels are important regulating factors in cerebral arteries; 2. NS1619, a selective KCa channel opener, abolished the contraction of basilar artery to erythrocyte lysate, a causative agent for cerebral vasospasm; 3. In rabbit basilar artery, NS1619 relaxed the contractions to IBX, erythrocyte lysate and KCI (20 and 60 mM), indicating that NS1619, besides opening KCa channels, possesses other vasodilating actions. We conclude that KCa channels are important factors in the regulation of cerebral vascular tension and KCa channel opener NS1619 may have dual relaxant actions in cerebral arteries. [Neurol Res 1999; 21: 705–711 ]     

Differential modulation of Ca2+-activated K+ channels in ovine pituitary gonadotrophs by GnRH, Ca2+ and cyclic AMP

Patch-clamp techniques were employed to examine the effects of cAMP in relation to gonadotrophin-releasing hormone (GnRH) action on Ca2+-activated K+ channels in pituitary gonadotrophs derived from ovine pars tuberalis. GnRH applied extracellularly increased channel openings in cell-attached patches similar to calcium ionophore (A23187), while raising intracellular cAMP concentration with dibutyryl cAMP or forskolin decreased the number of functional channels (Nf) and the open state probability (Po). Both cAMP and the catalytic subunit of cAMP-dependent protein kinase produced similar results when applied to the cytoplasmic membrane face of inside-out patches, and the effect of cAMP was abolished by the protein kinase inhibitor. Our results suggest that decreased permeability through these channels modulated by cAMP through a phosphorylation-dependent route can modulate luteinizing hormone release.     

Characteristics of a Ca2+/calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, to a postsynaptic density fraction isolated from canine cerebral cortex

Synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from the cerebral cortex (CTX) and cerebellum (CL) of the canine brain were found to contain one class of specific nitrendipine binding sites. The specific binding constants were: CTX-SM, Kd = 110 pM (Bmax = 126 fmol/mg protein); CTX-PSD, Kd = 207 pM (Bmax = 196 fmol/mg); CL-SM, Kd = 100 pM (Bmax = 65 fmol/mg); CL-PSD, Kd = 189 pM (Bmax = 80 fmol/mg). Treatment of the CTX-SM and CTX-PSD fractions with 0.5% deoxycholate and 1.0% N-lauroyl sarcosinate removed 88-91% and 42-51% of the nitrendipine binding, respectively, indicating that the major nitrendipine binding present in the SM fractions are of non-synaptic origin. Moreover, the percentages of total protein and specific nitrendipine binding removed from PSDs by these detergents were similar, indicating no preferential dissociation of the latter, and suggesting that the receptor protein is firmly bound and is probably an intrinsic component of the PSD fraction. Both Ca2+ and calmodulin were found to be important for the binding of nitrendipine to the CTX-SM and CTX-PSD fractions since: R24571, a calmodulin antagonist, was found to inhibit nitrendipine binding to the CTX-SM and CTX-PSD fractions with IC50 values of 1.1 microM and 0.9 microM, respectively; removal of Ca2+ from the CTX-SM and CTX-PSD fractions with 0.2 mM EGTA resulted in losses of specific nitrendipine binding of 80 and 90%, respectively; Ca2+ alone restored nitrendipine binding to EGTA-pretreated CTX-SM fractions and not to CTX-PSD fractions, with the latter needing both Ca2+ and calmodulin to restore nitrendipine binding; EGTA treatment removed 14-16% and 89-91% of nitrendipine bound to the CTX-SM and CTX-PSD fractions, respectively, suggesting that calmodulin (but not Ca2+) is needed to maintain the nitrendipine-nitrendipine receptor-calmodulin complex; Ca2+-reconstituted EGTA-pretreated CTX-SM fractions and the Ca2+ plus calmodulin-reconstituted EGTA-pretreated CTX-SM and CTX-PSD fractions were found to have similar binding constants to those for the corresponding native, untreated fractions; and the Ca2+/calmodulin dependency on nitrendipine binding was similar to the well-known Ca2+/calmodulin dependency on phosphorylation in EGTA-pretreated PSD fractions. It needed much less Ca2+ to saturate Ca2+/calmodulin-dependent phosphorylation of the pretreated CTX-PSD fractions than the nitrendipine binding. Yet, less calmodulin was needed to saturate nitrendipine binding than the phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of oxygen and glucose deprivation on vasoactivity in isolated bovine middle cerebral arteries

The effects of oxygen and glucose deprivation on vasoactivity were investigated using helical strips of bovine middle cerebral artery. Hypoxia, created by reducing the PO2 of the bath, or oxidative inhibition with 2,4 dinitrophenol (DNP) or sodium azide, significantly reduced contractions induced by serotonin. Normal tonic contractions induced with fresh and aged whole blood, or 5-HT became phasic and quickly relaxed to baseline in a hypoxic environment. Glucose elimination from the Krebs medium, or the inhibition of the glycolytic pathway with iodoacetic acid (IAA), did not significantly reduce serotonin-induced contractions. However, contractions were inhibited more with the combination of oxygen and glucose deprivation, or DNP + IAA, than with oxygen deprivation alone. Efforts to produce rigor in this preparation by oxygen/substrate reduction or metabolic inhibition were unsuccessful. Tonic contractions induced by 70 mM potassium became phasic as the Ca++ concentration was reduced. Contractions resulting from the readdition of Ca++ to arteries exposed to calcium-free high potassium solution were significantly reduced in the presence of oxidative and/or glycolytic inhibitors. The uptake of 45Ca++, as measured by the lanthanum technique, decreased as the bath PO2 was reduced in both serotonin stimulated and unstimulated arteries. Glucose deprivation alone did not affect 45Ca++ uptake. This study suggests that hypoxia has a direct inhibitory affect on cerebral vasoactivity mediated by reductions in sarcoplasmic Ca++ uptake.     

Effects of Na+ and Ca2+ gradients on intracellular free Ca2+ in voltage-clamped Aplysia neurons

Selected neurons of the abdominal ganglion of Aplysia californica were voltage-clamped and intracellular free Ca [( Ca2+]i) and Na [( Na+]i) concentrations were monitored with ion selective microelectrodes. Reducing [Na+]o from 500 mM (normal seawater, NSW) to 5 mM resulted in a decrease of the potential measured by the Ca electrode (VCa). Increasing [Ca2+]o from 10 to 50 mM increased [Ca2+]i two-fold, keeping [Ca2+]o at 50 mM and decreasing [Na+]o to 5 mM still led to a decrease in VCa. With 100 mM [Ca2+]o, which also increased [Ca2+]i, decreasing [Na+]o increased VCa in two of the eight cells tested. This indicates that in normal or moderately high resting [Ca2+]i, Ca2+ extrusion by Na/Ca exchange (forward mode) is not essential for [Ca2+]i buffering. [Na+]i was 12.9 +/- 3.6 mM (S.E.M., n = 7) in NSW; reducing [Na+]o to 5 mM decreased [Na+]i to 2.0 +/- 1.1 mM (S.E.M.). Keeping [Na+]o at 5 mM and increasing [Ca2+]o from 10 to 20 mM further decreased [Na+]i to about 1.0 mM, evidence of Na/Ca exchange operating in the reverse mode. Attempts to increase [Ca2+]i by bath application of the Ca ionophores A23187, X537A, ionomycin or ETH 1001 resulted in no measurable change of the resting [Ca2+]i. Application of Ouabain caused an apparent increase in [Ca2+]i in two of the six cells tested. In cells injected with the metallochromic indicator arsenazo III (AIII), the rate of the falling phase of the AIII absorbance increase, following a voltage-clamp pulse, was significantly slower in 5 mM [Na+]o. This indicates that in its forward mode Na-Ca exchange is active in clearing large submembrane increases in [Ca2+]i.     

The influence of Bay K 8644 treatment on (±)-epibatidine-induced analgesia

The purpose of this investigation was to determine if analogous to (−)-nicotine's analgesic effect, the analgesic effect of the recently characterized potent nicotinic acetylcholine receptor (nAChR) agonist (±)-epibatidine was altered in response to treatment with the calcium channel agonist (±)-Bay K 8644. In addition, the effects of the enantiomers, (+)-Bay K 8644, reported to be a calcium channel antagonist, and (−)-Bay K 8644, reported to be a calcium channel agonist were examined. (±)-Bay K 8644 (2.8 μmol/kg; i.p.) produced a large analgesic response in mice in the hot-plate paradigm that rapidly dissipated by 30 min after treatment. This analgesic effect of (±)-Bay K 8644 was not prevented by pre-treatment with the nicotinic antagonist mecamylamine (5 μmol/kg; i.p.). Treatment with non-analgesic doses of the calcium channel agonists (±)- and (−)-Bay K 8644 (1.4 μmol/kg; i.p.) significantly potentiated the analgesic effect of (±)-epibatidine (0.05 μmol/kg; i.p.). Potentiation of (±)-epibatidine's analgesic effect occurred when the agonists were administered prior to (±)-epibatidine or after (±)-epibatidine as long as analgesia testing was conducted 15 to 30 min after Bay K 8644 treatment. Pre-treatment with the calcium channel antagonist (+)-Bay K 8644 was found to attenuate (±)-epibatidine-induced analgesia. When given after (±)-epibatidine, (+)-Bay K 8644 had no effect on (±)-epibatidine's analgesic effect. These data provide additional in vivo evidence that altering calcium dynamics can modulate neuronal nAChR function.