The effects of calcium channel blockers on isoproterenol induced cyclic adenosine 3',5'-monophosphate generation in intact human lymphocytes

We have investigated the effects of clinically available calcium channel blockers (nifedipine, verapamil and diltiazem) on isoproterenol stimulated cyclic adenosine 3',5'-monophosphate (cyclic AMP) generation in intact human lymphocytes. After preincubation of various calcium antagonists with intact lymphocytes at 37 degrees C for 15 minutes, 10 microM nifedipine or verapamil partially inhibited isoproterenol induced cyclic AMP generation in the presence of cyclic AMP phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine) while they alone had no effect on cyclic AMP level at a concentration of up to 100 microM. In contrast, 10 nM-1.0 microM nifedipine, verapamil or diltiazem potentiated cyclic AMP generation induced by isoproterenol in a dose dependent manner. Similar results were observed in the time course studies of cyclic AMP generation. These effects are somewhat similar to the effect of phenothiazine, a calmodulin inhibitor, which, at 10 microM (close to IC50), also potentiated the effects of isoproterenol. In contrast, lanthanum chloride (LaCl3), an extracellular inorganic calcium antagonist, at 1.0 mM, inhibited isoproterenol induced cyclic AMP generation. The biochemical mechanisms underlying these potentiating effects are unknown. It may be partly related to the effect of calcium channel blockers (at least for nifedipine) on preventing beta 2 adrenergic receptor desensitization. This may provide a potential mechanism for the synergistic effect between calcium channel blockers and beta 2 adrenoceptor agonists on bronchial dilatation.     

Aging and the circadian rhythm of melatonin: a cross-sectional study of Chinese subjects 30-110 yr of age

Although previous reports indicate that nocturnal plasma melatonin secretion declines with age, some recent findings do not support this point. In the present cross-sectional study, we documented serum melatonin concentrations at two time points, 02:00 and 08:00 h, in 144 persons aged 30-110 yr and found a significant age-related decline. It began around the age of 60 and reached a very significantly lower level in subjects in their 70s and over 80 yr of age (P < 0.01, when compared with age <60 yr). Nocturnal melatonin levels were higher among (post-menopausal only) women than men overall (P < 0.05). In the older age-groups, nocturnal melatonin levels did not differ between healthy controls and subjects with high blood pressure or ischemic heart disease. To further check these results, we also assessed the circadian pattern of serum melatonin in four subgroups of healthy men, aged 30-39, 40-49, 50-59, and 60-69 yr: blood samples were taken at 2 h intervals from 08:00 to 22:00 h and hourly from 22:00 to 08:00 h. Our results showed generally similar circadian melatonin patterns that peaked at night with very low levels during the daytime. No significant difference was found among the three younger groups, but nocturnal melatonin levels were significantly lower in the men in their 60s.     

急性低氧下钙阻断剂对左,右心泵功能的影响

在20条麻醉开胸狗上,用RM-6000多道仪同步记录左心室内峰压(LVP)、左室压力变化率(L+dP/dt_(max))、右心室内峰压(RVP)、右室压力变化率(R±dp/dt_(max))、肺动脉压力(P_(Pa))、主动脉血流每搏峰值(Fa)、心率(HR)等各项生理指标,观察钙通道阻断剂Nife-dipine,Diltiazem和Verapamil对左、右心室功能影响。在钙通道阻断剂处理后,左室的LVP,L±dp/dt_(max)下降,而Fa增加;右室的RVF,R±dp/dt_(max)和P_(Pa)均有升高趋势,显示钙通道阻断剂对左、右心泵功能的影响不同。这可能提示左、右心室功能对钙离子的依赖程度不同。在急性低氧状态下,此三种钙阻断剂均使急性低氧引起LVP的增加反应消失,Fa增加明显,Verapamil和Diltiazem有减轻急性低氧引起的RVP和P_(Pa)的增压作用。从这些钙通道阻断剂对左右心泵功能影响的比较来看,Diltiazem比Verapamil和Nifedipine对急性低氧状态下的心泵功能有较好的作用。     

Effect of calcium entry blockers and adenosine on the relaxation of large and small coronary arteries

The mechanism(s) by which adenosine causes dilation of the vascular smooth muscle is not properly understood. Several mechanisms including the inhibition of calcium influx and intracellular translocation have been suggested for its action. This study is an attempt to further elucidate the site of action of adenosine in relation to calcium by making use of calcium entry blockers. Large (1 +/- 0.2 mm, o.d.) and small (0.5 +/- 0.2 mm, o.d.) branches of bovine left anterior descending coronary artery (LADCA) contracted with 50 mM K+ were used as a model for these studies. Concentration-response curves for various calcium entry blockers were obtained and the order of potency was found to be: D-600 greater than nifedipine greater than verapamil greater than diltiazem greater than lidoflazine for large branches and nifedipine greater than D-600 greater than verapamil greater than lidoflazine greater than diltiazem for small branches of LADCA. The concentration-response relationship for adenosine (10(-6)-10(-4) M) in the presence and absence of these drugs (10(-9)-10(-7) M) was unchanged. 8-phenyltheophylline (2 X 10(-5) M), an adenosine receptor antagonist was without an effect on the relaxations induced by various calcium entry blockers, however, it antagonized the relaxing response to adenosine. Lidoflazine at concentrations of 7 X 10(-7) M and 2 X 10(-7) M potentiated the effect of adenosine in relaxing the large and small LADCA, respectively. In summary, the data show an increased sensitivity of small coronary vessels to nifedipine, D-600 and lidoflazine. The data further suggest a different site of action for adenosine and calcium entry blockers.     

AGING AND THE CIRCADIAN RHYTHM OF MELATONIN: A CROSS-SECTIONAL STUDY OF CHINESE SUBJECTS 30–110 YR OF AGE

Although previous reports indicate that nocturnal plasma melatonin secretion declines with age, some recent findings do not support this point. In the present cross-sectional study, we documented serum melatonin concentrations at two time points, 02:00 and 08:00h, in 144 persons aged 30–110 yr and found a significant age-related decline. It began around the age of 60 and reached a very significantly lower level in subjects in their 70s and over 80 yr of age (P<0.01, when compared with age <60 yr). Nocturnal melatonin levels were higher among (post-menopausal only) women than men overall (P<0.05). In the older age-groups, nocturnal melatonin levels did not differ between healthy controls and subjects with high blood pressure or ischemic heart disease. To further check these results, we also assessed the circadian pattern of serum melatonin in four subgroups of healthy men, aged 30–39, 40–49, 50–59, and 60–69 yr: blood samples were taken at 2h intervals from 08:00 to 22:00h and hourly from 22:00 to 08:00h. Our results showed generally similar circadian melatonin patterns that peaked at night with very low levels during the daytime. No significant difference was found among the three younger groups, but nocturnal melatonin levels were significantly lower in the men in their 60s.     

Comparison of verapamil and nifedipine in thrombosis models

Calcium blockers and calmodulin antagonists have been reported to inhibit the aggregation of blood platelets in vitro. In the present study, the effects of two calcium blockers, verapamil and nifedipine, were compared in several rodent thrombosis models. In rat and mouse platelet-rich plasma, preincubation with either verapamil or nifedipine had a dose-dependent inhibitory effect on collagen-induced aggregation (P less than 0.01). The concentration required for 50% inhibition of rat platelet aggregation was 0.91 X 10(-4) M for verapamil and 1.77 X 10(-4) M for nifedipine. In in vivo thrombosis models in mice, acute pretreatment with nifedipine had a significant, dose-dependent protective effect (P less than 0.05). At a dose of 500 micrograms/kg, nifedipine inhibited thrombotic sudden death provoked by arachidonic acid, a thromboxane agonist (U46619), or a combination of collagen and epinephrine. In vivo platelet depletion induced by U46619 was also inhibited by this calcium blocker. Thus, nifedipine is protective against a variety of thrombotic stimuli, and its antiplatelet aggregatory effect apparently extends to the in vivo situation. In contrast, no in vivo antithrombotic activity was observed for verapamil. Two additional calcium blockers, perhexilene and diltiazem, and three calmodulin antagonists, W-7, chlorpromazine, and trifluoperazine, were also tested in the U46619-induced thrombotic sudden death model. Of these, only diltiazem (5 and 10 mg/kg) had an acute protective effect.     

Light/dark patterns of interleukin-6 in relation to the pineal hormone melatonin in patients with acute myocardial infarction

AIMS: Atherosclerosis is a chronic disease that, from its origin to its ultimate complications, involves inflammatory cells, inflammatory proteins, and inflammatory responses from vascular cells. It has been demonstrated that cytokine activities are under neuroendocrine control, in part exerted by the pineal gland through the circadian secretion of its main product melatonin. Melatonin is mainly released during the night, but the precise relationship between melatonin and the light/dark rhythm of interleukin-6 in patients with acute myocardial infarction is still unclear. METHODS AND RESULTS: The study included 60 patients diagnosed with acute myocardial infarction and 60 healthy volunteers whose venous blood samples were collected at 09:00 h (light period) and 02:00 h (dark period). Our results demonstrate that interleukin-6 concentrations presented a light/dark pattern with mean serum concentrations being higher in the acute myocardial infarction group than in the control group (101.26 +/- 13.43 and 52.67 +/- 7.73 pg/ml at 02:00 h, 41.93 +/- 5.90 and 22.98 +/- 4.49 pg/ml at 09:00 h, respectively, p < 0.05). Differences in the day/night changes in melatonin levels in control subjects (48.19 +/- 7.82 at 02:00 h, 14.51+/- 2.36 at 09:00 h, pg/ml) and acute myocardial infarction patients (25.97 +/- 3.90 at 02:00 h, 12.29 +/- 4.01 at 09:00 h, pg/ml) (p < 0.05) were a result of a reduced nocturnal elevation of melatonin in the acute myocardial infarction group. CONCLUSIONS: The current findings suggest that the circadian secretion of melatonin may be responsible at least in part for light/dark variations of endogenous interleukin-6 production in patients with acute myocardial infarction. In this study, the melatonin seems to have an anti-inflammatory effect.     

Attenuation of daunorubicin-augmented microsomal lipid peroxidation and oxygen consumption by calcium channel antagonists.

Daunorubicin (20 microM) stimulated NADPH-dependent microsomal lipid peroxidation about 2-fold over control values and enhanced the rate of oxygen utilization by microsomes. The calcium channel blockers tested inhibited daunorubicin-augmented lipid peroxidation and O2 consumption to varying degrees. Inhibition of daunorubicin-stimulated lipid peroxidation was found to be dose dependent; the IC50 (drug concentration producing 50% inhibition of lipid peroxidation) values for verapamil, nifedipine and diltiazem were approximately 150 microM, 200 microM, and 600 microM respectively. Our in vitro studies suggest that calcium channel antagonists may modulate the free radical-mediated, cardiotoxic effects of daunorubicin.     

Similarity of the nocturnal profile of serum melatonin at early puberty and early adulthood.

In the present study we determined the nocturnal profile of serum melatonin (MT) concentrations in 10 short normal children at Tanner stage I-II of pubertal development (12.5-14.9 yrs) and in 6 young adults (24-29 yrs). Blood was collected every 30 min from 00(00) to 06(00). We did not find any significant difference in the nocturnal profile of serum MT, as gauged by the comparison of MT concentrations at any time-point tested as well as of the transverse means (84.2 +/- 36.0 pg/ml [M +/- SD] in the children vs 78.7 +/- 10.8 pg/ml in the adults). Mean serum melatonin concentration was not correlated to sex hormone concentration or body surface area. Our findings do not support the view that MT concentrations fall at the beginning of pubertal development and that changes in body size may be the reason for age-dependent changes of serum MT concentrations.     

Calcium channels in PDGF-stimulated A172 cells open after intracellular calcium release and are not voltage-dependent.

Using laser image cytometry and Indo-1 fluorescence, we investigated the intracellular free Ca2+ concentration ([Ca2+]i) of confluent A172 human glioblastoma cells stimulated by the BB homodimer of platelet-derived growth factor (PDGF-BB). The shape of the calcium transients and the delay time between stimulation and the beginning of the transient varied considerably. The percentage of responsive cells, the peak [Ca2+]i and the duration of the response were directly related to PDGF-BB dose, while the delay time was inversely related; the maximal response occurred at a PDGF-BB concentration of 20 ng/ml. Studies with EGTA and inorganic calcium-channel blockers (Ni2+, La3+) showed that the increase of [Ca2+]i resulted from initial release of intracellular stores and subsequent calcium influx across the plasma membrane. Opening of calcium channels in the plasma membrane, monitored directly by studying Mn2+ quenching of Indo-1 fluorescence, was stimulated by PDGF-BB and blocked by La3+; the opening occurred 55 +/- 10 s after the initial increase in [Ca2+]i. Therefore, in these tumor cells, intracellular release always occurs before channel opening in the plasma membrane. Depolarization of cells with high extracellular [K+] did not generally induce calcium transients but did decrease calcium influx. L-type calcium-channel blockers (verapamil, nifedipine, and diltiazem) had little or no effect on the calcium influx induced by PDGF-BB. These results indicate that PDGF-BB induces calcium influx by a mechanism independent of voltage-sensitive calcium channels in A172 human glioblastoma cells.     

Protective Effect of L-type Calcium Channel Blockers Against Haloperidol-induced Orofacial Dyskinesia: A Behavioural, Biochemical and Neurochemical Study

Haloperidol is a classical neuroleptic drug that is still in use and can lead to abnormal motor activity such as tardive dyskinesia (TD) following repeated administration. TD has no effective therapy yet. There is involvement of calcium in triggering the oxidative damage and excitotoxicity, both of which play central role in haloperidol-induced orofacial dyskinesia and associated alterations. The present study was carried out to investigate the protective effect of calcium channel blockers [verapamil (10 and 20 mg/kg), diltiazem (10 and 20 mg/kg), nifedipine (10 and 20 mg/kg) and nimodipine (10 and 20 mg/kg)] against haloperidol induced orofacial dyskinesia and associated behavioural, biochemical and neurochemical alterations in rats. Chronic administration of haloperidol (1 mg/kg i.p., 21 days) resulted in a significant increase in orofacial dyskinetic movements and significant decrease in % retention, coupled with the marked increase in lipid peroxidation and superoxide anion generation where as significant decrease in non protein thiols and endogenous antioxidant enzyme (SOD and catalase) levels in rat brain striatum homogenates. All these deleterious effects of haloperidol were significantly attenuated by co-administration of different calcium channel blockers. Neurochemically, chronic administration of haloperidol resulted in significant decrease in levels of catecholamines (dopamine, serotonin) and their metabolites (HVA and HIAA) but increased turnover of dopamine and serotonin. Co-administration of most effective doses of verapamil, diltiazem, nifedipine and nimodipine significantly attenuated these neurochemical changes. Results of the present study indicate that haloperidol-induced calcium ion influx is involved in the pathogenesis of tardive dyskinesia and calcium channel blockers should be tested in clinical trials with nifedipine as the most promising one.     

Effects of Calcium Ions and of Calcium Channel Blockers on Galvanotaxis of Chlamydomonas reinhardtii

The effects of calcium ions and of the calcium channel blockers verapamil, diltiazem and nifedipine on galvanotaxis in Chlamydomonas have been investigated using a fully automated and computerized population system. Galvanotaxis is a function of the voltage applied to the cell population. However, the galvanotactic orientation also depends on the external calcium concentration. In a calcium-deprived nutrient medium which still contains 6 × 10^?7M calcium, galvanotactic orientation is about 20% of orientation at optimal calcium concentration of 10^?4 M at 9 V. The higher the external calcium concentration is, the lower is the voltage necessary for optimal galvanotactic orientation. The calcium channel blockers diltiazem and nifedipine likewise inhibit galvanotaxis of Chlamydomonas very specifically without impairing motility. Verapamil is effective, but also inhibits motility by causing detachment or shortening of the flagella. Nevertheless, inhibition of galvanotaxis by verapamil is not the only result of decreased motility, because the galvanotactic orientation is impaired to a greater extent than motility. The effectiveness of the three blockers tested in inhibiting galvanotaxis depends on the concentration and on the voltage applied. At 10^?5 M, verapamil causes maximal inhibition of galvanotaxis at 9 V. At increasing concentrations up to 10^?4 M, diltiazem inhibits galvanotaxis more strongly than the other blockers. If the voltage is varied at a constant blocker concentration of 2 × 10^?5 M, nifedipine causes maximal inhibition at 3 V–6 V, diltiazem at 9 V and verapamil above 12 V.     

Contraction of human colonic circular smooth muscle cells is inhibited by the calcium channel blocker pinaverium bromide

The effects of L-type calcium channel blockers (CCBs) selective for the gastrointestinal tract (pinaverium) or non-selective (nicardipine and diltiazem), were investigated on CCK-, CCh- or KCl-induced contraction of smooth muscle cells (SMC) isolated from the circular muscle layer of normal or of inflamed human colons. In the normal tissue colon, whatever the contractile agent used, CCK-8 (1nM), CCh (1nM) or KCl (20mM), a micromolar concentration of pinaverium significantly inhibited contraction (88.36%, 93.10%, 93.92% inhibition respectively); this effect was concentration-dependent for CCh (IC50 = 0.73 +/- 0.08nM) and for CCK (IC50 = 0.92 +/- 0.12nM). In parallel, both nicardipine and diltiazem inhibit significantly contraction of isolated SMC. In inflamed colons, pinaverium (1 microM) display a significant higher efficacy than diltiazem or nicardipine to reduce cell contraction induced by CCK-8 or by KCl. In addition, RT-PCR experiments were performed to evidence tissue specificity of the L-type calcium channel. They revealed the expression of the messenger of the a-1 subunit L-type calcium channel (binding site of such CCBs), consistent with the expression of the rbC-2 splice variant of the alpha1-C gene.In conclusion: (i) the inhibition by calcium channel blockers of agonist-induced contractile activity suggest a modulation of SMC contraction upon extracellular calcium via 'L-type' voltage-dependent calcium channel; (ii) this study provides a rationale for the clinical use of pinaverium in colonic motor disoders affecting the contractility of SMC, since it appeared to decrease the contraction even in pathological situation; and (iii) RT-PCR experiments confirms the presence in human colon SMC of the alpha-1 subunit mRNA of calcium channel.     

Isolated rat cardiac myocytes as an experimental model to study calcium overload: the effect of calcium-entry blockers

Calcium overload and the effect of a series of calcium-entry blockers were studied in isolated adult cardiac myocytes from the rat challenged with veratrine. The isolation procedure resulted in a high yield of individual rod shaped, calcium tolerant myocytes. After incubation with veratrine, an alkaloid which induces both sodium and calcium influx, 93% of the myocytes became calcium intolerant: the quiescent rod shaped cells vigorously contracted after 30 sec of contact with veratrine and contracture (round cells) ensued within 1 min. Exposure for 30 min to various doses of calcium-entry blockers prior to veratrine addition resulted in the prevention of contracture, the degree of protection depending on the type and the concentration of calcium-entry blocker. Among the different calcium-entry blockers tested, the diarylalkylpiperazines lidoflazine, cinnarizine and flunarizine were protective from the 10(-7) M concentration onwards. Nicardipine was protective at the 10(-6) M and 10(-5) M concentrations, verapamil at 10(-5)M only while other blockers of the "slow channel" type (diltiazem and nifedipine) were not protective in the concentration range tested. This study shows that isolated myocytes represent a valid model for pharmacological investigations. The results with the calcium-entry blockers stress the heterogeneity of the different series of calcium-entry blockers.     

Amlodipine inhibits the production of cytokines induced by ouabain

Recent studies have suggested that cytokines are capable of modifying cardiovascular function and that drugs used in the treatment of heart failure have various modulating properties on the production of cytokines. More recently, we have found that ouabain induces the production of cytokines. This study was performed to examine the effects of calcium channel blockers on the production of cytokines induced by a cardiac glycoside. Human peripheral blood mononuclear cells (PBMC) were obtained from healthy volunteers. PBMC were cultured in 0.1, 1, 10, and 30 micromol/l amlodipine, diltiazem, and nifedipine in presence of 1 micromol/l ouabain. After 24 h of incubation, IL-1alpha, IL-1beta, IL-6, and TNF-alpha were measured in the culture supernatants by enzyme-linked immunosorbent assay. Ouabain induced the production of IL-1alpha, IL-1beta and IL-6, but not of TNF-alpha. Induction of IL-1beta was most prominent. The production of IL-1alpha, and IL-6 was inhibited by amlodipine in a concentration-dependent manner and was significantly decreased at a concentration of 10 micromol/l. IL-1beta production was also inhibited by 30 micromol/l amlodipine. In contrast, neither diltiazem nor nifedipine inhibited the production of these cytokines. The unique property of amlodipine to inhibit the production of IL-1alpha, IL-1beta and IL-6 may contribute to its beneficial effects in heart failure patients.     

Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.     

General and unspecific damping by malignancy of the circadian amplitude of circulating human melatonin?

The circadian rhythm of serum melatonin of 39 cancer patients is compared with that of 28 healthy subjects matched by gender and age. Each subject provided 6 blood samples at 4-hour intervals for determination of melatonin by RIA. After log10-transformation, data series were analyzed by single and population-mean cosinor and compared between the two groups and among patients subgrouped by cancer site, stage and treatment. A circadian rhythm (P<0.001) is demonstrated for both groups, with a contributing 12-hour harmonic (P<0.001). In the absence of a difference in MESOR, the circadian amplitude of the cancer patients is smaller than that of the healthy subjects (P=0.003). Numerically, nocturnal (00:00 and 04:00) melatonin concentrations are lower and daytime (08:00-20:00) melatonin concentrations are higher in the cancer patients than in the healthy subjects (P=0.032 at 12:00 and P=0.058 at 16:00). In the age ranges examined, no differences are found with age in either group or by gender in health. No differences are found among cancer patients subgrouped either by site, stage (localized vs. metastasized) or treatment. If these results are validated, other Janus-like (two-faced: stimulation or inhibition, depending on chronome stage) effects of malignancy should be taken into consideration for screening and for timing treatment.     

Dependency of hypoxic chemotransduction in cat carotid body on voltage-gated calcium channels.

The hypothesis that the entry of extracellular calcium ions into some compartment, quite possibly the type I cells, through voltage-gated calcium channels (VGCC) is essential for hypoxic chemotransduction in the cat carotid body was tested using an in situ perfusion technique. The neural output of the carotid body of anesthetized, paralyzed, and artificially ventilated cats in response to perfusions with Krebs-Ringer bicarbonate solution (KRB), calcium-free KRB, KRB containing calcium channel blockers, or KRB containing BAY K 8644 was recorded. Selective perfusion of the carotid body with hypoxic calcium-free KRB significantly decreased carotid chemoreceptor activity, suggesting that extracellular calcium is essential for hypoxic chemotransduction. Selective perfusion of the carotid body with hypoxic KRB containing verapamil (10-100 microM), diltiazem (10-100 microM), or nifedipine (10-100 microM) dose dependently attenuated the increase in chemoreceptor activity produced by hypoxia, suggesting that VGCC need to be activated for hypoxic chemotransduction. The carotid body response to hyperoxic KRB containing the calcium channel agonist BAY K 8644 (10 microM) was 267 +/- 87% of hyperoxic control KRB, suggesting that an enhanced influx of calcium ions through VGCC stimulates carotid chemoreceptor activity. Selective perfusion of the carotid body with severely hypoxic KRB containing BAY K 8644 did not increase chemoreceptor activity above that produced by severe hypoxia alone. This suggests that severe hypoxia increases intracellular calcium in some compartment of the carotid body to achieve stimulatory maximum response and that further increase in intracellular calcium does not produce further elevation of neural activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the blockade of high voltage-activated calcium channels on in vitro pineal melatonin synthesis

The presence of high voltage-activated calcium channels in the rat pineal gland is well known. However, their role in pineal metabolism is not completely understood and is even controversial. Better to understand this matter, we investigated the effects of L-, N- or P/Q-type calcium channel blockers (nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, respectively) on melatonin content and arylalkylamine-N-acetyltransferase activity of denervated rat pineal glands kept for 48 h in culture and stimulated with norepinephrine. Melatonin was measured by high performance liquid chromatography with electrochemical detection and arylalkylamine-N-acetyltransferase activity was quantified by radiometric assay. Pre-incubation with any of these high voltage-activated calcium channel blockers reduced the melatonin production induced by norepinephrine although arylalkylamine-N-acetyltransferase activity was reduced only by the N-type calcium channel antagonist, omega-conotoxin GVIA. The results indicate that calcium influx through L-, N- or P/Q-type of high voltage-activated calcium channels is necessary for the full expression of the metabolic process leading to melatonin synthesis in the rat pineal glands. However, the mechanisms involved in this process are different for the L- or P/Q- and N-type calcium channels.