Inhibition by SK&F96365 of NO-mediated relaxation induced by Ca2(+) -ATPase inhibitors in rat thoracic aorta

Endocardial endothelium-dependent positive inotropy by Ca2+ pump inhibitors: possible involvement of store-operated Ca2+ entry

Positive inotropy by sarcoplasmic/endoplasmic reticulum Ca(2+) pump inhibitors was found and its mechanisms were analyzed pharmacologically. Thapsigargin and cyclopiazonic acid produced positive inotropy in isolated mouse left atria. The responses were inhibited by pretreatment of the endocardial surface with Triton X-100 or by indomethacin, which suggests that the inotropic responses were mediated by prostaglandin(s) released from the endocardial endothelium as well as acetylcholine-induced positive inotropy. The thapsigargin- and acetylcholine-induced positive inotropy was significantly inhibited by Gd(3+), La(3+) and lavendustin A, a tyrosine kinase inhibitor, but not by Ni(2+) and LOE908, a non-selective cation channel inhibitor. Gd(3+) and lavendustin A had no effect on the exogenously applied PGF(2)alpha-induced positive inotropy. In addition, acetylcholine did not induce any positive inotropy when applied after the application of thapsigargin. These results strongly suggest that thapsigargin- as well as acetylcholine-induced prostaglandin release from endocardial endothelium is mediated by store-operated Ca(2+) entry through Gd(3+)-sensitive channels and activation of tyrosine kinase.

Mechanisms underlying the programming of small artery dysfunction: review of the model using low protein diet in pregnancy in the rat

Human and animal studies have shown that unbalanced maternal nutrition is associated with the development of cardiovascular and metabolic disease in adulthood. In the Southampton maternal low protein model (SMLP), protein deprivation (50%) throughout pregnancy in rats leads to elevated blood pressure in adult offspring. Impaired peripheral arterial function may contribute to the cardiovascular dysfunction observed in these offspring. This review discusses the impact of such a dietary insult on the vascular function of resistance arteries from pregnant rats (pF(o)), their offspring (F(1)), the pregnant offspring (pF(1)) and the second generation (F(2)). At each stage, disturbances in endothelium-dependent relaxation were observed, implicating changes in endothelial nitric oxide (NO)-guanylate cyclase (GC) signalling pathway in the vascular adaptations to pregnancy and the programmed effects on offspring.

Involvement of Na(+)/Ca(2+) exchanger in endothelial NO production and endothelium-dependent relaxation

Endothelial nitric oxide (NO) synthase (eNOS) is controlled by Ca(2+)/calmodulin and caveolin-1 in caveolae. It has been recently suggested that Na(+)/Ca(2+) exchanger (NCX), also expressed in endothelial caveolae, is involved in eNOS activation. To investigate the role played by NCX in NO synthesis, we assessed the effects of Na(+) loading (induced by monensin) on rat aortic rings and cultured porcine aortic endothelial cells. Effect of monensin was evaluated by endothelium-dependent relaxation of rat aortic rings in response to acetylcholine and by real-time measurement of NO release from cultured endothelial cells stimulated by A-23187 and bradykinin. Na(+) loading shifted the acetylcholine concentration-response curve to the left. These effects were prevented by pretreatment with the NCX inhibitors benzamil and KB-R7943. Monensin potentiated Ca(2+)-dependent NO release in cultured cells, whereas benzamil and KB-R7943 totally blocked Na(+) loading-induced NO release. These findings confirm the key role of NCX in reverse mode on Ca(2+)-dependent NO production and endothelium-dependent relaxation.

Ca(2+) release from ryanodine-sensitive store contributes to mechanism of hypoxic vasoconstriction in rat lungs

Studies of thapsigargin, cyclopiazonic acid, and ryanodine in isolated pulmonary arteries and smooth muscle cells suggest that release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))- and/or ryanodine-sensitive sarcoplasmic reticulum Ca(2+) stores is a component of the mechanism of acute hypoxic pulmonary vasoconstriction (HPV). However, the actions of these agents on HPV in perfused lungs have not been reported. Thus we tested effects of thapsigargin and cyclopiazonic acid, inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, and of ryanodine, an agent that either locks the ryanodine receptor open or blocks it, on HPV in salt solution-perfused rat lungs. After inhibition of cyclooxygenase and nitric oxide synthase, thapsigargin (10 nM) and cyclopiazonic acid (5 microM) augmented the vasoconstriction to 0% but not to 3% inspired O(2). Relatively high concentrations of ryanodine (100 and 300 microM) blunted HPV in nitric oxide synthase-inhibited lungs. The results indicate that release of Ca(2+) from the ryanodine-sensitive, but not the IP(3)-sensitive, store, contributes to the mechanism of HPV in perfused rat lungs and that Ca(2+)-ATPase-dependent Ca(2+) buffering moderates the response to severe hypoxia.

Possible involvement of Ca2+ entry and its pharmacological characteristics responsible for endothelium-dependent, NO-mediated relaxation induced by thapsigargin in guinea-pig aorta

Thapsigargin, a specific inhibitor of Ca(2+)-pump Ca(2+)-ATPase in the sarcoplasmic/endoplasmic reticulum (SR/ER), produces an endothelium-dependent vascular relaxation. In the present study, pharmacological features of thapsigargin-induced endothelium-dependent relaxation were functionally characterized in the isolated guinea-pig aorta especially focusing on the Ca2+ mobilization mechanisms in endothelial cells. Thapsigargin-induced endothelium-dependent vascular relaxation was markedly suppressed by N(G)-nitro-L-arginine (L-NNA) and calmidazolium, suggesting that the vascular relaxation to thapsigargin is largely attributable to endothelium-derived nitric oxide (NO) produced as a result of the activation of Ca2+, calmodulin-dependent NO synthase (NOS). Removal of Ca2+ from the external solution abolished the endothelium-dependent relaxation of guinea-pig aorta in response to thapsigargin. Thapsigargin-induced endothelium-dependent relaxation was inhibited more strongly compared with the endothelium-independent relaxation to an NO donor, SIN-1 (3-(4-morpholinyl)-sydnonimine), when the artery preparation was preconstricted with a high concentration (80 mM) of KCl instead of agonistic stimulation. Endothelium-dependent relaxation induced by thapsigargin was not affected by diltiazem, a blocker of L-type voltage-gated Ca2+ channels. SK&F96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole) and Ni2+, both of which block capacitative Ca(2+) entry, did not show any appreciable inhibitory effects on the endothelium-dependent relaxation to thapsigargin. These findings suggest that in guinea-pig aorta, endothelium-dependent NO-mediated relaxation induced by thapsigargin is preceded by the increase in the cytosolic free Ca2+ concentrations ([Ca2+]cyt) following the depletion of stored Ca2+ in thapsigargin-sensitive store sites in endothelial cells. Although the increase in [Ca2+]cyt responsible for the activation of endothelium NOS leading to thapsigargin-induced vascular relaxation may be ascribed to the capacitative Ca2+ entry from extracellular space, the Ca2+ entry mechanism stimulated with thapsigargin is deficient in sensitivity to SK&F96365 and Ni2+ in the endothelium of guinea-pig aorta.

Agonist-stimulated calcium entry in primary cultures of human cerebral microvascular endothelial cells

Primary cultures of human cerebral microvascular endothelial cells (HCMEC) were loaded with fura-2. The intracellular free Ca2+ concentration ([Ca2+]i) was measured by digital imaging microscopy. Agonists ATP (100 micro), thrombin (10 units/ml), and histamine (25 microM) induced a transient [Ca2+]i increase. Histamine (100 microM) induced a biphasic [Ca2+]i increase with an initial [Ca2+]i peak followed by a [Ca2+]i plateau. The [Ca2+]i plateau was blocked by the receptor-operated Ca2+ channel (ROC) blockers SK&F 96365 and NCDC, indicating a contribution by Ca2+ influx through ROC to the [Ca2+]i plateau. However, this [Ca2+]i plateau was not blocked by the voltage-gated Ca2+ channel (VGC) blocker diltiazem (DTZ). Depolarization with 80K+ or application of the VGC agonist BAY K 8644 did not alter the resting [Ca2+]i; but 80K+ reduced the histamine (100 microM) induced [Ca2+]i plateau. These results show that HCMEC are devoid of functional VGC. Thus the membrane potential (Em) regulates Ca2+ entry mainly by enhancing the electrochemical Ca2+ gradient, such that hyperpolarization increases while depolarization decreases [Ca2+]i. Blockade of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) by CPA increased [Ca2+]i. This effect was dependent on extracellular Ca2+ and reduced by iberiotoxin (IBTX) blockade of Ca2+-activated K+ channels (Kca), suggesting a role for Kca in regulating Ca2+ influx. Ca2+ is the principal activator of endothelial nitric oxide synthase (eNOS), which stimulates cyclic GMP production. The final result that the eNOS inhibitor L-NAME enhanced the histamine (100 microM) induced [Ca2+]i plateau suggests a negative feedback loop (via cGMP) of endothelial NO on its own synthesis in the regulation of endothelial [Ca2+]i signal.

2,5-Di-t-butyl-1,4-benzohydroquinone induces endothelium-dependent relaxation of rat thoracic aorta

The aim of this work was to clarify the mechanism by which 2,5-di-t-butyl-1,4-benzohydroquinone (BHQ) induces relaxation of rat thoracic aorta. In particular, the role of endothelium-derived nitric oxide (NO) was investigated. BHQ concentration dependently (0.1-10 microM) relaxed rat aorta rings precontracted with phenylephrine. This effect was dependent on the intactness of the endothelium, suppressed by preincubation with 100 microM N(omega)-nitro-L-arginine methyl ester and antagonised by 3-30 microM methylene blue. The 10 microM BHQ-induced relaxation, however, was followed by the gradual and slow return to phenylephrine-induced tone. Superoxide dismutase (250 U/ml) increased the BHQ-induced relaxation, while preincubation with 3 mM diethyldithiocarbamate inhibited it in a time-dependent fashion. BHQ gave rise to superoxide anion formation which was markedly inhibited by the addition of superoxide dismutase (250 U/ml), either in the presence or in the absence of aorta rings. The non-specific blocker of Ca2+ channels, Ni2+, concentration dependently attenuated the BHQ relaxing effect. BHQ did not modify the relaxation induced by the NO donor 3-morpholino-sydnonimine in endothelium-deprived rings. In conclusion, BHQ induces endothelium-dependent relaxation and gives rise, by auto-oxidation, to the formation of superoxide anion. The former effect results from the enhanced synthesis of NO rather than from its enhanced biological activity; NO synthase is presumed to be stimulated by BHQ-induced activation of Ca2+ influx through Ni2+-sensitive Ca2+ channels.

Cardiovascular effects of CPU-23, a novel L-type calcium channel blocker with a unique molecular structure

1. The cardiovascular effects of CPU-23 (1-[1-[(6-methoxy)-naphth-2-yl]]-ethyl-2-(1-piperidinyl)-acetyl-6, 7-dimethoxy-1,2,3,4-tetrahydroisoquinoline), a cleavage product of tetrandrine, were investigated using the whole cell perforated patch-clamp technique, in vitro tension measurements and in vivo haemodynamic recordings. 2. CPU-23 (1 and 10 microM) dose-dependently reduced concentration-response curves for KCl and phenylephrine (PE) in the rat tail artery; inhibition of KCl-induced contraction was much more potent than for PE. At the same concentrations, CPU-23 inhibited the inward Ba2+ currents in single smooth muscle cells isolated from the rat tail artery, while CPU-23 (10 microM) produced 95% vasorelaxation of the rat middle cerebral artery preconstricted with BayK 8644. 3. CPU-23 (10 and 30 microM) inhibited the noradrenaline-induced phasic contraction of the rat tail artery in the absence of extracellular Ca2+ from 40% of control to 23% and 14%, respectively (P<0.01) and tonic contraction of the artery after addition of Ca2+ (2 mM) from 100% of control to 83% and 75%, respectively (P<0.01). In the presence of extracellular Ca2+ the PE-induced contraction was reduced by CPU-23 (30 and 100 microM) to 27% and 37%, respectively. 4. The haemodynamic profile of CPU-23 in the rat was very similar to diltiazem. At 5 mg kg(-1) CPU-23 induced a rapid onset and long-lasting decrease in left ventricular systolic pressure (LVSP), maximal velocity of pressure increase (dP/dt(max)), systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR). When haemodynamic actions of CPU-23, verapamil, diltiazem and nifedipine were compared at equidepressor doses, the order of potency for reducing LVSP and dP/dt(max) was verapamil > CPU-23 = diltiazem > nifedipine and the order of potency for decreasing HR was verapamil = CPU-23 = diltiazem > nifedipine. 5. These data indicate that CPU-23 is a novel calcium channel blocker with unique molecular structure, which exerts antihypertensive and cardiac depressant effects due primarily to its action on L-type voltage-gated calcium channels.

Estrogen augments cyclopiazonic acid-mediated, endothelium-dependent vasodilation

The modulatory effects of chronic estrogen treatment on the responses to cyclopiazonic acid, an endoplasmic reticulum Ca2(+)-ATPase inhibitor, were studied in rings of aorta and the isolated perfused kidney of the rat. Rings of aorta were obtained from the following groups of age-matched rats (i) male, (ii) female, and two groups of rats implanted with a subcutaneous pellet (iii) ovariectomized, placebo-treated, (iv) ovariectomized, 17beta-estradiol-treated (0.5 mg/pellet/21 days). In phenylephrine (2 microM) pre-contracted rings with intact endothelium, cyclopiazonic acid (10(-7) to 3 x 10(-5) M) produced endothelium-dependent relaxations in a concentration-dependent manner. The cyclopiazonic acid dilation as a percentage loss of phenylephrine tone was greater in aortic rings from female (72.9 +/- 2.4%) and estrogen-treated rats (65.5 +/- 4.8%) compared to those from male (51.5 +/- 3.4%) or ovariectomized rats (40.8 +/- 3.9%) (P < 0.05, one-way analysis of variance (ANOVA)). These relaxation responses of cyclopiazonic acid were converted to contractions by pre-treatment with an inhibitor of nitric oxide (NO) synthase, N(omega)-nitro-L-arginine methyl ester (L-NAME, 200 microM; 30 min). There were no differences in cyclopiazonic acid-induced contractions of aortas excised from either estrogen-treated or untreated-ovariectomized rats. In perfused kidneys, cyclopiazonic acid (10(-5) M) caused a larger decrease in perfusion pressure in kidneys from female rats (110 +/- 0.4 mmHg) than it did in kidneys from male rats (80 +/- 0.6 mmHg). These results demonstrate that cyclopiazonic acid causes a greater endothelium-dependent dilation in estrogen-treated ovariectomized and control female rats, possibly due to unmasking of estrogen-enhanced Ca2+ entry into the endothelial cells.

SK&F 96365 inhibits intracellular Ca2+ pumps and raises cytosolic Ca2+ concentration without production of nitric oxide and von Willebrand factor

The effects of the imidazole compound SK&F 96365 on Ca2+ movements and production of nitric oxide (NO) and von Willebrand factor (vWF) have been investigated in human endothelial cells. Changes in cytosolic Ca2+ concentration ([Ca2+]i) were measured with Fura-2. Real-time production of NO was monitored with a porphyrinic microsensor and the release of vWF with an enzyme-linked immunosorbent assay. Irrespective of the transmembrane Ca2+ gradient, 30 microM SK&F 96365 doubled [Ca2+]i suggesting a Ca2+ release from intracellular stores. The SK&F 96365-induced [Ca2+]i rise was not accompanied by detectable NO and vWF production, while 1 microM thapsigargin enhanced [Ca2+]i 2.5 times, doubled the secretion of vWF and increased the NO production to 10 +/- 4 nM (n = 5). Pretreatment with SK&F 96365 prevented thapsigargin from increasing [Ca2+]i, NO production and vWF secretion. To investigate the mechanism by which SK&F 96365 released Ca2+ from internal pools, its effect and that of thapsigargin on the ATP-dependent 45Ca2+ uptake into platelet membrane vesicles were compared. SK&F 96365 as thapsigargin, dose-dependently reduced the initial rate of 45Ca2+ uptake. In conclusion, we demonstrate that, in the absence of Ca2+ entry from the extracellular space, the [Ca2+]i increase elicited by SK&F 96365 or thapsigargin is not sufficient to initiate NO synthesis and vWF secretion. This confirms the important role of Ca2+ influx in endothelial secretion processes.