Spectrophotometric evaluation of calcium ion release from different calcium hydroxide preparations: an in-vitro study

Abstract: Pulp tissue conditions such as infections have long been treated with calcium hydroxide (CaOH). In the last decade, use of mineral trioxide aggregate (MTA) has gained ground. This study was carried out to comparatively evaluate the Ca release from CaOH powder with different vehicles and different types of MTA. Materials and Methods: 40 single rooted mandibular premolars were selected, decoronated and biomechanically prepared. They were randomly divided into four groups, consisting of 10 samples each. Root canals were packed with different preparations of CaOH and MTA. Calcium ion release was evaluated with an UV-spectrophotometer. Result: Amongst the CaOH preparations, using propylene glycol as a vehicle produced extended release of calcium ions (7.34 +/- 0.01) for a period of 14 days. Whereas, amongst MTA based products, MTA angelus produced the maximum release of calcium ions (2.42 +/- 0.010). A statistically significant difference was present between the four groups (p<0.05). Conclusion: Propylene glycol mixed with CaOH powder, produces a higher and extended release of calcium ions compared to distilled water. MTA angelus produces consistent calcium ion release.

Evolution of mammographic image quality in the state of Rio de Janeiro / A evolução da qualidade da imagem em mamografia no Estado do Rio de Janeiro

Objective: To evaluate the evolution of mammographic image quality in the state of Rio de Janeiro on the basis of parameters measured and analyzed during health surveillance inspections in the period from 2006 to 2011. Materials and Methods: Descriptive study analyzing parameters connected with imaging quality of 52 mammography apparatuses inspected at least twice with a one-year interval. Results: Amongst the 16 analyzed parameters, 7 presented more than 70% of conformity, namely: compression paddle pressure intensity (85.1%), films development (72.7%), film response (72.7%), low contrast fine detail (92.2%), tumor mass visualization (76.5%), absence of image artifacts (94.1%), mammography-specific developers availability (88.2%). On the other hand, relevant parameters were below 50% conformity, namely: monthly image quality control testing (28.8%) and high contrast details with respect to microcalcifications visualization (47.1%). Conclusion: The analysis revealed critical situations in terms of compliance with the health surveillance standards. Priority should be given to those mammography apparatuses that remained non-compliant at the second inspection performed within the one-year interval. .

Objetivo: Avaliar a evolução da qualidade da imagem de mamógrafos localizados no Estado do Rio de Janeiro, de 2006 a 2011, com base em parâmetros medidos e observados durante inspeções sanitárias. Materiais e Métodos: Estudo descritivo sobre a evolução de parâmetros que condicionam a qualidade da imagem focalizou 52 mamógrafos, inspecionados no mínimo duas vezes, com intervalo de um ano. Resultados: Dos 16 parâmetros avaliados, 7 apresentaram mais de 70% de conformidade: força do dispositivo de compressão (85,1%), processamento dos filmes (72,7%), resposta do filme do serviço (72,7%), detalhes lineares de baixo contraste (92,2%), visualização de massas tumorais (76,5%), ausência de artefatos de imagem (94,1%), existência de processadoras específicas para mamografia (88,2%). Importantes parâmetros apresentaram-se abaixo de 50% de conformidade: realização de testes mensais da qualidade de imagem pelo estabelecimento (28,8%) e detalhes de alto contraste, que dizem respeito à visualização de microcalcificações (47,1%). Conclusão: A análise revelou situações críticas da atuação da vigilância sanitária, cuja prioridade deveria ser dirigida aos estacionários, ou seja, os mamógrafos que permaneceram na situação de não conformidade nas inspeções realizadas com intervalo de um ano. .

Regulation of matrix metalloproteinase-9 protein expression by 1alpha,25-(OH)2D3 during osteoclast differentiation

To investigate 1alpha,25-(OH)2D3 regulation of matrix metalloproteinase-9 (MMP-9) protein expression during osteoclast formation and differentiation, receptor activator of nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) were administered to induce the differentiation of RAW264.7 cells into osteoclasts. The cells were incubated with different concentrations of 1alpha,25-(OH)2D3 during culturing, and cell proliferation was measured using the methylthiazol tetrazolium method. Osteoclast formation was confirmed using tartrate-resistant acid phosphatase (TRAP) staining and assessing bone lacunar resorption. MMP-9 protein expression levels were measured with Western blotting. We showed that 1alpha,25-(OH)2D3 inhibited RAW264.7 cell proliferation induced by RANKL and M-CSF, increased the numbers of TRAP-positive osteoclasts and their nuclei, enhanced osteoclast bone resorption, and promoted MMP-9 protein expression in a concentration-dependent manner. These findings indicate that 1alpha,25-(OH)2D3 administered at a physiological relevant concentration promoted osteoclast formation and could regulate osteoclast bone metabolism by increasing MMP-9 protein expression during osteoclast differentiation.

Antihypertensive and vasorelaxant effects of ethanol extract of stem barks from Zanthoxylum rhoifolium Lam in rats.

Administration of ethanol extract of stem bark from Z. rhoifolium (EEtOH-ZR) induced hypotension associated with a dual effect in heart rate in normotensive rats. This response was highlighted in spontaneously hypertensive rats (SHR). In rat superior mesenteric artery rings, the cumulative addition of EEtOH-ZR (0.1–750 µg/mL) on a phenylephrine-induced pre-contraction (10-5 M) promoted a vasorelaxant effect by a concentration-dependent manner and independent of vascular endothelium. A similar effect was obtained on KCl-induced pre-contractions (80 mM). EEtOH-ZR attenuated contractions induced by cumulative addition of CaCl2 (10-6–3 × 10-2 M) in depolarizing medium without Ca2+ only at 500 or 750 µg/mL. Likewise, on S-(–)-Bay K 8644-induced pre-contractions (10-7 M), the EEtOH-ZR-induced vasorelaxant effect was attenuated. EEtOH-ZR (27, 81, 243 or 500 µg/mL) inhibited contractions induced by cumulative addition of phenylephrine (10-9 - 10-5 M) in endothelium-denuded preparations or by a single concentration (10-5 M) in a Ca2+-free medium. The involvement of K+ channels was evaluated by tetraethylammonium (3 mM); the EEtOH-ZR-induced vasorelaxation was not attenuated. Thus, calcium influx blockade through voltage-operated calcium channels (CaVL) and inhibition of calcium release from intracellular stores are probably underlying EEtOH-ZR-induced cardiovascular effects.

A Case of Isolated Small Intestinal Wall Calcification on Patient with Continuous Ambulatory Peritoneal Dialysis / 대한소화기학회지

The metastatic calcification is defined as the deposition of calcium salt in normal tissue with an abnormal serum biochemical environment, such as chronic kidney disease, hyperparathyroidism, and hypercalcemia related with malignancy. Although the metastatic calcification can develop in any organs and tissues, presenting its symptoms and complications are rare. Thus a few cases have been reported. This case shows the metastatic calcification of the small intestine without any peritoneal and mesenteric vascular calcification which was early diagnosed by computed tomography and mesenteric angiography in a patient with abdominal pain, receiving continuous ambulatory peritoneal dialysis due to end stage renal disease. The clinician should early consider the metastatic calcification as differential diagnosis when unidentified calcifications are noted in simple abdominal X-ray such as in the present case, and promptly confirm it by using appropriate diagnostic tests in order to prevent its complications and progression.

Actividad inotrópica inducida por el derivado carbamacepina-alquino en un modelo de corazón aislado y perfundido a flujo constante / Inotropic activity induced by carbamazepine-alkyne derivative in an isolated heart model and perfused to constant flow

Introducción. Existen pocos datos con respecto a los efectos de la carbamacepina y sus derivados en el aparato cardiovascular; además, el mecanismo molecular y su sitio de acción celular no son claros. Objetivo. Evaluar los efectos inducidos por el derivado carbamacepina-alquino sobre la presión de perfusión, la resistencia vascular y la presión ventricular izquierda. Materiales y métodos. Los efectos de la carbamacepina y del derivado carbamacepina-alquino sobre la presión de perfusión, la resistencia vascular y la presión ventricular izquierda fueron evaluados en un modelo de corazón aislado de rata (Langendorff). Resultados. Se encontró que: 1) el derivado carbamacepina-alquino (1x10 -9 mM) incrementa la presión de perfusión y la resistencia vascular en comparación con la carbamacepina (1×10 -9 mM); 2) los efectos del derivado carbamacepina-alquino (1×10 -9 -1×10 -4 M) sobre la presión intraventricular no fueron inhibidos por metoprolol o prazosina (1×10 -6 mM); 3); el nifedipino (1×10 -6 mM) bloquea los efectos ejercidos por el derivado carbamacepina-alquino (1×10 -9 -1×10 -4 M) sobre la presión intraventricular, y 4) el efecto del derivado de carbamacepina (1×10 -9 mM) incrementa la concentración de calcio intracelular a través del tiempo (3 a 18 minutos); sin embargo, en presencia de nifedipino (1×10-6 mM), este efecto disminuye significativamente (p=0,005). Conclusiones. La actividad ejercida por el derivado carbamacepina-alquino sobre la presión de perfusión, la resistencia vascular y la presión intraventricular, involucra la activación del canal de calcio de tipo L, lo que trae como consecuencia indirecta cambios en los niveles de calcio intracelular y, subsecuentemente, induce un efecto inotrópico positivo.

Electrophysiological effects of hydrogen sulfide on human atrial fibers / 中华医学杂志(英文版)

<p><b>BACKGROUND</b>It has been reported that endogenous or exogenous hydrogen sulfide (H(2)S) exerts physiological effects in the vertebrate cardiovascular system. We have also demonstrated that H(2)S acts as an important regulator of electrophysiological properties in guinea pig papillary muscles and on pacemaker cells in sinoatrial nodes of rabbits. This study was to observe the electrophysiological effects of H(2)S on human atrial fibers.</p><p><b>METHODS</b>Human atrial samples were collected during cardiac surgery. Parameters of action potential in human atrial specialized fibers were recorded using a standard intracellular microelectrode technique.</p><p><b>RESULTS</b>NaHS (H(2)S donor) (50, 100 and 200 µmol/L) decreased the amplitude of action potential (APA), maximal rate of depolarization (V(max)), velocity of diastolic (phase 4) depolarization (VDD) and rate of pacemaker firing (RPF), and shortened the duration of 90% repolarization (APD(90)) in a concentration-dependent manner. ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide (Gli, 20 µmol/L) partially blocked the effects of NaHS (100 µmol/L) on human atrial fiber cells. The L-type Ca(2+) channel agonist Bay K8644 (0.5 µmol/L) also partially blocked the effects of NaHS (100 µmol/L). An inhibitor of cystathionine γ-lyase (CSE), DL-propargylglycine (PPG, 200 µmol/L), increased APA, V(max), VDD and RPF, and prolonged APD(90).</p><p><b>CONCLUSIONS</b>H(2)S exerts a negative chronotropic action and accelerates the repolarization of human atrial specialized fibers, possibly as a result of increases in potassium efflux through the opening of K(ATP) channels and a concomitant decrease in calcium influx. Endogenous H(2)S may be generated by CSE and act as an important regulator of electrophysiological properties in human atrial fibers.</p>

Protective effect of calcium channel agonist BayK8644 on lungs against ischemia-reperfusion injury in rabbits / 中南大学学报(医学版)

OBJECTIVE@#To explore the protective effect of calcium channel agonist BayK8644 preconditioning on the lungs against ischemia-reperfusion injury (I/R) and its mechanism in rabbits.@*METHODS@#Forty rabbits were randomly divided into 4 groups (10 in each group): a sham-operated group (Sham), an I/R group (I/R), an ischemic preconditioning (IP) group (IP), and a BayK8644 preconditioning group(BayK8644).The wet to dry weight (W/D) ratio, superoxide dismutase (SOD) activity, myleoperoxidase (MPO), and malondialdehyde (MDA) contents of the lung tissues were measured after the operation. Morphological and ultrastructural changes of the lung tissue were observed by light and electron microscope. The expression of pulmonary surfactant-associated protein-A (SP-A) was examined with immunohistochemistry.@*RESULTS@#The W/D ratio, MPO and MDA contents of the lung tissue in the BayK8644 group and IP group were significantly lower than those in the I/R group (P0.05).@*CONCLUSION@#Appropriate BayK8644 preconditioning can induce transient Ca²+ influx, and elicit strong protection against the lung ischemia-reperfusion injury, which can simulate the endogenous protective effect of ischemic preconditioning.

Role of PKG-L-type calcium channels in the antinociceptive effect of intrathecal sildenafil

Sildenafil increases the cyclic guanosine monophosphate (cGMP) by inhibition of a phosphodiesterase 5, thereby leading to an antinociceptive effect. The increased cGMP may exert the effect on an L-type calcium channel through the activation of protein kinase G (PKG). The purpose of this study was to examine the possible involvement of a PKG-L-type calcium channel on the effect of sildenafil at the spinal level. Catheters were inserted into the intrathecal space of male SD rats. Pain was induced by applying 50 microliter of a 5% formalin solution to the hindpaw. The sildenafil-induced effect was examined after an intrathecal pretreatment of a PKG inhibitor (KT 5823), or a L-type calcium channel activator (FPL 64176). Intrathecal sildenafil produced an antinociceptive effect during phase 1 (0~10 min interval) and phase 2 (10~60 min interval) in the formalin test. Intrathecal KT 5823 and FPL 64176 attenuated the antinociceptive effect of sildenafil during both phases. Sildenafil is effective against both acute pain and the facilitated pain state at the spinal level. In addition, the inhibition of an L-type calcium channel by activation of the PKG may contribute to the antinocieptive mechanism of sildenafil in the spinal cord.

Effects of glucocorticoid on calcium-activated chloride channel expression in nasal mucosa in allergic rhinitis rats / 中华耳鼻咽喉头颈外科杂志

<p><b>OBJECTIVE</b>To investigate the expression of CLCA3 and Muc5ac in nasal mucosa in allergic rhinitis rats and the effects of glucocorticoid on its expression.</p><p><b>METHODS</b>Thirty SD rats were randomly divided into allergic rhinitis group, dexamethasone group and control group. Expression of CLCA3 mRNA and Muc5ac protein in nasal mucosa were detected by RT-PCR and immunohistochemical assay, respectively.</p><p><b>RESULTS</b>CLCA3 mRNA and Muc5ac protein in allergic rhinitis group were significantly higher than those in control group (t = 8.565, 5.317, P < 0.01, respectively). The increased expression of CLCA3 mRNA in allergic rhinitis group was well correlated with the expression of Muc5ac protein and the correlation coefficient was 0.813 (P < 0.05). After treatment with dexamethasone, the expression of CLCA3 mRNA and Muc5ac protein was notably lower than that in allergic rhinitis group (t = 3.102, 2.226, P < 0.05, respectively).</p><p><b>CONCLUSIONS</b>The stronger gene expression of CLCA3 exists, complicated with mucus overproduction in the nasal mucosa of allergic rhinitis rats. CLCA3 expression may play a pivotal role in mucus overproduction in allergic rhinitis. Dexamethasone substantially downregulates the expression of CLCA3 mRNA and Muc5ac protein.</p>

Ginkgolide B inhibits carotid sinus baroreflex in anesthetized male rats / 生理学报

The effects of ginkgolide B on the carotid sinus baroreflex (CSB) were studied in the perfused isolated carotid sinus of 30 anesthetized Sprague-Dawley male rats. The results were as follows. (1) By perfusing with ginkgolide B (0.1, 1, 10 μmol/L), the functional curve of the baroreflex was shifted to the right and upward. There was a marked decrease in peak slope (PS) and reflex decrease (RD) in mean arterial pressure (P<0.01), while the threshold pressure (TP), equilibrium pressure (EP) and saturation pressure (SP) were significantly increased (P<0.05, P<0.01). Among the functional parameters of CSB, the changes in PS, RD, TP, EP and SP were dose-dependent. (2) Pretreatment with Bay K8644 (500 nmol/L), an agonist of L-type calcium channel, completely eliminated the effects of ginkgolide B (1 μmol/L) on the CSB. (3) Pretreatment with tetraethylammonium (TEA, 1 mmol/L), an inhibitor of potassium channel, completely abolished the above effects of ginkgolide B (1 μmol/L) on the CSB. These results suggest that ginkgolide B inhibits the CSB in anesthetized rats, which is mediated by decreased calcium influx and increased potassium efflux in baroreceptor nerve endings.

Resveratrol inhibits electrical activity of paraventricular nucleus neurons in rat hypothalamic slices / 生理学报

To study the role of resveratrol in the discharges of neurons in paraventricular nucleus (PVN) in hypothalamic slices, extracellular single-unit discharge recording technique was used. The effects of resveratrol were examined with glass microelectrodes in the rat PVN neurons at resting potential level. The results were as follows: (1) In response to the application of resveratrol (0.05, 0.5, 5.0 μmol/L, n=29) to the superfusate for 2 min, the spontaneous discharge rate (SDR) of neurons in 28/29 (96.6%) hypothalamic slices significantly decreased in a dose-dependent manner; (2) Pretreatment with L-glutamate (0.2 mmol/L) led to a marked increase in the SDR in all 8/8 (100%) slices in an epileptiform pattern. The increased discharges were suppressed by the application of resveratrol (5.0 mmol/L) in all 8 slices; (3) In 8 slices, perfusion of the selective L-type calcium channel agonist, Bay K8644 (0.1 μmol/L), induced a significant increase in the discharge rate in 8/8 (100%) slices. Resveratrol (5.0 μmol/L) significantly attenuated the increased SDR in all 8 slices; (4) Pretreatment with the nitric oxide synthase (NOS) inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME, 50 μmol/L) increased SDR in 7/8 (87.5%) slices, but did not affect the inhibitory effect of resveratrol (5.0 μmol/L). These results suggest that resveratrol inhibits the electrical activity of PVN neurons and exerts neuroprotective actions on central neurons. The inhibitory effect of resveratrol is possibly related to the blockade of L-type calcium channel, but not due to NO release.

Effects of ginkgolide B on neuronal discharges in paraventricular nucleus of rat hypothalamic slices / 神经科学通报·英文版

<p><b>OBJECTIVE</b>To study the central role of ginkgolide B (BN52021) in regulating cardiovascular function of nerve center by examining the effects of ginkgolide B on the electrical activity of rat paraventricular nucleus (PVN) neurons in hypothalamic slice preparation and to elucidate the mechanism involved.</p><p><b>METHODS</b>Extracellular single-unit discharge recording technique.</p><p><b>RESULTS</b>(1) In response to the application of ginkgolide B (0.1, 1, 10 micromol/L; n = 27) into the perfusate for 2 min, the spontaneous discharge rates (SDR) of 26 (26/27, 96.30%) neurons were significantly decreased in a dose-dependent manner. (2) Pretreatment with L-glutamate (L-Glu, 0.2 mmol/L) led to a marked increase in the SDR of all 8 (100%) neurons in an epileptiform pattern. The increased discharges were suppressed significantly after ginkgolide B (1 micromol/L) was applied into the perfusate for 2 min. (3) In 8 neurons, perfusion of the selective L-type calcium channel agonist, Bay K 8644 (0.1 micromol/L), induced a significant increase in the discharge rates of 8 (8/8, 100%) neurons, while ginkgolide B (1 micromol/L) applied into the perfusate, could inhibit the discharges of 8 (100%) neurons. (4) In 8 neurons, the broad potassium channels blocker, tetraethylammonium (TEA, 1 mmol/L) completely blocked the inhibitory effect of ginkgolide B (1 micromol/L).</p><p><b>CONCLUSION</b>These results suggest that ginkgolide B can inhibit the electrical activity of paraventricular neurons. The inhibitory effect may be related to the blockade of L-type voltage-activated calcium channel and potentially concerned with delayed rectifier potassium channel (K(DR)).</p>

Incidencia de hipocalcemia pos tiroidectomía total / Incidence of hypocalcemia after total thyroidectomy

Background: Postoperative hypocalcemia is one of the most common complications of thyroid surgery. It is related to the type of disease (malignant or benign), the number of identified parathyroid glands during the surgical procedure, and the surgeon's experience. Total thyroidectomy is the procedure of choice in our hospital for benign and malignant thyroid disease, but it can increase the incidence of complications. Aim: To evaluate the incidence of postoperative hypocalcemia in patients subjected to a total thyroidectomy. Material and methods: Two studies were performed. A retrospective review of medical records of 448 patients subjected to total thyroidectomy, looking for serum calcium levels of less than 8 mg/dl and clinical signs of hypocalcemia. In a second study, 45 patients were followed with measurements of preoperative and postoperative serum calcium levels. Results: In the retrospective study, only 136 records had reliable information. Clinical signs of hypocalcemia were registered in 14 percent of patients and a low serum calcium level was detected in 50 percent. In the prospective study, 42 percent of patients had a postoperative low serum calcium level and seven patients (15 percent) had symptoms. Patients were handled with oral calcium and calcitriol in some cases. Ninety nine percent of patients had normal serum calcium levels two moths after surgery. Conclusions: In this series, the rate of postoperative hypocalcemia after total thyroidectomy is similar to internaitonal reports.

Effect of Bay K 8644 on arteriole smooth muscle cell membrane potential in rats with severe hemorrhagic shock / 南方医科大学学报

<p><b>OBJECTIVE</b>To investigate the effect of Ca(2+) influx through L-type Ca(2+) channels on normal and hyperpolarized membrane potential of arteriole smooth muscle cells (ASMCs) in rats.</p><p><b>METHODS</b>The ASMCs isolated from normal rats and those with severe hemorrhagic shock were labeled with DiBAC4 (3) for membrane potential detection.</p><p><b>RESULTS</b>Ca(2+) influx caused hyperpolarization of the membrane potential in the normal ASMCs but depolarization in the cells from rats with hemorrhagic shock, and this effect could be inhibited by TEA.</p><p><b>CONCLUSION</b>Ca(2+)-activated potassium channels activated by Ca(2+) influx through L-type Ca(2+) channels in normal ASMCs to cause hyperpolarization but leads directly to membrane potential depolarization in ASMCs from rats with severe hemorrhagic shock. This finding can be meaningful for treatment of vascular hyporeactivity in advanced stage of severe shock.</p>

Resveratrol inhibits neuronal discharges in rat hippocampal CA1 area / 生理学报

The effects of resveratrol on the discharges of neurons in CA1 area of rat hippocampal slices were examined by using extracellular recording technique. The results are as follows: (1) In response to the application of resveratrol (0.05, 0.5, 5.0 micromol/L, n=52) into the superfusate for 2 min, the spontaneous discharge rate of 46/52 (88.5%) neurons was significantly decreased in a dose-dependent manner; (2) Application of L-glutamate (0.2 mmol/L) into the superfusate led to a marked increase in discharge rate of all 8 (100%) slices in an epileptiform pattern. The increased discharges were suppressed by application of resveratrol (5.0 micromol/L); (3) In 7 slices, perfusion of the selective L-type calcium channel agonist, Bay K8644 (0.1 micromol/L), induced a significant increase in the discharge rate of 6/7 (85.7%) slices. The increased discharges were suppressed by application of resveratrol (5.0 micromol/L); (4) In 9 slices, perfusion of nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 50 micromol/L) into the superfusate significantly augmented the discharge rate of 7/9 (77.8%) slices. Resveratrol (5.0 micromol/L) applied into the superfusate reduced the increased discharges of all 7/7 (100%) neurons; (5) In 10 units, the large-conductance Ca(2+)-activated K(+) channel blocker (tetraethylammonium chloride, TEA, 1 mmol/L) significantly increased the discharge rate of 9/10 (90%) slices. Resveratrol (5.0 micromol/L) applied into the superfusate inhibited the discharges of 8/9 (88.9%) slices. These results suggest that resveratrol inhibits the electrical activity of CA1 neurons. This effect may be related to the blockade of L-type calcium channel and a subsequent reduction of calcium influx, and probably has no association with large-conductance Ca(2+)-activated K(+) channel.

Electrophysiological effects of nitric oxide on spontaneous activity of rabbit atrioventricular node cells / 生理学报

The electrophysiological effects of nitric oxide (NO) on spontaneous activity of rabbit atrioventricular (AV) node cells were examined using intracellular microelectrode technique. The results obtained are as follows. (1) NO donors sodium nitroprusside (SNP, 1~1000 micromol/L) and 3-morpholinosydnonimine (SIN-1, 100, 1000 micromol/L) decreased the amplitude of action potential (APA), rate of spontaneous firing (RSF), velocity of diastolic (phase 4) depolarization (VDD), and maximal rate of depolarization (V(max)) in a concentration-dependent manner. (2) Pretreatment with L-type calcium channel agonist Bay K8644 (0.25 micromol/L) completely reversed the effects of SNP (100 micromol/L) on AV node cells. (3) Elevation of Ca(2+) concentration (5 mmol/L) in superfusate antagonized the effects of SNP on AV node cells. (4) Perfusion with Ca(2+)-free K-H solution, completely abolished the effects of SNP on AV node cells. (5) Application of methylene blue (50 micromol/L), a guanylyl cyclase inhibitor, failed to abolish the inhibitory effects of SNP (100 micromol/L). All these results suggest that NO exerts a negative effect on spontaneous activity of AV node cells in rabbits. These effects are likely due to reduction in calcium influx via a cGMP-independent mechanism.