Triphosphate-dependent Potassium Channels Research Articles

Antidiabetic Properties of Berberine: From Cellular Pharmacology to Clinical Effects

Berberine is an alkaloid that is highly concentrated in the roots, rhizomes, and stem bark of various plants. It affects glucose metabolism, increasing insulin secretion, stimulating glycolysis, suppressing adipogenesis, inhibiting mitochondrial function, activating the 5' adenosine monophosphate-activated protein kinase (AMPK) pathway, and increasing glycokinase activity. Berberine also increases glucose transporter-4 (GLUT-4) and glucagon-like peptide-1 (GLP-1) levels. On GLP-1 receptor activation, adenylyl cyclase is activated, and cyclic adenosine monophosphate is generated, leading to activation of second messenger pathways and closure of adenosine triphosphate–dependent potassium channels. Increased intracellular potassium causes depolarization, and calcium influx through the voltage-dependent calcium channels occurs. This intracellular calcium increase stimulates the migration and exocytosis of the insulin granules. In glucose-consuming tissues, such as adipose, or liver or muscle cells, berberine affects both GLUT-4 and retinol-binding protein-4 in favor of glucose uptake into cells; stimulates glycolysis by AMPK activation; and has effects on the peroxisome proliferator-activated receptor γ molecular targets and on the phosphorylation of insulin receptor substrate-1, finally resulting in decreased insulin resistance. Moreover, recent studies suggest that berberine could have a direct action on carbohydrate metabolism in the intestine. The antidiabetic and insulin-sensitizing effect of berberine has also been confirmed in a few relatively small, short-term clinical trials. The tolerability is high for low dosages, with some gastrointestinal complaints appearing to be associated with use of high dosages.

Модуляція збудливості кардіоміоцитів активатором аденозитрифосфатзалежних калієвих каналів флокаліном

Модуляція збудливості кардіоміоцитів активатором аденозитрифосфатзалежних калієвих каналів флокаліном

Early Anesthetic Preconditioning in Mixed Cortical Neuronal-Glial Cell Cultures Subjected to Oxygen-Glucose Deprivation: The Role of Adenosine Triphosphate Dependent Potassium Channels and Reactive Oxygen Species in Sevoflurane-Induced Neuroprotection

The purpose of the present study, on mixed cortical neuronal-glial cell cultures subjected to transient oxygen-glucose deprivation (OGD) was: i) to compare the neuroprotection afforded by sevoflurane added either before (preconditioning) or during (direct neuroprotection) the OGD and ii) to explore the possible involvement of adenosine triphosphate-sensitive potassium (KATP) channels and intracellular reactive oxygen species (ROS) levels in the mechanism of the early preconditioning effect of sevoflurane. Mature mixed cortical neuronal-glial cell cultures were exposed to 90-min OGD in an anaerobic chamber followed by reoxygenation. Sevoflurane (0.03-3.4 mM) was randomly administered for 90 min and discontinued 60 min before OGD (early preconditioning) or during the 90-min OGD (direct neuroprotection). Cell death was quantified 24 h after the OGD by lactate dehydrogenase release into the bathing medium. Intracellular ROS generation was assessed at the end of sevoflurane preconditioning using 2',7'-dichlorofluorescin diacetate. Sevoflurane preconditioning elicited a potent threshold-dependent neuroprotective effect at concentrations higher than 0.07 mM and sevoflurane added during OGD elicited a dose dependent neuroprotective effect. Blockers of KATP channels (glibenclamide 0.3 microM and 5 hydroxydecanoic acid 50 microM), or ROS-scavengers (N-2-mercaptopropionyl glycine 100 microM and N-acetylcysteine 50 microM), although they did not affect cell viability, counteracted the neuroprotection produced by early sevoflurane preconditioning. Sevoflurane exposure during preconditioning induced a significant increase in ROS levels which was prevented by both ROS scavengers and blockers of KATP channels. Early sevoflurane preconditioning induced a threshold-dependent protection of mixed cortical neuronal-glial cell cultures against OGD by mechanisms that seem to involve opening KATP channels, thereby leading to generation of ROS.

Downregulation of Kir6.1/SUR2B channels in the obese rat aorta

Objective This study was designed to evaluate the contribution of adenosine triphosphate–dependent potassium channels to the increase in blood pressure observed in obese rats. Methods The experiment was performed in male Sprague-Dawley rats. Glibenclamide-sensitive currents were measured in vascular smooth muscle cells by patch-clamp. Expressions of Kir6.1 and SUR2B were examined by reverse transcription polymerase chain reaction and western blot techniques, respectively. Results In the aortic vascular smooth muscle cells, pinacidil induced glibenclamide-sensitive currents. The current from obese rats was significantly lower (−10.55 ± 1.63 pA/pF) compared with that from the control rats (−20.18 ± 2.79 pA/pF). Expressions of Kir6.1 and SUR2B were downregulated in vascular smooth muscle cells of aortas from the obese rats. Conclusion These findings suggest that the adenosine triphosphate–dependent potassium channel is downregulated in smooth muscle cells from the aortas of obese rats, which may contribute to the increase in blood pressure in these rats.

Gatifloxacin-induced dysglycemia

The development of gatifloxacin-induced dysglycemia in 13 patients is described; the details of the 3 most severe cases are presented. Three elderly patients developed dysglycemia after initiation of gatifloxacin therapy. Both patients who developed hypoglycemia were receiving concomitant insulin or oral antidiabetic agents. Repeated doses of dextrose were required for management. The Naranjo et al. probability rating scale revealed that gatifloxacin was the probable cause in the majority of the 13 cases, primarily because of the temporal relationship with gatifloxacin and, in some instances, resolution of dysglycemia after drug discontinuation. Although the mechanism of gatifloxacin-induced hyperglycemia is not known, in vitro studies have found that certain quinolone antimicrobials can lower serum glucose levels by blocking adenosine 5'-triphosphate-dependent potassium channels in the pancreatic beta-cell, stimulating insulin release. It is difficult to unequivocally implicate gatifloxacin as the only cause of dysglycemia in the cases presented, as there are many explanations for poor glycemic control in hospitalized patients, such as stress, infection, decreased renal function, and concomitant drug therapies. However, the patients' medication regimens appeared to be stable before gatifloxacin administration. Thirteen patients developed dysglycemia after receiving gatifloxacin. Gatifloxacin was found to be the probable cause in the majority of cases.

Upstream signaling of protein kinase C-ε in xenon-induced pharmacological preconditioning: Implication of mitochondrial adenosine triphosphate dependent potassium channels and phosphatidylinositol-dependent kinase-1

Xenon elicits preconditioning of the myocardium via protein kinase C-ε. We determined the implication of (1) the mitochondrial adenosinetriphosphate dependent potassium (K ATP) channels and (2) the 3′phosphatidylinositol-dependent kinase-1 (PDK-1) in activating protein kinase C-ε. For infarct size measurements, anaesthetized rats were subjected to 25 min of coronary artery occlusion followed by 120 min of reperfusion. Rats received xenon 70% during three 5-min periods before ischaemia with or without the K ATP channel blocker 5-hydroxydecanoate or Wortmannin as PI 3K/PDK-1 inhibitor. For Western blot, hearts were excised at five time points after xenon preconditioning (Control, 15, 25, 35, 45 min). Infarct size was reduced from 42 ± 6% (mean ± S.D.) to 27 ± 8% after xenon preconditioning ( P < 0.05). Western blot revealed an increased activation of PKC-ε after 45 min and of PDK-1 after 25 min during xenon preconditioning. 5-hydroxydecanoate and Wortmannin blocked both effects. PKC-ε is activated downstream of mitochondrial K ATP channels and PDK-1. Both pathways are functionally involved in xenon preconditioning.

Gerard W. Ostheimer “What’s New in Obstetric Anesthesia” Lecture

Gerard W. Ostheimer “What’s New in Obstetric Anesthesia” Lecture

Nicorandil

Nicorandil is a nicotinamide nitrate used as an antianginal agent. It has two modes of action. First, by opening adenosine triphosphate–dependent potassium channels, nicorandil increases transmembrane potassium conductance and relaxes peripheral and coronary arterioles. Second, with its nitrate moiety, nicorandil increases intracellular concentrations of cGMP, resulting in peripheral vein and coronary artery dilation. Thus, because of its ability to dilate arteries and veins, nicorandil maximizes coronary flow while concomitantly reducing myocardial work through reductions in afterload. For these reasons, nicorandil has been successful in managing angina and hypertension. However, growing evidence suggests that this drug provides additional benefits that reach beyond its original therapeutic indications. Recently, the Impact of Nicorandil in Angina (IONA) study demonstrated significant improvement in outcomes in patients with angina when comparing a composite end point of morbidity and mortality attributable to coronary heart disease, nonfatal myocardial infarction, and unplanned hospital admission for chest pain.1 The consensus regarding the success of nicorandil in IONA purports an association between cardiac preservation and mitochondrial adenosine triphosphate–dependent potassium (KATP) channel activation.1 In light of what is already known about KATP channels, this is a germane conclusion, specifically, with regard to the pivotal role KATP channels play in cardiac preconditioning and …

Warm blood cardioplegic arrest induces mitochondrial-mediated cardiomyocyte apoptosis associated with increased urocortin expression in viable cells

ObjectivesThis study assesses the mechanisms of apoptosis in patients after on-pump coronary artery bypass graft surgery and the potential involvement of the endogenous cardiac peptide urocortin as a cardiomyocyte salvage mechanism. We have previously described the mechanisms of apoptosis after ischemia-reperfusion injury in the rat heart and shown that endogenous urocortin is cardioprotective. Here we extend these findings to the human heart exposed to ischemic-reperfusion injury. MethodsTwo sequential biopsy specimens were obtained from the right atriums of 24 patients undergoing coronary artery bypass grafting at the start of grafting and 10 minutes after release of the aortic clamp. Apoptosis was identified by means of immunocytochemical colocalization between terminal deoxynucleotidyl transferase–mediated nick end-labeling positivity and active caspase-3. Immunostaining for active caspase-9 and caspase-8 was performed to identify the pathways of apoptosis induction. Urocortin and adenosine triphosphate–dependent potassium channel expression was also assessed by means of immunocytochemistry. ResultsMyocyte apoptosis (<0.1% before coronary artery bypass grafting) was increased after coronary artery bypass grafting and reperfusion and was greater in patients with longer periods of cardioplegic arrest (3.3% ± 0.5% with <55 minutes and 5.1% ± 0.9% with 85-100 minutes, P < .001). Processing of caspase-9 was always more pronounced than that of caspase-8 (P < .05). Cardioplegic arrest was also associated with increased urocortin expression (up to 29% ± 3.5% vs <3% in samples obtained before coronary artery bypass grafting, P < .001) but only in nonapoptotic myocytes. These and surrounding viable myocytes also showed increased Kir6.1 adenosine triphosphate–dependent potassium channel expression. ConclusionsCardioplegic arrest and subsequent reperfusion result in cardiomyocyte apoptosis, largely through mitochondrial injury, as well as exclusive urocortin expression in viable cells. This finding might suggest a cardioprotective role for endogenous urocortin in human subjects.

Effects of Adenosine Triphosphate Dependent Potassium Channel Opener on Bladder Overactivity in Rats With Cerebral Infarction

We evaluated the effect of the adenosine triphosphate dependent K channel opener KRN2391 (N-cyano-N' -(2-nitroxyethyl)-3-pyridinecarboximidamide methanesulfonate) (Kirin Brewery Co., Gunma, Japan) on bladder overactivity induced by middle cerebral artery occlusion. At 7 days after implantation of a bladder catheter in male Sprague-Dawley rats a cannula for intracerebroventricular administration was implanted and the left middle cerebral artery was occluded with 4-zero monofilament nylon thread. At 22 hours after the induction of cerebral ischemia saline was infused into the bladder at a constant rate (200 microl. per minute) and a cystometrogram was obtained with the rat in the conscious state. KRN2391 (5 microl.) was administered in intracerebroventricular fashion at graded doses (0.15 to 15 microg.). Bladder capacity in conscious rats was significantly reduced after left middle cerebral artery occlusion. Intracerebroventricular administration of KRN2391 significantly increased bladder capacity in cerebral infarcted but not in sham operated rats. These results show that a adenosine triphosphate dependent K channel opener may be useful for neurogenic bladder overactivity after cerebral infarction via action on the central nervous system.

Effects of Adenosine Triphosphate Dependent Potassium Channel Opener on Bladder Overactivity in Rats With Cerebral Infarction

Effects of Adenosine Triphosphate Dependent Potassium Channel Opener on Bladder Overactivity in Rats With Cerebral Infarction

Long-term administration of nicorandil abolishes ischemic and pharmacologic preconditioning of the human myocardium: Role of mitochondrial adenosine triphosphate–dependent potassium channels

Background: Acute administration of mitochondrial adenosine triphosphate–dependent potassium channel openers preconditions the heart, but whether their long-term administration induces a permanent state of protection is unknown. These studies investigate the effect of long-term treatment with the mitochondrial adenosine triphosphate–dependent potassium channel opener nicorandil on the response of the human myocardium to ischemia and preconditioning. Methods: Right atrial tissue obtained from patients regularly treated with or without nicorandil (mean of 20 mg/d for 18.6 ± 2.5 months) and undergoing cardiac surgery was sliced and equilibrated for 30 minutes and then subjected to 90 minutes of simulated ischemia, followed by 120 minutes of reoxygenation. In study 1 the following groups were studied to investigate the effect of nicorandil on the susceptibility of the myocardium to ischemia and on the protective effect of ischemic and pharmacologic preconditioning: (1) aerobic control; (2) simulated ischemia and reoxygenation alone; (3) ischemic preconditioning with 5 minutes of simulated ischemia and 5 minutes of reoxygenation; and (4) phenylephrine (0.1 μmol/L) for 5 minutes and 5 minutes' washout before simulated ischemia and reoxygenation. In study 2 the following groups were studied to investigate the effect of nicorandil on the responsiveness of mitochondrial adenosine triphosphate–dependent potassium channels: (1) aerobic control; (2) simulated ischemia and reoxygenation; (3) ischemic preconditioning; (4) diazoxide (100 μmol/L) for 10 minutes before simulated ischemia and reoxygenation, and (5) 5-hydroxydecanoate (1 mmol/L) for 10 minutes before simulated ischemia and reoxygenation. In study 3 the following groups were included to investigate the effect of the long-term administration of nicorandil on the kinase pathway involved in preconditioning: (1) aerobic control; (2) simulated ischemia and reoxygenation alone; (3) ischemic preconditioning; (4) phorbol 12-myristate 13-acetate (1 μmol/L), a protein kinase C activator, for 10 minutes before simulated ischemia and reoxygenation; and (5) anisomycin (1 nmol/L), a p38 mitogen-activated protein kinase activator, for 10 minutes before simulated ischemia and reoxygenation. At the end of each protocol, the leakage of creatine kinase (in units per gram wet weight) and the reduction of 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide into insoluble formazan dye (in millimoles per gram wet weight) were measured. Results: In study 1 the leakage of creatine kinase and the reduction of 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide induced by simulated ischemia and reoxygenation were similar in the groups with or without nicorandil (creatine kinase, 3.4 ± 0.1 and 3.5 ± 0.2, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 74.6 ± 3.9 and 67.9 ± 7.3, respectively; P >.2 in each instance). Ischemic preconditioning and pharmacologic preconditioning protected the myocardium from patients without nicorandil (creatine kinase, 2.3 ± 0.1 and 2.4 ± 0.1, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 131.4 ± 4.9 and 128.4 ± 5.6, respectively; P < 0.001 vs simulated ischemia and reoxygenation alone in each instance) but not the myocardium from patients receiving nicorandil (creatine kinase, 3.3 ± 0.1 and 3.3 ± 0.2, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 89.7 ± 6.5 and 86.4 ± 5.2, respectively; P >.2 vs simulated ischemia and reoxygenation alone in each instance). In study 2 the administration of diazoxide had identical protection to that of ischemic preconditioning in the myocardium of patients not receiving nicorandil (creatine kinase, 2.1 ± 0.2 and 2.3 ± 0.1, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 141.4 ± 7.4 and 131.4 ± 4.9, respectively; P < 0.001 vs simulated ischemia and reoxygenation alone in each instance) but failed to precondition the myocardium from patients treated with nicorandil (creatine kinase, 3.3 ± 0.2 and 3.4 ± 0.1, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 90.1 ± 7.2 and 86.4 ± 5.2, respectively; P > 0.2 vs simulated ischemia and reoxygenation alone in each instance). In study 3 phorbol 12-myristate 13-acetate or anisomycin given for 10 minutes before simulated ischemia and reoxygenation afforded similar protection to that of ischemic preconditioning in the myocardium from patients with (creatine kinase, 1.5 ± 0.3 and 1.4 ± 0.1, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 147.0 ± 4.9 and 160.0 ± 16.1, respectively; P < 0.001 vs simulated ischemia and reoxygenation alone in each instance) and without nicorandil (creatine kinase, 1.7 ± 0.4 and 1.4 ± 0.2, respectively; 3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide, 160.3 ± 13.6 and 158.3 ± 11.8, respectively; P <.001 vs simulated ischemia and reoxygenation alone in each instance). Conclusion: The myocardium of patients chronically treated with nicorandil cannot be preconditioned either by ischemia or pharmacologically, and this is because of unresponsive mitochondrial adenosine triphosphate–dependent potassium channels. However, protection can be obtained by protein kinase C and p38 mitogen-activated protein kinase activation, which are downstream of mitochondrial adenosine triphosphate–dependent potassium channels in the signaling transduction pathway of preconditioning.J Thorac Cardiovasc Surg 2002;124:750-7

CONTRIBUTION OF CEREBRAL NITRIC OXIDE TO BLADDER OVERACTIVITY AFTER CEREBRAL INFARCTION IN RATS

We investigated the contribution of cerebral nitric oxide to neurogenic voiding dysfunction after cerebral infarction. The left mid cerebral artery in female Sprague-Dawley rats was occluded with 4-zero monofilament nylon thread. Bladder activity was monitored during infusion cystometrography. Time or dose dependent effects of intracerebral ventricular administration of the nonselective nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), were investigated in conscious, sham operated and cerebral infarcted rats. The selective neuronal nitric oxide synthase inhibitor 1-(2-trifluoromethylphenyl) imidazole was also administered to determine the participation of nitric oxide synthase subtypes. Cross-sectional infarct area was measured and infarct volume was calculated 12 hours after mid cerebral artery occlusion. Bladder capacity was reduced by 54% 30 minutes after mid cerebral artery occlusion. L-NAME significantly increased bladder capacity in a dose and time dependent manner in cerebral infarcted rats but had no effect on sham operated rats. L-NAME (50 microg./kg.) administered 3 or 5 hours after occlusion significantly increased bladder capacity. This effect of L-NAME was reversed by injecting 250 microg. L-arginine per rat, which alone did not produce any significant change in bladder capacity in cerebral infarcted rats. Administration of 1-(2-trifluoromethylphenyl) imidazole also significantly increased bladder capacity in these rats. On the other hand, 5 microg. of the nitric oxide donor FK-409 per rat reduced bladder capacity for 10 to 15 minutes. None of the drugs affected infarct volume. These results indicate that supraspinal nitric oxide has an important role in bladder overactivity after cerebral infarction but it does not affect normal micturition in rats. This finding suggests a central mechanism sensitive to nitric oxide for bladder overactivity after cerebral infarction.

Adenosine triphosphate–dependent potassium channel modulation and cardioplegia-induced protection of human atrial muscle in an in vitro model of myocardial stunning

Objectives: Although adenosine triphosphate–dependent potassium channel openers have been shown to enhance cardioplegic protection in animal myocardium, there is a lack of data on human cardiac tissues. We aimed at determining, on human atrial muscle, whether adenosine triphosphate– dependent potassium channels are involved in protection caused by high-potassium cardioplegia and whether adenosine triphosphate–dependent potassium channel activation might improve cardioplegic protection in an in vitro model of myocardial stunning. Methods: Human atrial trabeculae were obtained from adult patients undergoing cardiac operations. In an organ bath at 37°C, the preparations were subjected to 60 minutes of hypoxia at a high stimulation rate either in Tyrode solution (control, n = 17) or in St Thomas’ Hospital solution without additives (n = 6) or associated with 100 nmol/L bimakalim (n = 7) or 1 μmol/L glibenclamide (n = 7), followed by 60 minutes of reoxygenation and 15 minutes of positive inotropic stimulation with 1 μmol/L dobutamine. Results: Atrial developed tension was reduced by hypoxia to 27% ± 5% of baseline and incompletely recovered after reoxygenation to 38% ± 7%, whereas dobutamine restored contractility to 74% ± 7% of basal values. St Thomas’ Hospital solution with or without bimakalim improved developed tension after reoxygenation and dobutamine ( P < .0001 vs control), whereas glibenclamide inhibited these protective effects of cardioplegic arrest ( P = .001 vs St Thomas’ Hospital solution). After reoxygenation, the protective effect of bimakalim disappeared at a high pacing rate (400- and 300-ms cycle length) but recovered during dobutamine superfusion. Conclusions: Adenosine triphosphate–dependent potassium channels are likely involved in the cardioprotective effects of cardioplegia in human atrial trabeculae and adenosine triphosphate–dependent potassium channel activation with bimakalim used as an additive to cardioplegia enhanced protection. (J Thorac Cardiovasc Surg 2000;119:842-8)

Role of nucleotide cyclases in the inhibition of pregnant rat uterine contractions by the openers of potassium channels

Objective: Our objective was to study the involvement of adenylate and guanylate cylases in spontaneous uterine contractions and inhibition induced by the opening of potassium channels. Study Design: Uterine rings from rats at mid and term gestation and from rats at term gestation in labor were suspended in organ chambers for isometric tension recording. Concentration-response relationships to an opener of adenosine triphosphate–dependent potassium channels, levcromakalim, or to an opener of calcium-dependent potassium channels, NS 1619, were studied in the absence and presence of inhibitors of adenylate cyclase (MDL 12330 A, 2 × 10 –5 mol/L; SQ 22536, 10 –4 mol/L) or guanylate cyclase (LY 83583,3 × 10 –6 mol/L). Results: MDL 12330 A and SQ 22536 accentuated contractions in rings from rats at mid gestation but not at term gestation or at term gestation in labor. LY 83583 inhibited contractions in the rings from all 3 groups. Levcromakalim was equally effective in inhibiting contractions of rings from all 3 groups. MDL 12330 A, but not SQ 22536, decreased sensitivity and maximal inhibition induced by levcromakalim (term gestation greater than mid gestation greater than term gestation in labor). LY 83583 decreased the sensitivity to and maximal inhibition induced by levcromakalim in rings from pregnant rats at mid gestation. NS 1619 attenuated contraction of rings from rats at mid gestation and, to a lesser extent, at term gestation but accentuated contractions in rings from animals at term gestation in labor. MDL 12330 A, but not SQ 22536 or LY 83583, attenuated the changes induced by NS 1619 in rings from all 3 groups. Conclusions: (1) The influence of nucleotide cyclases on basal uterine contractility depends on gestational age. (2) The inhibition of uterine contractions that results from the opening of calcium-dependent potassium channels depends on adenylate cyclase, whereas that of adenosine triphosphate–dependent potassium channels depends on both adenylate and guanylate cyclases. 3. Activation of adenosine triphosphate–dependent potassium channels is more efficient than activation of calcium-dependent potassium channels. 4. The inhibition induced by calcium-dependent potassium channel openers, but not adenosine triphosphate–dependent potassium channel openers, decreases as pregnancy progresses, and at delivery the activation of spontaneous contractile activity is evident. (Am J Obstet Gynecol 2000;182:913-8.)

Roles of potassium channels and nitric oxide in modulation of uterine contractions in rat pregnancy

Objective: We sought to study the involvement of potassium channels in the inhibition by nitric oxide of spontaneous contractions in isolated uterine rings from midterm and term pregnant rats. Study Design: Uterine rings from Sprague-Dawley rats at midterm and term gestation were used for isometric tension recording. The inhibition of spontaneous contractile activity by potassium channel openers and nitric oxide was studied in the absence and presence of potassium channel inhibitors. Results: The adenosine triphosphate–dependent potassium channel opener levcromakalim inhibited spontaneous contractions in rings from both midterm and term pregnant rats in a concentration-dependent manner, and the effects were significantly attenuated by pretreatment with selective inhibitor of the adenosine triphosphate–dependent potassium channel inhibitor glibenclamide. The opener of calcium-dependent potassium channel NS 1619 inhibited spontaneous contractions in rings from midterm but significantly less so in rings from term pregnant rats in a concentration-dependent manner, and the effect was significantly attenuated by pretreatment with potassium channel inhibitors tetraethylammonium and tetrabutylammonium but not with glibenclamide. Rings from midterm and term pregnant rats were more sensitive to the inhibitory effect of levcromakalim compared with NS 1619. Nitric oxide donor diethylamine–nitric oxide inhibited spontaneous contractions in rings from midterm but significantly less in rings from term pregnant rats in a concentration-dependent manner, and the effect was attenuated by tetraethylammonium and tetrabutylammonium but not by glibenclamide. Conclusions: There is gestational age–dependent refractoriness to calcium-dependent potassium but not adenosine triphosphate–dependent potassium channel opener–induced inhibition of spontaneous contractile activity of isolated rat uterine rings. Nitric oxide inhibits uterine contractions by opening of calcium-dependent potassium channels in pregnant rat myometrium. Refractoriness to nitric oxide toward term may result from decreased probability to open or number of calcium-dependent potassium channels. (Am J Obstet Gynecol 1999;181:649-55.)