Ionic currents and inhibitory effects of glibenclamide in seminal vesicle smooth muscle cells

Acute Metabolic Stress Induces Lymphatic Dysfunction Through KATP Channel Activation

Lymphatic dysfunction is an underlying component of multiple metabolic diseases, including diabetes, obesity, and metabolic syndrome. We investigated the roles of KATP channels in lymphatic contractile dysfunction in response to acute metabolic stress induced by inhibition of the mitochondrial electron transport chain. Ex vivo popliteal lymphatic vessels from mice were exposed to the electron transport chain inhibitors antimycin A and rotenone, or the oxidative phosphorylation inhibitor/protonophore, CCCP. Each inhibitor led to a significant reduction in the frequency of spontaneous lymphatic contractions and calculated pump flow, without a significant change in contraction amplitude. Contraction frequency was restored by the KATP channel inhibitor, glibenclamide. Lymphatic vessels from mice with global Kir6.1 deficiency or expressing a smooth muscle-specific dominant negative Kir6.1 channel were resistant to inhibition. Antimycin A inhibited the spontaneous action potentials generated in lymphatic muscle and this effect was reversed by glibenclamide, confirming the role of KATP channels. Antimycin A, but not rotenone or CCCP, increased dihydrorhodamine fluorescence in lymphatic muscle, indicating ROS production. Pretreatment with tiron or catalase prevented the effect of antimycin A on wild-type lymphatic vessels, consistent with its action being mediated by ROS. Our results support the conclusion that KATP channels in lymphatic muscle can be directly activated by reduced mitochondrial ATP production or ROS generation, consequent to acute metabolic stress, leading to contractile dysfunction through inhibition of the ionic pacemaker controlling spontaneous lymphatic contractions. We propose that a similar activation of KATP channels contributes to lymphatic dysfunction in metabolic disease.

KATP channels in lymphatic function

KATP channels function as negative regulators of active lymphatic pumping and lymph transport. This review summarizes and critiques the evidence for the expression of specific KATP channel subunits in lymphatic smooth muscle and endothelium, the roles that they play in normal lymphatic function, and their possible involvement in multiple diseases, including metabolic syndrome, lymphedema, and Cantú syndrome. For each of these topics, suggestions are made for directions for future research.

Large-conductance calcium-activated K+ channels, rather than KATP channels, mediate the inhibitory effects of nitric oxide on mouse lymphatic pumping

BACKGROUND AND PURPOSE

K_ATP channels are negative regulators of lymphatic vessel excitability and contractility and are proposed to be targets for immune cell products that inhibit lymph transport. Previous studies in rat and guinea pig mesenteric lymphatics found that NO-mediated inhibition of lymphatic contraction was prevented or reversed by the K_ATP channel inhibitor, glibenclamide. We revisited this hypothesis using mouse lymphatic vessels and K_ATP channel knockout mice.

EXPERIMENTAL APPROACH

Mouse popliteal lymphatics were isolated, and contractility was assessed using pressure myography. K⁺ channel expression was determined by PCR analysis of FACS-purified lymphatic smooth muscle cells.

KEY RESULTS

The NO-producing agonist, ACh, and the NO donor, NONOate, both produced dose-dependent inhibition of spontaneous lymphatic contractions that were blocked by the soluble GC inhibitor, ODQ, or the PKG inhibitor, Rp-8-Br-PET-cGMPS. Surprisingly, the inhibitory effects of both were preserved in K_ir 6.1^-/- vessels, suggesting that K_ATP channels did not mediate NO-induced responses. We hypothesized a role for BK channels, given their prominence in arterial smooth muscle. Indeed, BK channels were expressed in mouse lymphatic smooth muscle and NS11021 (a BK channel activator) caused dilation and reduced contraction frequency, whereas iberiotoxin and penitrem A (BK channel inhibitors) produced right-ward shifts in NONOate concentration-response curves.

CONCLUSION AND IMPLICATIONS

Inhibition of mouse lymphatic contractions by NO primarily involves activation of BK channels, rather than K_ATP channels. Thus, BK channels are a potential target for therapeutic reversal of lymph pump inhibition by NO generated by immune cell activation of iNOS in chronic lymphoedema.

ATP-sensitive K+ channel inhibition in rats decreases kidney and skeletal muscle blood flow without increasing sympathetic nerve discharge

ATP-sensitive K⁺ (K_ATP) channels contribute to exercise-induced hyperemia in skeletal muscle either locally by vascular hyperpolarization or by sympathoinhibition and decreased sympathetic vasoconstriction. However, mean arterial pressure (MAP) regulation via baroreceptors and subsequent efferent activity may confound assessment of vascular versus neural K_ATP channel function. We hypothesized that systemic K_ATP channel inhibition via glibenclamide (GLI) would increase MAP without increasing sympathetic nerve discharge (SND). Lumbar and renal nerve SND were measured in anesthetized male rats with intact baroreceptors (n = 12) and sinoaortic denervated (SAD; n = 4) counterparts and blood flow (BF) and vascular conductance (VC) assessed in conscious rats (n = 6). GLI increased MAP (p < 0.05) and transiently decreased HR in intact (p < 0.05), but not SAD rats. Renal (-30 %) and lumbar (-40 %) ΔSND decreased in intact but increased in SAD rats (∼40 % and 20 %; p < 0.05). BF and VC decreased in kidneys and total hindlimb skeletal muscle (p < 0.05). Thus, because K_ATP inhibition decreases SND, GLI-induced reductions in blood flow cannot result from enhanced sympathetic activity.

Mucosa-Dependent, Stretch-Sensitive Spontaneous Activity in Seminal Vesicle

Seminal vesicles (SVs), a pair of male accessory glands, contract upon sympathetic nerve excitation during ejaculation while developing spontaneous phasic constrictions in the inter-ejaculatory storage phase. Recently, the fundamental role of the mucosa in generating spontaneous activity in SV of the guinea pig has been revealed. Stretching the mucosa-intact but not mucosa-denuded SV smooth muscle evokes spontaneous phasic contractions arising from action potential firing triggered by electrical slow waves and associated Ca²⁺ flashes. These spontaneous events primarily depend on sarco-endoplasmic reticulum (SR/ER) Ca²⁺ handling linked with the opening of Ca²⁺-activated chloride channels (CaCCs) resulting in the generation of slow waves. Slow waves in mucosa-intact SV smooth muscle are abolished upon blockade of gap junctions, suggesting that seminal smooth muscle cells are driven by cells distributed in the mucosa. In the SV mucosal preparations dissected free from the smooth muscle layer, a population of cells located just beneath the epithelium develop spontaneous Ca²⁺ transients relying on SR/ER Ca²⁺ handling. In the lamina propria of the SV mucosa, vimentin-immunoreactive interstitial cells including platelet-derived growth factor receptor α (PDGFRα)-immunoreactive cells are distributed, while known pacemaker cells in other smooth muscle tissues, e.g. c-Kit-positive interstitial cells or α-smooth muscle actin-positive atypical smooth muscle cells, are absent. The spontaneously-active subepithelial cells appear to drive spontaneous activity in SV smooth muscle either by sending depolarizing signals or by releasing humoral substances. Interstitial cells in the lamina propria may act as intermediaries of signal transmission from the subepithelial cells to the smooth muscle cells.

Role of mucosa in generating spontaneous activity in the guinea pig seminal vesicle

KEY POINTS

The mucosa may have neuron-like functions as urinary bladder mucosa releases bioactive substances that modulate sensory nerve activity as well as detrusor muscle contractility. However, such mucosal function in other visceral organs remains to be established. The role of mucosa in generating spontaneous contractions in seminal vesicles (SVs), a paired organ in the male reproductive tract, was investigated. The intact mucosa is essential for the generation of spontaneous phasic contractions of SV smooth muscle arising from electrical slow waves and corresponding increases in intracellular Ca²⁺ . These spontaneous events primarily depend on Ca²⁺ handling by sarco-endoplasmic reticulum Ca²⁺ stores. A population of mucosal cells developed spontaneous rises in intracellular Ca²⁺ relying on sarco-endoplasmic reticulum Ca²⁺ handling. The spontaneously active cells in the SV mucosa appear to drive spontaneous activity in smooth muscle either by sending depolarizing signals and/or by releasing humoral substances.

ABSTRACT

The role of the mucosa in generating the spontaneous activity of guinea-pig seminal vesicle (SV) was explored. Changes in contractility, membrane potential and intracellular Ca²⁺ dynamics of SV smooth muscle cells (SMCs) were recorded using isometric tension recording, intracellular microelectrode recording and epi-fluorescence Ca²⁺ imaging, respectively. Mucosa-intact but not mucosa-denuded SV preparations generated TTX- (1 μm) resistant spontaneous phasic contractions that were abolished by nifedipine (3 μm). Consistently, SMCs developed mucosa-dependent slow waves (SWs) that triggered action potentials and corresponding Ca²⁺ flashes. Nifedipine (10 μm) abolished the action potentials and spontaneous contractions, while suppressing the SWs and Ca²⁺ flashes. Both the residual SWs and spontaneous Ca²⁺ transients were abolished by cyclopiazonic acid (CPA, 10 μm), a sarco-endoplasmic reticulum Ca²⁺ -ATPase (SERCA) inhibitor. DIDS (300 μm) and niflumic acid (100 μm), blockers for Ca²⁺ -activated Cl^- channels (CACCs), or low Cl^- solution also slowed or prevented the generation of SWs. In SV mucosal preparations detached from the muscle layer, a population of mucosal cells generated spontaneous Ca²⁺ transients that were blocked by CPA but not nifedipine. These results suggested that spontaneous contractions and corresponding Ca²⁺ flashes in SV SMCs arise from action potential generation due to the opening of L-type voltage-dependent Ca²⁺ channels. Spontaneous Ca²⁺ transients appear to primarily result from Ca²⁺ release from sarco-endoplasmic reticulum Ca²⁺ stores to activate CACCs to develop SWs. The mucosal cells firing spontaneous Ca²⁺ transients may play a critical role in driving spontaneous activity of SV smooth muscle either by sending depolarizing signals or by releasing humoral substances.

Acute inhibition of ATP-sensitive K+ channels impairs skeletal muscle vascular control in rats during treadmill exercise

The ATP-sensitive K(+) (KATP) channel is part of a class of inward rectifier K(+) channels that can link local O2 availability to vasomotor tone across exercise-induced metabolic transients. The present investigation tested the hypothesis that if KATP channels are crucial to exercise hyperemia, then inhibition via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance during treadmill exercise. In 27 adult male Sprague-Dawley rats, mean arterial pressure, blood lactate concentration, and hindlimb muscle BF (radiolabeled microspheres) were determined at rest (n = 6) and during exercise (n = 6-8, 20, 40, and 60 m/min, 5% incline, i.e., ~60-100% maximal O2 uptake) under control and GLI conditions (5 mg/kg intra-arterial). At rest and during exercise, mean arterial pressure was higher (rest: 17 ± 3%, 20 m/min: 5 ± 1%, 40 m/min: 5 ± 2%, and 60 m/min: 5 ± 1%, P < 0.05) with GLI. Hindlimb muscle BF (20 m/min: 16 ± 7%, 40 m/min: 30 ± 9%, and 60 m/min: 20 ± 8%) and vascular conductance (20 m/min: 20 ± 7%, 40 m/min: 33 ± 8%, and 60 m/min: 24 ± 8%) were lower with GLI during exercise at 20, 40, and 60 m/min, respectively (P < 0.05 for all) but not at rest. Within locomotory muscles, there was a greater fractional reduction present in muscles comprised predominantly of type I and type IIa fibers at all exercise speeds (P < 0.05). Additionally, blood lactate concentration was 106 ± 29% and 44 ± 15% higher during exercise with GLI at 20 and 40 m/min, respectively (P < 0.05). That KATP channel inhibition reduces hindlimb muscle BF during exercise in rats supports the obligatory contribution of KATP channels in large muscle mass exercise-induced hyperemia.

Metabolic hyperemia requires ATP-sensitive K+ channels and H2O2 but not adenosine in isolated mouse hearts

We have previously demonstrated that adenosine-mediated H2O2 production and opening of ATP-sensitive K(+) (KATP) channels contributes to coronary reactive hyperemia. The present study aimed to investigate the roles of adenosine, H2O2, and KATP channels in coronary metabolic hyperemia (MH). Experiments were conducted on isolated Langendorff-perfused mouse hearts using combined pharmacological approaches with adenosine receptor (AR) knockout mice. MH was induced by electrical pacing at graded frequencies. Coronary flow increased linearly from 14.4 ± 1.2 to 20.6 ± 1.2 ml·min(-1)·g(-1) with an increase in heart rate from 400 to 650 beats/min in wild-type mice. Neither non-selective blockade of ARs by 8-(p-sulfophenyl)theophylline (8-SPT; 50 μM) nor selective A2AAR blockade by SCH-58261 (1 μM) or deletion affected MH, although resting flow and left ventricular developed pressure were reduced. Combined A2AAR and A2BAR blockade or deletion showed similar effects as 8-SPT. Inhibition of nitric oxide synthesis by N-nitro-l-arginine methyl ester (100 μM) or combined 8-SPT administration failed to reduce MH, although resting flows were reduced (by ∼20%). However, glibenclamide (KATP channel blocker, 5 μM) decreased not only resting flow (by ∼45%) and left ventricular developed pressure (by ∼36%) but also markedly reduced MH by ∼94%, resulting in cardiac contractile dysfunction. Scavenging of H2O2 by catalase (2,500 U/min) also decreased resting flow (by ∼16%) and MH (by ∼24%) but to a lesser extent than glibenclamide. Our results suggest that while adenosine modulates coronary flow under both resting and ischemic conditions, it is not required for MH. However, H2O2 and KATP channels are important local control mechanisms responsible for both coronary ischemic and metabolic vasodilation.

Control of pulmonary vascular tone during exercise in health and pulmonary hypertension

Despite the importance of the pulmonary circulation as a determinant of exercise capacity in health and disease, studies into the regulation of pulmonary vascular tone in the healthy lung during exercise are scarce. This review describes the current knowledge of the role of various endogenous vasoactive mechanisms in the control of pulmonary vascular tone at rest and during exercise. Recent studies demonstrate an important role for endothelial factors (NO and endothelin) and neurohumoral factors (noradrenaline, acetylcholine). Moreover, there is evidence that natriuretic peptides, reactive oxygen species and phosphodiesterase activity can influence resting pulmonary vascular tone, but their role in the control of pulmonary vascular tone during exercise remains to be determined. K-channels are purported end-effectors in control of pulmonary vascular tone. However, K(ATP) channels do not contribute to regulation of pulmonary vascular tone, while the role of K(V) and K(Ca) channels at rest and during exercise remains to be determined. Pulmonary hypertension is associated with alterations in pulmonary vascular function and structure, resulting in blunted pulmonary vasodilatation during exercise and impaired exercise capacity. Although there is a paucity of studies pertaining to the regulation of pulmonary vascular tone during exercise in idiopathic pulmonary hypertension, the few studies that have been performed in models of pulmonary hypertension secondary to left ventricular dysfunction suggest altered control of pulmonary vascular tone during exercise. Since the increased pulmonary vascular tone during exercise limits exercise capacity, future studies are needed to investigate the vasomotor mechanisms that are responsible for the blunted exercise-induced pulmonary vasodilatation in pulmonary hypertension.

Can smooth muscle represent a useful target for the treatment of rapid ejaculation?

Rapid ejaculation is probably the most common form of male sexual dysfunction. Current research into the treatment of the condition has focused on centrally acting or topical desensitizing agents; however, no treatment has yet been approved. An alternative approach could be to develop drugs that act directly upon the target organ itself and our increasing knowledge of the molecular biology of the accessory sex organs makes this a realistic possibility. This review analyzes the information in the literature that would support such a hypothesis. Particular emphasis has been placed on articles that have investigated smooth muscle cell relaxation. A critical review of the literature has revealed that there are potentially a myriad of targets through which rapid ejaculation can be treated.

A-type potassium currents in smooth muscle

A-type currents are voltage-gated, calcium-independent potassium (Kv) currents that undergo rapid activation and inactivation. Commonly associated with neuronal and cardiac cell-types, A-type currents have also been identified and characterized in vascular, genitourinary, and gastrointestinal smooth muscle cells. This review examines the molecular identity, biophysical properties, pharmacology, regulation, and physiological function of smooth muscle A-type currents. In general, this review is intended to facilitate the comparison of A-type currents present in different smooth muscles by providing a comprehensive report of the literature to date. This approach should also aid in the identification of areas of research requiring further attention.

Role of K(ATP)(+) channels in regulation of systemic, pulmonary, and coronary vasomotor tone in exercising swine

The role of ATP-sensitive K(+) (K(ATP)(+)) channels in vasomotor tone regulation during metabolic stimulation is incompletely understood. Consequently, we studied the contribution of K(ATP)(+) channels to vasomotor tone regulation in the systemic, pulmonary, and coronary vascular bed in nine treadmill-exercising swine. Exercise up to 85% of maximum heart rate increased body O(2) consumption fourfold, accommodated by a doubling of both cardiac output and body O(2) extraction. Mean aortic pressure was unchanged, implying that systemic vascular conductance (SVC) also doubled, whereas pulmonary artery pressure increased almost in parallel with cardiac output, so that pulmonary vascular conductance (PVC) increased only 25 +/- 9% (both P < 0.05). Myocardial O(2) consumption tripled during exercise, which was paralleled by an equivalent increase in O(2) supply so that coronary venous PO(2) was maintained. Selective K(ATP)(+) channel blockade with glibenclamide (3 mg/kg iv), decreased SVC by 29 +/- 4% at rest and by 10 +/- 2% at 5 km/h (both P < 0.05), whereas PVC was unchanged. Glibenclamide decreased coronary vascular conductance and hence myocardial O(2) delivery, necessitating an increase in O(2) extraction from 76 +/- 2% to 86 +/- 2% at rest and from 79 +/- 2% to 83 +/- 1% at 5 km/h. Consequently, coronary venous PO(2) decreased from 25 +/- 1 to 17 +/- 1 mmHg at rest and from 23 +/- 1 to 20 +/- 1 mmHg at 5 km/h (all values are P < 0.05). In conclusion, K(ATP)(+) channels dilate the systemic and coronary, but not the pulmonary, resistance vessels at rest and during exercise in swine. However, opening of K(ATP)(+) channels is not mandatory for the exercise-induced systemic and coronary vasodilation.

The influence of phenylephrine outward potassium currents in single smooth muscle cells from the rabbit mesenteric artery

In mesenteric artery smooth muscle cells, depolarizing voltage steps activated outward K+ currents whose amplitude was decreased by about 20% with phenylephrine (1-10 microM: n = 14 cells). Attenuation of outward current was only partly dependent on [Ca2+]i, because it persisted, although reduced, with 10 mM BAPTA in the patch pipette and was abolished in the presence of 3 mM 3,4-diaminopyridine (n = 13). In outside-out patches, phenylephrine did not exert any direct effect on the unitary current amplitude or open probability of large conductance K+ channels. Outward current was significantly increased (>100% in both cases) by 10 mM caffeine, presumably owing to the release of internal Ca2+ stores. With 10 mM BAPTA in the pipette, the only response to caffeine was a small decrease (9 +/- 3.7%, n = 10) in the K+ current. These observations show that a minor effect of phenylephrine is to reduce outward K+ current (probably Kv) in mesenteric cells.

Potentiation of quantal catecholamine secretion by glibenclamide: evidence for a novel role of sulphonylurea receptors in regulating the Ca(2+) sensitivity of exocytosis

Electrochemical detection of quantal catecholamine release from PC-12 cells revealed that glibenclamide, an inhibitor of ATP-sensitive K(+) channels, potentiated Ca(2+)-dependent exocytosis evoked by raised extracellular [K(+)] and by exposure of cells to caffeine. Glibenclamide was without effect on voltage-gated Ca(2+) currents, membrane potential, or rises of [Ca(2+)](i) evoked by either raised extracellular [K(+)] or caffeine. The dependence of K(+)-evoked secretion on extracellular Ca(2+) was shifted leftward in the presence of glibenclamide, with a small increase in the plateau level of release, suggesting that glibenclamide primarily increased the Ca(2+) sensitivity of the exocytotic apparatus. Enhancement of secretion by glibenclamide was reversed by pinacidil and cromakalim, indicating that the effects of glibenclamide were mediated via an action on a sulfonylurea receptor. These results demonstrate that sulfonylurea receptors can modulate Ca(2+)-dependent exocytosis via a mechanism downstream of Ca(2+) influx or mobilization.

Potentiation of quantal catecholamine secretion by glibenclamide: evidence for a novel role of sulphonylurea receptors in regulating the Ca(2+) sensitivity of exocytosis

Electrochemical detection of quantal catecholamine release from PC-12 cells revealed that glibenclamide, an inhibitor of ATP-sensitive K(+) channels, potentiated Ca(2+)-dependent exocytosis evoked by raised extracellular [K(+)] and by exposure of cells to caffeine. Glibenclamide was without effect on voltage-gated Ca(2+) currents, membrane potential, or rises of [Ca(2+)](i) evoked by either raised extracellular [K(+)] or caffeine. The dependence of K(+)-evoked secretion on extracellular Ca(2+) was shifted leftward in the presence of glibenclamide, with a small increase in the plateau level of release, suggesting that glibenclamide primarily increased the Ca(2+) sensitivity of the exocytotic apparatus. Enhancement of secretion by glibenclamide was reversed by pinacidil and cromakalim, indicating that the effects of glibenclamide were mediated via an action on a sulfonylurea receptor. These results demonstrate that sulfonylurea receptors can modulate Ca(2+)-dependent exocytosis via a mechanism downstream of Ca(2+) influx or mobilization.

Effect of divalent cations on KCl- and noradrenaline-induced contractile responses in rat aorta after nifedipine treatment

Nifedipine (1 microM) relaxed the sustained contractile responses induced by 1 microM noradrenaline or 60 mM KCl in rat aortic strips. After washing, a second addition of the spasmogens gave smaller tonic contractions than the first one. Even more, a third addition of KCl also gave a smaller contraction than the first one, but a complete recovery of the contractile response to noradrenaline was obtained by a third addition of this agonist. Application of cumulative amounts of Ca2+ or Ba2+ (2.4-24 mM) on the residual contraction in response to these agents after nifedipine treatment, but in the absence of the blocker, restored the magnitude of the contractile responses. Addition of cumulative amounts of Mg2+ (2.4-24 mM) did not modify or even relax the contractile responses to KCl and noradrenaline, respectively.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.