Pacemaker potentials in lymphatic smooth muscle of the guinea-pig mesentery

Lymphatic muscle: a review of contractile function

A recent surge in lymphangiogenesis research has led to a greater understanding of lymphatic endothelial cell biology. However, a general understanding of lymphatic muscle cell biology lags far behind its endothelial counterpart. Lymphatics at the level of the collecting vessels and higher contain muscular walls capable of both tonic and phasic contractions, which both generate and regulate lymph flow. Because lymphatic contraction is crucial to lymphatic function, a solid understanding of lymphatic muscle development and function is necessary to understand lymphatic biology. This review summarizes the current body of lymphatic muscle research and addresses important questions that are currently unanswered.

Proteinase-activated receptor 2 activation modulates guinea-pig mesenteric lymphatic vessel pacemaker potential and contractile activity

Lymphatic vessels rhythmically constrict to avoid fluid and protein accumulation in the interstitial space. This activity is critical during inflammation to prevent excessive oedema. Lymphatic pumping is intrinsic to the smooth muscle in the vessel wall and is due to the spontaneous occurrence of action potentials, the pacemaker of which is proposed to be spontaneous transient depolarizations (STDs). This function is highly susceptible to the fluid load and modulated by chemical agents, amongst which inflammatory mediators are important players. Activation of proteinase-activated receptors (PARs) has been involved in inflammation and affects vascular smooth muscle tone. The present study aims to investigate the role of PAR2, a member of the PAR family, in lymphatic vessel pumping. RT-PCR experiments revealed that PAR2 message is present in lymphatic vessels of the guinea-pig mesentery. Agonists of PAR2 such as trypsin and the activating peptide, SLIGRL-NH2, caused a decrease in the contractile activity of intraluminally perfused lymphatic vessels. Moreover, intracellular microelectrode recordings from isolated vessels revealed that PAR2 activation hyperpolarized the lymphatic smooth muscle membrane potential and altered STD amplitude and frequency. The decreases in constriction frequency and STD activity as well as the hyperpolarization were dependent on a functional endothelium, not affected by NO synthase or guanylyl-cyclase inhibition, but mimicked by PGE2 and iloprost and blocked by indomethacin (10 microM) and glibenclamide (1 microM). These results show that PAR2 activation alters guinea-pig lymphatic vessel contractile and electrical activity via the production of endothelium-derived cyclo-oxygenase metabolites.

Cyclopiazonic acid decreases spontaneous transient depolarizations in guinea pig mesenteric lymphatic vessels in endothelium-dependent and -independent manners

Guinea pig mesenteric lymphatic vessels exhibit vasomotion through a pacemaker mechanism that involves intracellular Ca2+release and resultant spontaneous transient depolarizations (STDs) of the smooth muscle membrane potential. This study presents a detailed characterization of the effects of cyclopiazonic acid (CPA) on this pacemaker activity. Microelectrode recordings from smooth muscle in vessel segments revealed that application of CPA (1–10 μM) caused a hyperpolarization accompanied by a decrease in the frequency and amplitude of STDs. The CPA-induced hyperpolarization was abolished after destruction of the endothelium and in the presence of NG-nitro-l-arginine (100 μM) or 1 H-[1,2,4]oxadiazolol-[4,3- a]quinoxaline-1-one (10 μM), which suggests a contribution of endothelium-derived nitric oxide (EDNO) in this response. In the absence of EDNO-induced effects, CPA decreased the frequency and amplitude of STDs recorded before and in the presence of the thromboxane A2mimetic U-46619, norepinephrine, or thimerosal. CPA abolished U-46619-induced vasomotion as determined by measurement of constriction-associated intracellular Ca2+concentration using the ratiometric Ca2+indicator fura-2. The endothelial actions of CPA were compared with those of ACh, which is known to cause EDNO release in this preparation. Although CPA and ACh both increased endothelial intracellular Ca2+concentration and depolarized the membrane potential, the kinetics of action for both parameters were markedly slower for CPA than ACh. These results suggest that CPA first hyperpolarizes the lymphatic smooth muscle and decreases STD frequency and amplitude through endothelial release of EDNO, and second, consistent with the action of CPA to inhibit sarcoplasmic reticulum Ca2+-ATPase and deplete Ca2+stores, it further reduces STD activity. Inhibition of the lymphatic smooth muscle pacemaker mechanism is thought to abolish agonist-induced vasomotion.

Stretch-induced calcium sensitization of rat lymphatic smooth muscle

The relationships between smooth muscle calcium and isometric tension generation to spontaneous lymphatic pump activity and its modulation by stretch equivalent from 0 to approximately 6 cmH2O were investigated. Excised preparations of the rat thoracic duct were mounted on a wire myograph and loaded with the calcium-sensitive fluorochrome indo-1. Calcium-dependent fluorescence and isometric force were simultaneously recorded. The thoracic duct segments developed spontaneous rhythmic contractile activity. Each contraction was preceded by an increase in intracellular calcium. When the vessels were normalized and stabilized at a preload equal to 3 cmH2O, the peak generation in tension occurred 0.70 +/- 0.11 s after that of calcium. Incremental stretch enhanced the frequency of the phasic activity and amplitude of isometric force generation but not the basal calcium level or the amplitude of the calcium transient. These findings suggest that stretch enhances lymphatic pump activity by increasing the pacemaker activity and the calcium sensitivity of the contractile apparatus.

ET-1-associated vasomotion and vasospasm in lymphatic vessels of the guinea-pig mesentery

In vitro experiments were performed to investigate the actions of endothelin-1 (ET-1) on vasomotion and vasospasm in guinea-pig mesenteric lymphatics. ET-1 modulated lymphatic vasomotion independent of the endothelium, with lower concentrations (<or=10 nm) increasing lymphatic vasomotion and higher concentrations (>or=100 nm) causing vasospasm. ET-1-induced increases in vasomotion were accompanied by an increase in tonic [Ca2+]i. These actions were inhibited by the ETA receptor antagonist BQ-123 (1 microm), the phospholipase C (PLC) inhibitor U73122 (5 microm), removal of extracellular Ca2+, chelation of intracellular Ca2+ with BAPTA/AM (10 microm), the store Ca2+-ATPase inhibitor thapsigargin (1 microm), caffeine (10 mm) and the inositol 1,4,5-trisphosphate (IP3) receptor blocker heparin and 2-APB (30 microm). In contrast, the ETB receptor antagonist BQ-788 (1 microm), ryanodine (1 & 20 microm), pertussis toxin (PTx) or Cs+ had no significant actions on vasomotion or the magnitude of increase in tonic [Ca2+]i. ET-1-induced vasospasm was accompanied by a transient increase in smooth muscle [Ca2+]i followed by a sustained plateau, an action that was abolished by removal of extracellular Ca2+, but only marginally inhibited by nifedipine (1 microm). Caffeine (10 mm), SKF 96165 (30 microm) or U73122 (5 microm) together with nifedipine (1 microm) abolished ET-1-induced vasospasm and increase in [Ca2+]i. These results indicate that ET-1 increases lymphatic vasomotion by acting on smooth muscle ETA receptors and activation of G-protein-PLC-IP3 cascade, which is known to cause pacemaker Ca2+ release and resultant pacemaker potentials. High concentrations of ET-1 cause a failure in Ca2+ homeostasis causing vasospasm, triggered by excessive Ca2+ influx primarily through store-operated channels (SOCs) with l-Ca2+ voltage-operated channels (VOCs) also contributing, but to a much lesser extent.

Ionic basis for the regulation of spontaneous excitation in detrusor smooth muscle cells of the guinea-pig urinary bladder

(1) The regulatory mechanisms of spontaneous excitation in detrusor smooth muscles of the guinea-pig urinary bladder were investigated using intracellular microelectrode and muscle tension recording techniques. (2) Detrusor smooth muscle cells exhibited nifedipine-sensitive spontaneous action potentials. Their frequency was highly sensitive to membrane polarization and was reduced by lowering the temperature. Lowering the temperature also reduced the frequency of spontaneous contractions and increased their amplitude. (3) Charybdotoxin (50 nm) and iberiotoxin (0.1 microm) increased the amplitude and duration of action potentials, and abolished after hyperpolarizations (AHPs). Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. Apamin (0.1 microm) did not change the shape of action potentials but often converted individual action potentials into bursts. It also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 4-aminopyrideine (4-AP, 1 mm) increased the frequency of action potentials without affecting their shape, and increased the amplitude and frequency of spontaneous contractions. (4) Cyclopiazonic acid (CPA, 10 microm) and ryanodine (50 microm) increased the amplitude of action potentials, and suppressed AHPs. Both agents also increased the amplitude and duration of spontaneous contractions, and reduced their frequency. 1,2-(Bis (2-aminophenoxy) ethane-N,N,N', N'-tetraacetic acid tetrakis (acetoxymethyl ester) (50 microm) dramatically increased the amplitude and duration of the action potential, and abolished AHPs. (5) Spontaneous action potentials in detrusor smooth muscles cells result from the opening of L-type Ca2+ channels, and their frequency is regulated by voltage-dependent mechanisms and by some metabolic process. Both the activation of large conductance Ca2+-activated K+ (BK) channels and Ca2+-mediated inactivation of the Ca2+ channels are involved in the repolarizing phase of action potentials. The Ca2+ influx through L-type Ca2+ channels triggers calcium-induced calcium release via ryanodine receptors and activates BK channels to generate AHPs. Both small conductance Ca2+-activated K+ channels and voltage-sensitive K+ channels may contribute to the resting membrane potential and regulate the frequency of action potentials. The regulatory mechanisms of action potentials are closely related to the regulation of spontaneous contractions.

5-HT decreases contractile and electrical activities in lymphatic vessels of the guinea-pig mesentery: role of 5-HT 7-receptors

1 Constriction measurements and intracellular microelectrode recordings were performed in vitro on lymphatic vessels isolated from the guinea-pig mesentery to investigate whether 5-hydroxytryptamine (5-HT) affected lymphatic pumping and smooth muscle membrane potential. 2 5-HT decreased in a concentration-dependent manner the frequency of constrictions induced by intraluminal vessel perfusion. In nonperfused vessels, 5-HT hyperpolarized the lymphatic smooth muscle membrane potential and decreased the frequency and amplitude of spontaneous transient depolarizations (STDs). 3 The actions of 5-HT were significantly reversed by the 5-HT(7) receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine (SB269970, 0.5 micro M) and by the 5-HT(1/2/5/7) receptor antagonists methysergide (0.5 micro M), and were mimicked by the 5-HT(1/7)-receptor agonist, 5-CT. 4 The 5-HT(4)-receptor antagonists 1-methyl-1H-indole-3-carboxylic acid [1-2-[(methyl sulfonyl) amino] ethyl-4-piperidinyl] methyl ester (GR113808, 1 micro M) and (1-piperidinyl) ethyl 1H-indole 3-carboxylate (SB203186, 1 micro M) did not significantly affect the 5-HT-induced responses. The 5-HT(4)-receptor agonist 1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1-(2-methylsulfonylamino) ethyl-4-piperidinyl]-1-propanone hydrochloride (RS67506) decreased the constriction frequency, albeit only at 50 micro M and without affecting the smooth muscle membrane potential. 5 Responses to 5-HT were attenuated by the nitric oxide synthase inhibitor N(G)-nitro L-arginine (100 micro M), whereas indomethacin (10 micro M) and tetrodotoxin (1 micro M) were without effects. 6 5-HT-induced responses were inhibited by the ATP-sensitive K(+) channel blocker, glibenclamide (10 micro M) and the cAMP-dependent protein kinase inhibitor N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89, 10 micro M) blocked the hyperpolarization. 7 These results suggest that 5-HT modulates the rate of lymphatic vessel pumping by eliciting K(ATP) channel-mediated smooth muscle hyperpolarization and decrease in STD activity, which appear to be mediated by activation of 5-HT(7) receptors coupled to cAMP production.

Molecular and functional analyses of the contractile apparatus in lymphatic muscle

Lymphatics are necessary for the generation and regulation of lymph flow. Lymphatics use phasic contractions and extrinsic compressions to generate flow; tonic contractions alter resistance. Lymphatic muscle exhibits important differences from typical vascular smooth muscle. In this study, the thoracic duct exhibited significant functional differences from mesenteric lymphatics. To understand the molecular basis for these differences, we examined the profiles of contractile proteins and their messages in mesenteric lymphatics, thoracic duct, and arterioles. Results demonstrated that mesenteric lymphatics express only SMB smooth muscle myosin heavy chain (SM-MHC), whereas thoracic duct and arterioles expressed both SMA and SMB isoforms. Both SM1 and SM2 isoforms of SM-MHC were detected in arterioles and mesenteric and thoracic lymphatics. In addition, the fetal cardiac/skeletal slow-twitch muscle-specific beta-MHC message was detected only in mesenteric lymphatics. All four actin messages, cardiac alpha-actin, vascular alpha-actin, enteric gamma-actin, and skeletal alpha-actin, were present in both mesenteric lymphatics and arterioles. However, in thoracic duct, predominantly cardiac alpha-actin and vascular alpha-actin were found. Western blot and immunohistochemical analyses corroborated the mRNA studies. However, in arterioles only vascular alpha-actin protein was detected. These data indicate that lymphatics display genotypic and phenotypic characteristics of vascular, cardiac, and visceral myocytes, which are needed to fulfill the unique roles of the lymphatic system.

Ca2+ phase waves: a basis for cellular pacemaking and long-range synchronicity in the guinea-pig gastric pylorus

Ca2+ imaging and multiple microelectrode recording procedures were used to investigate a slow wave-like electrical rhythmicity in single bundle strips from the circular muscle layer of the guinea-pig gastric pylorus. The 'slow waves' (SWs) consisted of a pacemaker and regenerative component, with both potentials composed of more elementary events variously termed spontaneous transient depolarizations (STDs) or unitary potentials. STDs and SW pacemaker and regenerative potentials exhibited associated local and distributed Ca2+ transients, respectively. Ca2+ transients were often larger in cellular regions that exhibited higher basal Ca2+ indicator-associated fluorescence, typical of regions likely to contain intramuscular interstitial cells of Cajal (ICCIM). The emergence of rhythmicity arose through entrainment of STDs resulting in pacemaker Ca2+ transients and potentials, events that exhibited considerable spatial synchronicity. Application of ACh to strips exhibiting weak rhythmicity caused marked enhancement of SW synchronicity. SWs and underlying Ca2+ increases exhibited very high 'apparent conduction velocities' ('CVs') orders of magnitude greater than for sequentially conducting Ca2+ waves. Central interruption of either intercellular connectivity or inositol 1,4,5-trisphosphate receptor (IP3R)-mediated store Ca2+ release in strips caused SWs at the two ends to run independently of each other, consistent with a coupled oscillator-based mechanism. Central inhibition of stores required much wider regions of blockade than inhibition of connectivity indicating that stores were voltage-coupled. Simulations, made using a conventional store array model but now including depolarization coupled to IP3R-mediated Ca2+ release, predicted the experimental findings. The linkage between membrane voltage and Ca2+ release provides a means for stores to interact as strongly coupled oscillators, resulting in the emergence of Ca2+ phase waves and associated pacemaker potentials. This distributed pacemaker triggers regenerative Ca2+ release and resultant SWs.

Characterization of properties underlying rhythmicity in mouse portal vein

We have used sharp intracellular and patch clamp electrophysiology, together with mechanical recordings and immunohistochemistry to characterize some of the properties underlying spontaneous rhythmicity in isolated murine portal vein. Mechanical recordings revealed that isolated whole portal veins were spontaneously active and generated regular contractions every 5-15-s that persisted in the presence of cyclopiazonic acid (CPA) (10 microM) or thapsigargin (100 nM). Intracellular recordings from smooth muscle cells revealed spontaneous depolarizations (SDs) in membrane potential, which were abolished by nifedipine (1 microM). Whole cell patch clamp recordings from isolated smooth muscle cells revealed an inward "pacemaker" current (I(H)) at negative potentials. Immunohistochemical studies failed to detect the presence of Kit-immunoreactive cells in portal veins of wild type mice, but were consistently observed in the small intestine. Furthermore, portal veins obtained from W/W(v) mutant mice, which lack full expression of the tyrosine-kinase, c-Kit, were also rhythmically active and were not different from wild type mice, in either their electrical or mechanical properties. These results show that both the wild type and W/W(v) mutant mouse portal vein are rhythmically active in vitro. However, pacemaker activity in this blood vessel occurs in the absence of Kit-immunoreactive cells; and is not critically dependent upon release of Ca(2+) from intracellular stores. The rhythmic pacemaker activity of mouse portal vein does involve L-type Ca(2+) currents, and possibly pacemaker conductances intrinsic to the smooth muscle.

Multiple conductance states of single Ca2+-activated Cl- channels in rabbit pulmonary artery smooth muscle cells

Ca2+-activated Cl- channels contribute to agonist-evoked contraction and spontaneous activity in some smooth muscle preparations. Patch pipette techniques were used to study the properties of single Ca2+-activated Cl- channels in freshly dispersed rabbit pulmonary artery myocytes. In the cell-attached recording mode, two conductance states of 3.5 and 1.8 pS were recorded either spontaneously or in response to increasing [Ca2+]i. With inside-out patches, the 3.5 pS channel current predominated at 50 nM [Ca2+]i, but at 500 nM [Ca2+]i most channels opened to the 1.8 pS level and an additional 1.2 pS channel conductance was resolved. At 1 microM [Ca2+]i all of the Cl- channels opened either to the 1.8 pS or 1.2 pS level. In 0 [Ca2+]i, no channel activity was observed at -100 mV to +100 mV, but with 10-250 nM [Ca2+]i the total single channel open probability (NP(o)) increased with depolarisation. This voltage dependence was not seen at higher values of [Ca2+]i. The plot of NPo vs. [Ca2+]i yielded Ca2+ affinity constants of 8 and 250 nM and Hill slopes of 1.3 and 2.3 at +100 and -100 mV, respectively. The distribution of open times was fitted by two exponentials of about 5 and 30 ms, which were neither voltage nor Ca2+ dependent. Replacement of external Cl- by I- shifted the reversal potential by about -30 mV and lengthened the longer of the two mean open times without significant effects on other kinetic parameters. Based on these data, a model for the activation of Ca2+-activated Cl- channels is proposed.

Physiologic aspects of lymphatic contractile function: current perspectives

The lymphatic system plays an important role in fluid/macromolecular balance, lipid absorption, and immune functions, and is involved in many different pathologic conditions, like inflammation, spread of cancer cells, and lymphedema. There are several forces that drive lymph centripetally. Extrinsic driving forces, or the passive lymph pump, include lymph formation, arterial pulsations, skeletal muscles contractions, fluctuations of central venous pressure, gastrointestinal peristalsis, and respiration. Intrinsic forces, or the active lymph pump, are the result of coordinated contractions of lymphangions, the morpho-functional units of the lymphatic vessels, which include the valve and portion of the vessel extending to the next valve. The contractions of the lymphangions are initiated by the pacemaker activity of the smooth muscle cells of lymphangion wall. Transmural pressure is an important hydrodynamic factor that modulates pacemaking. Under conditions of low filling, lymphangions might produce negative intraluminal pressures and a suction effect. Because of the complicated hydrodynamic conditions in lymphatic beds, the passive and active lymph pumps sometimes work together to propel lymph centripetally. In other cases (i.e., under conditions of enhanced lymph flow), flow-mediated inhibition of the active lymph pump could serve to decrease lymphatic outflow resistance and save metabolic energy when the driving force of the passive lymph pump is enough to propel lymph. We have recently found that there are profound differences in the pressure and flow sensitivities of lymphatic vessels derived from different tissues, such as the thoracic duct and mesenteric lymphatics. Such results, when considered in light of the controversy surrounding some studies performed in different animals, lead to the idea that the active lymph pumps in humans may have greater regional differences in contractile function than has been seen in animals, because of the upright posture in bipedal humans. This posture creates an additional outflow resistance for lymphatics of the lower part of the body. Thus, despite the ongoing attempts to determine the mechanisms of lymphatic diseases and useful therapies to treat them, there are many disputable or unknown issues regarding the physiology of lymph transport in humans.

ATP-induced endothelium-independent enhancement of lymphatic vasomotion in guinea-pig mesentery involves P2X and P2Y receptors

1. The present study has investigated mechanisms underlying ATP-induced endothelium-independent enhancement of vasomotion in guinea-pig mesenteric lymphatic vessels. 2. Lymphatic vasomotion, vessel tone and smooth muscle [Ca(2+)](i) showed similar ATP concentration-response curves. 3. ATP, at 0.1 mM, caused a biphasic increase in tonic [Ca(2+)](i) and superimposed vasomotion-associated Ca(2+) transients. All ATP-induced [Ca(2+)](i) changes were abolished by incubating the smooth muscle with suramin (0.1 mM). 4. alpha,beta-MeATP (0.1 mM) and UTP (0.1 mM) caused similar changes in [Ca(2+)](i) but the responses to these agonists were smaller than to ATP. 5. The actions of alpha,beta-MeATP (0.1 mM) were inhibited by suramin (0.1 mM) and PPADS (30 micro M) but not by reactive blue 2 (30 micro M). 6. In the presence of alpha,beta-MeATP (0.1 mM), the increases in tonic [Ca(2+)](i) and vasomotion-associated Ca(2+) transients induced by ATP (0.1 mM) were inhibited by U73122 (5 micro M), CPA (20 micro M) and heparin, whereas U73343 (5 micro M) and pre-treatment with PTx (100 ng ml(-1)) had no significant effects. 7. Depletion of the intracellular stores with CPA (20 micro M) caused an increase in [Ca(2+)](i), which was not blocked by desensitization of P(2X) receptors with alpha,beta-MeATP. 8. The data indicate that ATP, at relatively high concentrations increases lymphatic smooth muscle [Ca(2+)](i) and vasomotion through activation of P(2X1) and P(2Y2) purinoceptors present on lymphatic smooth muscle. The increase in [Ca(2+)](i) is likely to result from Ca(2+) release from inositol-1,4,5-trisphosphate-sensitive stores as well as Ca(2+) influx through store-operated channels and P(2X)-gated channels.

Effects of histamine on the contractile and electrical activity in isolated lymphatic vessels of the guinea-pig mesentery

1 The effect of histamine on the rate of lymphatic vessel constrictions and lymphatic smooth muscle membrane potential was examined in the guinea-pig mesentery. 2 Histamine (0.01-5 micro M) increased the frequency and decreased the amplitude of constrictions in lymphatic vessels under intraluminal perfusion. This response was accompanied by a depolarization of the smooth muscle membrane potential, an increase in the activity of spontaneous transient depolarizations (STDs), the proposed pacemaker for constrictions in these vessels, and an increase in the occurrence of action potentials. 3 Responses to histamine were inhibited by the H(1) receptor antagonist pyrilamine (0.2 micro M), but unaffected by NO synthase inhibition with N(G)-nitro L-arginine (L-NOARG, 100 micro M) and lysis of the endothelium. 4 In about 50% of the vessels, a decrease in constriction frequency, STD activity and a smooth muscle hyperpolarization were observed in response to dimaprit (10 micro M), suggesting the presence of H(2) receptors. These vessels had also a significantly lower basal contractile rate. Lymphatic vessel pumping was not affected by R-alpha-methylhistamine (10-50 micro M), ruling out a role for H(3) receptor stimulation in the histamine response. 5 The present results suggest a direct action of histamine on the lymphatic smooth muscle via stimulation of H(1) (and in some vessels H(2)) receptors. H(1) receptors enhance and H(2) receptors slow down lymphatic pumping, the dominant effect being an increased contractile activity. Correlation of these effects with histamine-induced changes in membrane potential and STD activity suggests the involvement of these electrical changes in the initiation of the contractile response.

Voltage independence of vasomotion in isolated irideal arterioles of the rat

The cellular mechanisms underlying vasomotion of irideal arterioles from juvenile rats have been studied using electrophysiological methods, ratiometric calcium measurements and video microscopy. Vasomotion was not affected by removal of the endothelium. Spontaneous contractions were preceded by spontaneous depolarizations. Both were abolished by the intracellular calcium chelator, BAPTA AM (20 microM), but not by ryanodine (10 microM), suggesting a dependence on the cyclical release of calcium from intracellular stores, other than those operated by ryanodine receptors. Oscillations were little changed when the membrane potential of short segments of arteriole was either depolarized or hyperpolarized. When the segments were voltage clamped, oscillating inward currents were recorded, indicating that the changes in membrane potential were voltage independent. Vasomotion was preceded by intracellular calcium oscillations and both were abolished by inhibitors of phospholipase C (U73122, 10 microM), phospholipase A(2) (AACOCF(3), 30 microM) and protein kinase C (chelerythrine chloride, 5 microM, and myristoylated protein kinase C peptide, 10 microM). Inhibition of vasomotion by the dual lipoxygenase and cyclo-oxygenase inhibitor, NDGA (10 microM), the lipoxygenase inhibitor, ETI (1 microM) but not by the cyclo-oxygenase inhibitors, aspirin (10 microM) and indomethacin (10 microM), or the cytochrome P450 inhibitor 17-ODYA (10 microM), suggested an involvement of the lipoxygenase pathway. The observations suggest that vasomotion of iris arterioles is voltage independent and results from the cyclical release of calcium from IP(3)-sensitive stores which are activated by cross talk between the phospholipase C and phospholipase A(2) pathways in vascular smooth muscle.

Modulators of internal Ca2+ stores and the spontaneous electrical and contractile activity of the guinea-pig renal pelvis

1. The role of internal Ca(2+) stores in the generation of the rhythmic electrical and contractile activity in the guinea-pig proximal renal pelvis was examined using intracellular microelectrode and muscle tension recording techniques. 2. Ryanodine (30 microM) transiently increased contraction amplitude, while caffeine (0.5 - 3 mM) reduced contraction amplitude and frequency. Contractility was also reduced by 2-aminoethoxy-diphenylborate (2-APB 60 microM), xestospongin C (1 microM), U73122 (5 microM) and neomycin (4 mM), blockers of IP(3)-dependent release from Ca(2+) stores. 3. 60 mM K(+) saline-evoked contractions were reduced by caffeine (1 mM), U73122 (5 microM) and neomycin (4 mM), but little affected by ryanodine or 2-APB (60 microM). 4. Spontaneous action potentials consisting of an initial spike followed by a long plateau were recorded (frequency 8.6+/-1.0 min(-1)) in small urothelium-denuded strips of proximal renal pelvis. 5. Action potential discharge was blocked in 75 and 35% of cells by 2-APB (60 microM) and caffeine (1 mM), respectively. In the remaining cells, only a truncation of the plateau phase was observed. 6. Cyclopiazonic acid (CPA 10 microM for 10 - 180 min), blocker of CaATPase, transiently increased contraction frequency and amplitude. Action potential durations were increased 3.6 fold. Contraction amplitude and frequency slowly declined during a prolonged (>60 min) CPA exposure. 7. We conclude that the action potential in caffeine-sensitive cells and the shoulder component of caffeine-insensitive action potential arise from the entry of Ca(2+) through Ca(2+) channels. The inhibitory actions of modulators of internal Ca(2+) release were partially explained by a blockade of Ca(2+) entry.

Review article: lymphatic vessel pumping and inflammation--the role of spontaneous constrictions and underlying electrical pacemaker potentials

The lymphatic circulation is important in maintaining tissue fluid homeostasis. It removes fluid, proteins and other particles from tissue spaces and returns them to the blood stream. This function is achieved by rhythmical contractions of the collecting lymphatic vessels. The contractile mechanism is intrinsic to the smooth muscles present in the vessel wall and consequent to action potentials. The underlying electrical mechanism has been proposed to be due to rhythmic synchronization of Ca2+-dependent spontaneous transient depolarizations. The lymphatic pumping activity adapts to changes in fluid load and has been observed to augment during inflammatory reactions to help resolve the associated oedema. This beneficial action has been generally attributed to the increase in interstitial pressure consequent to the oedema. However, little attention has been paid to the possible role inflammatory mediators that are present in the lymphatic vessel environment, could play in directly affecting the lymphatic contractile mechanism. This review article discusses our current knowledge on the mechanism and initiation of lymphatic pumping and how these events are modulated during inflammatory conditions.

Nitric oxide decreases pacemaker activity in lymphatic vessels of guinea pig mesentery

Intracellular microelectrode recordings were used to determine whether nitric oxide (NO), affects the pacemaker events that initiate vasomotion in lymphatic vessels of the guinea pig mesentery. This pacemaker activity is recorded as spontaneous transient depolarizations (STDs) and is likely to arise through synchronized Ca2+ release from intracellular stores. We show here that acetylcholine-induced endothelium-derived NO and exogenous NO released by sodium nitroprusside (SNP; 100 microM) and DEA-NONOate (500 microM) reduced the frequency and amplitude of STDs. This inhibition of STD frequency and amplitude was independent of the NO-induced hyperpolarization of the smooth muscle. The SNP-induced inhibition of STD frequency and amplitude was abolished during superfusion with the soluble guanylyl cyclase inhibitor ODQ (10 microM) and was diminished in the presence of cGMP and cAMP-dependent protein kinase inhibitors. The data are consistent with the hypothesis that NO inhibits vasomotion primarily by production of cGMP and activation of both cGMP- and cAMP-dependent protein kinases, which reduce the size and frequency of STDs, probably by acting on the underlying synchronized Ca2+ release from intracellular stores.

Origin and propagation of spontaneous excitation in smooth muscle of the guinea-pig urinary bladder

The origin and propagation of waves of spontaneous excitation in bundles of smooth muscle of the guinea-pig bladder were examined using intracellular recording techniques and visualization of the changes in the intracellular calcium concentration ([Ca2+]i). Bladder smooth muscle cells exhibited spontaneous transient increases in [Ca2+]i which originated along a boundary of each smooth muscle bundle and then spread to the other boundary with a conduction velocity of 2.0 1r1r>mm1> s-1. Spontaneous increases in [Ca2+]i were always preceded by action potentials. Nifedipine (10 microM) abolished increases in both [Ca2+]i and action potentials. Caffeine (10 1s1sFmM1F), ryanodine (50 microM) and cyclopiazonic acid (10 microM reduced the amplitude of the associated increases in [Ca2+]i without preventing the generation of action potentials. Spontaneous action potentials had conduction velocities of 40 1t1t>mm 1> s-1 in the axial direction and 1.3 1u1u>mm 1> s-1 in the transverse direction. The electrical length constants of the bundles of muscle were 425 microM in the axial direction and 12.5 microM in the transverse direction. Neurobiotin, injected into an impaled smooth muscle cell, spread more readily to neighbouring cells located in the axial direction than those located in the transverse direction. The spread of neurobiotin was inhibited by 18beta-glycyrrhetinic acid (18beta-GA, 40 microM), a gap junction blocker. Immunohistochemistry for Connexin 43 showed abundant punctate staining on the smooth muscle cell membranes. These results suggested that spontaneous action potentials and associated calcium waves occur almost simultaneously along the boundary of bladder smooth muscle bundles and then propagate to the other boundary probably through gap junctions.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Properties of spontaneous electrical activity in smooth muscle of the guinea-pig renal pelvis

In the guinea-pig renal pelvis, most smooth muscle cells examined (>90%), using a conventional microelectrode, had a resting membrane potential of about -50 mV and produced spontaneous action potentials with initial fast spikes and following plateau potentials. The remainder (<10%) had a resting membrane potential of about -40 mV and produced periodical depolarization with slow rising and falling phases. Experiments were carried out to investigate the properties of spontaneous action potentials. The potentials were abolished by nifedipine, suggesting a possible contribution of voltage-gated Ca(2+) channels to the generation of these potentials. Niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), inhibitors of Ca(2+)-activated Cl(-) channels, showed different effects on the spontaneous action potentials, and the former but not the latter inhibited the activities, raised the question of an involvement of Cl(-) channels in the generation of these activities. Depleting internal Ca(2+) stores directly with caffeine or indirectly by inhibiting Ca(2+)-ATPase at the internal membrane with cyclopiazonic acid (CPA) prevented the generation of spontaneous activity. Chelating intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) increased the amplitude of the spike component of spontaneous activity. Indomethacin inhibited the spontaneous activity, whereas prostaglandin F(2 alpha) enhanced it. The results indicate that in smooth muscle of the renal pelvis, the generation of spontaneous activity is causally related to the activation of voltage-gated Ca(2+) channels through which the influx of Ca(2+) may trigger the release of Ca(2+) from the internal stores to activate a set of ion channels at the membrane. Endogenous prostaglandins may be involved in the initiation of spontaneous activity.

Lymphatic vasomotion

1. Experimental findings in the past decade have greatly advanced present understanding of electrical/mechanical rhythmicities in smooth muscle, including vasomotion. Lymphatic vessels show strong vasomotor activity and have provided a key experimental system to study these processes. 2. Evidence from lymphatic vessels, blood vessels and other smooth muscles indicates that rhythmical contractions arise through a Ca2+ store-controlled pacemaker mechanism, which can function to cause smooth muscle constriction. 3. Such a model fits with observations that vasomotion can be near synchronous over large vessel lengths involving many cells. 4. The alternative interpretation that smooth muscle rhythmicities are generated by a cardiac-like electrical pacemaker mechanism has not been substantiated in any smooth muscle preparation under normal physiological conditions. However, elements of this latter mechanism are likely to be present at least in some smooth muscles, serving to modulate pacemaking.

Voltage dependency of the frequency of slow waves in antrum smooth muscle of the guinea-pig stomach

The effects of membrane depolarization on the frequency of spontaneous activities were investigated in circular smooth muscle of the guinea-pig antrum attached with (intact tissue) or without longitudinal muscles (circular tissue). Both types of tissue were spontaneously active; the intact tissues generated slow wave and circular tissues generated regenerative potential. The latter but not the former was abolished by caffeine. Increasing K(+) concentrations depolarized the membrane and reduced the amplitude and interval between spontaneous activities in both tissues; the amplitude was reduced linearly with depolarization and disappeared at about -35 mV; the interval was reduced successively with depolarization and reached a stable value (about 8 s) at about -45 mV. The depolarization and reduction in amplitude and interval of spontaneous activities induced by high K(+) solution were not altered by atropine, nitroarginine, or apamin in either tissue, suggesting that these changes did not involve the effects of neurotransmitters. The depolarization of the membrane by electrical stimulation also reduced the amplitude and interval of spontaneous activities in both tissues, in a potential-dependent way. The absolute refractory period for generation of the evoked regenerative potential was about 8 s, and the relative refractory period was 8--12 s. The results indicate that the frequency of slow waves increases with a depolarization of the membrane up to -45 mV, irrespective of the presence of caffeine-insensitive components. A depolarization of the membrane above -45 mV does not further increase the frequency of slow waves, possibly because of the refractory period for the generation of slow waves.

Ca(2+)-activated Cl(-) current in sheep lymphatic smooth muscle

Freshly dispersed sheep mesenteric lymphatic smooth muscle cells were studied at 37 degrees C using the perforated patch-clamp technique with Cs(+)- and K(+)-filled pipettes. Depolarizing steps evoked currents that consisted of L-type Ca(2+) [I(Ca(L))] current and a slowly developing current. The slow current reversed at 1 +/- 1.5 mV with symmetrical Cl(-) concentrations compared with 23.2 +/- 1.2 mV (n = 5) and -34.3 +/- 3.5 mV (n = 4) when external Cl(-) was substituted with either glutamate (86 mM) or I(-) (125 mM). Nifedipine (1 microM) blocked and BAY K 8644 enhanced I(Ca(L)), the slow-developing sustained current, and the tail current. The Cl(-) channel blocker anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing inward and tail currents. Application of caffeine (10 mM) to voltage-clamped cells evoked currents that reversed close to the Cl(-) equilibrium potential and were sensitive to 9-AC. Small spontaneous transient depolarizations and larger action potentials were observed in current clamp, and these were blocked by 9-AC. Evoked action potentials were triphasic and had a prominent plateau phase that was selectively blocked by 9-AC. Similarly, fluid output was reduced by 9-AC in doubly cannulated segments of spontaneously pumping sheep lymphatics, suggesting that the Ca(2+)-activated Cl(-) current plays an important role in the electrical activity underlying spontaneous activity in this tissue.

Specialised pacemaking cells in the rabbit urethra

1. Collagenase dispersal of strips of rabbit urethra yielded, in addition to normal spindle-shaped smooth muscle cells, a small proportion of branched cells which resembled the interstitial cells of Cajal dispersed from canine colon. These were clearly distinguishable from smooth muscle in their appearance under the phase-contrast microscope, their immunohistochemistry and their ultrastructure. They had abundant vimentin filaments but no myosin, a discontinuous basal lamina, sparse rough endoplasmic reticulum, many mitochondria and a well-developed smooth endoplasmic reticulum. 2. Interstitial cells were non-contractile but exhibited regular spontaneous depolarisations in current clamp. These could be increased in frequency by noradrenaline and blocked by perfusion with calcium-free solution. In voltage clamp they showed abundant calcium-activated chloride current and spontaneous transient inward currents which could be blocked by chloride channel blockers. 3. The majority of smooth muscle cells were vigorously contractile when stimulated but did not show spontaneous electrical activity in current clamp. In voltage clamp, smooth muscle cells showed very little calcium-activated chloride current. 4. We conclude that there are specialised pacemaking cells in the rabbit urethra that may be responsible for initiating the slow waves recorded from smooth muscle cells in the intact syncitium.

Cellular mechanisms of myogenic activity in gastric smooth muscle

In many regions of the intestine, a thin layer of interstitial cells of Cajal (ICC) lie in the myenteric region, between the circular and longitudinal muscle layers. ICC are connected by gap junctions to surrounding ICC and also with circular and longitudinal smooth muscle cells, forming a large electrical syncytium. Damage of the ICC causes a disorder in the patterns of rhythmic activity. Isolated ICC produce a rhythmic oscillation of the membrane potential. All these observations have led to the suggestion that ICC may be the pacemaker cell responsible for intestinal activity. Gastric smooth muscles generate slow oscillatory membrane potential changes (slow waves) and spike potentials. The activity is considered to be linked to the metabolism in the cell. Three types of cells located in the gastric wall (circular and longitudinal smooth muscle cells and ICC) produce synchronized electrical responses with different shapes. The electrical responses appear to originate in ICC and then spread to the smooth muscle layers, indicating that ICC may also be the pacemaker cells responsible for gastric activity. However, isolated circular smooth muscle tissues spontaneously generate regenerative potentials, suggesting that there are at least two sites for the initiation of spontaneous activity in the stomach. Regenerative potentials persist in the presence of Ca-antagonists and are inhibited by agents which disrupt intracellular Ca(2+) homeostasis. Depolarization of the membrane elicits regenerative potentials after a long delay and the potentials have long refractory periods. This suggests that an unidentified 2nd messenger may be formed during the delay between membrane depolarization and the initiation of a regenerative potential. In gastric muscles of mutant mice which do not express inositol trisphosphate (InsP(3)) receptors, spike potentials but not slow waves are generated, suggesting the possible involvement of InsP(3) in the initiation of spontaneous activity.

Properties of gastric smooth muscles obtained from mice which lack inositol trisphosphate receptor

Membrane potential recordings, made from the circular smooth muscle layer of the gastric antrum taken from mutant mice which lacked the inositol trisphosphate (InsP3) type 1 receptor, were compared with those obtained from the stomach of control (wild-type) mice. Immunostaining of gastric muscles indicated that the distribution and form of c-kit positive cells were similar in wild-type and mutant mice. Smooth muscles from wild-type mice generated slow waves that in turn initiated spike potentials, while those from mutant mice were either quiescent or generated irregular bursts of spike potentials. In the presence of nifedipine, slow waves with reduced amplitude were generated in wild-type mice, while all electrical activity was abolished in mutant mice. Acetylcholine depolarized and sodium nitroprusside hyperpolarized the membrane in muscles from both types of mice, being more effective in wild-type mice. Noradrenaline produced similar hyperpolarizations in both types of mice. Transmural nerve stimulation evoked inhibitory junction potentials (IJPs) in both wild-type and mutant mice. In wild-type mice, the IJPs were reduced in amplitude by nitroarginine and converted to a cholinergic excitatory junction potential (EJP) by apamin. In mutant mice, the IJPs were unaffected by nitroarginine or atropine but were abolished by apamin. It is concluded that in antral smooth muscle, the expression of InsP3 type 1 receptors may be causally related to the generation of slow waves but not to the generation of action potentials. A lack of InsP3 receptors attenuates cholinergic excitatory and nitrergic inhibitory responses but does not alter the response to noradrenaline.

Mechanisms of excitatory neuromuscular transmission in the guinea-pig urinary bladder

1. In smooth muscle of the guinea-pig bladder, either membrane potential recordings or [Ca2+]i measurements were made simultaneously with isometric tension recordings. 2. Single transmural stimuli initiated excitatory junction potentials (EJPs) which triggered action potentials, transient increases in [Ca2+]i and associated contractions. These responses were abolished by alpha, beta-methylene ATP, suggesting that they resulted from the activation of purinoceptors by neurally released ATP. 3. Nifedipine abolished action potentials leaving the underlying EJPs and reduced the amplitude of both nerve-evoked increases in [Ca2+]i and associated contractions. The subsequent co-application of caffeine and ryanodine inhibited the residual responses without inhibiting EJPs. These results indicate that stimulation of purinoceptors activates both Ca2+ influx through L-type Ca2+ channels and Ca2+ release from intracellular Ca2+ stores. 4. In the presence of alpha, beta-methylene ATP, trains of stimuli failed to initiate EJPs but increased the frequency of action potentials. Trains of stimuli also initiated oscillatory increases in [Ca2+]i and associated contractions. These responses were abolished by hyoscine, indicating that they resulted from the activation of muscarinic receptors by neurally released ACh. 5. Oscillatory increases in [Ca2+]i and associated contractions were inhibited by either nifedipine or caffeine, indicating that the stimulation of muscarinic receptors activates both Ca2+ influx through L-type Ca2+ channels and Ca2+ release from intracellular Ca2+ stores.

Role of calcium stores and membrane voltage in the generation of slow wave action potentials in guinea-pig gastric pylorus

1. Intracellular recordings made in single bundle strips of a visceral smooth muscle revealed rhythmic spontaneous membrane depolarizations termed slow waves (SWs). These exhibited 'pacemaker' and 'regenerative' components composed of summations of more elementary events termed spontaneous transient depolarizations (STDs). 2. STDs and SWs persisted in the presence of tetrodotoxin, nifedipine and ryanodine, and upon brief exposure to Ca2+-free Cd2+-containing solutions; they were enhanced by ACh and blocked by BAPTA AM, cyclopiazonic acid and caffeine. 3. SWs were also inhibited in heparin-loaded strips. SWs were observed over a wide range of membrane potentials (e.g. -80 to -45 mV) with increased frequencies at more depolarized potentials. 4. Regular spontaneous SW activity in this preparation began after 1-3 h superfusion of the tissue with physiological saline following the dissection procedure. Membrane depolarization applied before the onset of this activity induced bursts of STD-like events (termed the 'initial' response) which, when larger than threshold levels initiated regenerative responses. The combined initial-regenerative waveform was termed the SW-like action potential. 5. Voltage-induced responses exhibited large variable latencies (typical range 0.3-4 s), refractory periods of approximately 11 s and a pharmacology that was indistinguishable from those of STDs and spontaneous SWs. 6. The data indicate that SWs arise through more elementary inositol 1,4,5-trisphosphate (IP3) receptor-induced Ca2+ release events which rhythmically synchronize to trigger regenerative Ca2+ release and induce inward current across the plasmalemma. The finding that action potentials, which were indistinguishable from SWs, could be evoked by depolarization suggests that membrane potential modulates IP3 production. Voltage feedback on intracellular IP3-sensitive Ca2+ release is likely to have a major influence on the generation and propagation of SWs.

Mechanisms underlying spontaneous rhythmical contractions in irideal arterioles of the rat

1. Mechanisms underlying spontaneous rhythmical contractions have been studied in irideal arterioles of the rat using video microscopy and electrophysiology. 2. Rhythmical contractions (4 min-1) were more common during the second and third postnatal weeks and were always preceded by large, slow depolarizations (5-40 mV). 3. Spontaneous contractions were unaffected by tetrodotoxin (1 microM), neurotransmitter receptor antagonists, the sympathetic neurone blocker, guanethidine (5 microM) or sensory neurotoxin, capsaicin (1 microM). 4. Stimulation of sensory nerves inhibited spontaneous activity and this was not prevented by L-NAME (10 microm). 5. L-NAME (10 microm) caused an increase in frequency of spontaneous contractions, while forskolin (30 nM), in the presence of L-NAME, abolished spontaneous, but not nerve-mediated, contractions. 6. Spontaneous activity was not affected by felodipine (1 nM) or nifedipine (1 microM), but was abolished by cadmium chloride (1 microM) or superfusion with calcium-free solution. 7. Caffeine (1 mM), thapsigargin (2 microM) and cyclopiazonic acid (3 microM), but not ryanodine (3 microM), abolished spontaneous and nerve-mediated contractions. After preincubation in L-NAME (10 microM), cyclopiazonic acid abolished spontaneous contractions only. 8. Spontaneous depolarizations and contractions were abolished by 18alpha-glycyrrhetinic acid (20 microM). 9. Results suggest that spontaneous rhythmical contractions are myogenic and result from the cyclical release of calcium from intracellular stores, without a contribution from voltage-dependent calcium channels. Intercellular coupling through gap junctions appears to be essential for co-ordination of these events which could be modulated by nitric oxide and increases in cAMP. The possibility that different intracellular stores underly spontaneous and nerve-mediated contractions is discussed.

Hyperpolarisation-activated inward current in isolated sheep mesenteric lymphatic smooth muscle

1. Freshly isolated sheep lymphatic smooth muscle cells were studied using the perforated patch-clamp technique. Hyperpolarisation with constant-current pulses caused a time-dependent rectification evident as a depolarising 'sag' followed by an anode-break overshoot at the end of the pulse. Both sag and overshoot were blocked with 1 mM Cs+. 2. Cells were voltage clamped at -30 mV and stepped to -120 mV in 10 mV steps of 2 s duration. Steps negative to -60 mV evoked a slowly activating, non-inactivating inward current which increased in size and rate of activation with increasing hyperpolarisation. 3. The slowly activating current was reduced in Na+-free bathing solution but enhanced when the extracellular K+ concentration was increased to 60 mM. The current was significantly reduced by 1 mM Cs+ and 1 microM ZD7288 but not by 1.8 mM Ba2+. 4. The steady-state activation curve of the underlying conductance showed a threshold at -50 mV and half-maximal activation at -81 mV. Neither threshold nor half-maximal activation was significantly affected by increasing the external K+ concentration to 60 mM. 5. The frequency of spontaneous contractions and fluid propulsion in isolated cannulated segments of sheep mesenteric lymphatics were decreased by 1 mM Cs+ and by 1 microM ZD7288. 6. We conclude that sheep lymphatics have a hyperpolarisation-activated inward current similar to the If seen in sinoatrial node cells of the heart. Blockade of this current slows spontaneous pumping in intact lymphatic vessels suggesting that it is important in normal pacemaking.

Identification of the cells underlying pacemaker activity in the guinea-pig upper urinary tract

1. The varying profile of cell types along the muscle wall of the guinea-pig upper urinary tract was examined electrophysiologically, using intracellular microelectrodes, and morphologically, using both electron and confocal microscopy. 2. Simple 'pacemaker' oscillations (frequency of 8 min-1) of the membrane potential were recorded in both the pelvi-calyceal junction (83 % of cells) and the proximal renal pelvis (15 % of cells), but never in the distal renal pelvis or ureter. When filled with the cell marker, neurobiotin, 'pacemaker' cells were spindle shaped and approximately 160 microm in length. 3. In most cells of the ureter (100 %) and in both the proximal (75 %) and distal (89 %) renal pelvis, spontaneous action potentials (frequency of 3-5 min-1) consisted of an initial spike, followed by a number of potential oscillations superimposed on a plateau phase. When filled with neurobiotin, cells firing these 'driven' action potentials, were spindle shaped and > 250 microm in length. 4. Greater than 80 % of smooth muscle cells in the pelvi-calyceal junction were 'atypical', having < 40 % of their sectional areas occupied by loosely packed contractile filaments. Most of the smooth muscle cells in the ureter (99.7 %) and both the proximal (83 %) and distal (97.5 %) renal pelvis were of 'typical' appearance in that they contained cytoskeletal and contractile elements occupying > 60 % of cross-sectional area. 5. A third type of spontaneously discharging cell fired 'intermediate' action potentials (3-4 min-1), consisting of a single spike followed by a quiescent plateau and an abrupt repolarization. These cells were morphologically similar to interstitial cells of Cajal (ICC). However, these 'ICC-like' cells were not immuno-reactive for c-Kit, the proto-oncogene for tyrosine kinase. 6. In summary, 'atypical' smooth muscle cells were predominant in the pelvi-calyceal junction and fired 'pacemaker' potentials at a frequency significantly higher than 'driven' action potentials recorded in 'typical' smooth muscle cells throughout the renal pelvis and ureter. 'Intermediate' action potentials were recorded in 'ICC-like' cells in both the pelvi-calyceal junction and renal pelvis. We suggest that these 'ICC-like' cells act as a preferential pathway, conducting and amplifying pacemaker signals to initiate action potential discharge in the driven areas of the upper urinary tract.

Evidence that the ATP-induced increase in vasomotion of guinea-pig mesenteric lymphatics involves an endothelium-dependent release of thromboxane A2

1. Experiments were made to investigate mechanisms by which adenosine 5'-trisphosphate (ATP) enhanced vasomotion in mesenteric lymphatic vessels isolated from young guinea-pigs. 2. ATP (10-8 - 10-3 M) caused a concentration-dependent increase of perfusion-induced vasomotion with the endothelium mediating a fundamental role at low ATP concentrations (10-8 - 10-6 M). 3. The response to 10-6 M ATP showed tachyphylaxis when applied at intervals of 10 min but not at intervals of 20 or 30 min. 4. Suramin (10-4 M) or reactive blue 2 (3x10-5 M) but not PPADS (3x10-5 M) abolished the excitatory response to 10-6 M ATP confirming an involvement of P2 purinoceptors. 5. The excitatory response to 10-6 M ATP was abolished by treatment with either pertussis toxin (100 ng ml-1), antiflammin-1 (10-9 M), indomethacin (3x10-6 M) or SQ29548 (3x10-7 M), inhibitors of specific G proteins, phospholipase A2, cyclo-oxygenase and thromboxane A2 receptors respectively. 6. ATP simultaneously induced a suramin-sensitive inhibitory response, which was normally masked by the excitatory response. ATP-induced inhibition was mediated by endothelium-derived nitric oxide (EDNO) as the response was abolished by NG-nitro-L-arginine (L-NOARG; 10-4 M), an inhibitor of nitric oxide synthase. 7. We conclude that ATP modulates lymphatic vasomotion by endothelium-dependent and endothelium-independent mechanisms. One of these is a dominant excitation caused through endothelial P2 purinoceptors which because of an involvement of a pertussis toxin sensitive G-protein may be of the P2Y receptor subtype. Their stimulation increases synthesis of phospholipase A2 and production of thromboxane A2, an arachidonic acid metabolite which acts as an endothelium-derived excitatory factor.

Tonic and phasic activity in smooth muscle

We have studied the electrical and mechanical behaviour of two very different smooth muscle preparations, mesenteric lymphatic ducts and proximal urethra. These tissues generate different patterns of spontaneous contraction adapted to fulfil their contrasting functions. While lymphatics undergo regular phasic contractions and relaxations, suited to their role in propelling lymph, the urethra remains in a state of contracture to maintain urinary continence. The challenge is to understand how both of these achieve their respective roles. Interestingly, electrical activity of lymphatics resembles that in the heart in having a one to one relationship between the action potential and phasic contraction. Patch clamp studies have shown that lymphatic cells express 3 ionic currents that are not present in urethral cells, but are shared with cardiac muscle. These are, i) fast Na+ current, ii) T-type Ca2+ current and iii) a hyperpolarization-activated cation current, Ir. The fast Na+ current is ideally suited to the propagation of the action potential over large distances, as required by a vessel capable of generating a rapid well co-ordinated contraction along its length. The T-current and Ir, on the other hand, appear to be involved in electrical pacemaking as they are in the heart. The urethra does not usually undergo regular phasic contractions and it lacks these currents. Instead, urethral tone may depend on an interaction between L-type Ca2+ current and a large Ca(2+)-activated Cl- current. Activation of Cl- channels (perhaps by spontaneous release of Ca2+ from intracellular stores) would depolarize the membrane potential to within the 'window current' range for L-type Ca2+ channels and result in Ca2+ influx and contraction. This process may be maintained for a time by positive feedback whereby the influx of Ca2+ continues to activate the Cl- channels.

Interactions of noradrenalin and tumour necrosis factor alpha, interleukin 4 and interleukin 6 in the control of lipolysis from adipocytes around lymph nodes

The contributions of inflammatory and immunosuppressive cytokines and noradrenalin to the control of lipolysis in adipocytes surrounding and remote from lymph nodes were investigated in healthy adult guinea-pigs. A few hours after excision from fasting animals, spontaneous lipolysis in adipocytes from around the popliteal and mesenteric lymph nodes and omental "milky spots" was significantly lower than in those from elsewhere in the same depots, and much lower than in perirenal, epididymal or parametrial adipocytes. The perinodal adipocytes were consistently more sensitive to noradrenalin at 10(-8), 10(-7)and 10(-5) M, and their maximum rate of lipolysis was higher. They also responded more strongly to pre-incubation for 24 h with tumour necrosis factor alpha interleukin 6 and interleukin 4 than those elsewhere in the same depots. Tumour necrosis factor alpha and interleukin 6 applied alone stimulated lipolysis, but combined with interleukin 4, they suppressed glycerol release, especially in perinodal adipocytes, thereby creating large within-depot differences. These cytokines had minimal effects on lipolysis in perirenal or gonadal adipocytes. The authors conclude that adipocytes surrounding lymph nodes contribute little to whole-body energy supply during fasting, but are more sensitive than all others to cytokines and to noradrenalin, having higher maximum but lower minimum rates of lipolysis. These properties equip perinodal adipocytes for local interactions with lymphoid tissue.

Identification of rhythmically active cells in guinea-pig stomach

1. When intracellular recordings were made from the antral region of guinea-pig stomach, cells with different patterns of electrical activity were detected. 2. One group of cells, slow-wave cells, generated slow waves which consisted of initial and secondary components. When filled with either Lucifer Yellow or neurobiotin, the cells identified as smooth muscle cells lying in the circular muscle layer. 3. A second group of cells, driving cells, generated large, rapidly rising, potential changes, driving potentials. They had small cell bodies with several processes. With neurobiotin, a network of cells was visualized that resembled c-kit positive interstitial cells of the myenteric region. 4. A third group of cells generated sequences of potential changes which resembled driving potentials but had smaller amplitudes and slow rates of rise. These cells resembled smooth muscle cells lying in the longitudinal muscle layer. 5. When simultaneous recordings were made from the driving and slow-wave cells, driving potentials and slow waves occurred synchronously. Current injections indicated that both cell types were part of a common electrical syncytium. 6. The initial component of slow waves persisted in low concentrations of caffeine, but the secondary component was abolished; higher concentrations shortened the duration of the residual initial component. Driving potentials continued in the presence of low concentrations of caffeine; moderate concentrations of caffeine shortened their duration. 7. Hence three different types of cells were distinguished on the basis of their electrical activity, their responses to caffeine and their structure. These were smooth muscle cells, lying in the longitudinal and circular layers, and interstitial cells in the myenteric region. The observations suggest that interstitial cells initiate slow waves.

Spontaneous and neurally activated depolarizations in smooth muscle cells of the guinea-pig urethra

1. Membrane potential recordings were made from longitudinal smooth muscle cells of the guinea-pig urethra using conventional microelectrode techniques. 2. Smooth muscle cells of the urethra developed spontaneous transient depolarizations (STDs) and slow waves. Single unit STDs had amplitudes of approximately 5 mV and slow waves seemed to occur as amplitude multiples of single unit STDs. 3. STDs and slow waves were abolished by niflumic acid or low chloride solution and also by cyclopiazonic acid (CPA), BAPTA or high concentrations of caffeine. Lower concentrations of caffeine abolished slow waves but not STDs. Nifedipine inhibited slow waves but not STDs. 4. When stochastic properties of STDs were examined, it was found that the intervals between occurrences were not well modelled by Poisson statistics, instead the STDs appeared to be clustered. 5. Transmural stimulation evoked excitatory junctional potentials (EJPs) and triggered slow waves which were abolished by either alpha,beta-methylene-ATP or tetrodotoxin. Evoked slow waves were also abolished by caffeine, co-application of caffeine and ryanodine or by CPA which left EJPs unaffected. 6. In conclusion, smooth muscle cells of urethra exhibit STDs which are clustered rather than random events, and are the result of spontaneous Ca2+ release from intracellular stores and subsequent activation of Ca2+-activated chloride channels. STDs sum to activate L-type Ca2+ channels which contribute to the sustained phase of slow waves. Stimulation of purinoceptors by neurally released ATP initiates EJPs and also causes the release of Ca2+ from intracellular stores to evoke slow waves.

ATP-sensitive K+ channels in smooth muscle cells of guinea-pig mesenteric lymphatics: role in nitric oxide and beta-adrenoceptor agonist-induced hyperpolarizations

1. Intracellular microelectrode recordings were performed to investigate the membrane K+ conductances involved in smooth muscle hyperpolarization of lymphatic vessels in the guinea-pig mesentery. 2. Nitric oxide (NO), released either by the endothelium after acetylcholine (ACh; 10 microM) stimulation or by sodium nitroprusside (SNP; 50-100 microM), hyperpolarized lymphatic smooth muscle. These responses were inhibited with the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole [4,3-a]quinoxalin-1-one (ODQ, 10 microM). 3. ACh and SNP-induced hyperpolarizations were inhibited (by about 90%) upon application of the ATP-sensitive K+(K(ATP)) channel blocker, glibenclamide (10 microM), or with 4-aminopyridine (2.5 mM), but were not affected by the Ca2+-activated K+ channels blocker, penitrem A (100 nM). 4. Hyperpolarization caused by the K+ channel opener, cromakalim (0.1-10 microM), isoprenaline (0.1 microM) or forskolin (0.5 microM) were all significantly blocked by glibenclamide. 5. Hyperpolarization evoked by ACh and SNP were inhibited with N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dich loride (H89, 10 microM), suggesting the involvement of cyclic AMP dependent protein kinase (PKA). 6. These results suggest that K(ATP) channels play a central role in lymphatic smooth muscle hyperpolarization evoked by a NO-induced increase in cyclic GMP synthesis, as well as by beta-adrenoceptor-mediated production of cyclic AMP. Interestingly, both pathways lead to K(ATP) channels opening through the activation of PKA.

Ca2+ current and Ca(2+)-activated chloride current in isolated smooth muscle cells of the sheep urethra

1. Isolated sheep urethral cells were studied using the perforated patch clamp technique (T = 37 degrees C). Depolarizing steps ranging from -40 to -10 mV evoked an inward current that peaked within 10 ms and a slower inward current. Stepping back to the holding potential of -80 mV evoked large inward tail currents. All three currents were abolished by nifedipine (1 microM). Substitution of external Ca2+ with Ba2+ resulted in potentiation of the fast inward current and blockade of the slow current and tails. 2. Changing the chloride equilibrium potential (ECl) from 0 to +27 mV shifted the reversal potential of the tail currents from 1 +/- 1 to 27 +/- 1 mV (number of cells, n = 5). Chloride channel blockers, niflumic acid (10 microM) and anthracene-9-carboxylic acid (9AC, 1 mM), reduced the slow current and tails suggesting that these were Ca(2+)-activated Cl- currents, ICl(Ca). 4. Caffeine (10 mM) induced currents that reversed at ECl and were blocked by niflumic acid (10 microM). 5. In current clamp mode, some cells developed spontaneous transient depolarizations (STDs) and action potentials. Short exposure to nifedipine blocked the action potentials and unmasked STDs. In contrast, 9AC and niflumic acid reduced the amplitude of the STDs and blocked the action potentials. 6. In conclusion, these cells have both L-type ICa and ICl(Ca). The former appears to be responsible for the upstroke of the action potential, while the latter may act as a pacemaker current.

Functional electrical properties of the endothelium in lymphatic vessels of the guinea-pig mesentery

1. The resting and agonist-stimulated properties of endothelial cells and electrical communication between the endothelium and smooth muscle were investigated in open segments of guinea-pig mesenteric lymphatic vessels using intracellular microelectrodes. 2. Endothelial cells had a mean resting membrane potential (RMP) of -71.5 +/- 0.5 mV (n = 100) which was significantly different from the value of -60.8 +/- 1.1 mV (n = 75) recorded in smooth muscle. 3. Acetylcholine (ACh, 5-10 microM) generally evoked an initial hyperpolarization followed by depolarization (mean 3.4 +/- 0.5 mV and 15.4 +/- 1.0 mV, respectively, n = 75). 4. Ca(2+)-activated K+ channels were likely to underlie the ACh-induced hyperpolarization as this response exhibited an increased in membrane conductance, was larger in 0.5 mM K+ solution and was blocked by charybdotoxin (50 nM). 5. The endothelium did not exhibit a response to nitric oxide (NO) as the NO-donor sodium nitroprusside did not alter the RMP and the electrical responses to ACh were not affected by the NO-synthase inhibitor N omega-nitro L-arginine at a concentration which markedly inhibited smooth muscle hyperpolarization. 6. Electrical coupling between the endothelium and smooth muscle was not functional as there was extremely limited electrical continuity (1 in 12, endothelial/smooth muscle cell simultaneous recordings) and bradykinin, noradrenaline and isoprenaline caused different electrical responses in the two cell types. 7. These results provide the first description of RMP and electrical responses to various agonists in the lymphatic endothelium and its lack of functional electrical coupling with the smooth muscle.

Outward currents in smooth muscle cells isolated from sheep mesenteric lymphatics

1. The patch-clamp technique was used to measure membrane currents in isolated smooth muscle cells dispersed from sheep mesenteric lymphatics. Depolarizing steps positive to -30 mV evoked rapid inward currents followed by noisy outward currents. 2. Nifedipine (1 microM) markedly reduced the outward current, while Bay K 8644 (1 microM) enhanced it. Up to 90% of the outward current was also blocked by iberiotoxin (Kd = 36 nM). 3. Large conductance (304 +/- 15 pS, 7 cells), Ca(2+)- and voltage-sensitive channels were observed during single-channel recordings on inside-out patches using symmetrical 140 mM K+ solutions (at 37 degrees C). The voltage required for half-maximal activation of the channels (V1/2) shifted in the hyperpolarizing direction by 146 mV per 10-fold increase in [Ca2+]i. 4. In whole-cell experiments a voltage-dependent outward current remained when the Ca(2+)-activated current was blocked with penitrem A (100 nM). This current activated at potentials positive to -20 mV and demonstrated the phenomenon of voltage-dependent inactivation (V1/2 = -41 +/- 2 mV, slope factor = 18 +/- 2 mV, 5 cells). 6. Tetraethylammonium (TEA; 30 mM) reduced the voltage-dependent current by 75% (Kd = 3.3 mM, 5 cells) while a maximal concentration of 4-aminopyridine (4-AP; 10 mM) blocked only 40% of the current. TEA alone had as much effect as TEA and 4-AP together, suggesting that there are at least two components to the voltage-sensitive K+ current. 7. These results suggest that lymphatic smooth muscle cells generate a Ca(2+)-activated current, largely mediated by large conductance Ca(2+)-activated K+ channels, and several components of voltage-dependent outward current which resemble 'delayed rectifier' currents in other smooth muscle preparations.

Ca(2+)-activated K+ channels regulate action potential repolarization in urinary bladder smooth muscle

The goal of this study was to examine the role of large conductance Ca(2+)-activated K+ channels in the regulation of cell excitability in urinary bladder smooth muscle from the guinea pig. Ca(2+)-activated K+ channels were studied with single-channel recording techniques and found to be intracellular Ca2+ and voltage dependent and sensitive to external tetraethylammonium and blocked by nanomolar concentrations of iberiotoxin (apparent dissociation constant of 4 nM). Spontaneous action potentials recorded from intact tissue strips depended on external Ca2+ and were inhibited by Ca2+ channel blockers. Iberiotoxin (100 nM) significantly altered the configuration of the action potential by increasing the duration and peak amplitude of the action potential and decreasing the rate of decay. Iberiotoxin also increased the action potential frequency from 0.11 to 0.29 Hz. This study suggests that Ca(2+)-activated K+ channels play a significant role in the repolarization of the action potential and in the maintenance of the resting membrane potential of the urinary bladder smooth muscle.

Co-ordination of contractile activity in guinea-pig mesenteric lymphatics

1. Intraluminally perfused lymphatic vessels from the mesentery of the guinea-pig were examined in vitro to investigate their contractile activity and the co-ordination of this activity between adjacent lymphangions. 2. Lymphangions constricted at fairly regular intervals and exhibited 'refractory' periods of up to 3 s during which constrictions did not occur. 3. The contractile activity of adjacent lymphangions was highly co-ordinated. 4. The smooth muscle was found to be continuous between the adjacent lymphangions for the majority of valve regions examined morphologically (52 of 63 preparations). 5. Mechanical and electrical coupling between adjacent lymphangions was indicated, as some lymphangions underwent transient dilatations just prior to constriction, whereas direct electrophysiological measurements showed that the smooth muscle of most adjacent lymphangions was electrically coupled across the valve (15 out of 20 pairs of lymphangions). 6. It is concluded that perfused lymphangions of guinea-pig mesenteric lymphatic vessels rhythmically constrict, with the contractile activity of adjacent lymphangions highly co-ordinated. The findings also indicate that transmission of both mechanical and electrical signals between the adjacent lymphangions contribute to the co-ordination of their contractile activity.

Properties of spontaneous depolarizations in circular smooth muscle cells of rabbit urethra

1. Intracellular microelectrode recordings were made from circular smooth muscle of rabbit urethra. 2. The smooth muscle of urethra was spontaneously active exhibiting large, regularly occurring depolarizations, termed slow waves (SWs), 1-3 s in duration, up to 40 mV in amplitude and generated every 3-15 s and small irregularly occurring events (or summations there of) termed spontaneous transient depolarizations (STDs) of < 1 s in duration. 3. The SWs and STDs were not sensitive to 10(-6) M atropine, 10(-6) M phentolamine, 10(-5) M guanethidine or 10(-6) M tetrodotoxin, indicating that they were myogenic in origin. 4. Application of 3 x 10(-6) M nifedipine or 5 x 10(-5) M GdCl3 did not inhibit the generation of SWs or STDs, indicating that activation of L-type Ca2+ channels and non-selective cation channels are not essential for their generation. However, the duration of SWs but not STDs was reduced by nifedipine, indicating L-type Ca2+ channels contribute to the plateau-like potential of SWs. 5. Application of low chloride solution (6.4 mM), niflumic acid (10(-5) - 10(-4) M) or 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 10(-4) -5 x 10(-4) M) inhibited the generation of SWs and STDs, suggesting an involvement of chloride channels. 6. Application of nominally Ca2+ free solution, 5 x 10(-5) M BAPTA-AM, 10(-5) M cyclopiazonic acid, 10(-2) M caffeine or 10(-3) M procaine inhibited the generation of SWs and STDs, indicating that Ca2+ released from intracellular stores was required for the generation of SWs and STDs. 7. Exogenously applied noradrenaline (10(-7) - 10(-5) M) increased the frequency of SWs through stimulation of alpha-adrenoceptors which was inhibited by sodium nitroprusside (SNP, 10(-4) M). SNP also reduced the frequency of SWs without altering the membrane potential, an effect mimicked by 8-bromocyclic GMP (10(-3) M) indicating that SNP acted by elevating the production of cyclic GMP. 8. It is concluded that smooth muscle cells of the rabbit urethra exhibit SWs and STDs which are likely to be induced by stimulation of Ca(2+)-activated chloride channels evoked by release of Ca2+ from intracellular stores.

Endothelium-dependent modulation of pacemaking in lymphatic vessels of the guinea-pig mesentery

1. Endothelial control of the rate of constrictions and the underlying pacemaker potentials has been studied in vitro in guinea-pig mesenteric lymphatic vessels. 2. ACh stimulated 60% of intraluminally perfused vessels to slow or abolish lymphatic constrictions. This action was inhibited by atropine and was likely to be due to the release of endothelium-derived nitric oxide (EDNO) as the effect was absent after endothelial lysis, mimicked by sodium nitroprusside (SNP), blocked by N omega-nitro L-arginine (NOLA) and partially inhibited by Methylene Blue (MB). 3. The remaining 40% of perfused vessels did not mechanically respond to ACh or SNP. In four of seven such vessels this appeared to be due to excessive perfusion-associated release of EDNO, as incubation with NOLA restored the response to SNP. 4. Application of NOLA or MB in perfused vessels significantly increased constriction frequency, further indicating perfusion-associated release of EDNO. 5. ACh induced a marked increase in endothelial [Ca2+]i of both mechanically responding and non-responding vessels. This ACh-induced increase could be repetitively induced when Ca2+ was present in the perfusate, but rapidly ran down when a Ca(2+)-free EGTA perfusate was used. 6. Intracellular recordings from the smooth muscle of non-perfused vessel segments demonstrated an ACh-induced hyperpolarization and decrease in membrane resistance, changes which were prevented by atropine, NOLA, MB and endothelial lysis and mimicked by SNP. 7. ACh directly reduced the size of the underlying pacemaker potentials termed spontaneous transient depolarizations (STDs). 8. NOLA and MB enhanced STDs in non-perfused vessel segments indicating an endogenous release of EDNO. 9. It is concluded that the lymphatic endothelium produces and releases EDNO endogenously, during perfusion or after stimulation with ACh, to decrease the efficacy of STDs to generate action potentials and resultant constrictions.

Mediation of excitatory neurotransmission by the release of ATP and noradrenaline in sheep mesenteric lymphatic vessels

1. Spontaneous isometric contractions were measured in rings of sheep mesenteric lymphatics. Field stimulation at short pulse widths increased the frequency of spontaneous contractions and this response was blocked by 3 x 10(-7) M omega-conotoxin and by 10(-6) M guanethidine. 2. Rings that had been incubated with [3H]noradrenaline release 3H in response to field stimulation in a frequency-dependent manner. 3. Exogenous ATP mimicked the response to field stimulation and this was blocked by 10(-4) M suramin but not by prior desensitization with 10(-6) M alpha, beta-methylene ATP. Exogenous noradrenaline was not blocked by 10(-4) M suramin. 4. The excitatory response to field stimulation was not blocked by 10(-4) M suramin but a combination of 10(-4) M suramin and 3 x 10(-6) M phentolamine did block the response. 5. In rings taken from sheep that had been pretreated with reserpine, 10(-4) M suramin alone blocked the response to field stimulation. 6. The results of this study suggest that the excitatory response to stimulation of intramural nerves in sheep mesenteric lymphatics is mediated by the release of both ATP and noradrenaline.