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D F Van Helden

Publications and source records attributed to D F Van Helden.

At least 19 recordsLinked to original sources
PubMed2001

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.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed2000

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.

Animals↗
PubMed1999

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.

Adenosine Triphosphate↗
PubMed1997

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.

Acetylcholine↗
PubMed1997

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.

Animals↗
PubMed1997

Evidence that the substance P-induced enhancement of pacemaking in lymphatics of the guinea-pig mesentery occurs through endothelial release of thromboxane A2.

1. In vitro studies were performed to examine the mechanisms underlying substance P-induced enhancement of constriction rate in guinea-pig mesenteric lymphatic vessels. 2. Substance P caused an endothelium-dependent increase in lymphatic constriction frequency which was first significant at a concentration of 1 nM (115 +/- 3% of control, n = 11) with 1 microM, the highest concentration tested, increasing the rate to 153 +/- 4% of control (n = 9). 3. Repetitive 5 min applications of substance P (1 microM) caused tachyphylaxis with tissue responsiveness tending to decrease (by an average of 23%) and significantly decreasing (by 72%) for application at intervals of 30 and 10 min, respectively. 4. The competitive antagonist of tachykinin receptors, spantide (5 microM) and the specific NK1 receptor antagonist, WIN51708 (10 microM) both prevented the enhancement of constriction rate induced by 1 microM substance P. 5. Endothelial cells loaded with the Ca2+ sensing fluophore, fluo 3/AM did not display a detectable change in [Ca2+]i upon application of 1 microM substance P. 6. Inhibition of nitric oxide synthase by NG nitro-L-arginine (L-NOARG; 100 microM) had no significant effect on the response induced by 1 microM substance P. 7. The enhancement of constriction rate induced by 1 microM substance P was prevented by the cyclooxygenase inhibitor, indomethacin (3 microM), the thromboxane A2 synthase inhibitor, imidazole (50 microM), and the thromboxane A2 receptor antagonist, SQ29548 (0.3 microM). 8. The stable analogue of thromboxane A2, U46619 (0.1 microM) significantly increased the constriction rate of lymphangions with or without endothelium, an effect which was prevented by SQ29548 (0.3 microM). 9. Treatment with pertussis toxin (PTx; 100 ng ml-1) completely abolished the response to 1 microM substance P without inhibiting either the perfusion-induced constriction or the U46619-induced enhancement of constriction rate. 10. Application of the phospholipase A2 inhibitor, antiflammin-1 (1 nM) prevented the enhancement of lymphatic pumping induced by substance P (1 microM), without inhibiting the response to either U46619 (0.1 microM) or acetylcholine (10 microM). 11. The data support the hypothesis that the substance P-induced increase in pumping rate is mediated via the endothelium through NK1 receptors coupled by a PTx sensitive G-protein to phospholipase A2 and resulting in generation of the arachidonic acid metabolite, thromboxane A2 this serving as the diffusible activator.

Androstanes↗
PubMed1996

Prevention of high blood pressure by reducing sympathetic innervation in the spontaneously hypertensive rat.

It has previously been reported that the increase in blood pressure in the spontaneously hypertensive rat (SHR) occurs concurrently with a marked increase in thickness of the arterial wall and an increase in vascular innervation, particularly for the small muscular arteries. The purpose of the present study was to determine whether prevention of the increase in vascular innervation could prevent elevation of blood pressure in the SHR. We found that intraperitoneal injection of a single dose of an antiserum to nerve growth factor (anti-NGF) into young SHRs (postnatal day 19-24) caused a marked reduction in mean blood pressure at age 3-4 months from the raised value of 24.2 +/- 0.5 kPa to 18.9 +/- 0.8 kPa. By comparison, treated Wistar Kyoto rats (WKYs) maintained normal blood pressures. The treatment reduced the amplitude of the intracellularly recorded excitatory junction potential and the NA content of mesenteric arteries in the SHR, leaving the values similar to those of control WKYs. The NA content of these vessels was also reduced in treated WKYs. Importantly, the thickness of the vessel wall, which was greater in the SHR than the WKY, was not significantly altered by anti-NGF treatment. It is concluded that anti-NGF treatment during late neonatal development inhibits the increase in the functional levels of vascular innervation observed in the SHR. Furthermore, this increase in the functional levels of vascular innervation is necessary for the development of hypertension in this rat strain.

Animals↗
PubMed1996

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.

Acetylcholine↗
PubMed1996

The sigma receptor ligand, reduced haloperidol, induces apoptosis and increases intracellular-free calcium levels [Ca2+]i in colon and mammary adenocarcinoma cells.

The sigma receptor ligand reduced haloperidol (50 and 100 microM), potently inhibited cell proliferation, and induced apoptosis in WIDr colon and MCF-7 adenocarcinoma cell lines. Apoptosis was confirmed after drug treatment of the cells by the presence of nuclear fragmentation after staining of the cells with Hoechst 33258 and cellular DNA fragmentation ELISA and by condensation of the heterochromatin using transmission electron microscopy. However, internucleosomal DNA cleavage was not detected using gel electrophoresis. Reduced haloperidol (100 microM) increased the intracellular free calcium levels [Ca2+]i in both cell lines, which was independent of extracellular calcium, suggesting that the rise in [Ca2+]i was from intracellular stores and that an increase in [Ca2+]i may act as a "trigger" for apoptosis in these cell lines.

Adenocarcinoma↗
PubMed1996

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.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1996

Modulation of frog skeletal muscle Ca2+ release channel gating by anion channel blockers.

Effects of niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) on frog skeletal muscle ryanodine receptors have been studied by incorporating sarcoplasmic reticulum vesicles into planar lipid bilayers. Niflumic acid increased the mean open probability (Po) at 10 microM and decreased Po at 100 microM with no change in open time constants, unitary conductance, and reversal potential. The Po was augmented by DIDS at 5-200 microM without affecting either unitary conductance or reversal potential. DIDS induced a new third open time constant, probably contributing to a long-lived open state. Channels modified by niflumic acid or DIDS still responded to Ca2+ release channel modulators. These results provide evidence that niflumic acid and DIDS modify the gating mechanism of ryanodine receptors without affecting binding sites to the modulators and the physical pathway of the conducting pore. p-Chloromercuriphenyl sulfonic acid (pCMPS) transiently increased the Po. The channel modified by DIDS responded to pCMPS, whereas that by ryanodine did not. The long open state of the channel induced by DIDS is produced by a quite different mechanism(s) from that by ryanodine. Contrary to cardiac ryanodine receptors, Po of skeletal muscle channels was independent of voltage after DIDS modification.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1996

Beta-adrenoceptor-mediated hyperpolarization in lymphatic smooth muscle of guinea pig mesentery.

Intracellular microelectrode recordings were performed to investigate the consequences of beta-adrenoceptor activation in smooth muscle of guinea pig mesenteric lymphatic vessels. Isoproterenol (Iso) hyperpolarized the membrane with an associated increase in membrane conductance and decreased the amplitude of spontaneous transient depolarizations. Iso effects were mimicked by forskolin (FSK), 3-isobutyl-1-methylxanthine, and two adenosine 3',5'-cyclic monophosphate (cAMP) derivatives. Iso- and FSK-induced hyperpolarizations were inhibited by H89, an inhibitor of cAMP-dependent protein kinase A, increased in K+-free solution, but were not affected by ouabain or by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine. They were partially inhibited by 20 mM tetraethylammonium (approximately 40%) or by 2.5 mM 4-aminopyridine (approximately 55%). The-Iso-induced hyperpolarization was partially inhibited by iberiotoxin (20 nM) and charybdotoxin (40 nM), whereas the FSK-induced hyperpolarization was less affected. In cells where the Iso-induced hyperpolarization was decreased by 40 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, acetoxymethyl ester form, the FSK-induced hyperpolarization was little changed. Our results indicate that in guinea pig mesenteric lymphatic vessels, beta-adrenoceptor stimulation activates a protein kinase A-dependent K+ conductance, involving more than one channel type.

Animals↗
PubMed1995

Enhanced excitatory junction potentials in mesenteric arteries from spontaneously hypertensive rats.

Excitatory junction potentials (EJPs) were examined using intracellular recording techniques in mesenteric arteries isolated from 12- to 15-week-old spontaneously hypertensive (SHR), Wistar Kyoto (WKY) and Sprague Dawley (SD) rats. The amplitudes of EJPs evoked by single supramaximal stimuli were larger in arteries from SHRs (12.9 +/- 0.7 mV, n = 16) than in arteries from either WKYs (5.2 +/- 0.5 mV, n = 24) or SDs (8.6 +/- 0.8 mV, n = 15). The time constant of decay of EJPs did not differ significantly, suggesting that the passive electrical properties of the vascular smooth muscle are similar in the three rat strains. Spontaneous EJPs recorded in tissues from SHRs and WKYs had similar amplitude frequency distributions, suggesting that the quantal size is also similar between strains. In some arteries from SHRs, EJPs evoked by single stimuli triggered muscle action potentials (MAPs). Visible constriction only occurred following a MAP. In tissues from all three strains, summation of EJPs triggered MAPs. As EJPs are generated by the sympathetic co-transmitter adenosine 5'-triphosphate (ATP), the findings of the present study indicate that purinergic transmission is enhanced in mesenteric arteries from SHRs, probably as a result of an increase in quantal release. A consequence is that when nerves are activated SHR arteries more readily undergo constriction that is dependent on voltage-activated Ca2+ influx.

Action Potentials↗
PubMed1993

Ultrastructural analysis of sympathetic neuromuscular junctions on mesenteric veins of the guinea pig.

This study reports on the detailed ultrastructure of sympathetic postganglionic varicose axon terminals on mesenteric veins leading from the ileum of the guinea pig and in particular the structural arrangement of the varicosities with venous smooth muscle cells. The response to nerve stimulation in veins has a long time course and it has been suggested that this reflects a wide separation between the site of transmitter release and the receptors on the effector cell membrane. The aim of this study was to determine the distance between individual sympathetic varicosities and smooth muscle cells in mesenteric veins. Fluorescent histochemical preparations of the sympathetic innervation of the different branches of mesenteric veins indicate the branching network of varicose axons around the vessel to be relatively dense. Electron micrographs show the innervation to be confined to the adventitia close to the medio-adventitial border and to be predominantly catecholaminergic. A serial section ultrastructural analysis of the relationship of the varicosities with the outer smooth muscle cells showed that almost all (98%) of the exposed axon varicosities in the adventitia formed neuromuscular junctions. Three-dimensional reconstructions from serial sections of individual varicosities have shown that the junctions have structural specialisations identical to neuromuscular junctions described on arterial vessels and similar to those found at skeletal neuromuscular junctions. The density of neuromuscular junctions on the veins was found to be similar to that on the corresponding artery in the same animal. We suggest that in veins, noradrenaline is released focally at neuromuscular junctions.

Animals↗
PubMed1993

Edge detection at multiple locations using a 'radar' tracking algorithm as exemplified in isolated guinea-pig lymphatic vessels.

An algorithm based on the Kalman filter and used for tracking a target in a noisy environment has been applied to monitoring the position of moving boundaries as recorded by a video camera. The method, termed the Probabilistic Data Association Filter, is computationally efficient, allowing simultaneous monitoring at many positions along the boundary at video frame rates of 25 Hz. The present study uses the tracking procedure to investigate and correlate constrictions along lymphatic vessels. The procedure is well suited for real time tracking of visualised biological events including the simultaneous monitoring of constrictions in networks of blood vessels, the movement of cells or the trajectory of intracellular events monitored by intracellular fluorescent markers.

Algorithms↗
PubMed1993

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

1. Intracellular microelectrode recordings have been made from lymphatic smooth muscle (SM) to investigate pacemaker mechanisms underlying lymphatic pumping. 2. The SM of small lymphangions or that of short segments, cut from lymphangions of any length, behaved similarly. Both preparations exhibited spontaneous transient depolarizations (STDs) and exhibited simplified electrical characteristics approximating those of a spherical cell. 3. STDs were found to underlie activation of action potentials and hence constrictions. 4. The level of STD activity correlated to the pumping activity of lymphangions, the SM from more active chambers exhibiting increased STD activity. 5. Lymphatic SM exhibited STDs with properties similar to the STDs of mesenteric veins. STDs appeared to be of myogenic origin as they were present despite denervation or substantial destruction of the endothelium. 6. Noradrenaline enhanced the size and frequency of STDs. 7. STD activity was abolished by chelation of cytosolic Ca2+. 8. It is proposed that STDs provide a mechanism for pacemaking in the lymphatic SM studied here. Furthermore, it is postulated that STDs are the consequence of Ca(2+)-dependent pulsatile release of an intracellular messenger, probably Ca2+ itself. This mechanism provides a novel means for pacemaking.

Action Potentials↗
PubMed1991

Spontaneous and noradrenaline-induced transient depolarizations in the smooth muscle of guinea-pig mesenteric vein.

1. Recordings of membrane current were made in the smooth muscle of short segments of mesenteric vein before or during stimulation with noradrenaline (NA). 2. Small veins (diameter less than 150 microns) when cut into short segments (of length less than 250 microns) had the passive electrical characteristics of short cables both before and during activation with NA. 3. Spontaneous transient depolarizations (STDs) or the underlying inward currents (STICs) were recorded in these preparations. STDs were of myogenic origin as they were not blocked by tetrodotoxin or antagonists to the alpha-adrenoreceptor and persisted after either denervation or disruption of the endothelium. 4. STDs had time courses similar to the underlying currents and were generally slow compared to the membrane time constant of the short segments. 5. STDs and the underlying currents showed large variability in frequency and amplitude both within and between short segments. Currents were typically less than 0.3 nA, were characteristic in shape, had half-durations normally in the range 0.1-0.7 s and reversed at about -25 mV. 6. STDs persisted, but at markedly reduced frequencies, after exposure (3-10 min) to a solution in which cobalt ions had been used to substitute for Ca2+. STDs were also substantially suppressed by exposure to low-chloride solution. 7. Caffeine induced excitatory and inhibitory conductances. An initial component of the caffeine-induced responses showed similar voltage dependence to STDs and was also suppressed by exposure to low-chloride solution. 8. NA, through activation of alpha-adrenoreceptors, caused a sustained depolarization or inward current (under voltage clamp) with considerable membrane potential or current noise often in the form of agonist-induced spontaneous transient depolarizations (ASTDs) or currents (ASTICs). There were marked increases in amplitude and frequency of ASTDs with increase in NA concentrations. 9. ASTDs appeared to be generated within the smooth muscle as they were activated in preparations which had been denervated or in which the endothelium had been disrupted. 10. Except for the pathway of activation, ASTDs were indistinguishable from STDs having half-durations in the same range (0.1-2 s with the majority less than 0.7 s). The underlying currents again showed large variation in amplitude (typically less than 0.3 nA; maximum recorded 0.9 nA). They reversed at about -25 mV, could still be elicited in cobalt solution (but at reduced intensity for long exposures to this low-Ca2+ solution) and were reduced by long term exposure to low-chloride solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Electrophysiology of neuromuscular transmission in guinea-pig mesenteric veins.

1. Neuromuscular transmission in the smooth muscle of mesenteric veins has been investigated by recording intracellular potential changes resulting from stimulation of the sympathetic nerves and comparing these potential changes with responses obtained by ionophoresis of noradrenaline. 2. Neural stimulation or exogenous noradrenaline acted similarly to cause two excitatory depolarizations, a slow response reported previously (Suzuki, 1981) and a separate fast depolarization. 3. The fast depolarization was distinct from the slow depolarizing response in that it had a different dependence on the level of stimulation, was readily desensitized and was more suppressed in low-chloride solution. 4. The fast but not the slow depolarization shared certain characteristics with constriction. The fast depolarization and constriction both increased with the intensity of stimulation; inactivation in both was dependent on the recovery interval between trains of stimuli and both were suppressed to a similar degree by antagonists to alpha-adrenoceptors. The fast depolarization was, however, not a prerequisite for constriction to occur. 5. The fast and slow depolarizations were activated after a long latency which had a high temperature coefficient consistent with the postulate that these responses are rate limited by intracellular biochemical reactions. 6. The fast depolarization was preferentially suppressed by prazosin, an antagonist to the alpha 1-adrenoceptor subtype. Suppression of the slow depolarization required relatively higher concentrations of antagonist, indicating that these responses were mediated by receptor interactions involving a different alpha-adrenoceptor subtype. 7. It is concluded that neuromuscular transmission in mesenteric veins occurs through activation of alpha-adrenoceptors. A number of responses result, including voltage-independent constriction and two distinct excitatory depolarizations which can lead to voltage-dependent constriction.

Action Potentials↗