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M A Khoyi

Publications and source records attributed to M A Khoyi.

At least 19 recordsLinked to original sources
PubMed2003

Expression and function of KCNH2 (HERG) in the human jejunum.

Previous studies suggest that ether-a-go-go related gene (ERG) KCNH2 potassium channels contribute to the control of motility patterns in the gastrointestinal tract of animal models. The present study examines whether these results can be translated into a role in human gastrointestinal muscles. Messages for two different variants of the KCNH2 gene were detected: KCNH2 V1 human ERG (HERG) (28) and KCNH2 V2 (HERG(USO)) (13). The amount of V2 message was greater than V1 in both human jejunum and brain. The base-pair sequence that gives rise to domains S3-S5 of the channel was identical to that previously published for human KCNH2 V1 and V2. KCNH2 protein was detected immunohistochemically in circular and longitudinal smooth muscle and enteric neurons but not in interstitial cells of Cajal. In the presence of TTX (10(-6) M), atropine (10(-6) M). and l-nitroarginine (10(-4) M) human jejunal circular muscle strips contracted phasically (9 cycles/min) and generated slow waves with superimposed spikes. Low concentrations of the KCNH2 blockers E-4031 (10(-8) M) and MK-499 (3 x 10(-8) M) increased phasic contractile amplitude and the number of spikes per slow wave. The highest concentration of E-4031 (10(-6) M) produced a 10-20 mV depolarization, eliminated slow waves, and replaced phasic contractions with a small tonic contracture. E-4031 (10(-6) M) did not affect [(14)C]ACh release from enteric neurons. We conclude that KCNH2 channels play a fundamental role in the control of motility patterns in human jejunum through their ability to modulate the electrical behavior of smooth muscle cells.

Acetylcholine↗
PubMed2001

Beta2-adrenoceptor-mediated prejunctional facilitation and postjunctional inhibition of sympathetic neuroeffector transmission in the guinea pig vas deferens.

This study examines the role of prejunctional and postjunctional beta-adrenoceptors in the modulation of sympathetic cotransmission in the guinea pig vas deferens. The prejunctional involvement of beta-adrenoceptors was evaluated by testing the effects of several agonists and antagonists on the nerve stimulation-evoked overflow of ATP and norepinephrine (NE) from the "in vitro" vas deferens. The nonsubtype-selective beta-adrenoceptor agonist isoproterenol and the beta2-subtype-selective agonist clenbuterol increased, to a similar degree, the overflow of ATP and NE, while the beta1-subtype-selective agonist xamoterol and the beta3-subtype-selective agonist BRL 37 344 had no effect. Pretreatment with ICI 118, 551, a beta2-subtype-selective antagonist, abolished the facilitation of cotransmitter release by isoproterenol and clenbuterol, while the beta1-subtype-selective antagonist atenolol had no effect. Activation of beta-adrenoceptors by either isoproterenol or clenbuterol, but not by xamoterol and BRL 37 344, reduced the amplitude of contractions evoked by exogenously applied ATP. Pretreatment with propranolol or ICI 118, 551, but not atenolol, prevented these inhibitory effects. Isoproterenol in lower concentrations produced dose-dependent reduction of the purinergic but not the adrenergic phase of nerve stimulation-induced contraction of the guinea pig vas deferens. When applied in concentrations greater than 1 microM, isoproterenol, but not clenbuterol, actually produced a concentration-dependent facilitation of contractions evoked by both nerve stimulation and exogenously applied ATP. Antagonists of alpha-adrenoceptors blocked these facilitatory effects. Together, these results demonstrate that beta2-adrenoceptors can influence sympathetic neuroeffector transmission both prejunctionally, where they facilitate equally well the release of sympathetic cotransmitters and postjunctionally, where they inhibit smooth muscle contractions evoked by ATP.

Adenosine Triphosphate↗
PubMed1999

Species-dependent effects of adenosine receptor agonists on contractile responses of vas deferens to ATP.

1. Experiments were carried out to examine the postjunctional actions of adenosine receptor agonists on the smooth muscle of the vas deferens of the guinea-pig and rabbit. 2. Although they produced neither contraction nor relaxation by themselves, adenosine analogues enhanced contractions of the guinea-pig vas deferens induced by 10 microm ATP. The rank order of potency was N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > adenosine > CGS 21680. Dose-response curves for NECA were shifted to the right by the nonselective adenosine receptor antagonist 8(p-sulphophenyl)theophylline (8-SPT; 100 microM) and by the selective A1-receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 1 mM). 3. In the rabbit vas deferens, contractions induced by ATP (1 mM) were inhibited rather than facilitated by NECA. Neither CPA, R(-)-N6-(2-phenyl isopropyl)-adenosine (R-PIA) nor CGS 21680 had any effect. 4. The results indicate that the smooth muscle of the guinea-pig vas deferens expresses facilitatory adenosine A1 receptors but not adenosine A2 receptors. In contrast, in rabbit there are postjunctional inhibitory adenosine A2A receptors but not adenosine A1 receptors.

Adenosine↗
PubMed1999

An unusual Ca(2+) entry pathway activated by protein kinase C in dog splenic artery.

The characteristics of the Ca(2+) entry pathways that are activated by protein kinase C (PKC) in canine splenic artery were investigated. Phorbol 12, 13-dibutyrate (PDB) contracted tissues and increased Ca(2+) influx. PDB-induced contraction was reduced by preincubation of tissues in Ca(2+)-free Krebs' solution (1 mM EGTA) but was unaffected when Ca(2+)-free solution was applied after contraction was initiated with PDB. In contrast, (45)Ca influx and contraction induced by PDB were resistant to nifedipine, Cd(2+), Gd(3+), La(3+), or Ni(2+) whether added before or during exposure to PDB. Indeed, Cd(2+) reduced (45)Ca(2+) efflux and potentiated Ca(2+) influx, but not PDB-induced contraction. Norepinephrine (NE)-induced contractions were inhibited by preincubation in Ca(2+)-free Krebs' solution (1 mM EGTA). Nifedipine (10 microM) led to a small reduction in the NE-induced contraction but was without effect on (45)Ca(2+) influx. Pretreatment for 16 min with Cd(2+), Gd(3+), or La(3+) (each 1 mM) reduced or abolished NE-induced contraction and Ca(2+) influx. Application of these cations after exposure to NE did not affect (45)Ca(2+) influx but reduced tension. The Q(10) for the increase in (45)Ca(2+) influx was approximately 2 for high K(+) and NE, but 4 for PDB. The results suggest that stimulation of PKC in dog splenic artery activates a Ca(2+) entry pathway that is resistant to di- and trivalent cations. The inhibition of Ca(2+) influx by preincubating with cations during short-term exposure to NE may represent an action on Ca(2+) turnover that precedes activation of PKC.

Animals↗
PubMed1998

Modulation of cholinergic neuromuscular transmission by nitric oxide in canine colonic circular smooth muscle.

This study examines the effect of nitric oxide (NO) on cholinergic transmission in strips of canine colonic circular muscle in which neural plexus-pacemaker regions had been removed. Electrical field stimulation gave rise to atropine- and TTX-sensitive excitatory junction potentials (EJPs), the amplitude of which were frequency dependent. In 47% of control muscles, the EJP was followed by an inhibitory junction potential (IJP), whereas in the presence of atropine all preparations exhibited only IJPs. The NO synthase inhibitor Nomega-nitro-L-arginine (L-NNA), the guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (ODQ), and the protein kinase G (PKG) antagonist Rp-8-bromo-PET-cGMPS all significantly increased EJP amplitude and reduced or abolished IJPs. The potentiation of EJPs by L-NNA was reversed by the NO donors sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine in a manner blocked by ODQ. [14C]ACh overflow was also measured to evaluate the possible prejunctional effects of NO. Both norepinephrine and TTX significantly decreased [14C]ACh overflow; however, L-NNA, ODQ, and SNP were without effect. These data suggest that both cholinergic and nitrergic motoneurons functionally innervate the interior of the circular muscle layer. The inhibitory actions of NO on cholinergic transmission appear to be post- rather than prejunctional and to involve guanylyl cyclase as well as possibly PKG.

Acetylcholine↗
PubMed1996

Hyperpolarization and inhibition of contraction mediated by nitric oxide released from enteric inhibitory neurones in guinea-pig taenia coli.

1. Inhibition of nitric oxide synthase by NG-nitro-L-arginine (L-NNA) reduced the neurogenic relaxation of precontracted taenia coli only in the absence of atropine. The membrane hyperpolarization associated with the neurogenic relaxation was also reduced by inhibition of NOS only when atropine was absent. 2. The membrane hyperpolarization associated with the neurogenic relaxation of the taenia coli was inhibited by oxyhaemoglobin only in the absence of atropine. In the presence of atropine, oxyhaemoglobin did not reduce the i.j.p. or nerve evoked relaxation. 3. Inhibition of NOS by L-NNA did not affect the overflow of [3H]-ACh in response to electrical field stimulation (EFS), suggesting that, under the conditions of our experiments, endogenous NO did not modulate release of ACh. Sodium nitroprusside also had no effect on the neurogenic overflow of [3H]-ACh; however, noradrenaline significantly reduced [3H]-ACh overflow. 4. In summary, the postjunctional effects of neurally-released NO are not apparent in guinea-pig taenia coli when atropine is present. This implies muscarinic regulation of NO release or muscarinic regulation of another excitatory substance, such as tachykinin(s), that, when blocked, masks the postjunctional effects of NO. These data, together with previous studies, suggest a possible regulatory role for NO in enteric neurotransmission that may be more prominent in some species or tissues than others.

Acetylcholine↗
PubMed1996

Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens.

The present study investigates how changes in intracellular Ca2+ concentration modulate the influx of 45Ca2+ in isolated rat vasa deferentia. Raising extracellular K+ concentration ([K+]0) to > or = 32 mM increased 45Ca2+ influx during the 1st min in solutions containing 0.03-1.5 mM extracellular Ca2+ concentration ([Ca2+]0). During the 6th min in [K+]0 > or = 50 mM, 45Ca2+ influx was less than during the 1st min. This decline in 45Ca2+ influx occurred for [Ca2+]0 > or = 0.4 mM. Procaine potentiated K(+)-stimulated 45Ca2+ influx in 1.5 mM [Ca2+]0 and eliminated the decline of 45Ca2+ influx in low [Ca2-]0. Ryanodine and norepinephrine reduced K(+)-stimulated 45Ca2+ influx. 45Ca2+ content changed with time in accordance with the changes observed in 45Ca2+ influx. In isolated cells, voltage-dependent inward currents inactivated more rapidly with 1.5 mM Ca2+ as the charge carrier than with 1.5 mM Ba2+, and the steady-state inactivation relationship was shifted in the hyperpolarizing direction. Inward current was reduced with either caffeine, ryanodine, or norepinephrine. The inhibitory effects of norepinephrine were abolished by depletion of intracellular Ca2+ stores. These results are compatible with the hypothesis that K(+)-stimulated 45Ca2+ influx declines with time due to Ca(2+)-induced inhibition of Ca2- channels. Ca(2+)- and inositol 1,4,5-trisphosphate-induced releases of Ca2+ from the sarcoplasmic reticulum appear to play an important role in this process.

Animals↗
PubMed1993

[Ca2+]i-sensitive, IP3-independent Ca2+ influx in smooth muscle of rat vas deferens revealed by procaine.

1. The actions of procaine (10 mM) on noradrenaline-induced effects on 45Ca-influx, 45Ca-efflux, 45Ca-content, total inositol phosphates, inositol-1,4,5-trisphosphate, and contractile status of the rat was deferens were examined. 2. Noradrenaline alone had no effect on 45Ca-influx or 45Ca-content, but released Ca2+ from intracellular stores as indicated by an increased 45Ca-efflux and increased total inositol phosphates, specifically inositol-1,4,5-trisphosphate, leading to contraction of the rat vas deferens. 3. Noradrenaline, in the presence of 10 mM procaine, increased 45Ca-influx and 45Ca-content. Procaine blocked the noradrenaline-induced 45Ca-efflux, the increase in total inositol phosphates, the increase in inositol-1,4,5-trisphosphate, and contraction. 4. The noradrenaline-induced increase in 45Ca influx which was observed in the presence of procaine was abolished by phentolamine and nifedipine but was not altered significantly by propranolol suggesting that, in the presence of procaine, noradrenaline activates dihydropyridine-sensitive calcium channels through alpha-adrenoceptors. 5. These findings indicate that, in the rat vas deferens, noradrenaline induces contraction by releasing intracellularly stored Ca2+. The effects of procaine appear to be due to its ability to block the release of Ca2+ from intracellular stores. Furthermore, the simultaneous increase in 45Ca influx and inhibition of inositol-1,4,5-trisphosphate formation in tissues treated with procaine plus noradrenaline indicates that Ca2+ influx is independent of inositol-1,4,5-trisphosphate formation.

Animals↗
PubMed1993

Time-dependent increase in Ca2+ influx in rabbit abdominal aorta: role of Na-Ca exchange.

Exposure of the rabbit abdominal aorta to the combination of high K+ and norepinephrine resulted in a time-dependent increase in the rate of 45Ca influx and 45Ca and 22Na content over that observed after stimulation with either K+ or norepinephrine alone. The increase in 45Ca influx, but not the increase in 22Na content, was extracellular Ca2+ (Cao2+) dependent. This time-dependent increase in 45Ca influx was prevented by incubating the tissue in Na(+)-free medium. Nifedipine inhibited both the initial depolarization-induced 45Ca influx and time-dependent increase in 45Ca influx and 22Na content. The effect of nifedipine on time-dependent fluxes was prevented by ouabain. Phorbol dibutyrate mimicked the effects of norepinephrine on 22Na retention and 45Ca fluxes. The effects of phorbol dibutyrate and norepinephrine were not additive. It is concluded that, in rabbit abdominal aorta, norepinephrine plus K+ causes 22Na retention (possibly through inhibition of the sodium pump) and a Cao(2+)- and intracellular Na+ (Nai+)-dependent increase in 45Ca influx. This latter effect is possibly the result of increased Nai(+)-Cao2+ exchange.

Animals↗
PubMed1991

Norepinephrine increases Na-Ca exchange in rabbit abdominal aorta.

Na-Ca exchange was measured as intracellular Na+ (Na+i)-dependent 45Ca uptake in rabbit abdominal aortic rings. The amount of Na+i-dependent 45Ca uptake was proportional to both the concentration of Na+ in the Na(+)-loading solution and the concentration of Ca2+ in the assay medium. Na+i-dependent 45Ca uptake was inhibited by incorporation of Na+ in the assay medium and by amiloride analogues. Norepinephrine significantly enhanced the rate of 45Ca uptake in Na(+)-loaded tissue but had no effect on Na+i-independent 45Ca uptake. The effect of norepinephrine was prevented by phentolamine but not by propranolol. The stimulatory effect of norepinephrine was absent when the concentration of extracellular Ca2+(Ca2+o) was 0.3 mM or lower but became significant at 0.6 mM and higher. Na-Ca exchange was also increased by phorbol 12,13-dibutyrate but not by its inactive analogue (4 alpha-phorbol 12,13-didecanoate). 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of protein kinase C, blocked the stimulatory effect of norepinephrine on Na-Ca exchange. It is suggested that alpha-adrenoceptor stimulation increases Na-Ca exchange in rabbit abdominal aorta in a Na+i- and Ca2+o-dependent fashion. This effect is possibly mediated through the activation of protein kinase C.

Animals↗
PubMed1991

Effect of membrane potential on H1-receptor-mediated 45Ca influx in coronary artery.

These studies were undertaken to determine whether agonist-induced 45Ca influx in the coronary artery is modified under conditions in which Na+ pump activity (and consequently membrane potential) is altered. 45Ca influx and contraction were measured in the rabbit coronary artery with the H1-receptor agonist 2-(2-aminoethyl)pyridine (AEP, 10(-4) M) after prolonged Na+ pump block (leading to profound depolarization) and after enhanced Na+ pump activity (leading to hyperpolarization). AEP contracted vessels, enhanced 45Ca influx into the smooth muscle, and increased 45Ca efflux. The AEP-induced contraction was reduced but not abolished with either nifedipine (10(-6) M) or Ca(2+)-free solution. In tissues subjected to prolonged Na+ pump block AEP caused contraction but 45Ca influx was not increased over the unstimulated tissue. Ca(2+)-free solution and nifedipine reduced but did not abolish these contractions. In tissues with enhanced Na+ pump activity AEP caused contraction but 45Ca influx was not increased. Ca(2+)-free solution reduced these contractions but nifedipine did not. We conclude that the measurable 45Ca influx changes in a manner that is compatible with the characteristics of voltage-gated Ca2+ channels that have a low open probability during hyperpolarization and are inactivated by a period of long slow depolarization.

Animals↗
PubMed1989

Factors involved in the generation of tension during contraction to high potassium in the rat vas deferens.

A large number of studies indicate that K(+)-induced contractions of smooth muscle depend on extracellular calcium. If these contractions depend exclusively on extracellular calcium then contractile responses to 140 mM K+, which are larger than the response to 35 mM K+, should be associated with a larger influx of 45Ca. This is not the case in the vas deferens from reserpine pretreated rats. During a 2 min interval, 45Ca influx induced by 140 mM K+ was identical to that produced by 35 mM K+. This suggests that a second mechanism may be involved in responses to high K+. Indeed, 140 mM K+ caused an approximately 300% increase above control in the formation of inositol trisphosphate (IP3) in tissues prelabelled with 3H-myoinositol whereas 35 mM K+ did not increase IP3. IP3 is thought to cause the release of calcium from internal stores which is consistent with our finding of an increase in 45Ca efflux into calcium-free medium from tissues prelabelled with 45Ca and stimulated with 140 mM K+. Stimulation with 35 mM K+ did not influence 45Ca efflux. We conclude that in the rat vas deferens high K+ promotes tension development by smooth muscle by a dual mechanism: influx of extracellular calcium and release of calcium from internal stores via an IP3 mechanism.

Animals↗
PubMed1988

Inhibition of tracheal smooth muscle contraction and myosin phosphorylation by ryanodine.

Previous studies have shown that muscarinic activation of airway smooth muscle in low Ca++ solutions increases myosin phosphorylation without increasing tension. Blocking Ca++ influx reduced phosphorylation, but not to basal levels. It was proposed that release of intracellular Ca++ contributed to dissociation of phosphorylation and contraction. To test this hypothesis the effects of ryanodine were studied under similar conditions. Ryanodine (10(-7) to 10(-5) M) antagonized caffeine-induced contraction of canine tracheal smooth muscle. Ryanodine also reduced carbachol-induced contractions and carbachol-induced myosin phosphorylation. The effect of ryanodine on potassium and serotonin-induced contractions was also investigated to test for a nonspecific inhibitory effect. In contrast to the effect on carbachol responses, ryanodine (10(-5) M) potentiated the contractile response to low concentrations of serotonin and potassium, but had no effect on the maximum response to either stimulant. Carbachol (10(-6) M) and ryanodine (10(-5) M) both significantly decreased 45Ca++ content of tracheal muscle. The effect of ryanodine and carbachol together on 45Ca++ content was not greater than either drug alone suggesting that ryanodine reduces the caffeine and carbachol responses by depleting releaseable Ca++ stores. Ryanodine significantly reduced Ca++-induced contraction and myosin phosphorylation in carbachol-stimulated muscle, suggesting that some of the Ca++ responsible for elevated phosphorylation is released from the sarcoplasmic reticulum.

Alkaloids↗
PubMed1988

Norepinephrine and potassium induced calcium translocation in rat vas deferens.

To understand calcium regulation in smooth muscle, we studied both potassium- and norepinephrine-mediated alterations in the movement of calcium in the smooth muscle of rat vas deferens. We employed 45Ca to measure agonist-mediated calcium influx and efflux, as well as tissue calcium content. In addition we labeled tissues with [3H]myoinositol to measure the effect of norepinephrine on inositol phosphate generation. Stimulation of the vas deferens with 50 mM potassium caused a rapid influx of 45Ca (6-fold). Norepinephrine stimulation, even at a concentration maximal for contraction of the tissue (1 mM), did not result in any alteration in 45Ca influx by itself but inhibited potassium-stimulated 45Ca influx (IC50 = 3 microM). This alpha receptor-mediated effect of norepinephrine was not diminished by either pretreatment with reserpine or adrenergic denervation. Studies of the efflux of 45Ca from vas deferens revealed that efflux was not affected by potassium but was significantly stimulated by norepinephrine. Alpha receptor stimulation of vas deferens smooth muscle caused a marked elevation in the appearance of inositol phosphates, particularly inositol trisphosphate, that was not dependent on extracellular calcium. We conclude that norepinephrine does not stimulate calcium influx in vas deferens smooth muscle but leads to the release of calcium from intracellular stores via formation of inositol trisphosphate and that the resulting increase in intracellular calcium may lead to inactivation of the potential-dependent calcium channel.

Animals↗
PubMed1987

Effects of potassium and norepinephrine on calcium influx in guinea-pig vas deferens.

The effects of potassium (K+) and adrenoceptor agonists on 45Ca influx and contraction of isolated vas deferens from reserpine-treated guinea-pigs has been investigated. K+ (18-84 mM) increased 45Ca influx with the maximum influx occurring at 50 mM K+. The rate of influx upon exposure to K+ was highest in the first 2 min and decreased thereafter. In contrast to K+, norepinephrine, methoxamine and xylazine did not increase the rate of 45Ca influx. Norepinephrine and methoxamine, but not xylazine, inhibited 45Ca influx induced by high K+. The inhibitory effect of norepinephrine was also present in denervated tissues. Prazosin, but not yohimbine or propranolol, blocked the inhibitory effect of norepinephrine. Potassium-induced contractions were significantly potentiated in the presence of norepinephrine, especially at low calcium concentrations. We conclude that K+ contracts the guinea-pig vas deferens by increasing the influx of calcium whereas stimulation of alpha 1-adrenoceptors prevents 45Ca influx elicited by high potassium.

Adrenergic alpha-Agonists↗
PubMed1985

Effect of the dihydropyridine Bay K 8644 on the release of [3H]-noradrenaline from the rat isolated vas deferens.

The effects of Bay K 8644 on the release of [3H]-noradrenaline evoked by potassium, electrical stimulation or tyramine from the rat isolated vas deferens labelled with [3H]-noradrenaline were investigated. Bay K 8644 (1 microM) by itself did not affect the spontaneous release of tritium from the rat isolated vas deferens. However, it increased the calcium-dependent release of tritium elicited by both high potassium (59 mM) and electrical field stimulation. The exposure of rat vas deferens to phentolamine (10 microM) increased the release of tritium induced by potassium (59 mM) and electrical field stimulation. Bay K 8644 (1 microM) failed to increase further the release of tritium elicited by both stimuli in preparations previously treated with phentolamine (10 microM). The calcium-independent release of [3H]-noradrenaline evoked by tyramine (10 microM) was not affected by Bay K 8644 (1 microM). The results of our study support the view that alpha2-adrenoceptors modulate noradrenaline release by restricting calcium influx into sympathetic nerve terminals through voltage-dependent channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗