The effect of apamin on non-adrenergic, non-cholinergic vasodilator mechanisms in the intestines of the cat

Submucosal secretomotor and vasodilator reflexes

The presence of neuronal reflexes within the intestine that modulate mucosal ion secretion and blood flow have been recognized for many years, but the organization of these reflexes was unclear. This review highlights important findings from recent in vitro guinea-pig studies which have shown that both intrinsic primary afferent neurones (IPANs) and extrinsic primary afferent neurones (EPANs) can respond to chemical and/or mechanical stimuli to activate pathways, the afferent and efferent elements of which are confined to the walls of the intestine. Enteric neuronal pathways involve both myenteric and submucosal plexus neurones whereas capsaicin-sensitive afferent nerves evoke secretion by stimulating submucosal secretomotor neurones and vasodilation by direct actions on the submucosal arterioles. In this review, the cellular mechanisms involved in these pathways are described and the implications of these findings are discussed.

Apamin-sensitive K+ channels mediate an endothelium-dependent hyperpolarization in rabbit mesenteric arteries

1. Vascular endothelial cells release a variety of substances which affect the membrane potential and tone of underlying vascular smooth muscle. In the presence of N omega-nitro-L-arginine to inhibit nitric oxide synthase and indomethacin to inhibit cyclo-oxygenase, acetylcholine (ACh; EC50 approximately 1 microM) elicited the release of an endothelium-derived hyperpolarizing factor (EDHF) in rabbit mesenteric arteries. 2. The hyperpolarization due to EDHF was blocked by apamin (IC50 approximately 0.3 nM), and by other inhibitors of the apamin-sensitive K+ channel (10 nM scyllatoxin, 100 microM d-tubocurarine, 300 microM gallamine) in the presence of indomethacin and N omega-nitro-L-arginine. The hyperpolarization was not blocked by glibenclamide (5 microM), iberiotoxin (10 nM), tetraethylammonium (1 mM), barium (500 microM), 4-aminopyridine (500 microM), ouabain (10 microM), bumetanide (10 microM), or nimodipine (100 nM). 3. In the presence of apamin and N omega-nitro-L-arginine, but the absence of indomethacin, ACh triggered a hyperpolarization that was blocked by glibenclamide, an inhibitor of ATP-sensitive K+ (KATP) channels. A similar glibenclamide-sensitive hyperpolarization was caused by Iloprost, a stable analogue of prostacyclin. 4. In experiments which distinguished the effects of EDHF, prostanoids and nitric oxide, hyperpolarizations and/or relaxations triggered by ACh were antagonized by muscarinic antagonists, the relative potencies (atropine approximately 4-DAMP > pirenzepine) of which indicated that the release of all three endothelium-derived factors was mediated by M3 receptors. 5. Our results suggest that ACh stimulates M3 receptors on endothelial cells, triggering the release of nitric oxide and prostanoids, which hyperpolarize underlying smooth muscle by activation of KATP channels, and the release of an EDHF, which hyperpolarizes smooth muscle through the activation of apamin-sensitive K+ (KAS) channels.

Nitric oxide hyperpolarizes rabbit mesenteric arteries via ATP-sensitive potassium channels

1. Nitric oxide (NO) relaxes vascular smooth muscle (VSM) by mechanisms which are not fully understood. One possibility is that NO hyperpolarizes membranes, thereby diminishing Ca2+ entry through voltage-dependent Ca2+ channels. In the current study, the effects of NO on membrane potential of rabbit mesenteric arteries were recorded using intracellular microelectrodes. 2. NO, released by 3-morpholinosydnonimine (SIN-1, 3 microM), reversibly hyperpolarized arteries by -9.5 +/- 4.0 mV (means +/- S.D., n = 97) from a resting membrane potential of -53.1 +/- 5.7 mV. The hyperpolarization was blocked by oxyhaemoglobin (20 microM), and only occurred in arteries pre-treated with N omega-nitro-L-arginine (100 microM) or denuded of endothelium. 3. The effect of SIN-1 was concentration dependent (EC50 approximately 0.4 microM) and its dose response was shifted to the left by zaprinast (100 microM), an inhibitor of cGMP-specific phosphodiesterases. 4. The hyperpolarization due to SIN-1 was modified by changes in extracellular K+ concentration, but not by changes in Ca2+, Na+ or Cl-. The hyperpolarization was blocked by glibenclamide (IC50 approximately 0.15 microM), but not by apamin (3-300 nM), barium (5-150 microM), tetraethylammonium (0.1-10 mM), or 4-aminopyridine (5-500 microM). The hyperpolarization due to lemakalim (0.03-3 microM), an activator of ATP-sensitive potassium channels (KATP), displayed the same sensitivities to these K+ channel blocking agents, whereas the endothelium-derived hyperpolarizing factor, triggered by the addition of acetylcholine (3 microM), caused a hyperpolarization (-15.3 +/- 6.2 mV) that was blocked by apamin, but not by any other agent. 5. These results suggest that NO hyperpolarizes VSM in rabbit mesenteric arteries by activating KATP channels, with the accumulation of cGMP as an intermediate step.

Rhythmic contractions in isolated small arteries of rat: role of K+ channels and the Na+,K(+)-pump

Small mesenteric arteries from Wistar rats display rhythmic tension oscillations, associated with oscillations in membrane potential, when stimulated with noradrenaline. The purpose of this study was to investigate the role of potassium conductance and Na+, K(+)-pump activity in the generation of these oscillations. The effect on the rhythmic contractions of several agents, interacting with K+ channels, was studied. Application of apamin, pinacidil or glibenclamide did not affect the rhythmic activity. Tetraethylammonium (TEA) increased the frequency of the rhythmic contractions, while application of 4-aminopyridine (4-AP) increased the amplitude by approximately 50%, with no changes in frequency. Ba2+, on the other hand, impaired the rhythmic contractions or converted them to irregular oscillations in the presence of functional endothelium, but did not affect oscillations in endothelium-denuded vessels. Ouabain or exposure to K(+)-free solution, procedures known to inhibit the Na+,K(+)-pump, abolished the rhythmic contractions. This effect was immediate, suggesting that it was due to elimination of the electrogenic action of the Na+,K(+)-ATPase, rather than to a change in intracellular ion concentrations. Exposure to an extracellular potassium concentration of more than 20 mM also inhibited the oscillation activity. The results suggest that the oscillations are not caused by, but may be modulated by, variations in potassium conductance. The Na+,K(+)-pump seems to play an important role in the generation of rhythmic contractions in these vessels.

Effect of apamin on the responses to VIP, ATP and NANC neurone stimulation in the rat and cat gastric fundus

1. The influence of apamin on non-adrenergic non-cholinergic (NANC) relaxation and on the effect of the putative transmitters VIP and ATP was examined in the rat and cat gastric fundus. 2. In longitudinal muscle strips of the rat gastric fundus, ATP induced a biphasic effect, relaxation followed by contraction. The relaxant effect of ATP was blocked by apamin, whereas the relaxations induced by VIP and NANC neurone stimulation were not influenced by apamin. 3. In circular and longitudinal muscle strips of the cat gastric fundus, ATP only induced relaxation at high concentrations. The ATP-induced relaxation was increased in the presence of apamin, whereas the VIP-induced and NANC relaxations were not influenced. 4. It is concluded that the relaxant effect of ATP might be related to activation of apamin-sensitive Ca(2+)-dependent K(+)-channels in the rat gastric fundus. In the cat gastric fundus, apamin did not antagonize the relaxant effect of ATP so that it cannot be used to investigate an ATP involvement in inhibitory NANC neurotransmission in this tissue. No evidence for a presynaptic inhibitory action of apamin on VIP-ergic NANC neurones was obtained.

Mechanical, electrical and cyclic nucleotide responses to peptide VIP and inhibitory nerve stimulation in rat stomach

1. Effects of apamin on electrical and mechanical activities and cyclic nucleotide accumulation in response to vasoactive intestinal peptide (VIP) and intramural nerve stimulation were investigated in isolated circular strips of the rat stomach in the presence of atropine and guanethidine. 2. Circular muscles generated rhythmic contractions and slow waves in the antrum but not in the fundus. Intramural nerve stimulation and VIP caused frequency- and dose-dependent relaxation of fundic strips and inhibition of spontaneous contractions of antral strips. Apamin partly reduced the responses to intramural nerve stimulation but not those to VIP. 3. In the antrum, apamin reduced inhibitory junction potentials (IJPs) evoked at the nadir of slow waves but not at their zenith. In the fundus, apamin partly decreased the amplitude of IJPs. Repetitive nerve stimulation was associated with an apamin-sensitive hyperpolarization and apamin-resistant decrease in the slow wave amplitude in the antrum. 4. VIP caused a dose-dependent hyperpolarization of fundic circular muscle membrane. In the antrum, VIP inhibited spike potentials superimposed on slow waves and it decreased the slow wave amplitude in about half of the preparations. These electrical responses to VIP were resistant to apamin. 5. Intramural nerve stimulation evoked an apamin-resistant output of VIP from muscle strips, which no longer occurred after tetrodotoxin or removal of extracellular Ca2+. 6. Intramural nerve stimulation and VIP elicited apamin-resistant increases in cyclic AMP and cyclic GMP accumulations. The effects of VIP on cyclic AMP were greater than those on cyclic GMP. The effects of intramural nerve stimulation on cyclic GMP were faster in onset than those of cyclic AMP. 7. It is suggested that VIP is a neurotransmitter of the intramural inhibitory nerves concerned in the apamin-resistant relaxation of the rat stomach.

VIP antisera inhibit the relaxatory motor responses of the feline sphincter of Oddi and gall-bladder induced by VIP or vagal nerve stimulation

Regional administration of VIP elicited a dose-dependent relaxation of the feline sphincter of Oddi and gall-bladder. Relaxatory motor responses of these regions at efferent electrical stimulation of the vagal nerves were unmasked after atropine (resistant to propranolol but sensitive to hexamethonium). These findings in combination with the presence of a rich VIP-ergic innervation, including intrinsic VIP neurons, have made VIP a tentative post-ganglionic non-adrenergic, non-cholinergic neurotransmitter to these regions. The relaxatory motor responses elicited by VIP or vagal activation were selectively antagonized using regional administration of specific VIP antisera in support of this hypothesis.

A novel neurogenic vasodilator mechanism in bovine mesenteric artery

The presence of a neurogenic vasodilator mechanism was investigated in isolated bovine mesenteric arteries (BMAs) that were precontracted with phenylephrine. Electrical field stimulation induced tetrodotoxin-sensitive relaxations in guanethidine-pretreated BMAs. The relaxation occurred after a delay of about 5-8 seconds and amounted to 25-35% in different sets of experiments. The relaxation was not affected by classical receptor antagonists such as atropine (1 microM), cimetidine (3.9 microM), clemastine (2.8 microM), naloxone (1.2 microM), 8-phenyltheophylline (1 microM), propranolol (3.4 microM), ritanserin (5 microM), or droperidol (13 microM). The nicotinic acetylcholine-receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium iodide (10 microM) was without effect on the relaxation, and removal of the endothelium of the arteries also had no effect. The bee venom component apamin (1 microM), which has been shown to block the nonadrenergic, noncholinergic relaxation in intestinal and vascular smooth muscle from other species, was also found to be without effect on the relaxation induced by electrical field stimulation in BMAs. Pretreatment of the arteries with capsaicin (1 microM) had no effect per se and did not affect the relaxation induced by a subsequent stimulation. Capsaicin has been suggested to release neurotransmitter and eventually deplete neurons containing substance P and calcitonin gene-related peptide. Furthermore, exogenously applied calcitonin gene-related peptide (1-100 nM), substance P (10 nM-1 microM), and vasoactive intestinal peptide (0.3-30 nM) gave relaxations amounting to less than 10%. It is postulated that electrical field stimulation induces a neurogenic relaxation of a nonadrenergic, noncholinergic nature in BMAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Apamin and nonadrenergic inhibition of guinea pig trachealis

Apamin has been shown to antagonize the nonadrenergic, noncholinergic (NANC) inhibitory system in guinea pig taenia coli. We have examined the effects of apamin on the nonadrenergic noncholinergic inhibitory system and its putative transmitters in isolated guinea pig trachea. Electrical field stimulation (ES) of isolated trachea pretreated with atropine and propranolol evoked reproducible relaxations that were blocked by tetrodoxin, but were unaffected by apamin. Vasoactive intestinal peptide (VIP), adenosine (AD), and adenosine triphosphate (ATP) produced concentration-dependent inhibition of histamine (H)-induced contractions of isolated trachea but the inhibitory actions of these agents were not significantly affected by apamin. In contrast, apamin virtually abolished ES-evoked relaxations in guinea pig isolated taenia caeci, and reduced the inhibition of H-induced contraction by ATP from 40% to 1%. We conclude that neither the NANC inhibitory system in the guinea pig trachea nor its putative mediators VIP, AD, and ATP are antagonized by apamin, in contrast to taenia caeci.

Vasodilator responses to enflurane in the small intestine

Local effects of enflurane on intestinal vascular resistance were studied in vivo in cats. A jejunal segment was prepared and perfused at constant flow with blood from the femoral arteries. The intestine was either: (1) left with intact sympathetic innervation, (2) denervated and exposed to electrical post-ganglionic vasoconstrictor nerve stimulation, or (3) excluded from neurogenic remote control by post-ganglionic denervation. Enflurane dissolved in lipid and intra-arterially administered to the jejunal segment in doses comparable to those clinically encountered, decreased intestinal vascular resistance in relation to the intra-arterial concentration of the drug. The vasodilator response was, at the highest enflurane doses studied (blood concentration: 400 and 800 mg X 1(-1], most pronounced in the intestine with intact sympathetic innervation. Otherwise, no differences were observed in vasodilator responses between the three different investigated modes of neurogenic influence on the intestine. In vitro enflurane (-in-lipid) did not affect the vasoconstrictor response to electrical field stimulation in the rat mesenteric arterioles. Enflurane, however, dose-dependently reduced spontaneous contractile activity in the rat portal veins.

Comparison of the effects of apamin, a Ca2+-dependent K+ channel blocker, and arylazido aminopropionyl ATP (ANAPP3), a P2-purinergic receptor antagonist, in the guinea-pig vas deferens

Apamin, which blocks Ca2+-dependent increases in K+ permeability, antagonizes ATP-induced relaxation of several smooth muscles. The ATP photoaffinity label arylazido aminopropionyl ATP (ANAPP3), following its photolysis in the presence of the guinea-pig vas deferens, antagonizes contractile responses to ATP. This study was conducted to determine whether apamin antagonizes ATP-induced responses in the guinea-pig vas deferens, and also to evaluate whether ANAPP3 antagonizes responses to ATP by interfering with Ca2+-dependent K+ permeability changes. Apamin (10(-6) M) potentiated ATP-induced contractions. This potentiation was nonspecific in that responses to norepinephrine, histamine and acetylcholine also were enhanced; responses to KCl were unaffected. To evaluate the possible interactions between the two agents at the same cellular site, the effect of apamin was examined in ANAPP3-treated tissues. In such tissues apamin did not potentiate the residual responses to ATP; however, apamin was nevertheless able to potentiate responses of ANAPP3-treated tissues to norepinephrine, histamine and acetylcholine, and responses to KCl remained unaffected. These studies provide additional support for the view that ANAPP3 antagonizes ATP-induced responses of the guinea-pig vas deferens by blocking P2-purinergic receptors. The antagonism by ANAPP3 is not attributable to a blockade of Ca2+-dependent K+ permeability changes.

Effect of apamin on release of vasoactive intestinal polypeptide (VIP) from the cat intestines

The effect of apamin, a polypeptide from bee venom, on the release of vasoactive intestinal polypeptide (VIP) during active neurogenic vasodilatation in the intestines was studied in vivo in anesthetized cats. Three non-adrenergic, non-cholinergic mechanisms were investigated, i.e. the vasodilatation seen upon transmural electrical field stimulation, pelvic nerve activation and stimulation of the intramural nerves with 5-hydroxytryptamine (5-HT) infused i.a. Apamin given close i.a. abolished the three vasodilator responses. Concomitantly, the increase of VIP release was also markedly diminished although apamin increased the rate of VIP release seen in the "resting" control period. The results are in agreement with the hypothesis that VIP is the neurotransmitter in the three investigated vasodilator mechanisms.