Effect of lanthanum on the release of acetylcholine from the myenteric plexus and on its activation by ouabain and electrical stimulation

Presynaptic and postsynaptic effects of mercuric ions on guinea-pig ileum longitudinal muscle strip preparation

The toxic effect of mercuric ions on intestinal cholinergic neurotransmission was investigated in vitro. Hg2+ inhibited the evoked release and enhanced the resting release of ACh. Smooth muscle contraction was irreversibly inhibited by Hg2+ in a concentration-dependent manner, and Na2EDTA did not antagonize this effect. We also investigated if Hg2+ enters the nerve terminal through Ca(2+)-channels, or Na(+)-channels, or both. The effects of mercuric ions obtained in our study were completely abolished by the combined administration of TTX and Co2+. It is suggested that the site of the action of mercuric ions is intracellular. We concluded that Hg2+ may interfere with cholinergic transmission by blocking [Ca2+]o-dependent release of ACh and by enhancing [Ca2+]o-independent resting release of ACh. The effect of Hg2+ was not only presynaptic since it also inhibited the effect of ACh on smooth muscle.

Effect of lanthanum ions on the release of acetylcholine induced by tityustoxin, K+ and ouabain from myenteric plexus and brain cortical slices

Exposure of myenteric plexus longitudinal muscles to 2 mM LaCl3 increased the rate of spontaneous [3H]-ACh release. In brain cortical slices La3+ did not affect the spontaneous release of ACh. The release of ACh induced by tityustoxin, ouabain or K+ 50 mM in brain cortex and myenteric longitudinal muscles slices was inhibited by La3+ treatment. It is suggested that tityustoxin, ouabain and K+ release ACh from the same pool that is inhibited by La3+ pretreatment.

Activation of protein kinase C suppresses the gamma-aminobutyric acidB receptor-mediated inhibition of the vesicular release of noradrenaline and acetylcholine

Modulation of the gamma-aminobutyric acidB (GABAB) receptor-mediated response by protein kinase C (PKC) was examined with regard to inhibition by stimulation of the GABAB receptor of stimulation-evoked release of noradrenaline (NA) from slices of cerebellar cortex and of acetylcholine (ACh) from strips of ileum. 12-O-Tetradecanoylphorbol 13-acetate (TPA) potentiated the high K(+)-evoked Ca2+-dependent release of NA and ACh, but not the ouabain-evoked release, even in the presence of external Ca2+. The potentiating effect was antagonized by sphingosine, thereby suggesting that PKC participates in the exocytotic-vesicular release of neurotransmitters, but does not do so in case of a nonvesicular release. GABA inhibited the high K(+)-evoked release of NA and ACh, but not the ouabain-evoked Ca(2+)-independent release. The effect of GABA was mimicked by baclofen and was antagonized by phaclofen, thereby suggesting that stimulation of the GABAB receptor inhibits the vesicular but not the nonvesicular release of neurotransmitters. TPA suppressed the GABAB receptor-mediated inhibition of high K(+)-evoked release of NA and ACh. The effect of TPA was antagonized by sphingosine. These results indicate that stimulation of the GABAB receptor inhibits the stimulation-evoked Ca(2+)-dependent release of neurotransmitters and that activation of PKC suppresses the GABAB receptor-mediated response.

Influence of ouabain on the tracheal musculature of the dog

The effects of ouabain on the smooth muscles of the airway were investigated in anesthetized, paralyzed and artificially ventilated mongrel dogs. Ouabain (30 micrograms/kg, i.v.) caused a constriction of the tracheal smooth muscle which was followed by bradycardia. When ouabain was infused at a rate of 2 micrograms/kg/min (i.v.), the tracheal constriction was induced by a total dose of 45.0 +/- 5.5 micrograms/kg, while the bradycardia appeared with a total dose of 54.4 +/- 6.1 micrograms/kg. The ouabain-induced tracheal constriction was inhibited by bilateral vagotomy. The tracheal constriction induced by i.a. infusion of 10 microM ouabain into the bilateral cranial thyroid arteries was inhibited by bilateral vagotomy, but it was not completely blocked. With bilateral vagotomy, the tracheal constriction induced by i.a. infusion of ouabain was unaffected by 3 microM hexamethonium, but it was significantly inhibited by 1 microM atropine. These results suggest that ouabain may induce tracheal constriction by a neurogenic action in addition to its action via the augmentation of the vagal reflex, and the neurogenic action of ouabain may be related, in large part, to the release of acetylcholine from the presynapses of vagus nerves in dogs.

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH-5183), tityustoxin and ouabain on the release of acetylcholine and its mobilization from cytoplasmic and vesicular pools of rat brain cortical slices

The effect of vesicular acetylcholine (ACh) transport blocker 2-(4-phenylpiperidino)cyclohexanol (AH-5183) on the subcellular storage and release of ACh was studied in rat brain cortical slices. AH-5183 reduced the release of ACh from cortical slices stimulated by tityustoxin and ouabain. Tissue stimulated in the presence of AH-5183 contained more ACh in both the nerve terminal synaptic vesicles and cytoplasmic fraction than did tissue stimulated in drug's absence. Thus, AH-5183 blocked the tityustoxin and ouabain induced release of ACh from both cytoplasmic and vesicular pools. AH-5183 also depressed the spontaneous release of ACh from incubated slices and, in this condition, the drug had no effect in the subcellular distribution of ACh. It is suggested that AH-5183 interferes with the process of ACh release independent of its blocking action on ACh transport into the synaptic vesicles.

Interactions between alpha-latrotoxin and trivalent cations in rat striatal synaptosomal preparations

The interactions between alpha-latrotoxin (alpha-LTx), a neurosecretagogue purified from the venom of the black widow spider, and the trivalent cations Al3+, Y3+, La3+, Gd3+, and Yb3+ were investigated in rat striatal synaptosomal preparations. All trivalent cations tested were inhibitors of alpha-LTx-induced [3H]dopamine [( 3H]DA) release (order of potency: Yb3+ greater than Gd3+ approximately Y3+ greater than La3+ greater than Al3+). Only with Al3+ could inhibition of [3H]DA release be attributed to a block of 125I-alpha-LTx specific binding to synaptosomal preparations. The inhibitory effect of trivalent ions was reversible provided synaptosomes were washed with buffer containing EDTA. Trivalent ions also inhibited alpha-LTx-induced [3H]DA release at times when alpha-LTx-stimulated release was already evident. alpha-LTx-induced synaptosomal membrane depolarization was blocked by La3+, but not affected by Gd3+, Y3+, and Yb3+. alpha-LTx-stimulated uptake of 45Ca2+ was inhibited by all trivalent cations tested. These results demonstrate that there exist at least three means by which trivalent cations can inhibit alpha-LTx action in rat striatal synaptosomal preparations: (1) inhibition of alpha-LTx binding (Al3+); (2) inhibition of alpha-LTx-induced depolarization (La3+); and (3) inhibition of alpha-LTx-induced 45Ca2+ uptake (Gd3+, Y3+, Yb3+, La3+).

Involvement of protein kinase C in the Ca2+-dependent vesicular release of GABA from central and enteric neurons of the guinea pig

The involvement of protein kinase C (PKC) in the release of endogenous gamma-aminobutyric acid (GABA) was studied using slices of deep cerebellar nucleus and strips of small intestine from the guinea pig. 12-O-tetradecanoylphorbol 13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD), potentiated the high K+-evoked release of GABA from both preparations in the presence of tetrodotoxin. Ouabain evoked the release of GABA from both preparations, and this release was not altered by TPA. Therefore, the activation of protein kinase C potentiates the Ca2+-dependent vesicular release of GABA from nerve terminals of the central and enteric GABAergic neurons of the guinea pig.

Role of protein kinase C in the vesicular release of acetylcholine and norepinephrine from enteric neurons of the guinea pig small intestine

The involvement of protein kinase C in the release of [3H]acetylcholine (ACh) and [3H]norepinephrine (NE) was studied in strips of guinea pig small intestine. 12-O-tetradecanoyl phorbol 13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate (4 alpha-PDD) potentiated the A23187-evoked release of [3H]ACh and [3H]NE from the strips of small intestine preloaded with [3H]choline and [3H]NE, and the potentiating effect of TPA was inhibited by polymyxin B. High K+-evoked releases of [3H]ACh and [3H]NE in the presence of tetrodotoxin were also potentiated by TPA. These TPA-induced potentiations of the evoked release were greater at a low concentration of external Ca2+ (0.5 mM) than at a high concentration (2 mM). Ouabain induced the release of these neurotransmitters both in the absence and presence of the low concentration of external Ca2+. The ouabain-evoked release was not altered by TPA. These results indicate that the activation of protein kinase C potentiates the vesicular release of ACh and NE at low Ca2+ concentration from the nerve terminals of enteric neurons in the guinea pig small intestine.