Effects Of Exogenous Acetylcholine Research Articles

The effect of exogenous acetylcholine and other cholinergic agents on photoperiodic flower induction of Pharbitis nil

Exogenous acetylcholine (ACh), acetylcholinesterase (AChE) inhibitors, as well as agonists and antagonists of ACh receptors applied on the cotyledons of 5-day-old seedlings of <em>Pharbitis nil</em> during a 16-hour long inductive night or during a 12-hour long subinductive night, do not essentially influence the flower bud formation. Also the application of above mentioned substances to the seedlings growing under the conditions of 72 hours of darkness, 24 hours of light and then 24 hours of darkness does not influence in an essential way flowering of <em>P. nil</em>. On the other hand, applying these substances on the cotyledons of <em>P. nil</em> during 24-hour-long inductive night, preceded by 72 hours of darkness and then 24 hours of light of lowered intensity finished by 15-minute-long impulse of far red light which inhibit flowering, caused the reversion of the effect of far red light irradiation and stimulated the flowering. The obtained results suggest that endogenous system ACh/AChE could participate in the mechanism of a phytochrome controlled flowering of short day plants.

Angiotensin AT 1 receptor antagonism normalizes retinal blood flow and acetylcholine-induced vasodiliation in normotensive diabetic rats

The renin angiotensin system is emerging as a potential therapeutic target for diabetic retinopathy. This study examines the effects of angiotensin-converting-enzyme inhibition by captopril and angiotensin AT(1) receptor antagonism using candesartan-cilexetil on retinal blood flow and acetylcholine-stimulated vasodilatation in normotensive diabetic rats. Non-diabetic or streptozotocin-induced diabetic rats were treated for 2 weeks with captopril (100 mg/kg/day) or candesartan cilexetil (2 mg/kg/day). Retinal haemodynamics were measured using video fluorescein angiography. Effects of exogenous acetylcholine on retinal haemodynamics were examined following intravitreal injection. Total retinal diacylglycerol was labelled using diacylglycerol kinase, separated by thin-layer chromatography, and quantified using autoradiography. Diabetic rats had prolonged retinal mean circulation time and decreased retinal blood flow compared with non-diabetic rats. Treatment of diabetic rats with either captopril or candesartan blocked the development of these blood flow abnormalities. Intravitreal injection of acetylcholine (10(-5) mol/l) in non-diabetic rats increased retinal blood flow by 53.9+/-22.0% relative to baseline whereas this response to acetylcholine was blunted in diabetic rats (4.4+/-19.6%, p<0.001). Candesartan treatment of diabetic rats restored the acetylcholine-stimulated retinal blood flow response to 60.0+/-18.7% compared with a 56.2+20.1% response in candesartan-treated non-diabetic rats. Total retinal diacylglycerol levels were increased in diabetic rats (3.75+/-0.98 nmol/mg, p<0.05) compared with non-diabetic rats (2.13+/-0.25 nmol/mg) and candesartan-treatment of diabetic rats normalized diacylglycerol levels (2.10+/-0.25 nmol/mg, p<0.05). This report provides evidence that angiotensin-converting enzyme inhibition and AT(1) receptor antagonism ameliorates retinal haemodynamic dysfunctions in normotensive diabetic rats.

Effect of exogenous acetylcholine on neuromuscular transmission in the stimulated rat diaphragm

Nerve stimulation is performed in rat phrenicodiaphragmal preparations with armine-inhibited acetylcholinesterase. Acetylcholine (1×10−7 M) is added to the saline for 15 min, and as it is washed off (during 1–2 h), the amplitude of isometric contractions and of the total action potential increases in the continuously stimulated muscle. Contractions in response to direct muscle stimulation remain unchanged. The membrane resting potential of muscle fibers exposed to acetylcholine shifts by 2–3 mV toward hyperpolarization and remains at this level for 2 h after the removal of acetylcholine from the saline.

Endothelium-mediated and direct actions of acetylcholine on rabbit aortic baroreceptors.

Effects of exogenous acetylcholine on single-unit aortic baroreceptors were studied using a rabbit in vitro aortic arch/aortic nerve preparation. The arch was perfused with Krebs-Henseleit solution at constant pressure while simultaneously recording arch diameter and baroreceptor discharge frequency. Administration of acetylcholine over a wide range of concentrations (10(-9) to 10(-4) mol/L) evoked multiple dose-dependent changes in baroreceptor activity. "Low" concentrations (< 10(-6) mol/L) consistently dilated arch and increased baroreceptor discharge (n = 11). These responses were prevented by prerelaxing smooth muscle with sodium nitroprusside (10(-7) mol/L, n = 6) and were augmented by precontracting smooth muscle with norepinephrine (0.5 x 10(-7) mol/L, n = 11). Thus, the baroreceptor responses were dependent quantitatively on the level of preexisting vasoactive tone. The responses also were antagonized by atropine (10(-7) mol/L, n = 6), which inhibits release of endothelium-derived relaxing factor (EDRF), and by N-monomethyl-L-arginine (10(-5) mol/L, n = 6), a specific blocker of nitric oxide formation (primary relaxing factor responsible for acetylcholine vasorelaxation). Hexamethonium was ineffective (n = 6). These results indicate that "low-dose" responses to acetylcholine were mediated solely by release of EDRF. In contrast, "high" concentrations of acetylcholine (> 10(-6) mol/L) constricted arch and reduced baroreceptor discharge in most units (66%). These responses were blocked by atropine, but not by hexamethonium, and were attributed to direct contraction of the smooth muscle. In the remaining baroreceptors (33%), discharge increased irrespective of changes in diameter or preexisting vasoactive tone, but not in the presence of hexamethonium. Therefore, some baroreceptors were stimulated chemically by acetylcholine (or an intermediary substance), probably through activation of nicotinic receptors on the sensory endings. The effects of acetylcholine on the baroreceptor pressure-frequency curve also were examined by injecting slow pressure ramps before and during acetylcholine treatment. Low concentrations that relaxed smooth muscle shifted the curve to lower pressures; high concentrations that contracted smooth muscle shifted the curve to higher pressures. Last, when acetylcholine was given with the diameter rather than the pressure held constant, the baroreceptor reciprocal responses to smooth muscle relaxation and contraction were directionally reversed but remained consistent with the changes in wall tension.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of exogenous acetylcholine on potassium currents in the frog motor nerve ending during acetylcholinesterase inhibition

The effect of exogenous acetylcholine (ACh) on potassium currents in the motor nerve ending (NE) has been studied in neuromuscular preparations of the frog cutaneous-sternal muscle by extracellular recording of evoked electrical potentials from the NE. The investigation was performed during inhibition of acetylcholinesterase (AChE) activity by specific inhibitors and AChE removal from the synaptic cleft by collagenase. After AChE inhibition by either armine or proserine, or after treatment of the preparation with collagenase, no effect of exogenous ACh in concentrations of 1·10−4–6·−4 mole/liter was observed, in contrast to results from preparations with intact AChE. However, under the same conditions, as in the case of active AChE, ACh in concentrations of 7·10−4–2·10−3 mole/liter inhibited Ca-activated potassium current of the NE membrane. Experiments with dipyroxim, a synaptic AChE reactivator, have shown that the ACh effect on the potential-dependent potassium current is mediated by specific AChE. The role of AChE is discussed in respect to its significance for realization of the ACh action on potential-dependent potassium current in NE.

Effect of exogenous acetylcholine on ionic currents of the frog and mouse motor nerve ending

Effect of exogenous acetylcholine on ionic currents of the frog and mouse motor nerve ending

Is there functional cholinergic innervation in the frog duodenum ( Rana catesbiana)?

1. To determine if functional cholinergic innervation occurs in the frog duodenum or not, the effects of exogenous acetylcholine and electrical transmural stimulation, the contractile activity of an acid extract from the frog duodenum, and the distribution of acetylcholinesterase (AChE) activity in the wall of the frog duodenum were investigated. 2. Acetylcholine caused non-sustained contraction in a dose-dependent manner (100nM−1 mM). The ed 50) value was 17 ± 2.4 μM. Atropine (500 nM) shifted the dose-response curve for acetylcholine parallel to the right. 3. Transmural stimulation of the frog duodenum caused frequency-dependent (0.5–50 Hz) contraction which was not decreased by atropine (500 nM) at all. 4. The acid extract from the frog duodenum caused contraction of a longitudinal muscle strip of guinea-pig ileum but atropine (500 nM) had no significant effect on the contraction. 5. Only a little AChE activity was found in Auerbach's plexus of the frog duodenum compared with that of the rat ileum. 6. These results suggest that a cholinergic nerve is present in the frog duodenum but its physiological significance is very small.

Presynaptic regulation by ACh of the NE mediated responses in the rat vas deferens.

In the isolated electrically stimulated vas deferens preparation the effect of exogenous acetylcholine (ACh) was studied. It was possible to differentiate two separate sites for the action of ACh. A postsynaptic effect (M1) which is revealed as a sudden decrease in the basal tension of the muscular twitch, antagonized competitively by atropine (pA2 = 8.44 +/- 0.79), potentiated at a ratio of 10.24 by neostigmine, and not altered by hexamethonium, yohimbine, clonidine, theophylline or prazosin. Treatment with reserpine or 6-OH-DA induced a supersensitivity of this effect. The second action is a presynaptic effect (M2), which is manifested by a rapid increase in the muscular twitch, is dose-dependent to ACh, and is potentiated significantly by reserpine, neostigmine and clonidine. This latter effect was not modified by atropine, hexamethonium, yohimbine, prazosin or theophylline. The normal efficacy of ACh is 185 times greater in presynaptic over postsynaptic receptors. The increase in the twitch is not considered to be by the release of endogenous ACh or ATP. The evidence presented here indicates that the presynaptic effect of ACh (M2) is due to participation of alpha 2-adrenoceptors in enhancement of the release of norepinephrine (NE) from the nerve terminals.

Modification by strychnine and d-tubocurarine of cardiac vagal inhibition in dogs

Intravenous inufison of strychinine or d-tubocurarine chloride attenuated cardiac depressant action and fall in blood pressure produced by electrical stimulation of the right vagus nerve in dogs anaesthetized with pentobarbitone sodium. The effects of exogenous acetylcholine on the heart and blood pressure (B.P.) were not modified by strychnine or d-turbocurarine. Physostigmine sulphate potentiated the effects of injected acetylcholine without modifying the effects of strychnine and d-tubocurarine on vagal transmission. Dimethylphenylpiperazinium bromide also produced its characteristic effects on the heart and B.P. in the presence of strychnine and d-tubocurarine. The results indicate that strychnine and d-tubocurarine inhibition of vagal transmission is not a result of blockade of peripheral cholinergic receptors or cardiac vagal ganglia. This inhibition occurs presumably via a preganglionic presynaptic action.

Inhibition by acetylcholine of the stimulation-evoked release of [ 3H]acetylcholine from the guinea-pig myenteric plexus

The longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum was incubated with [ 3H]choline and then superfused with Tyrode's solution. Exposure to [ 3H]choline resulted in the formation of [ 3H]acetylcholine which was released upon electrical field stimulation. The effects of exogenous acetylcholine, physostigmine and scopolamine on the stimulation-evoked release of [ 3H]acetylcholine were studied. In the absence of a cholinesterase inhibitor exogenous acetylcholine (10 −5 M) only slightly inhibited (by 26%) the evoked release of [ 3H]acetylcholine. If the cholinesterase activity of the preparation was reduced by about 50% in the presence of 10 −7M physostigmine, exogenous acetylcholine caused a concentration-dependent depression of the release evoked at 1 Hz. At a concentration of 10 −5 M acetylcholine the release was reduced by 76%. Scopolamine (10 −9 M) shifted the concentration-response curve for the inhibitory action of acetylcholine in a parallel manner to the right. From the dose ratio a pA 2 value of 9.8 for scopolamine against the release-inhibitory effect of acetylcholine was calculated. Physostigmine also inhibited the stimulation-evoked release of [ 3H]acetylcholine in a concentration-dependent manner, the maximal effect measured being an 85% reduction by 10 −5 M physostigmine. In the absence of a cholinesterase inhibitor scopolamine enhanced the evoked release of [ 3H]acetylcholine. The facilitatory effect was more marked at a stimulation frequency of 3 Hz (2-fold increase) than at 1 Hz (1.4-fold increase). It is concluded that extracellular acetylcholine decreases the stimulation-evoked release of neuronal acetylcholine. This inhibition is specifically mediated by a stimulation of presynaptic muscarinic receptors. The increase by scopolamine of the evoked release of [ 3H]acetylcholine suggests that previously liberated acetylcholine may trigger the negative feedback mechanism of acetylcholine release even if the cholinesterase activity is not inhibited, and that the presynaptic muscarinic receptors of the myenteric plexus have a physiological role in regulating the release of acetylcholine.

Effects of exogenous acetylcholine upon adrenal 17-hydroxycorticosteroid secretion of intact and head X-irradiated dogs.

Adrenocortical response to exogenous acetylcholine (Ach) was investigated, under anesthetized conditions, in intact, hypophysectomized and head X-irradiated dogs. Intravenous injection of Ach (1 mg/kg b.w.) to intact dogs resulted in marked increases in the secretion of 17-hydroxycorticosteroids (17-OHCS) by the adrenal gland. The maximum response was seen at 10 min after the injection and a return to preinjection levels tended to occur by 60 min. This effect was abolished completely by hypophysectomy. In dogs whose heads had been irradiated with 200 and 1,000 R of X-rays 1 day previously, a considerably lower response to Ach was found; 17-OHCS output at the time when the secretion had been maximum was 44--53% less than that in non-irradiated dogs.

The effects of pentobarbitone and chloralose anaesthesia on the vagal component of bronchoconstriction produced by histamine aerosol in the anaesthetized dog.

1 Total lung resistance (R(L)) and dynamic lung compliance (C(dyn)) were measured in dogs anaesthetized with pentobarbitone or chloralose and subjected to aerosols of histamine during 4 successive inspirations.2 Histamine caused concentration-dependent increases in R(L) and decreases in C(dyn). A significant vagal component was involved, but only when chloralose was employed and then only in the R(L) response.3 The resting values of R(L) and C(dyn) were similar regardless of which anaesthetic was used and remained essentially the same if the vagi were cooled.4 Electrical stimulation of the efferent vagi caused large increases in R(L) of dogs given chloralose and these effects were attenuated by the administration of pentobarbitone. Such stimulation was relatively ineffective in dogs given pentobarbitone alone.5In vitro, electrical field stimulation caused contractions of dog trachealis muscle. The responses were reduced by pentobarbitone in concentrations approximating to plasma levels in the anaesthetized dogs (1 to 5 x 10(-4) M), but the effects of exogenous acetylcholine were unaltered. The inhibition was dose-dependent, reversed by washing and unaltered by hexamethonium.6 The results suggest that pentobarbitone inhibits the vagal component of histamine-induced bronchoconstriction in the dog by an action on the efferent pathway. Furthermore, pentobarbitone acts either by blocking transmission along postganglionic parasympathetic nerves or by preventing the release of acetylcholine from the nerve endings in the lung.