Genetic Analysis of Susceptibility to Radiation-induced Apoptosis of Thymocytes in Mice

Genetic control of thymocyte susceptibility to radiation-induced apoptosis was investigated in BALB/cHeA, STS/A and five other strains of mice by counting pyknotic cells in a selected area of thymic cortex on histological specimens after whole-body X-irradiation. Number of dead cells increased almost linearly with doses (range 0.25-0.75 Gy) in BALB/cHeA and STS/A mice. However, dead cell counts in BALB/cHeA mice were more than twice those in STS/A mice at each dose. Of five other strains of mice, C57BL/6N and B10.BR mice exhibited a sensitive phenotype similar to BALB/cHeA mice, while C3H/HeAMsNrs and NFS/N mice showed a resistant phenotype similar to STS/A mice. A/J mice seemed to be rather resistant. A sex difference was not recognized in BALB/cHeA and STS/A mice. Resistance was dominant over susceptibility in the progenies of reciprocal crosses between the two strains, indicating an autosomal inheritance and no maternal effect. Segregation ratio of susceptible phenotype to resistant one in the backcrosses of female (BALB/cHeA x STS/A)F1 mice with male BALB/cHeA mice was not significantly different from 1:1 and all backcrosses of female (BALB/cHeA x STS/A)F1 mice with male STS/A mice exhibited a resistant phenotype. Results suggested that thymocyte susceptibility to radiation-induced apotosis is controlled by one major autosomal allele.

Ca(2+)-independent fusion of synaptic vesicles with phospholipase A2-treated presynaptic membranes in vitro

To clarify the mechanism of exocytosis in neurotransmitter release, the fusion of synaptic vesicles with presynaptic membranes prepared from rat brain synaptosomes and concomitant acetylcholine (ACh) release induced by fusion of them were studied in vitro. Fusion of the synaptic vesicles with presynaptic membranes was measured by a fluorescence-dequenching assay with octadecyl rhodamine B. Synaptic vesicles fused with presynaptic membranes which had been pretreated with porcine phospholipase A2 (PLA2) in the presence of 20 microM Ca2+ and released ACh, whereas synaptic vesicles did not interact with non-pretreated membranes. The fusion followed by ACh release depended (i) on the activity of PLA2 during the membrane pretreatment, (ii) on the amount of pretreated membrane and (iii) on the duration of the pretreatment. The presence of Ca2+ ions during the pretreatment was essential for inducing a fusogenic activity of the membranes, but Ca2+ ions were not required for the fusion itself because the fusion experiment was carried out in the presence of 5mM EGTA without added Ca2+. The presence of quinacrine, an antagonist of PLA2, during the membrane pretreatment inhibited their fusogenic activity, suggesting the importance of activation of PLA2. Presence of albumin during the pretreatment, which is an adsorbent of free fatty acids, also inhibited the fusogenic activity. Arachidonic acid, when added during the pretreatment, potentiated the fusogenic activity of the membrane. These findings suggest that the conformational change in the presynaptic membrane phospholipids induced by PLA2 and the presence of arachidonic acid produced by PLA2 are important in the process of fusion of synaptic vesicles with the presynaptic membranes of rat brain, and that the fusion process itself is independent of Ca2+.

Effects of H-89, an inhibitor of protein kinase A, on the acetylcholine release from myenteric plexus of guinea pig ileum

In order to clarify the involvement of cyclic AMP-dependent protein kinase (protein kinase A) in acetylcholine (ACh) release from myenteric plexus of guinea pig ileum, the effect of H-89, a specific inhibitor of protein kinase A, on the ACh release was investigated. H-89 (0.1-10 microM) inhibited the spontaneous and nicotine-induced release of ACh in a concentration dependent manner. It at 1 microM decreased both kinds of release of ACh to almost half of the control, but it did not affect the ACh release evoked by electrical field stimulation and by 5-hydroxytryptamine. H-89 had no significant effect on the indomethacin (IND), an inhibitor of PG synthesis, -insensitive component of the spontaneous and nicotine-induced release of, ACh. OP-41483, an analog of PGI2 and forskolin, an activator nicotine-induced release of, ACh. OP-41483, an analog of PGI2 and forskolin, an activator of adenylate cyclase, reversed the inhibitory effect of IND on the ACh release. H-89 at 1 microM completely inhibited the reverse effects of OP-41483 and forskolin. These results suggest that activation of protein kinase A is essential for modulation of the nicotine-induced and spontaneous ACh release from myenteric plexus of guinea pig ileum and the activity of protein kinase A is regulated by endogenous PGs via intracellular cyclic AMP level.

Factors influencing the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction

1. The effects of several conditions and agents on the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction in low-Ca2+ high-Mg2+ bathing solutions were examined. 2. Twin-pulses gave two endplate potential (epps) with first (m2) and second (m2) quantal contents. The ratio of m2/m1 was taken as a measure of the degree of facilitation. 3. The mean value of this ratio was > 1. Individual ratios fluctuated widely at junctions with smaller values of m1 but were focused around 1 at junctions with larger values of m1. Thus, some populations of junctions with smaller values of m1 contributed to an increment in the mean ratio. 4. The mean ratio was virtually constant irrespective of changes in the spontaneous and evoked release of transmitter at temperatures between 20 and 36 degrees C and at external concentrations of Ca2+ from 0.4 to 0.8 mM. 5. 4-Aminopyridine(4-AP) slightly but significantly increased this ratio with increases in m1 and m2 at temperatures of 24 and 36 degrees C. Ouabain slightly but significantly reduced the ratio, with increases in m1 and m2. The steadiness of the ratio was maintained in the presence of caffeine, high K+, neomycin or omega-conotoxin irrespective of changes in m1 and m2, except in the case of omega-conotoxin. 6. Spontaneous output at 36 degrees C increased in the presence of 4-aminopyridine, ouabain, caffeine, high K+ or neomycin. 7. These results indicate that maintenance of a stable value of the ratio of m2 to m1 is a dominant feature.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of a reduction in temperature on the quantal release of transmitter at the mouse neuromuscular junction

1. The effects of a reduction in temperature were examined on evoked and spontaneous release of transmitter quanta and on presynaptic negative signals, blocked by Cd2+, measured externally at neuromuscular junctions in mouse diaphragm muscles in low-Ca2+, high-Mg2+ Krebs-Ringer solutions. 2. The evoked release was enhanced with lowering of the temperature, whereas the extent of spontaneous release was reduced. Cooperativity of Ca2+ in the evoked release was slightly reduced by lowering the temperature. 3. The presynaptic negative signals increased in duration with lowering of the temperature. 4. These results support the hypothesis that the effect of a reduction in temperature reflects the improved efficacy of the calcium-mediated mechanism of transmitter release, manifested as a prolongation of the inflow of Ca2+. The process involved in the evoked release is probably attributable to an almost passive mechanism.

Role of prostacyclin in acetylcholine release from myenteric plexus of guinea-pig ileum

The roles of metabolites of arachidonic acid in spontaneous and agonist-induced acetylcholine release from a longitudinal muscle preparation with myenteric plexus of guinea-pig ileum were studied. Indomethacin significantly decreased both spontaneous acetylcholine release and its release induced by nicotine and substance P. We had found that prostaglandin E2 (PGE2) partly reversed this inhibition. We now found that a stable prostacyclin analog, OP-41483 at 100 nM, completely reversed the inhibition of acetylcholine release by indomethacin. On the other hand, PGD2, PGF2 alpha and ONO-11113, a thromboxane A2 analog, did not have any significant effect on the inhibition by indomethacin. OP-41483 had no effect on acetylcholine release induced by nicotine or substance P in the absence of indomethacin. To confirm the modulatory role of endogenous prostaglandins on acetylcholine release, we also studied the release of 6-keto-PGF1 alpha, a metabolite of prostacyclin, and PGE2 from longitudinal muscle preparations. The preparations released appreciable amounts of 6-keto-PGF1 alpha continuously during the experiments. Indomethacin inhibited release, while nicotine did not affect it so significantly. Our results suggest that endogenous prostacyclin modulates acetylcholine release from cholinergic nerve terminals in the myenteric plexus of guinea-pig ileum.

Mediators of nonadrenergic, noncholinergic inhibition in the proximal, middle and distal regions of rat colon

1. The mediators of non-adrenergic non-cholinergic (NANC) relaxation of the longitudinal muscle of rat proximal, middle and distal colon were examined in vitro. 2. Electrical transmural stimulation (TMS) of proximal, middle and distal segments of rat colon induced NANC relaxations which were inhibited by tetrodotoxin (1 microM), but not by atropine (1 microM) or guanethidine (4 microM). 3. In the proximal colon, L-nitro-arginine (N5-nitroamidino-L-2,5-diaminopentanoic acid) inhibited the TMS-induced NANC relaxation and L-arginine (1 mM) reversed this inhibition. Nitric oxide (0.3-10 microM) induced relaxation of the proximal segment. 4. NANC relaxation of the proximal segments was still evident after desensitization to vasoactive intestinal peptide (VIP). A VIP antagonist (VIP 10-28, 10 microM) had no effect on the TMS-induced NANC relaxation, which was also resistant to alpha-chymotrypsin (2 units ml-1) and a substance P antagonist ([D-Pro2, D-Trp7,9]substance P, 1 microM). 5. In the middle colon, L-nitro-arginine did not inhibit the TMS-induced NANC relaxation in 6 of 9 preparations tested and partially inhibited the relaxation in the other 3 preparations. L-Arginine did not reverse the partial inhibition. 6. Complete desensitization to VIP was not achieved in the middle colon. The VIP antagonist had no effect on the TMS-induced NANC relaxation. After alpha-chymotrypsin treatment of the segment, desensitization of the segments to substance P, or in the presence of the substance P antagonist, the TMS-induced NANC relaxation was augmented. 7. In the distal colon, L-nitro-arginine did not have any significant effect on the TMS-induced relaxation and nitric oxide did not induce relaxation. The VIP antagonist significantly inhibited TMS-induced NANC relaxation. Alpa-Chymotrypsin-treatment of the distal segments resulted in significant inhibition of NANC relaxation. No desensitization to substance P was achieved. Treatment with the substance P antagonist had no effect. 8. These results suggest that nitric oxide is the mediator of the NANC inhibitory response in the proximal region of rat colon; in the middle colon, substance P acts as an excitatory neurotransmitter, antagonizing the NANC relaxation caused by the mediator of the response, which is still uncertain. Our results suggest that that VIP is the most likely candidate as a NANC transmitter in the distal colon.

Role of cyclic AMP in prostaglandin-induced modulation of acetylcholine release from the myenteric plexus of guinea pig ileum

Prostaglandins (PGs) have modulatory effects on spontaneous and nicotine-induced release of acetylcholine (ACh) from the myenteric plexus of guinea pig ileum. To determine whether cyclic AMP is involved in the mechanisms of these effects, we studied ACh release under conditions that inhibit PG synthesis. Indomethacin (IND), a cyclooxygenase inhibitor, inhibited ACh release concentration-dependently. The effect of the maximally inhibitory concentration of IND (2.8 microM) on nicotine-induced ACh release were reversed concentration-dependently by PGE2, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and 8-bromo cyclic AMP. These compounds caused concentration-dependent reversal of the inhibition of spontaneous ACh release by IND, but their concentrations for restoration of spontaneous release were higher than those for restoration of nicotine-induced release. The effects of PGE2 and forskolin or IBMX were not additive in reversing the inhibition of nicotine-induced ACh release by IND. Neither forskolin nor 8-bromo cyclic AMP alone had any significant effect on either release. These results showed that increase in the level of cyclic AMP in myenteric cholinergic neurons restored ACh release from the tissue whose PG level had been lowered by IND and indicated that endogenous PGs may modulate the level of intraneuronal cyclic AMP.

Selective inhibitory effects of calcium channel antagonists on the two components of the neurogenic response of guinea pig vas deferens

The effects of L-type calcium channel antagonists and omega-conotoxin on the contractile responses of guinea pig vas deferens were examined in vitro. Electrical stimulation of the postganglionic hypogastric nerve induced biphasic contraction consisting of rapid phasic and delayed tonic components. L-type calcium channel antagonists, such as diltiazem, verapamil and nicardipine, mainly inhibited the delayed tonic component, whereas omega-conotoxin mainly inhibited the rapid phasic component. Stimulations in the presence of prazosin and alpha, beta-methylene ATP induced rapid transient and delayed contraction, respectively, which were inhibited by omega-conotoxin and L-type calcium channel antagonists, respectively. Short-term stimulation with five pulses induced a small fast phasic contraction. This contraction, which could be desensitized by alpha, beta-methylene ATP, was inhibited by omega-conotoxin, but not by L-type calcium channel antagonists. At the concentrations used in the present study, none of the calcium channel antagonists inhibited the contractions induced by exogenously added ATP or norepinephrine. These findings suggest that L-type calcium channel antagonists and omega-conotoxin inhibit the neurotransmissions mediated by norepinephrine and ATP, respectively, from the postganglionic nerve to the vas deferens of the guinea pig. Inhibition of the voltage-dependent calcium channel is discussed in relation to the mechanism of cotransmission in this preparation.

Key roles of nitric oxide and cyclic GMP in nonadrenergic and noncholinergic inhibition in rat ileum

Nonadrenergic and noncholinergic (NANC) inhibitory responses in circular and longitudinal muscles of the rat ileum were studied separately in vitro. Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region. Nitro-arginine inhibited the relaxation and L-arginine counteracted the effect of nitro-arginine. Treatment of the preparation with superoxide dismutase (SOD) and methylene blue resulted in enhancement and inhibition, respectively, of the relaxation induced by distension. Nitric oxide caused relaxation of the circular muscle in a dose-dependent manner. 8-Bromo cyclic GMP (cGMP) caused relaxation of the circular muscle. Electrical transmural stimulation caused relaxation followed by a rebound contraction of the longitudinal muscle. Nitro-arginine inhibited the relaxation and L-arginine counteracted this inhibition. Similar results to those in the circular muscle were obtained in the longitudinal muscle with SOD, methylene blue, nitric oxide and 8-bromo cGMP. Electrical field stimulation increased the cGMP content of the longitudinal muscle preparation. Nitric oxide also increased the cGMP content of smooth muscle cells obtained from circular and longitudinal muscles of rat ileum. Preincubation of smooth muscle cells with methylene blue inhibited the effect of nitric oxide on the cGMP content. These results suggest a key role of cGMP in NANC inhibitory responses in rat ileum. The factors mediating the responses are discussed.

Diversity among mouse motor nerve terminals with respect to release transmitter quanta

1. The aim of this work was to reexamine whether a positive correlation exists between the frequency (F, sec-1) of miniature endplate potentials (m.e.p.ps) and the quantal content (m) of endplate potentials (e.p.ps) or between quantal content, frequency and twin-pulse facilitation of transmitter release at a large number of neuromuscular junctions in the mouse. 2. The values of F and m were both measured intracellularly at endplates of mouse diaphragm in a high Mg2+/low Ca2+ bathing solution. 3. Values of both F and m varied from junction to junction. Smaller values of F were correlated with smaller values of m, and vice versa, resulting in a linear relationships. Histograms of F and m were skewed towards smaller values. 4. E.p.ps evoked by twin pulses gave the quantal contents of the first (m1) and second (m2) responses. 5. The ratio of m2 to m1 varied from junction to junction. A histogram of this ratio was skewed towards smaller values. 6. The ratio of m2 to m1 showed larger fluctuations at junctions with smaller values of F or m1 but was focused around 1 at junctions with larger values of F or m1. 7. The skewed parts of the histograms of F, m and m2/m1 accounted for the major population of junctions. 8. These results support the hypothesis that an intrinsic ability to release transmitter plays a role in regulation of the evoked output of transmitter at neuromuscular junctions in the mouse. 9. Such an ability is not correlated with the twin-pulse facilitation of transmitter release.

Role of PGE2 in neurotransmission from pre- to post-ganglionic hypogastric nerves of guinea pigs

The hypogastric nerve to guinea pig vas deferens was stimulated pre- or post-ganglionically by adjusting the position of the suction electrode. Both stimulations induced a biphasic contraction consisting of a rapid transient phase and a delayed tonic phase. Indomethacin partially inhibited the contraction induced by pre-ganglionic stimulation, but did not inhibit that induced by post-ganglionic stimulation. Prostaglandin (PG) E2 counteracted the inhibitory effect of indomethacin. Mepacrine also inhibited the contraction induced by pre-ganglionic stimulation. Arachidonic acid and PGE2 both reversed the inhibition. The PGE2-receptor antagonist SC-19220 inhibited the contraction induced by pre-ganglionic, but not post-ganglionic nerve stimulation. These results suggested that endogenous PGE2 is important in neurotransmission in the pelvic ganglion of guinea pigs.

The different activation of int genes in mammary carcinomas developed in three mouse strains harboring mouse mammary tumor viruses derived from DD/Tbr

RNA expressions of common integration site (int) genes and several oncogenes were investigated in mammary carcinomas spontaneously developed in different three strains of mice; DD/Tbr, NIH Swiss and BALB/c which harbor DD-MMTV derived from DD/Tbr mouse. Latter two strains of mice were designated NIH/Mtv+ and BALB/Mtv+, respectively. An increased expression of int-1 (wnt-1) and int-2 genes was observed in 56% (9/16) and 50% (8/16) of mammary carcinomas of DD/Tbr mice, respectively. Either int-1 or int-2 RNAs were expressed in 81% (13/16) of the carcinomas of DD/Tbr mice. IN NIH/Mtv+ mice, activation of int-1 and int-2 was observed in 41% (7/17) and 24% (4/17) of mammary carcinomas, respectively. Either int-1 or int-2 RNAs were expressed in 47% (8/17) of the carcinomas examined in this strain. In BALB/Mtv+ mice, on the other hand, either int-1 or int-2 gene were transcribed into RNAs at low frequency (33%: 3/9). These results suggest that the frequency of activation of int genes in mammary carcinomas induced by the same DD-MMTV in three strains of mice is genetically defined characteristics of these strains, and that the involvement of int-1 and int-2 genes in virus-induced mammary carcinogenesis may be influenced by genetic properties of animals. The activation of int-1 and int-2 genes did not clearly correlate with an increase in the expression of oncogenes examined; H-ras, K-ras, N-ras, myc, raf, fgr, fms, erB, mos, and src genes.

Prostaglandin E2 selectively affects purinergic transmission in guinea pig vas deferens

The effect of prostaglandin E2 (PGE2) on the contractile response of the guinea pig vas deferens was examined. Postganglionic hypogastric nerve stimulation for 7 sec at 20 Hz induced a biphasic contractile response, consisting of fast phasic and delayed tonic components. Prostaglandin E2 delayed the onset and increased the maximum contractile responses. Stimulation in the presence of prazosin induced only a fast phasic contraction. Treatment with PGE2, in the presence of prazosin, delayed the onset of this response and increased its maximum. The delayed contraction, observed on stimulation in the presence of alpha,beta-methylene adenosine triphosphate (ATP), was enhanced moderately and concentration-dependently by PGE2. Short-term stimulation with 5 pulses induced a small fast phasic contraction. This contraction, which could be desensitized by alpha,beta-methylene ATP, was inhibited by PGE2 but not by prazosin. Prostaglandin E2 significantly enhanced the transient phasic contraction, induced by addition of exogenous ATP to the organ bath and had a similar but somewhat smaller effect on the tonic contraction induced by the addition of exogenous norepinephrine (NE). These findings suggest that PGE2 selectively delayed neurotransmission, mediated by ATP and enhanced contractions of the smooth muscle of guinea pig vas deferens, elicited by ATP or NE.

The differential contribution of endogenous prostaglandins to the release of acetylcholine from the myenteric plexus of the guinea-pig ileum

1. Prostaglandin E (PGE) may be essential for maintaining the sensitivity of the myenteric plexus of guinea-pig ileum to nicotine. The contributions of prostaglandins to nervous activity evoked by different stimuli have now been investigated by measuring the amount of acetylcholine (ACh) released from the myenteric plexus of the guinea-pig ileum. 2. The amount of ACh released in response to dimethylphenylpiperazinium (DMPP) or substance P was depressed to about 40% of control by 2.8 microM indomethacin (Ind), whereas the release of ACh induced by 5-hydroxytryptamine (5-HT) was not affected. The inhibitory effects of Ind were overcome by 14.3 nM PGE2. 3. Mepacrine 5 microM, an inhibitor of phospholipase A2, depressed the release of ACh in response to DMPP and substance P to the same extent as Ind. These inhibitory effects of mepacrine were overcome by arachidonic acid (10 microM), but not by arachidonic acid plus Ind. The release of ACh evoked by 5-HT or electrical field stimulation (EFS) was also inhibited to about 60% of control by mepacrine but these inhibitions were overcome by arachidonic acid (10 microM) either in the absence or the presence of Ind. 4. The results suggest that endogenous prostaglandins and arachidonic acid contribute to the maintenance of the excitability of the myenteric plexus by DMPP and substance P. By contrast, the release of ACh induced by 5-HT and EFS may be regulated by arachidonic acid and not by prostaglandins.

Oxygen consumption stimulated by increases in permeability of Na+ of mouse diaphragm muscle in vitro

1. The effects on oxygen consumption of agents that modify Na(+)-permeability were examined in mouse diaphragm muscle perfused with a bathing solution that contained (+)-tubocurarine in a flow-through mode. Twitch tension and levels of Na+ and K+ in the muscle were also measured. 2. Unstimulated preparations decreased the concentration of oxygen in the bathing solution, indicating a basal level of oxygen consumption. Electrical stimulation of the muscle further decreased the concentration of oxygen. Potassium cyanide eliminated both the basal and the stimulated consumption of oxygen. 3. Veratridine facilitated the effect of stimulation at 0.1 Hz on both the consumption of oxygen and the twitch tension. Ouabain antagonized those effects. 4. Twitch contractions were blocked in the presence of dantrolene sodium or by pretreatment with ethylene glycol. Electrical stimulation of such preparations still caused a residual but considerably decreased consumption of oxygen. Ouabain and tetrodotoxin reduced the residual consumption of oxygen. 5. Ouabain significantly increased the levels of Na+ in the tissue, while veratridine alone did not. The effect of ouabain was further potentiated by the simultaneous presence of veratridine. 6. These results indicate that the enhancement of the sarcolemmal permeability to Na+ increases the rate oxygen consumption. This concept supports the hypothesis that Na+ homeostasis depends on energy consumption.

The biochemical and ultrastructural examinations in central cholinergic damage of the rat induced by the intraperitoneal administration of AF64A

Ethylcholine mustard aziridinium ion (AF64A), a synthesized cholinergic neurotoxin, was administered via intraperitoneal injection to the rat to study its effect on the central cholinergic nervous system. A single or consecutive daily injection of AF64A for 10 days resulted in a persistent reduction of acetylcholine (ACh) content in the several tested regions of the brain in the following order: hippocampus greater than cerebral cortex = striatum, the degree was the greatest in the hippocampus. Both resting and K(+)-stimulated release of ACh from the hippocampus were also significantly reduced 24 hr after a single injection of AF64A. Furthermore, daily injection of AF64A for 10 days induced a significant reduction of choline acetyltransferase (ChAT) activity in the homogenate obtained from the hippocampus but not from the cerebral cortex and striatum. ChAT activity in the crude synaptosomal fraction of the cerebral cortex was also significantly decreased. These results suggest that intraperitoneal administration of AF64A could induce cholinergic hypofunction more selectively in the nerve terminals. The high affinity choline uptake, which is located mainly on cholinergic nerve terminals, was not affected by the administration of AF64A. Any notable changes of ultrastructure in the cholinergic nerve terminals after the administration were not observed in all three regions examined. The present findings suggested that intraperitoneal administration of AF64A induces a specific damage of cholinergic nerve terminals by inhibiting ChAT activity. The cholinergic damage was most prominent in the hippocampus.

Differences in control of descending inhibition in the proximal and distal regions of rat colon

1. Descending inhibition in the proximal and distal portions of rat colon was studied separately, in vitro. 2. In the proximal colon, localized distension with a small balloon caused three types of response (contraction; relaxation; relaxation, then contraction) of the circular muscle on the anal side of the distended region. 3. Distension caused descending relaxation of circular muscle in all segments of the proximal colon, although for this prostaglandin F2 alpha (PGF 2 alpha) was necessary in some segments to increase muscle tone. 4. Atropine and guanethidine did not inhibit this descending relaxation, but tetrodotoxin did. 5. Hexamethonium inhibited the descending relaxation in 14 of 17 preparations of proximal colon tested, but not in the others. 6. In the distal colon, distension consistently caused an increase in the tone of the circular muscles. Descending relaxation was observed only after development of higher tone. Atropine and guanethidine did not inhibit the relaxation, but tetrodotoxin did. 7. Hexamethonium did not inhibit the descending relaxation in most of the preparations of distal colon examined. 8. AF64A, an inhibitor of choline uptake, inhibited the response mediated by cholinergic neurons in vitro to electrical transmural stimulation of the longitudinal muscle of proximal colon. 9. Treatment of colonic preparations with AF64A in vitro resulted in inhibition of descending relaxation in those of proximal, but not those of distal, colon. 10. The participation of intrinsic cholinergic neurones in the descending neuronal pathway is strongly suggested by the results in the proximal colon, but less so in the distal colon. 11. The tone and spontaneous contractile activity of colonic circular muscles are discussed in relation to their neuronal control.

Essential role of nitric oxide in descending inhibition in the rat proximal colon

Possible mediators of descending inhibition in the rat proximal colon were studied. Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region. This relaxation was still observed after the colonic segment had been desensitized to ATP, neurotensin and vasoactive intestinal peptide, so these compounds seem unlikely to mediate descending inhibition. Nitro-arginine inhibited the relaxation induced by the distension, and L-arginine counteracted the effect of nitro-arginine. Nitric oxide, isoamylnitrate and sodium nitroprusside caused relaxation. These results strongly suggest an essential role of nitric oxide in descending relaxation in the rat proximal colon.

Cooperation of ATP and norepinephrine in inducing contractile responses in guinea pig vas deferens

Stimulation of the hypogastric nerve to the guinea pig vas deferens induced biphasic contraction consisting of a rapid transient phase (mediated by ATP) and a delayed tonic phase (mediated by norepinephrine, NE), whereas stimulations in the presence of selective antagonists caused each contractile phase separately. Stimulation in the absence of antagonist induced a larger rapid transient contraction than that induced by stimulation in the presence of alpha 1-antagonist. Results obtained in separate or simultaneous additions of exogenous ATP and NE showed synergism in the rapid transient contraction. These findings indicate that NE assisted ATP in inducing the hypogastric nerve-mediated contractile response in guinea pig vas deferens, but not vice versa.

Effects of indomethacin and prostaglandin E2 on amylase secretion by rat parotid tissue

Indomethacin increased the secretion by low concentrations of isoproterenol (IPR, 30-100 nM) and decreased the secretion by a high concentration of IPR (1 microM). Indomethacin also had opposite effects on the secretions induced by low and high concentrations of norepinephrine (NE). PGE2 did not affect the secretions induced by IPR and NE, but it reversed the stimulatory and inhibitory effects of indomethacin. These findings suggest that PG has a role in modulating the amylase secretory response of the rat parotid gland.

Ryanodine facilitates calcium-dependent release of transmitter at mouse neuromuscular junctions

1. Quantal release of transmitter was measured intracellularly at mouse neuromuscular junctions in the presence and absence of ryanodine (Rnd). 2. Rnd at concentrations up to 1 microM did not significantly alter the frequency of miniature endplate potentials (m.e.p.ps) in the presence or absence of Ca2+, suggesting that Rnd is unlikely to alter the internal concentration of Ca2+ ([Ca2+]i) at rest. 3. In a high-K+ (10 mM) bathing solution, Rnd further potentiated the facilitatory effect of Ca2+ on the frequency (F, s-1) of m.e.p.ps. Rnd shifted the relationship between 1n(F) and 1n[Ca2+]o to lower concentrations. 4. In a high-Mg2+ bathing solution, Rnd did not affect the frequency of m.e.p.ps at any value of [Ca2+]o. However, Rnd slightly but significantly increased the quantal content (m) of e.p.ps. It shifted the relationship between 1n(m) and 1n[Ca2+]o to lower concentrations. These results suggest that Rnd potentiates the quantal release of transmitter after depolarization of the membrane or nerve impulse, in keeping with the cooperativity of Ca2+ at the active site. 5. A series of two closely spaced nerve impulses produced a facilitation of transmitter release, as judged by the quantal content (m2) of the second response in relation to that of the first one (m1), m2/m1. Rnd did not change the ratio m2/m1. Thus Rnd is unlikely to affect the rapid phase of the sequestration of Ca2+ inside the nerve terminal. 6. High levels of K+ (5 mM) and caffeine (2 mM) potentiated both modes of transmitter release, in a manner dependent on [Ca2+]o. Caffeine did not potentiate facilitation of transmitter release. 7. These results indicate that Rnd facilitates the quantal release of transmitter presumably via an increase in [Ca2 ]i by a manner different from that of high-K+ or caffeine. The results suggest that Rnd probably affects calcium turnover in neuronal cells.

Effects of Ca2+ blockers on the various types of stimuli-induced acetylcholine release from guinea pig ileum myenteric plexus

In the present study, we investigated the effects of various Ca2+ blockers on the release of acetylcholine (ACh) induced by nicotine, electrical field stimulation (EFS) and high-K+. Cd2+ markedly depressed the ACh release due to these stimuli. Verapamil inhibited the nicotine-induced ACh release remarkably and the EFS- or the high-K(+)-induced ACh release to a lesser extent. Since the nicotine- and the EFS-induced ACh releases were inhibited by procaine, the local anesthetic property of verapamil likely contributes in part to the inhibition. Diltiazem abolished the nicotine-induced ACh release completely but did not affect the EFS-induced release and significantly increased the high-K(+)-induced ACh release. These results suggest the absolute requirement for extracellular Ca2+ in the release of ACh induced by nicotine as well as EFS and high-K+. In addition, these stimuli may open the same Ca2+ channel to evoke ACh release.

Re-evaluation of the stimulatory effect of norepinephrine on the secretion of amylase in the parotid gland of the rat

The effect of the neurotransmitter norepinephrine(NE) in stimulating the secretion of amylase from the parotid gland of rats was studied by use of selective alpha- and beta-adrenergic antagonists. Its secretory response, mediated through beta-adrenoceptors, was slight during a short period of incubation, but rapidly increased after incubation for 10 min, showing a supersensitization phenomenon. Norepinephrine alone did not induce this phenomenon, but it induced the phenomenon in the presence of the alpha-adrenergic antagonist phentolamine or the alpha 1-antagonist prazosin. Isoproterenol-induced supersensitization was prevented by methoxamine. While, the accumulation of cyclic AMP in the tissue during incubation with isoproterenol and NE was not significantly affected by the presence of methoxamine and phentolamine, respectively. Phorbol dibutyrate did not inhibit the secretion induced by NE in the presence of phentolamine. These findings indicate that stimuli, mediated through alpha- and beta-adrenoceptors, induced secretion of amylase in parotid gland of the rat but that the alpha-effect inhibited the beta-effect when both stimuli were applied simultaneously and that the overall response of the tissue to NE resulted from the interaction of the two adrenoceptors.

Sodium salicylate facilitates calcium-dependent release of transmitter at mouse neuromuscular junctions

1. The effects of sodium salicylate on the frequency of miniature endplate potentials (m.e.p.ps) and on the quantal content of endplate potentials (e.p.ps) in mouse diaphragm muscles at 36 degrees C and 24 degrees C were studied by conventional microelectrode techniques. 2. At 36 degrees C, salicylate (10 mM) elevated the frequency of m.e.p.ps in a manner which was insensitive to [Mg2+]0 and independent of the presence of [Ca2+]0. Lowering the temperature to 24 degrees C abolished the stimulating effect. 2,4-Dinitrophenol (10 microM) had similar effects at 36 degrees C and these disappeared at 24 degrees C. All subsequent experiments were performed at 24 degrees C. 3. Salicylate (10 mM) further increased the frequency (F) of m.e.p.ps stimulated by [Ca2+]0 in a depolarizing solution. When [Ca2+]0 was varied in the absence of salicylate, a linear relationship between ln(F) and ln([ CA2+]0) was obtained. Salicylate shifted this relationship to the left, with respect to the control, without altering the slope. 4. Salicylate (5 mM) also increased the quantal content (m) of e.p.ps in a solution that contained 5 mM Mg2+, in a concentration-dependent fashion. As [Ca2+]0 was varied in the absence of salicylate, a linear relationship between ln(m) and ln[(Ca2+]0) was observed. Salicylate shifted this linear relationship to the left, with respect to the control, without altering the slope. Doubling the concentration of [Mg2+]0 antagonized this effect of salicylate on the quantal content of e.p.ps. 5. These results indicate that salicylate enhances the effect of changing [Ca2+]0. Salicylate probably facilitates the entry of Ca2+ into the nerve terminal or sensitizes the process that is regulated by Ca2 , thereby stimulating the release of transmitter. Surface negative charges may have an important role in the effect of [Ca22]0.

Contribution of external Ca2+ to the modulation by prostaglandin E2 of the release of acetylcholine from the myenteric plexus of guinea pig ileum

The effects of external concentration of Ca2+ [( Ca2+]o) on the modulation by indomethacin (IND) or prostaglandin E2 (PGE2) of the evoked release of acetylcholine (ACh) were investigated in the myenteric plexus of guinea pig ileum. An increase in the [Ca2+]o from 0.45 to 3.6 mM depressed both the spontaneous and the nicotine-induced release of ACh, while the releases of ACh induced by electrical-field stimulation (EFS) or by high K+ were augmented. IND and PGE2 did not modify the release of ACh induced by EFS and high K+, at any given [Ca2+]o tested. The nicotine-induced release of ACh was significantly inhibited by IND with no relation to [Ca2+]o, and these inhibitory effects of IND were prevented by PGE2. At 0.45 mM [Ca2+]o, the extent of the recovery by PGE2 was less potent than those at other [Ca2+]o. IND inhibited the spontaneous release of ACh at lower [Ca2+]o, and such inhibition was not observed in the presence of PGE2. Thus, the modulatory actions of PGE2 on the spontaneous and the nicotine-induced release of ACh may depend partially on [Ca2+]o.

The influence of ouabain on twitch potentiation by anticholinesterases in the phrenic nerve-diaphragm muscles of mice

1. (+)-Tubocurarine, hexamethonium, atropine, ouabain, and removal of potassium from the bathing medium were examined for their effects on indirectly evoked twitches (IT) of mouse phrenic nerve-diaphragm muscles in the presence or absence of neostigmine. 2. Neostigmine increased the amplitude of IT. The twitch potentiation was reduced by (+)-tubocurarine at low concentrations that had no inhibitory effect on normal IT. Hexamethonium (10-100 microM), but not atropine (0.1-1 microM), partially inhibited the twitch potentiation. Neither hexamethonium nor atropine had an inhibitory effect on IT in the absence of neostigmine. 3. Ouabain (5 microM) abolished the twitch potentiation by neostigmine while having no inhibitory effect on directly evoked twitches in the presence of neostigmine and (+)-tubocurarine together. 4. The potentiating effect of neostigmine was less in a potassium-free bathing solution. The inhibitory effect of ouabain disappeared in this solution. 5. Reinclusion of KCl at 2.5 mM restored both the potentiating effect of neostigmine and the antagonistic effect of ouabain. This reinclusion did not potentiate IT in the absence of neostigmine. 6. An interaction resembling that between ouabain and neostigmine was obtained between ouabain and physostigmine or paraoxon. 7. Both endplate potentials (e.p.ps) and miniature e.p.ps increased in terms of their amplitude and duration in the presence of neostigmine. Ouabain did not reduce the enhanced endplate responses. 8. These results indicate that the potentiation of IT by anticholinesterases may occur via nicotinic receptors which are sensitive to both (+)-tubocurarine and hexamethonium, and that the interaction between anticholinesterases and ouabain depends on the presence of K+. It appears that the mechanisms of twitch potentiation are dependent on the ionic gradients maintained by Na+-K+-ATPase.

Effects of AF64A on cholinergic neurotransmission in the sixth abdominal ganglion of the cockroach

1. The effects of ethylcholine mustard aziridinium ion (AF64A) on the cholinergic neurotransmission in the sixth abdominal ganglion of the cockroach were studied electrophysiologically and morphologically. 2. The pre- and post-synaptic compound action potentials (CAPs) elicited via electrical stimulation of the presynaptic fibers were recorded extracellularly. 3. The amplitude of both CAPs was depressed by AF64A (50-400 microM) in a concentration- and time-dependent manner. 4. At a high concentration, they were abolished but 100 microM of carbachol still evoked the postsynaptic event. 5. Electron microscopic observation of AF64A-treated ganglia showed that nerve terminals containing small lucent vesicles could not be observed but those containing dense core or large granular vesicles changed only slightly in shape. 6. These results suggest that AF64A is selectively neurotoxic for the presynaptic cholinergic neurons in the sixth abdominal ganglion of the cockroach.

The influence of ouabain on twitch contractions in the presence of veratridine

1. Direct stimulation evoked twitches in mouse diaphragm muscles in presence of 10 microM D-tubocurarine in vitro. Effects of ouabain and their dependence on K+ were examined on the twitch responses and action potentials in the presence and absence of twitch potentiators. 2. Ouabain inhibited twitch contractions only in the presence of veratridine, aconitine and monensin while it had no inhibitory effect on control twitches. The interactions between ouabain and these twitch potentiators depended on the presence of external K+, except in the case of monensin. 3. Removal of Ca2+ from a bathing solution accelerated the potentiating effect of veratridine and the antagonizing effect of ouabain. 4. Caffeine further potentiated the twitches which had been attenuated by ouabain combined with veratridine. 5. Ouabain combined with veratridine consistently decreased resting membrane potentials, action potentials and overshoot potentials and prolonged time to peak of and duration of the muscle action potentials. 6. Tetraethylammonium, 4-aminopyridine, and caffeine produced twitch potentiation which was insensitive to ouabain or the removal of K+. 7. These results suggest that twitch contractions in the presence of activators of sodium channels link with activation of Na+-K+-ATPase. Accumulation of Na+ inside the muscle fibres may uncouple the excitation-contraction system. 8. This uncoupling may not include the caffeine-sensitive process that controls the release of Ca2+ from the sarcoplasmic reticulum. Na+ accumulation may decrease transmembraneous gradient of this ions, thereby causing a reduction in excitation coupled with twitch contraction.

[Effects of ethylcholine mustard aziridinium ion (AF64A) administered intraperitoneally on cholinergic nerves in rat brain]

Rats injected i.p. with AF64A at a dose of 1/3 to 11/25 LD50 showed transient signs of motor disturbance such as ataxia, squatting down and respiratory distress and also longer lasting signs consisting of adipsia, aphagia and loss of body weight. Twenty-four hours after a single injection with AF64A (129 mumol/kg), ACh contents in the cerebral cortex, hippocampus and striatum decreased significantly, and the content in the cerebral cortex still remained at a lowered level even 10 days after injection. When rats received AF64A i.p. with an initial dose of 129 mumol/kg and nine consecutive daily doses of 12.9 mumol/kg and thereafter they were kept with no medication for at least 20 days, the ACh contents in the three brain regions were still significantly reduced. ACh release by electrical stimulation or K+, but not spontaneous release, was significantly reduced in the AF64A treated rats. The T-maze spontaneous alternation behavior in rats treated with AF64A was impaired. The present results indicate that AF64A administered i.p. can induce a long lasting defect in central cholinergic transmission.

Modulation of acetylcholine release from guinea-pig Ileum myenteric plexus by arachidonic acid cascade inhibitors

Effects of arachidonic acid and mepacrine on ACh release from guinea-pig ileum myenteric plexus were investigated. Mepacrine (1-8 microM) inhibited the ACh release in a concentration-dependent manner. Arachidonic acid counteracted the inhibitory effect of mepacrine, but PGE2 did not. The inhibition induced by a combination of mepacrine and indomethacin on nicotine-induced ACh release was prevented by arachidonic acid, while that on spontaneous ACh release was prevented by arachidonic acid and PGE2 added simultaneously. The roles of arachidonic acid and PGs in the ACh release will be discussed.

Effects of hypoxia, 2,4-dinitrophenol and ouabain on twitch potentiation elicited following conditioning pulses in the mouse phrenic nerve-diaphragm preparation

Hypoxia is suggested to increase intracellular Ca2+, thereby affecting cellular functions. If this is the case, the effect would be modified by other treatments which also elevate intracellular levels of Ca2+. To test this possibility, in mouse diaphragm preparations, the effects of hypoxia were examined on twitch potentiations after application of neural conditioning pulses which are considered to elevate intracellular Ca2+. The effects were compared with those of 2,4-dinitrophenol (DNP) and ouabain. Hypoxia increased the tension of twitches elicited directly or neurally with 0.1 or 0.5 Hz pulses at 36 degrees C but not at 24 degrees C. Paired pulses to the nerve induced a complex response profile in which the amplitude of the second twitch was enhanced (twitch facilitation). Tetanic pulses (50 Hz) to the nerve were followed by post-tetanic twitches of increased amplitude (post-tetanic twitch potentiation, PTP). Hypoxia little affected the twitch facilitation but abolished PTP at both temperatures. These effects differed from those of DNP and ouabain in some ways. Thus, present experiments indicate that hypoxia selectively influences the process which is responsible for the PTP phenomenon rather than for the twitch facilitation. It is possible that the mechanism by which hypoxia would accumulate intracellular Ca2+ may be included in the process through which PTP occurs.

Role of adrenergic alpha-receptors in regulation of acetylcholine release evoked by distension of guinea pig ileum

The significance of adrenergic nerves in the regulation of acetylcholine (ACh) release evoked by distension of the guinea pig ileum was evaluated by pharmacological manipulations. ACh release was measured by bioassay. Release in response to distension was completely abolished by epinephrine and norepinephrine. Tyramine also suppressed the distension-evoked ACh release, while dopamine was ineffective. The release of ACh in the anal segment, adjacent to the distended part, was abolished by epinephrine and norepinephrine. Dibenamine and phentolamine abolished ACh release anal to the distension, but augmented release orally, while dichloroisoproterenol and propranolol were ineffective. The present results give direct evidence that adrenergic nerves modulate cholinergic transmission in the myenteric plexus through alpha-receptors.

Effects of sulfhydryl agents on neuromuscular transmission in the presence or absence of cadmium

To test whether thiol groups are involved in neuromuscular transmission and its blockade by cadmium, the influence of sulfhydryl reagents were investigated on neuromuscular transmission, in the presence or absence of cadmium, using isolated mouse phrenic nerve-diaphragm muscles. Cadmium attenuated twitches evoked by indirect shocks but had almost no effect on twitches by direct shocks in the presence of d-tubocurarine (2.5 microM). The inhibitory effect was reversed by cysteine or by an increase in external calcium. The sulfhydryl oxidizing agent DTNB 5,5'-dithiobis(2-nitrobenzoic acid) potentiated the inhibitory effect of cadmium. Conversely, the reducing agent dithiothreitol (DTT) inhibited this effect. The potentiation by DTNB could be reversed by subsequent application of DTT. The sulfhydryl reagents had no influence on neuromuscular transmission in the absence of cadmium. Neuromuscular transmission was inhibited by the removal of calcium. This inhibition could be reversed by readmission of calcium. This restoration was neither potentiated nor reduced in the presence of these sulfhydryl reagents: It is suggested that an interaction of cadmium with thiol groups is independent of its calcium antagonistic action on neuromuscular transmission. Thiol groups may protect the transmission from the effect of cadmium.

Effect of external Ca2+ on the spontaneous and the various stimuli-induced acetylcholine release from guinea-pig ileum myenteric plexus

Effect of external Ca2+ concentration ([Ca2+]o) on spontaneous and various types of stimuli-induced acetylcholine (ACh) release from guinea-pig ileum myenteric plexus was studied. Electrical field stimulation- or high-K+-induced ACh release increased with the increment of [Ca2+]o. On the other hand, the spontaneous and the nicotine-induced ACh release increased up to 0.45 mM [Ca2+]o and then declined progressively as [Ca2+]o was raised. Qualitatively similar results were obtained with dimethylphenylpirerazinium-, 5-hydroxytryptamine- and substance P-induced ACh release. These results were discussed in terms of the stabilizing effect of Ca2+.

[Contribution of prostaglandins to the activity of guinea-pig phrenic and chicken esophageal parasympathetic nerves]

In order to clarify the role of prostaglandins (PGs) in the activity of cholinergic neurones other than the ileal myenteric plexus, the effects of indomethacin (IND) and PGE2 on the contractile responses and the release of acetylcholine (ACh) induced by electrical stimulation were investigated in isolated guinea-pig phrenic nerve-diaphragm and chicken parasympathetically innervated oesophagus preparations. In the guinea-pig phrenic nerve-diaphragm preparations, IND at 56 microM did not affect the twitch responses induced by direct or indirect electrical stimulation. PGE2 at 1 microgram/ml augmented the twitch responses induced by direct or indirect stimulation of submaximal, but not of supramaximal intensity. The amounts of ACh released from the phrenic nerve by electrical stimulation were unaffected by IND (56 microM). PGE2 (0.01-1 microgram/ml) inhibited the ACh release by the nerve stimulation of high frequency (50 Hz) in a concentration-dependent manner. The inhibitory effect of PGE2 was less clear on the ACh release by lower frequency (1 Hz). Neither IND nor PGE2 affected the ACh release induced by 40 mM-K+. In the chicken parasympathetically innervated oesophagus preparation, IND (2.8-5.6 5.6 microM) augmented the twitch responses induced by the nerve stimulation at a frequency of 0.017 Hz, but not those produced by train pulse (5 sec at a frequency of 1 or 10 Hz). This augmentation was reversed by the application of PGE2 at 10 ng/ml. PGE2 at 10 ng/ml produced a transient inhibition of the twitch responses induced by 0.017 Hz or train pulses.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of adenylate cyclase inhibitors, phosphodiesterase inhibitors, and dibutyryl cyclic AMP on spontaneous and various stimuli-induced acetylcholine release from guinea pig ileum myenteric plexus

In order to examine a possible contribution of cyclic AMP to acetylcholine (ACh) release from guinea pig ileum myenteric plexus, effects of adenylate cyclase inhibitors, phosphodiesterase (PDE) inhibitors and dibutyryl cyclic AMP on the spontaneous and the various stimuli-induced ACh release were investigated. A PDE inhibitor, theophylline (1 mM) increased the ACh release induced by nicotine (6.16 microM) significantly. Another PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 1 mM) and dibutyryl cyclic AMP (4 mM) had no effect. The adenylate cyclase inhibitors dithiobisnitrobenzoic acid (DTNB, 1 mM) and alloxan (4 mM) both decreased the nicotine-induced ACh release remarkably. PDE inhibitors increased and adenylate cyclase inhibitors decreased the high-K+-induced ACh release. Dibutyryl cyclic AMP brought about a slight but significant increase of the high-K+-induced ACh release. All the drugs failed to alter the ACh release induced by electrical field stimulation (EFS) at 10 Hz. Effects of all drugs except dibutyryl cyclic AMP on the spontaneous ACh release were the same as those on the nicotine-induced one. Dibutyryl cyclic AMP decreased it significantly. These results suggest that the cyclic AMP system is involved in the spontaneous, the nicotine-induced and the high-K+-induced ACh release and that the EFS-induced ACh release is independent of cyclic AMP.

The role of sialic acid in the nerve terminal for the release of transmitter

The role of sialic acid in the frequency of miniature endplate potentials (MEPPs) was examined using neuraminidase and gangliosides in the mouse diaphragm. Neuraminidase increased and decreased MEPP frequency in normal K+ and high K+ solution, respectively. The effects were dependent on the presence of Ca2+ in extracellular medium. Neuraminidase liberated sialic acid from and lowered Ca2+-binding capacity of synaptosomal membrane. Gangliosides treatment of the tissue partially restored the effects of neuraminidase on the frequency of MEPP and Ca2+-binding capacity. It is possible that sialic acid in the nerve endings provides a functional storage site which supply intracellular Ca2+ to cause a transmitter release.

Transmitter release at the mouse neuromuscular junction stimulated by cadmium ions

Cadmium ion (Cd2+) was examined for effects on frequency of miniature end-plate potentials (m.e.p.p.s) under several conditions in mouse diaphragm muscles. Cd2+ depressed potassium (K+)-stimulated rise in m.e.p.p. frequency, acting antagonistically to Ca2+, whereas in the absence of K+ stimulation Cd2+ produced a time- and concentration-dependent rise in m.e.p.p. frequency without affecting the resting membrane potential of muscle fibers. Such a presynaptic effect was observed with or without external Ca2+ [( Ca]0). In the absence of [Ca]0, Cd2+ frequently produced twitching in several fibers, which caused dislodgement of the micro-electrode. Therefore, 10 mM KCl was added to the Ca-free solution to overcome such difficulty. The following experiments were performed in the Ca-free 10 mM KCl solution. Prolonged exposure to Cd2+ led to an eventual decline in m.e.p.p. frequency within 20-30 min. After decline of the effect, m.e.p.p. frequency in the presence of Cd2+ was not restored by adding Ca2+ combined with elevated K+ and with A23187 or by carbonyl cyanide p-trifluoromethoxyphenyl hydrazine and hypoxic condition, each of which was separately effective in enhancing transmitter release. Exposure to 50 microM Cd2+ for 60 min decreased acetylcholine content in diaphragm muscle to approximately 50% of control. Tetrodotoxin and caffeine did not modify the presynaptic stimulatory effect of Cd2+. The effect of Cd2+ on m.e.p.p. frequency was partially reduced by trifluoperazine and phenytoin each of which depressed m.e.p.p. frequency rise induced by Ca2+ with elevated K+. The possible mechanism of Cd2+ action and of its interaction with intracellular Ca2+ stores is discussed.

Contribution of endogenous prostaglandins to excitation of the myenteric plexus of guinea-pig ileum: are adrenergic factors involved?

The role of endogenous prostaglandins (PGs) in the nicotine-induced contraction and release of ACh was investigated in the isolated guinea-pig ileum. A low concentration of indomethacin (IND 2.8 microM) inhibited the contraction and ACh release induced by nicotine. These inhibitory effects of IND were reversed by PGE2 at concentrations which are thought to be released spontaneously. SC-19220, a PG receptors antagonist, also inhibited the contraction and ACh release induced by nicotine. Unlike the nicotine-induced release of ACh, the potassium-induced release of ACh was unaffected by IND and SC-19220. IND was as potent in inhibiting the responses to cholinergic nerve stimulation by nicotine after treatment of the preparations with antiadrenergic agents. It is concluded that the inhibitory effect of IND does not depend on the functional integrity of adrenergic neurons and that endogenous PGs contribute directly to the modulation of myenteric plexus excitability by nicotine.

Involvement of adrenergic and serotonergic nervous mechanisms in allethrin-induced tremors in mice

Oral or intravenous administration of allethrin, a synthetic derivative of the pirethrin-based insecticides, produces neurotoxic symptoms consisting of mild salivation, hyperexcitability, tremors and convulsions which result in death. Intracerebroventricular injection of allethrin to mouse at about one-nineth the dose of intravenous administration, produced qualitatively identical but less prominent symptoms, indicating that at least some of the symptoms may be originated in the central nervous system. To investigate the mechanism of action of the compound, we studied the ability of agents which alter neurotransmission to prevent or potentiate the effect of convulsive doses of technical grade (15.5% cis, 84.5% trans) allethrin. Intraperitoneal pretreatment with drugs which block noradrenergic receptors or norepinephrine synthesis, such as pentobarbital, chlorpromazine, phentolamine, phenoxybenzamine and reserpine, depressed the tremor induced by allethrin. The inhibitory effect of reserpine was reversed by phenylephrine. Both the serotonergic blocker, methysergide, and the serotonin depletor, rho-chlorphenylalanine, potentiated the effect of allethrin. The potentiating effect of methysergide was antagonized by 5-hydroxytryptamine. However, intracerebroventricular administration of methysergide was ineffective in potentiating the effect of allethrin. alpha 2- and beta-adrenoceptor blockers, muscarinic antagonists, GABA mimenergics and morphine had no effect. These results suggest that allethrin produces its neurotoxic responses in mice by acting on the brain and spinal levels. Furthermore, adrenergic excitatory and serotonergic inhibitory mechanisms may be involved in the neural pathway through which the allethrin-induced tremor is evoked.

Acetylcholine release from the myenteric plexus of guinea-pig ileum by prostaglandin E1

Release of acetylcholine (ACh) by prostaglandin E1 from the nerve terminals of the guinea-pig longitudinal muscle strip was studied in order to reveal the effect of PGE1 on myenteric plexus activity. The ACh released was collected in the presence of physostigmine (2.1 microgram ml-1) and choline (0.1 microgram ml-1) at 38 degrees C. Five to 100 ng ml-1 PGE1 enhanced the release dose-dependently. The effect was maintained during the presence of PGE1 in the organ bath, while rapid tachyphylaxis was observed with the ACh-releasing action of nicotine. Tetrodotoxin or morphine almost completely inhibited the effect of PGE1 on ACh release. Hexamethonium, in a concentration which completely blocked the effect of nicotine, partially inhibited the effect of PGE1. In the late phase of nicotine action, the tissue was still sensitive to PGE1 despite the continued exposure to nicotine. These data suggest the presence in the myenteric plexus of PG receptors which can increase ACh release.

Release of prostaglandins from the passively distended wall of guinea pig small intestine

The effects of the radial distension of the intestinal wall and the increased intraluminal pressure on the liberation of prostaglandins (PGs) were studied with the isolated guinea pig ileum. Both procedures were accompanied by an increased output of a prostaglandin-like substance and the release was proportional to the degree of distension or intraluminal pressure. Thin-layer chromatography coupled with bioassay suggested that this substance may be a mixture of PGF2 alpha, PGE2 and PGE1. The existence of the former two types of PGs were prominent. The distension-induced release of PG-like substance was assumed to be not mediated by nerve excitation since tetrodotoxin failed to inhibit the release. The increased output of the PG-like substance was not maintained after distension of the intestinal wall, despite continued application of stimulus, indicating that actual tissue deformation is an essential condition leading to increased PG output. It is concluded that distension of the intestinal wall to its circumferential direction is an appropriate stimulus for the release of PG-like substance from the small intestine. The present results favor the view that prostaglandins may participate in the peristaltic activity.

[Oral propagation of the circular muscle contraction induced by local distension of the isolated guinea pig ileum (author's transl)]

The local distension of the intestinal wall was carried out by inflating a thin rubber balloon which had been fixed in the lumen and the contractions of the circular muscle were recorded at the points of 5 mm (PO1) and 20 mm (PO2) oral to the fixed balloon. The contractions elicited by local distension was blocked by tetrodotoxin and atropine, or removing longitudinal muscle, with the myenteric plexus adhering to it, from the intestine at the distending region. When 2 to 3 mm length of longitudinal muscle was stripped off around the intestine between PO1 and PO2, the contraction initiated at PO1 never reached to PO2. That is, the oral contraction did not propagate beyond the myenteric plexus-free region of the segment. When drugs were applied exclusively to the region around PO2, tetrodotoxin and hexamethonium, but not atropine, abolished the contraction at PO2, while the contraction at PO1 was clearly observed. It is likely that the contraction observed at the point of PO2 was evoked by non-cholinergic nervous mechanisms.

Descending release of acetylcholine from the locally distended guinea pig ileum

The effects of local distension of the intestinal wall on the release of acetylcholine (ACh) from the adjacent non-distended part were studied with the segment os isolated guinea pig ileum. Local distension of the intestinal wall induced the increased release of ACh in the distended part and in its anal side but not in its oral side. Such aboral release of ACh by local distension was abolished by tetrodotoxin or atropine in the concentrations which did not block the release in the distended part. When hexamethonium was applied exclusively to the distending part, significant increase of ACh release was observed in both the regions oral to and anal to the distended part. It is suggested that distension stimuli applied to the myenteric plexus are transmitted aborally along the network of the Auerbach's plexus to the anal direction. The release of ACh from the intestine by nicotine or DMPP differed from the occurring during local distension in that the release was localized to the part of the intestine to which the drug was applied.