The potentiating effects of 4-aminopyridine on adrenergic transmission in the rabbit vas deferens

Perspective on the role of P2X-purinoceptor activation in human vas deferens contractility

The contractile actions of α,β-methylene ATP (α,β-meATP) and ATP and the effects of K(+) channel blockers in longitudinal and circular muscles of human vas deferens were investigated with a view to clarifying the functional importance of P2X(1)-purinoceptor activation and K(+) channels in modulating contractility of the tissues. The results provide an experiment-based perspective for resolving differing reports on purinergic activation of the tissues and uncertain roles of large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated delayed rectifier K(+) (K(V)) channels. α,β-Methylene ATP (3-100 μm) evoked suramin-sensitive contractions of longitudinal muscle but rarely of circular muscle. ATP (0.1-3 mm) less reliably activated only longitudinal muscle contractions. These were enhanced by ARL 67156 (100 μm), but a different ectonucleotidase inhibitor, POM 1, was ineffective. Both muscle types were unresponsive to ADP-βS (100 μm), a P2Y-purinoceptor agonist. Longitudinal muscle contractions in response to α,β-meATP were enhanced by FPL 64176 (1 μm), an L-type Ca(2+) agonist, TEA (1 mm), a non-specific K(+) channel blocker, 4-aminopyridine (0.3 mm), a selective blocker of K(V) channels, and iberiotoxin (0.1 μm), a selective blocker of BK(Ca) channels. Quiescent circular muscles responded to α,β-meATP reliably in the presence of FPL 64176 or iberiotoxin. Apamin (0.1 μm), a selective blocker of small conductance Ca(2+)-activated K(+) (SK(Ca)) channels had no effect in both muscle types. Y-27632 (1-10 μm) reduced longitudinal muscle contractions in response to α,β-meATP, suggesting involvement of Rho-kinase-dependent contractile mechanisms. The results indicate that P2X(1)-purinoceptor stimulation elicits excitatory effects that: (a) lead to longitudinal muscle contraction and secondary activation of 4-aminopyridine-sensitive (K(V)) and iberiotoxin-sensitive (BK(Ca)) K(+) channels; and (b) are subcontractile in circular muscle due to ancillary activation of BK(Ca) channels. The novel finding of differential action by P2X(1)-purinoceptor agonists in the muscle types has functional implication in terms of the purinergic contribution to overall contractile function of human vas deferens. The modulatory effects of K(V) and BK(Ca) channels following P2X(1)-purinoceptor activation may be pivotal in providing the crucial physiological mechanism that ensures temporal co-ordination of longitudinal and circular muscle contractility.

Effects of 4-methyl-2-aminopyridine on [3H]-noradrenaline overflow and contractility of isolated rabbit arteries

1. The effects of 4-methyl-2-aminopyridine (4M2AP) and 4-aminopyridine (4AP) on spontaneous and evoked [3H]-noradrenaline overflow were compared in rabbit ear artery strips. The effects of 4M2AP on smooth muscle contractility were also investigated in isolated perfused ear arteries. 2. Both 4M2AP and 4AP enhanced spontaneous [3H] overflow from arterial strips in a concentration-dependent manner (10-1000 microM). A bell-shaped dose-response relation was obtained for evoked [3H] overflow over the same concentration range, with maximum effects occurring at 10 microM for 4M2AP (163 +/- 31% increase) and 100 microM for 4AP (154 +/- 16% increase). 3. 4M2AP did not significantly affect evoked tension in the 1-to 100-microM range but clearly depressed it at 1,000 microM (by 65 +/- 11%). In contrast, 4AP enhanced evoked tension in the 10- to 100-microM range (by 30-50%). 4. 4M2AP (10-100 microM) enhanced vasoconstrictor responses to exogenous noradrenaline injections in isolated perfused rabbit ear arteries, whereas higher concentrations (1,000 microM) caused significant depression. 5. 4M2AP (1,000 microM) markedly potentiated vasoconstrictor responses induced by perfusion with a high extracellular K+ solution. When 4M2AP was present during the reloading of noradrenaline-sensitive Ca2+ stores, it enhanced the subsequent vasoconstrictor responses to noradrenaline obtained in a Ca(2+)-free medium. 6. The results show that 4M2AP, like 4AP, enhances [3H] overflow from sympathetic nerve terminals and has complex effects on vascular smooth muscle contractility, indicating the ability of these compounds to affect the Ca2+ permeability of both extracellular and intracellular membrane systems.

The mechanisms of enhancement and inhibition of field stimulation responses of guinea-pig vas deferens by prostacyclin analogues

1. In the guinea-pig isolated vas deferens preparation bathed in Tyrode's solution, the prostacyclin analogues, cicaprost, TEI-9063, iloprost, taprostene and benzodioxane-prostacyclin, enhanced twitch responses to submaximal electrical field stimulation (20%-EFS). The high potency of cicaprost (EC150 = 1.3 nM) and the relative potencies of the analogues (equi-effective molar ratios = 1.0, 0.85, 1.6, 17 and 82, respectively) suggest the involvement of a prostacyclin (IP-) receptor. 2. Maximum enhancement induced by cicaprost in 2.5 mM K+ Krebs-Henseleit solution was similar to that in Tyrode solution (2.7 mM K+), but was progressively reduced as the K+ concentration was increased to 3.9, 5.9 and 11.9 mM. There was also a greater tendency for the other prostacyclin analogues to inhibit EFS responses in 5.9 mM standard K+ Krebs-Henseleit solution; this may be attributed to their agonist actions on presynaptic EP3-receptors resulting in inhibition of transmitter release. 3. The EFS enhancing action of cicaprost was not affected by the alpha1-adrenoceptor antagonist prazosin (100 and 1000 nM). Cicaprost (20 and 200 nM) did not affect contractile responses of the vas deferens to either ATP (5 microM) or alpha,beta-methylene ATP (1 microM) in the presence of tetrodotoxin (TTX, 100 nM). In addition, enhancement by cicaprost of responses to higher concentrations of ATP (30 and 300 microM) in the absence of TTX, as shown previously by others, was not seen. Prostaglandin E2 (PGE2, 10 nM) and another prostacyclin analogue TEI-3356 (20 nM) enhanced purinoceptor agonist responses. Unexpectedly, TTX (0.1 and 1 microM) partially inhibited contractions elicited by 10-1000 microM ATP; contractions elicited by 1-3 microM ATP were unaffected. Further studies are required to establish whether a pre- or post-synaptic mechanism is involved. 4. In a separate series of experiments, cicaprost (5-250 nM), TEI-9063 (3-300 nM), 4-aminopyridine (10-100 microM) and tetraethylammonium (100-1000 microM) enhanced both 20%-EFS responses and the accompanying overflow of noradrenaline to a similar extent. In further experiments with the EP1-receptor antagonist AH 6809, TEI-3356 (1.0-100 nM) and the EP3-receptor agonist, sulprostone (0.1-1.0 nM) inhibited both maximal EFS responses and noradrenaline overflow, thus confirming previous reports of the high activity of TEI-3356 at the EP3-receptor. Cicaprost had no significant effect on noradrenaline overflow at 10 and 100 nM, but produced a modest inhibition at 640 nM. 5. In conclusion, our studies show that prostacyclin analogues (particularly TEI-3356) can inhibit EFS responses of the guinea-pig vas deferens by acting as agonists at presynaptic EP3-receptors. Prostacyclin analogues (particularly cicaprost and TEI-9063) can also enhance EFS responses through activation of IP-receptors. The mechanism of the enhancement has not been rigorously established but from our results we favour a presynaptic action to increase transmitter release.

Pyridine derivatives stimulate phosphatidylcholine secretion in primary cultures of rat type II pneumocytes

We have examined the effects of pyridine derivatives on phosphatidylcholine secretion in primary cultures of rat type II pneumocytes. Of 12 pyridine derivatives, 4-aminopyridine, 4-dimethylaminopyridine and 4-pyrolidinopyridine had a stimulatory effect on phosphatidylcholine secretion, whereas other derivatives had little effect. The stimulatory effect of 4-aminopyridine was concentration- and time-dependent, and was inhibited by the acetoxymethyl ester of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (3 microM), an intracellular Ca2+ chelator. In addition, the stimulatory effect of 4-aminopyridine was suppressed by W-7(N-(6-aminohexyl)-5-chloro-1-napthalene-sulphonamide)(10 microM), a calmodulin inhibitor, and sphingosine (10 microM) and staurosporine (0-1 microM), protein kinase C inhibitors. These results indicate that several pyridine derivatives stimulate phosphatidylcholine secretion in type II pneumocytes.

Prejunctional actions of K+ channel blockers in rat vas deferens

In studies of isometric contractions in prostatic portions of rat vas deferens evoked by single pulse electrical stimulation, the K+ channel blockers 4-aminopyridine, tetraethylammonium and charybdotoxin, but not apamin, significantly reduced the prejunctional inhibitory potency and the maximum inhibitory effect of the alpha 2-adrenoceptor agonist xylazine. The protein kinase C activator phorbol dibutyrate had similar effects to 4-aminopyridine against xylazine. However, 4-aminopyridine, tetraethylammonium, charybdotoxin and phorbol dibutyrate, but not apamin, significantly increased the magnitude of the isometric contraction to a single stimulus. 4-Aminopyridine and phorbol dibutyrate significantly reduced, while tetraethylammonium did not affect, isometric contractions to noradrenaline, and 4-aminopyridine failed to affect contractions to alpha,beta-methylene-ATP, so that the effects of these agents on the isometric contraction to a single stimulus were presumably by a prejunctional action. The Ca2+ entry facilitator Bay K 8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyrid ine carboxylic acid methylester) increased stimulation-evoked contractions by a postjunctional action and reduced the inhibitory effects of xylazine. When the isometric contraction following 4-aminopyridine was reduced by decreasing the stimulation voltage or by reducing the Ca2+ concentration from 2.5 to 0.9 mM, 4-aminopyridine significantly reduced the potency of xylazine. However, tetraethylammonium and Bay K 8644 failed to affect the inhibitory potency of xylazine in low Ca2+. It is concluded that the K+ channel blocker 4-aminopyridine reduces the prejunctional inhibitory potency of xylazine, and this action is independent of increased neurotransmitter release. These results suggest that prejunctional alpha 2-adrenoceptor-mediated inhibition in rat vas deferens involves K+ channels sensitive to block by 4-aminopyridine.

Effects of cyclopiazonic acid on contractility and ecto-ATPase activity in guinea-pig urinary bladder and vas deferens

1. Cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic ATPase, was tested on guinea-pig urinary bladder and vas deferens for its ability: (1) to modify contractile responses to electrical field stimulation (EFS), exogenous ATP, alpha,beta-methylene ATP (alpha,beta-MeATP), carbachol, noradrenaline (NA), histamine, and KCl; (2) to affect ecto-ATPase activity; (3) to modify the release of ATP evoked by EFS. 2. In the urinary bladder, CPA (10 microM) potentiated contractile responses to EFS, exogenous ATP (100 microM), alpha,beta-meATP (1 microM), carbachol (0.5 microM), histamine (30 microM) and KCl (30 mM). In the vas deferens, CPA (10 microM) potentiated responses to EFS, ATP, alpha,beta-meATP, NA (100 microM) and KCl. CPA at a concentration of 1 microM had no effect on ATP-induced relaxation of carbachol-precontracted guinea-pig taenia coli, and at a concentration of 10 microM it markedly increased spontaneous contractile activity of taenia. 3. Ecto-ATPase was estimated to have Vmax and Km values of 0.98 nmol Pi 30 min-1 mg-1 wet tissue and 881 microM ATP in the urinary bladder, and 0.75 nmol Pi 30 min-1 mg-1 wet tissue and 914 microM ATP in the vas deferens, respectively. CPA at a concentration of 10 microM significantly inhibited ecto-ATPase activity by 18% in the urinary bladder and by 24% in the vas deferens. 4. In the guinea-pig vas deferens, CPA significantly potentiated ATP release evoked by EFS from 2.2 +/- 0.8 (6) pmol ATP min-1 g-1 wet tissue to 35.2 +/- 4.8 (6) pmol ATP min-1 g-1 wet tissue (P < 0.01). 5. In conclusion, the potentiation of contractile responses of the guinea-pig urinary bladder and vas deferens by CPA has a non-specific character. CPA inhibited ecto-ATPase activity and increased ATP release, but these effects do not appear to contribute to the potentiation of Pu-purinoceptor-mediated responses since the contractile actions of all the agonists studied were potentiated to the same extent.

Hemodynamic effects of 3,4-diaminopyridine in a swine model of verapamil toxicity

STUDY HYPOTHESIS

3,4-Diaminopyridine (3,4-DAP) may reverse the hemodynamic effects of severe verapamil toxicity.

DESIGN

A nonblinded acute animal preparation.

INTERVENTIONS

Eighteen chloralose-anesthetized and instrumented swine were poisoned with verapamil at 10 mg/kg/hr for five minutes and then 5 mg/kg/hr until a systolic blood pressure of 55 mm Hg was achieved. Heart rate, lead II ECG, mean arterial pressure, left ventricular dP/dT max, and cardiac index were monitored. Nine control animals received 0.2 mL/kg/min infusion of normal saline, and nine treatment animals received similar volumes of 1 mg/kg/min 3,4-DAP until systolic blood pressure reached 100 mm Hg, an elapsed time of 30 minutes, or death.

RESULTS

Verapamil toxicity was produced in all animals following an average dose of 1.38 +/- 0.44 mg/kg verapamil, and resulted in diminished mean arterial pressure, dP/dT max, cardiac index, and heart rate to average values of 47%, 32%, 46%, and 88% of baseline values, respectively. Transient atrioventricular dissociation was noted in only 22% of cases. 3,4-DAP treatment (with an average dose of 14 +/- 5.6 mg/kg) resulted in significant increases in mean arterial pressure, dP/dT max, cardiac index, and heart rate to 136%, 298%, 149%, and 158% of baseline values, respectively. Mortality was unchanged (22% in both groups). 3,4-DAP treatment was complicated by muscle twitching, tachycardia (rate of more than 180) and hypertension (diastolic blood pressure of more than 110 mm Hg) each in 44% of cases.

CONCLUSION

Although 3,4-DAP reversed the hypotensive and negative inotropic effects of verapamil toxicity, it failed to improve survival and resulted in several complications including muscle twitching, tachycardia, and hypertension.

Prejunctional action of the venom from the Indian red scorpion Mesobuthus tamulus on adrenergic transmission in vitro

Venom (RSV) from the Indian red scorpion, Mesobuthus tamulus (Buthus tamulus), can cause increased peripheral sympathetic activity with consequent enhancement of adrenergic responses. We have therefore investigated the effects of RSV on adrenergic transmission in the rat isolated anococcygeus muscle. The effects of phentolamine (5 microM), tetrodotoxin (2 microM), guanethidine (5 microM), desipramine (1 microM) and reserpine pretreatment in vivo (5 mg/kg s.c. x 24 hr and 5 mg/kg i.p. x 3 hr) on the contractile responses of the rat anococcygeus muscle to RSV (1.5 microgram/ml), field stimulation, noradrenaline (NA, 1 microM or 3 microM) and tyramine (15 microM) were compared. The contractile responses to RSV and to field stimulation were completely blocked by phentolamine, tetrodotoxin, guanethidine and reserpine pretreatment, but the responses were potentiated by desipramine. The contractile responses to tyramine were completely blocked by phentolamine, reserpine pretreatment as well as desipramine. The responses to NA were completely blocked by phentolamine, but were potentiated by guanethidine, desipramine and reserpine. Relatively low concentrations (0.1 microgram/ml x 4) of RSV which did not produce any observable increase in tone of the anococcygeus muscle, potentiated the contractile response of the anococcygeus muscle to field stimulation, but not the responses to exogenous NA; 4-aminopyridine (25 microM x 2) also potentiated the muscle responses to field stimulation. HPLC measurements revealed only very low concentrations (0.10 +/- 0.03 mumol/g venom) of NA in RSV. Thus, the adrenergic agonist action of RSV in the rat isolated anococcygeus muscle can be attributed to the involvement of some prejunctional mechanism(s) of action that stimulates the release of neurotransmitter which differs from the indirect action mediated by tyramine.

Prejunctional modulation of the nitrergic innervation of the canine ileocolonic junction via potassium channels

1. The effects of different K+ channel blockers were studied on nitric oxide (NO)-mediated non-adrenergic non-cholinergic (NANC) relaxations in the canine ileocolonic junction. 2. The non-selective blockers of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA) and the blocker of large conductance Ca(2+)-activated K+ channels, charybdotoxin, potently enhanced the NANC relaxations induced by low frequency stimulation. The blocker of small conductance Ca(2+)-activated K+ channels, apamin, had no effect on electrically-induced NANC relaxations. 3. NANC nerve-mediated relaxations induced by adenosine 5'-triphosphate (ATP), acetylcholine (ACh) and gamma-aminobutyric acid (GABA) were significantly enhanced by 4-AP and charybdotoxin but not by apamin. TEA significantly enhanced the NANC relaxations in response to GABA and ATP while that in response to ACh was abolished. 4. None of the K+ channel blockers had an effect on the dose-response curve to NO, on the noradrenaline-induced contraction or on the relaxation to nitroglycerine (GTN). 5. From these results we conclude that inhibition of prejunctional K+ channels increases the nitrergic relaxations induced by electrical and chemical receptor stimulation of NANC nerves and thus suggests a regulatory role for these prejunctional K+ channels in the release of NO from NANC nerves in the canine ileocolonic junction.

Ranitidine inhibits adrenergic transmission in the rat isolated anococcygeus muscle

1. The effects of ranitidine on adrenergic transmission in the rat isolated anococcygeus muscle were investigated. 2. Cumulative doses (2-8 mmol/L) or ranitidine produced a concentration-dependent inhibition of motor responses of the rat isolated anococcygeus muscle evoked by field stimulation (20-25 V, 10 Hz for 10 s, 1 ms pulse width) every 2 min, but also potentiated the contractile response to exogenous noradrenaline (5 mumol/L). The inhibited motor responses recovered rapidly and completely after washing out ranitidine. 3. 4-Aminopyridine (100 mumol/L) effectively reversed the partially inhibited (55% or greater) motor responses. 4. The results strongly suggest that ranitidine can inhibit adrenergic transmission in the anococcygeus muscle by a prejunctional mechanism with, presumably, consequent development of supersensitivity of the effector cells to noradrenaline.

Interactions of 3,4-diaminopyridine and choline in stimulating acetylcholine release and protecting membrane phospholipids

We investigated the effects of 3,4-DAP on ACh release from rat striatal slices superfused with or without choline, at rest and during electrical stimulation. In a choline-free medium, 3,4-DAP increased basal and stimulated ACh release while lowering the net efflux of choline; thus while the sum of ACh plus choline released remained constant, the ratio of released ACh to that of choline was increased. The drug failed to affect tissue ACh, choline or membrane phospholipid levels (including those of phosphatidylcholine). In a choline-containing medium, 3,4-DAP potentiated the enhancement by choline of both basal and electrically stimulated ACh release. Electrical stimulation alone increased ACh release from the slices without altering choline efflux or depleting tissue choline or ACh stores; however, this treatment did deplete membranes of phosphatidylcholine and of other major phospholipids. Superfusion of the slices with 3,4-DAP protected the slices from stimulation-induced phospholipid depletion. Calcium-dependent activation of high-affinity choline uptake may underlie the observed effects of 3,4-DAP.

Noradrenergic, purinergic, and cholinergic transmission in the mouse vas deferens: influence of field-stimulation parameters, reserpinization, 6-hydroxydopamine and 4-aminopyridine

In the field-stimulated mouse vas deferens the twitch inhibiting potency of prazosin (1 microM) and alpha,beta-methylene ATP (MeATP, 10 microM) was studied, using two types of stimulation-response curves, (a) variation of frequency from 3 to 100 Hz at a constant pulse width of 0.1 ms and (b) variation of pulse width from 0.04 to 0.8 ms at a constant frequency of 15 Hz. Prazosin and MeATP reduced the twitch response by eliminating the noradrenergic and purinergic component, respectively. After the combined application of both compounds a small third twitch component remained that was most prominent at high frequencies. Reserpinization reduced the effect of prazosin but enhanced that of MeATP and increased the cholinergic component. 6-Hydroxydopamine enhanced the effects of prazosin and MeATP to the same extent, but left the cholinergic component intact. In vasa pre-loaded with [3H]-noradrenaline, field stimulation induced a larger release of tritium at high frequency and short pulse duration (100 Hz, 0.1 ms) than at lower frequency and long pulse duration (15 Hz, 0.3 ms). Prazosin (1 microM) augmented both the spontaneous and the stimulation-induced overflow of tritium, whereas MeATP (10 microM) had only a negligible negative effect on the outflow of label. In conclusion, the twitch contraction of the mouse vas deferens has a small cholinergic component in addition to the noradrenergic and purinergic components. Adrenergic and purinergic transmission seem not to run strictly in parallel: the purinergic component dominates during stimulation at low frequency and long pulse duration, and after reserpinization; 4-aminopyridine enhances the adrenergic mechanism more than the purinergic one.

The effect of 4-aminopyridine on micturition reflex in normal or capsaicin-desensitized rats

4-aminopyridine (4-AP) produced a dose-related (0.15-2 mg/kg i.v.) potentiation of the voiding cycle of the urinary bladder and increased frequency of micturition in urethane-anesthetized rats. In bladders containing a subthreshold amount of fluid for eliciting reflex micturition 4-AP (1-3 mg/kg i.v.) activated a series of high-amplitude, hexamethonium-sensitive rhythmic bladder contractions. In rats desensitized to capsaicin as newborns, reflex micturition was almost abolished: in these animals i.v. 4-AP did not affect bladder voiding unless at high doses (1-2 mg/kg), at which a reversal from anesthesia occurred. This was accompanied by a prompt micturition. In unanesthetized rats, neither the 4-AP-induced convulsions nor the behavioral response (assessed in an open field) to 4-AP were affected by neonatal capsaicin desensitization. Daily urine production of capsaicin-pretreated animals did not differ from that of controls. However, when measurements were made during daytime, almost no spontaneous urine emission was found in capsaicin-treated rats. On the rat isolated urinary bladder, 4-AP potentiated the response to field stimulation in preparations from both vehicle- and capsaicin-pretreated animals. These findings indicate that 4-AP has a potent excitatory action on bladder voiding in rats and support the hypothesis that in this species 'conscious' bladder voiding can be initiated through capsaicin-resistant mechanisms.

The rat brain phencyclidine (PCP) receptor. A putative K+ channel

Receptor binding studies were carried out to test whether the rat brain phencyclidine (PCP) receptor is part of a K+ channel. [3H]PCP, and two analogs, [3H]TCP and m-amino[3H]PCP, labeled a single receptor on rat brain synaptic membranes. Each compound bound to a similar number of sites (Bmax = 2.7 pmol bound/mg protein); the apparent dissociation constants for these compounds (KD less than 0.3 microM) decreased with increasing temperature. The following observations indicate that the PCP receptor is part of a K+ channel: (1) aminopyridines (AP) and tetraalkylammonium ions blocked [3H]PCP binding; their respective orders of potency, 4-AP = 3,4-diAP much greater than 3-AP, and tetrabutylammonium (TBA) greater than tetraethylammonium much greater than tetramethylammonium, paralleled their abilities to block K+ channels, (2) the order of potency of PCP and its analogs for binding to the PCP receptor, TCP greater than PCE greater than m-amino-PCP greater than PCP greater than PCPY greater than m-nitro-PCP, paralleled their rank order for blocking brain K+ channels, and (3) the stereospecific displacement of [3H]PCP binding by the isomers of the "sigma" ligands, (+)N-allyl-normetazocine (NANM) greater than (-)NANM, and (-)cyclazocine greater than (+)cyclazocine, and of the dioxolanes, dexoxadrol much greater than levoxadrol, paralleled their abilities to block brain K+ channels. Reciprocal plot and Schild plot analyses indicated that TBA, (+)NANM and dexoxadrol were competitive inhibitors at the PCP receptor, whereas 4-AP had an allosteric interaction.

Transmitter releasing action of selegiline ((-)-deprenyl) from peripheral sympathetic nerves under different experimental conditions

A high concentration of selegiline ((-)-deprenyl; 10(-4) M) potentiated low frequency (2 Hz) nerve stimulation-evoked release of [3H]noradrenaline from the isolated main pulmonary artery of the rabbit in the presence of neuronal (cocaine, 3 X 10(-5) M) and extraneuronal (corticosterone, 5 X 10(-5) M) uptake blockers, and inhibited the postsynaptic response. The transmitter-releasing action of 10(-4) M selegiline was inhibited by a moderate increase of external K+ (23.6 mM). Excess K+ by itself abolished the nerve-evoked release of [3H]noradrenaline but did not increase the resting outflow of radioactivity. Excess Ca2+ (7.5 mM) increased the stimulation-evoked transmitter release. In the presence of excess Ca2+, selegiline (10(-4) M) was effective in increasing the [3H]noradrenaline release in response to nerve-stimulation. Excess Ca2+ partly antagonized the postsynaptic inhibitory action of selegiline. In Ca2+-free, 1 mM EGTA-containing Krebs solution both the nerve-evoked 3H release and the transmitter releasing action of selegiline were abolished in agreement with the 'Ca-hypothesis'. The voltage-dependent K+-channel blocker, 4-aminopyridine (10(-5) M), increased the nerve-stimulation-evoked release of tritium from arteries. If selegiline was also present in the perfusion medium the nerve-evoked transmitter release further increased. 4-Aminopyridine completely antagonized the inhibitory action of selegiline on the postsynaptic contraction.

Effect of mono- and diaminopyridines on release of [3H]norepinephrine from isolated guinea-pig atrium

Neurochemical evidence has been obtained that 4-aminopyridine, 3,4-diaminopyridine and 3,3-dimethyl-1-(4-amino-3-pyridyl)urea HBr (LF-14), concentration-dependently enhanced the stimulation-evoked release of [3H]norepinephrine ([3H]NE) from isolated guinea-pig atrium. The effects of aminopyridines, compounds known to inhibit potassium channels, were Ca0-dependent. High pressure liquid chromatography, combined with radiochemical detection, indicated that the increased stimulated release of radioactivity was due to [3H]NE. Since the aminopyridines studied also enhanced the release of [3H]NE from atrium treated with cocaine, a blocker of uptake1, it seems likely that the increased release of NE caused by the aminopyridines is due to the enhanced release of NE from sympathetic axon terminals and not to the inhibition of reuptake. It is probable that the sympathomimetic cardiac effects (positive inotropic and chronotropic effect) of aminopyridines observed in animal experiments is due to the increased release of NE, caused by these compounds.

Differential effects of Ca antagonists on the noradrenaline release and contraction evoked by nerve stimulation in the presence of 4-aminopyridine

We previously reported that verapamil, nicardipine and diltiazem inhibited both neurotransmitter release and contraction evoked by transmural nerve stimulation (TNS) in the canine saphenous vein. To evaluate whether the three Ca antagonists act on the nerve endings by inhibiting Ca2+ influx, the effects of the three antagonists were studied in the presence of 4-aminopyridine (4-AP) 3 X 10(-4) M on the TNS-evoked tritium overflow and contraction of canine saphenous veins preloaded with [3H]-noradrenaline. 4-AP increased both tritium overflow and contraction evoked by TNS, but did not enhance the contraction induced by exogenous noradrenaline (10 nmol). In the veins pretreated with 4-AP, verapamil (3 X 10(-5) M) and nicardipine (10(-5) M and 3 X 10(-5) M) caused no significant effects on the TNS-evoked tritium overflow, but they still inhibited the contraction. Diltiazem (10(-5) M and 3 X 10(-5) M) significantly inhibited both responses to TNS in the veins pretreated with 4-AP, the effects being nearly equipotent to those in the absence of 4-AP. The (-)-cis isomer of diltiazem (10(-5) M and 3 X 10(-5) M), which is about 100 times less potent than diltiazem in inhibiting Ca2+ influx, inhibited both responses to TNS in the presence of 4-AP to almost the same degree as diltiazem. When 4-AP was added after the Ca antagonists, it reversed the TNS-evoked tritium overflow inhibiting actions of verapamil (3 X 10(-5) M) and nicardipine (3 X 10(-5) M) much more effectively than that of diltiazem (3 X 10(-5) M). Tetracaine (4 X 10(-6) M) significantly inhibited the TNS-evoked tritium overflow and contraction, which were unaffected by 4-AP. Sodium salicylate (10(-2) M) failed to modify the inhibition of TNS-evoked tritium overflow following diltiazem (3 X 10(-5) M), but it enhanced that of tetracaine (4 X 10(-6) M). Verapamil but not diltiazem and nicardipine significantly increased the spontaneous tritium overflow from veins pretreated with 4-AP. The present study together with previous results suggests that diltiazem but not verapamil and nicardipine may inhibit the TNS-evoked neurotransmitter release through an action other than inhibition of Ca2+ influx into the adrenergic nerve endings, allowing an inhibition of the resulting contraction.

Effects of 4-aminopyridine on cat blood pressure in relation to the sympathetic nervous system

The pressor effect of 4-aminopyridine (4-AP) was studied in anesthetized cats and in isolated cat aortic ring preparations. A significant increase in blood pressure (38.9 +/- 11.4 mmHg) was observed following intravenous administration of 4-AP (0.3 mg/kg). The elevated blood pressure lasted for 1.3 h and returned to the control level after 1.5 h. The pressor effect of 4-AP was not blocked by prior administration of phentolamine (2 mg/kg i.v.) or by pretreatment with reserpine (2 mg/kg i.m., once a day for two consecutive days). In vitro, a dose-dependent tonic contraction of aortic ring was noted after addition of 4-AP into the bathing fluid. The ED50 of 4-AP was significantly greater than that of norepinephrine (NE) (8 X 10(-4) M vs. 10(-7) M). Phentolamine (10(-6) M) antagonized the contractile response to NE but not that to 4-AP, whereas verapamil (10(-5) M) attenuated the 4-AP induced contractile response. A diminished contractile effect was also obtained when aortic ring preparations were bathed in calcium-free solution. These data suggest that the sympathetic nervous system plays a minor role in the pressor effect of 4-AP and that the increased calcium influx may account, at least partially, for the vasoconstricting effect of 4-AP.

Effective treatment of verapamil intoxication with 4-aminopyridine in the cat

To study the value of 4-aminopyridine as an antidote to verapamil intoxication, we subjected 12 adult cats to verapamil poisoning by administering doses of 4.0-25.0 mg/kg verapamil by intravenous infusion. Six animals were given 4-aminopyridine 2 X 0.5 mg/kg i.v. after the verapamil infusion was stopped and the other six animals (the control group) were not. Verapamil caused profound cardiovascular depression and also partial neuromuscular block, both of which were completely reversed by 4-aminopyridine within 50 min, in spite of extremely high serum verapamil concentrations (ranging between 3,700 and 13,500 ng/ml). The six animals that received 4-aminopyridine survived the verapamil intoxication, whereas four of the six animals in the control group died. The results suggest that 4-aminopyridine may be useful in the treatment of verapamil intoxication.

The inhibitory action of PGF2 alpha on release of [3H]noradrenaline enhanced by alpha 2-adrenoceptor blockade, sodium-pump inhibition and 4-aminopyridine in the main pulmonary artery of the rabbit

Large concentrations of prostaglandin PGF2 alpha inhibited the stimulation (2 Hz) evoked release of [3H]noradrenaline from the isolated main pulmonary artery of the rabbit (the inhibition caused by 3 X 10(-5) M PGF2 alpha was 62%). Furthermore, PGF2 alpha inhibited the release evoked by stimulation when it was enhanced by different procedures. During blockade of presynaptic alpha 2-adrenoceptors by 3 X 10(-7) M yohimbine, which by itself enhanced the overflow of [3H]NA in response to stimulation, the inhibitory action of PGF2 alpha was more pronounced (78.2%). In tissue in which the Na+-pump was inhibited (K+-free treatment) where the overflow of 3H was markedly increased, PGF2 alpha exerted nearly equal inhibition of transmitter release to that observed in control experiments (64.3%). The inhibitory effect of PGF2 alpha on the stimulation-evoked release of [3H]NA was less pronounced (32.1%) in the presence of 10(-4) M 4-aminopyridine (a blocker of K+-channels).

Antagonistic effects of taurine and 4-aminopyridine on guinea pig ileum

The effects of 4-aminopyridine (4-AP) and of taurine on the guinea pig ileum were examined. Four-AP increases the basal activity from 1.3 to 6.45 cm2/min. This effect is atropine-sensitive and calcium-dependent. Taurine counteracts the effects of 4-AP on muscle contracture in a concentration-dependent manner. In muscles preincubated with 20 mM taurine, the effect of 4-AP decreased about 65%. Taurine added at the time of maximal effect of 4-AP reduced the muscle contracture induced by the drug to the basal levels. Glycine and beta-alanine did not influence the contracture induced by 4-AP, whereas 25 mM gamma-aminobutyric acid (GABA) reduced it by 30%. Taurine reduced the basal muscle activity by 50% but did not affect the contracture induced by exogenously applied acetylcholine.

NaI prevents and reverses transmission failure in low Ca2+ Ringer

Synaptic transmission in the bullfrog sympathetic ganglion fails progressively as Ca2+ is reduced from the normal (1.8 mM) to 0.47 mM in an NaCl Ringer. This Ca2+ dependence of synaptic transmission is shifted to the left (lower Ca2+ levels) when NaCl is replaced by NaI. Replacement of NaCl by NaI also reverses pre-existing transmission failure in low Ca2+. As reported previously by us, NaI also causes repetitive postganglionic responses to each single preganglionic stimulus. It is of interest that iodide's ability to cause repetitive postganglionic responses and to shift the Ca2+ dependence of synaptic transmission duplicate the effects of aminopyridines, notably 3,4-diaminopyridine.

The actions of 3,4-diaminopyridine in bullfrog sympathetic ganglia

The effects of 3,4-diaminopyridine (3,4-DAP) on isolated sympathetic ganglia were studied by means of intracellular and extracellular recording techniques. 3,4-DAP in micromolar concentrations caused a single orthodromic stimulus to generate a brief burst of repetitive postganglionic discharges. Such stimulus-bound repetition (SBR) appeared to represent a presynaptic action of 3,4-DAP, as no repetitive firing could be evoked by antidromic or direct stimulation of ganglion cells. 3,4-DAP also increased the latency to the onset of the synaptic potential at concentrations paralleling those responsible for SBR. The actions of 3,4-DAP on synaptic transmission extended over a 10,000-fold concentration range, beginning with synaptic facilitation at micromolar concentrations (1-500 microM), and proceeding to depressant effects at millimolar concentrations (0.5-10 mM). The postganglionic repetitive discharges (SBR) induced by 3,4-DAP could be selectively suppressed by D-tubocurarine (D-Tc) or tetraethylammonium (TEA), at concentrations of these antagonists below those required to block transmission. 3,4-DAP had significant anti-curare effects, producing a four-fold shift of the D-Tc concentration-effect curve for transmission block. In contrast, 3,4-DAP neither antagonized nor enhanced the transmission block produced by TEA. These results are incompatible with traditional concepts of ganglionic blockade by D-Tc and TEA, and suggest the possibility of presynaptic sites of action in ganglion block.

A comparison of 3,4-diaminopyridine and 4-aminopyridine in the anaesthetized cat

3,4-Diaminopyridine and 4-aminopyridine were compared in the anaesthetized cat and found to be equiactive in their anti-curare activity. This in vivo similarity is at variance with previous in vitro studies which demonstrate 3,4-diaminopyridine to be more active than 4-aminopyridine at the neuromuscular junction. Possible reasons for the similarity between the two aminopyridines at the in vivo neuromuscular junction are discussed and it is concluded that 3,4-diaminopyridine has only marginal advantages over 4-aminopyridine as a potential anti-curare agent.

Ion-pair high-performance liquid chromatographic assay of 4-aminopyridine in serum

An assay for the quantitative estimation of 4-aminopyridine in biological fluids has been developed using 2-aminopyridine as internal standard and ion-pair reversed-phase (C18) high-performance liquid chromatography with detection at 263 nm. A 7.5% solution o acetonitrile in water containing tetrabutylammonium iodide and sodium heptanesulfonate buffered at pH 3.0 provided excellent separation of the analytes from each other and from an interfering peak that was occasionally observed in the outdated human sera used in these studies. Sensitivity, specificity, precision, accuracy and reproducibility all were judged sufficient for the routine use of this assay for pharmacokinetic and pharmacodynamic studies.

4-aminopyridine-a review

4-aminopyridine is the first of the aminopyridines to be used in clinical practice. It blocks potassium channels and thereby increases acetylcholine, and possibly noradrenaline, release at nerve terminals. In man the drug has a significant action at the neuromuscular junction, but has little effect on the autonomic nervous system or muscle (smooth, skeletal, or cardiac) although such actions have been demonstrated in animals. It may be clinically useful in the reversal of nondepolariser blockade and a role in antibiotic associated block has been proposed. It may be used effectively as an analeptic agent. It appears to be a useful therapeutic agent in myasthenia gravis and Eaton Lambert syndrome, although of limited use in botulism. It effects on the central nervous system are considerable. These account for the major side effects of the drug which include tremor, excitability and convulsions.

Dual effect of the muscarinic agonist McN-A-343 on vascular neuroeffector transmission

The site and mechanism of action of McN-A-343 (4-m-chlorophenylcarbamoyloxy)-2-butynyltrime-thylammonium chloride) on sympathetic neuroeffector transmission in the rabbit isolated pulmonary artery was studied. Low concentrations (10(-6) - 3 x 10(-5) M) of McN-A-343 and cocaine enhanced (up to 210 and 236%, respectively) the contractions evoked by electrical-field stimulation, while higher concentrations (10(-4) -3 x 10(-4)M) inhibited them. McN-A-343 (10(-4)M) caused an initial transitory potentiation (222% of control) of the stimulation-evoked contractions followed by an inhibition. In the presence of cocaine (3 x 10(-5)M), the potentiation caused by McN-A-343 was prolonged and the secondary inhibitory phase was thus abolished. Physostigmine (10(-5) - 10(-4)M), hexamethonium (10(-5) M), atropine (3 x 10(-7) M), suprofen (10(-5) M), and 4-aminopyridine did not alter the effect of McN-A-343 (1-(-4)M). Cocaine (3 x 10(-5)M) and (+)-amphetamine (10(-5) M) reversed the McN-A-343-evoked block, while they did not alter the inhibition caused by tetracaine (3.2 x 10(-5) M). Atropine (3 x 10(-7) M) had no effect on the McN-A-343-induced block, while 4-aminopyridine (10(-4) M) caused a partial and transitory reversal. On the aorta McN-A-343 (10(-4)M) did not alter the contractile concentration-response curve of (-)-noradrenaline (10(-9) - 3 x 10(-4)M), while that of serotonin (10(-8) - 3 x 10(-5)M) was moved to the right in a competitive manner. McN-A-343 (10(-4)M) did not alter the contractions evoked by noradrenaline (10(-7) M) during the period corresponding to the stimulation-evoked enhancement and subsequent inhibition. McN-A-343 (10(-4)M) slightly antagonized the contractions caused by tyramine (10(-6) - 10(-3) M) and carbachol (10(-6) - 10(-3) M). It is concluded that McN-A-343 enhances stimulation-evoked transmitter release by a presynaptic facilitatory action mediated via receptors localized on the outer surface of adrenergic neurones and to a lesser extent by inhibition of noradrenaline re-uptake. The enhancement does not involve presynaptic nicotine or muscarine receptors. Furthermore, McN-A-343 inhibits transmitter release by acting as an adrenergic neurone blocking agent at an intraneuronal site. The inhibition does not involve presynaptic muscarine inhibitory receptors and is prostaglandin-independent.

Effects of 4-aminopyridine on contractile response and action potential of rabbit papillary muscle

The effects of 4-aminopyridine (4-AP) were studied on isolated papillary muscles from the heart of reserpinized rabbits (at 37 degrees C). The preparations were paced to contract at 0.67 Hz under isometric conditions and the muscle length was adjusted to 95% of the length for optimum force production. Simultaneous recordings of isometric force and membrane potentials were performed. 4-AP (50 microM) increased peak force by approximately 20% of the control and prolonged the action potential by 20%. Higher concentrations of 4-AP (800 microM) resulted in further increments of force and action potential duration (60 and 70% of controls, respectively). Prolongation of the action potential and potentiation of the isometric force was still present one hour after withdrawal of the drug from the perfusate. The results are consistent with the view that 4-AP prolongs the action potential by inhibiting the late repolarizing potassium current. It is suggested that the calcium uptake by the ventricular cell during the prolonged action potential is increased and that this leads to the positive inotropic effect.

Effects of 4-aminopyridine on insulin secretion and plasma glucose levels in intact and adrenalectomized-chemically sympathectomized mice

The effects of 4-aminopyridine (4-AP) on basal and glucose-induced insulin secretion and on plasma glucose concentrations were investigated in vivo in intact mice and in mice subjected to surgical adrenalectomy plus chemical sympathectomy induced by 6-hydroxydopamine. In normal intact mice, an i.v. injection of 4-AP, 26 micro mol/kg, induced an elevation of plasma glucose concentrations from 6.4 +/- 2 to 10.0 +/- 0.3 mmol/1 (P less than 0.001) seen after 30 min. Thereafter the plasma glucose concentration gradually returned to the normal level. Despite this marked elevation of plasma glucose levels, no change in plasma insulin concentrations was seen. In intact normal mice, the insulin secretion induced by a half-maximal dose of glucose was partially inhibited by 4-AP, 0.26 micro mol/kg. No further inhibition was observed after a larger dose, 26 micro mol/kg. In adrenalectomized-chemically sympathectomized mice, 4-AP, 26 micro mol/kg, did not affect plasma glucose concentrations or plasma levels of insulin. In these animals, 4-AP potentiated glucose-induced insulin secretion by approximately 65% at the two dose levels studied. This potentiation of glucose-induced insulin secretion was not affected by muscarinic receptor blockade. In summary, 4-AP increased plasma glucose levels in normal intact mice, but had no effect on plasma glucose levels in adrenalectomized-chemically sympathectomized mice. In addition, 4-AP inhibited glucose-induced insulin secretion in normal intact mice, but potentiated glucose-induced insulin secretion in adrenalectomized-chemically sympathectomized mice. It is concluded that the effect of 4-AP on insulin secretion and plasma glucose levels in normal mice is exerted indirectly, through stimulation of the sympatho-adrenal system. Further, from the results obtained in animals deprived of their sympatho-adrenal system it is suggested that the drug also has the capability to stimulate insulin secretion by acting directly on the insulin secreting cells.

Cholinergic effect of 4-aminopyridine and adrenergic effect of 4-methyl-2-aminopyridine in cardiac muscle

The effect of 4-aminopyridine (4AP) and 4-methyl-2-aminopyridine (4M2AP), 10(-3) M, was studied on isolated spontaneously beating right atria and electrically driven left atria of the rabbit and the cat. In the rabbit preparations, 4AP had a positive inotropic effect which was unaffected by propranolol or atropine, and a negative chronotropic effect which was abolished by atropine. In the cat, 4AP decreased both force and rate, and these effects were reversed when 4AP was repeated in the presence of atropine. In contrast, 4M2AP increased the rate and force of contraction in both species. These effects were reduced but not abolished when 4M2AP was repeated in the presence of propranolol. It is concluded that 4AP has a cholinergic effect presumably mediated by acetylcholine release which predominates over its direct positive inotropic effect in the rabbit but not in the cat, and which is responsible for its effect on rate in both species. 4M2AP also has a direct positive inotropic action, but its marked effects on force and rate of contraction are largely due to noradrenaline release. The selectivity of 4AP for cholinergic and 4M2AP for noradrenergic transmission suggests that there are differences in potassium channels or in calcium uptake or release mechanisms in the respective nerve terminals.

Actions of 4-methyl-2-aminopyridine on neuromuscular transmission and contractility of skeletal muscle

4-Methyl-2-aminopyridine (4M2AP) increased responses of isolated rat diaphragm and chick biventer cervicis nerve-muscle preparations to nerve stimulation but depressed responses to direct stimulation. Responses to acetylcholine were also increased while responses to carbachol were depressed. When tested after inhibition of acetylcholinesterase activity with neostigmine the effect of 4M2AP on responses to indirect stimulation was greatly reduced and responses to acetylcholine were depressed. It is concluded that, in contrast to other aminopyridines, 4M2AP facilitates neuromuscular transmission by inhibition of cholinesterase rather than augmenting release of acetylcholine and depresses rather than enhances muscles contractility.

Effect of 4-aminopyridine on cardiovascular functions in the dog

The drug 4-aminopyridine (4-AP) at a dose of 0.2 mg . kg-1 body weight intravenously is an effective antagonist of non-depolarizing neuromuscular blockade. We studied its cardiovascular effects in the canine heart using a right-heart bypass with extracorporeal circulation in seven dogs. This study demonstrates that 4-aminopyridine significantly augments arterial blood pressure, left ventricular dp/dt maximum, as well as left ventricular pressure at dp/dt mx. The highest values were obtained between two and 20 minutes after administration of 4-AP. Left ventricular end-diastolic pressure diminished slightly, but this was not statistically significant. Although the exact mechanism of action of 4-AP is not known, its positive inotropic effects may be of value in the reversal of non-depolarizing neuromuscular blockade in patients with impaired myocardial function of diverse aetiologies and it would be contra-indicated in patients with arterial hypertension and/or coronary artery disease.

Factors influencing the magnitude and duration of responses to endogenous and exogenous noradrenaline in rabbit pulmonary artery and portal vein

1 In the rabbit portal veins, cocaine potentiated responses to (-)-noradrenaline, and prolonged relaxation following transmural stimulation. Neither process was altered by hydrocortisone and U-0521, oxytetracycline or 4-aminopyridine alone or in combination with cocaine. These findings indicated that neuronal uptake was principally responsible for the inactivation of noradrenaline in this tissue. 2 In the rabbit pulmonary artery, the rate of relaxation following transmural stimulation was prolonged by cocaine and by hydrocortisone and U-0521, and further prolonged by the combination of all three drugs. These findings suggest that both neuronal and extra-neuronal uptake are responsible for the inactivation of noradrenaline in this tissue. 3 4-Aminopyridine potentiated responses to (-)-noradrenaline and prolonged relaxation after transmural stimulation in pulmonary artery possibly as a consequence of a postjunctional action in this tissue. 4 The rabbit portal vein was less sensitive to (-)-noradrenaline and relaxed more after transmural stimulation than the pulmonary artery. It is suggested that this reflects the denser noradrenergic innervation and smaller neuromuscular distance present in the portal vein.

Calcium- and potassium-channel blockers interact with alpha-adrenoceptors

The so-called calcium antagonist D-600 and agents that enhance the influx of calcium ions into nerve terminals during action potentials (4-aminopyridine, tetraethylammonium, guanidine) interact with alpha-adrenoceptors. The interaction was revealed by binding experiments in vitro with the alpha 2- and alpha 1-specific ligands [3H]clonidine and [3H]prazosin, resp. D-600 binds in a competitive manner to alpha 2- and alpha 1-adrenoceptors in rat-brain membranes. [3H]Prazosin was used to identify alpha 1-adrenoceptors in rat-heart-membrane fragments. D-600 inhibited the binding of the tritiated antagonist, in a manner similar to that seen in rat-brain membranes. 4-Aminopyridine, guanidine and tetraethylammonium blocked no-competitvely the binding of [3H]clonidine in brain membranes. There ws little or no effect of these agents on the binding of [3H]prazosin to alpha 1-adrenoceptors in rat heart or brain. The results indicate that D-600 binds to a region common to alpha 1- and alpha 2-adrenoceptors, whereas the potassium-channel blockers reveal a structural feature of the alpha 2-adrenoceptors in brain membranes which is also present in voltage-dependent potassium channels but is not shared by alpha 1-adrenoceptors.

Comparative study of the effects of 4-aminopyridine and tetraethylammonium on neuro-effector transmission in the guinea-pig vas deferens

1 Effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) on the neuro-effector junction of the guinea-pig vas deferens were investigated by microelectrode and double sucrose gap techniques. 2 4-AP (0.05 to 0.5 mM) or TEA (0.5 to 1 mM) did not alter the membrane potential, or the membrane input resistance of the smooth muscle cell. 3 The amplitude and frequency of the miniature junction potentials (m.e.j.ps) were not modified by treatment with 4-AP (0.05 to 0.5 mM) or TEA (1 mM). 4 4-AP (1 mM) increased the membrane input resistance, enhanced the spike amplitude of the smooth muscle cells and thereby augmented the amplitude of twitch contraction. 5 4-AP (.05 to 0.5 mM) or TEA (1 mM) markedly increased the amplitude of excitatory junction potentials (e.j.ps), but the facilitation phenomena produced by repetitive stimulation were not affected. 6 The duration of the extracellularly recorded action potential from the small nerve bundle was prolonged by 4-AP (0.5 mM). 7 The amplitude of the e.j.p. was dependent on the external concentration of calcium. A straight line was produced when the amplitude of the e.j.p. and [Ca]o was plotted on a double log scale. Application of 4-AP resulted in a parallel shift of this line to the left. 8 These results indicate that 4-AP (0.05 to 0.5 mM) and TEA (0.5 to 1.0 mM) prolonged the action potential generated from the sympathetic nerve terminal thus enhancing the amplitude of the e.j.p. due to an increase in the Ca-influx. However, in the concentrations used, these compounds did not modify the Ca-mobilization in the nerve terminal or the postsynaptic membrane during the resting state.

Effects of 4-aminopyridine on pacemaker activity of frog sinus venosus

The effects of low concentrations of 4-aminopyridine (4-AP) were studied on the frog sinus venosus by means of intracellular microelectrodes. 4-AP depressed spontaneous automaticity. This effect was not reversed by atropine. 4-AP also prolonged the sinus action potential repolarization and slowed the rate of diastolic depolarization. These effects are attributed to blockade of potassium conductance.

Effects of 4-aminopyridine on the turnover of monoamines in the central nervous system of the rat

The effects of 4-aminopyridine (4-AP) on the turnovers of 5-hydroxytryptamine, dopamine and noradrenaline in the central nervous system of rats were studied by means of amine disappearance following inhibition of tryptophan or tyrosin hydroxylase. 4-AP (3 mg/kg i.p.) did not change the utilizations of 5-hydroxytryptamine or dopamine but it markedly accelerated that of noradrenaline in the brain and in the spinal cord. This stimulatory effect of 4-AP was completely dependent on nerve impulses since no effect was observed following an acute section of the noradrenaline nerves to the spinal cord. The effect of 4-AP was blocked by the alpha-adrenoreceptor stimulating agent clonidine. Pentylenetetrazole, at a dose producing similar behavioural changes as 4-AP, caused only a slight stimulation of the noradrenaline turnover. 4-AP might enhance the flux of calcium ions into nerve terminals during depolarization, and thus increase the release of noradrenaline, whereas this process might be of less importance in the dopamine and the 5-hydroxytryptamine nerves.

Potentiation of neostigmine and pyridostigmine by 4-aminopyridine in the rat

The interaction between 4-aminopyridine and neostigmine or pyridostigmine was studied in vivo in the rat sciatic nerve-anterior tibialis preparation using the constant infusion of pancuronium technique. The ED50 (dose of drug which produced a 50% antagonism) of neostigmine, pyridostigmine and 4-aminopyridine were 18, 49 and 440 microgram kg(-1) respectively. The addition of 100 microgram kg(-1) of 4-aminopyridine, which produced no antagonism by itself, decreased the neostigmine ED50 to 7.4 microgram kg(-1). The addition of 200 microgram kg(-1) of 4-aminopyridine, which produced a 30% antagonism by itself, decreased the ED50 of pyridostigmine to 11 microgram kg(-1). We conclude that both neostigmine and pyridostigmine interact with 4-aminopyrine synergistically.

Potentiation of vasoconstrictor responses by 3- and 4-aminopyridine

1. In concentrations that are known to reduce potassium conductance in many excitable membranes, 3 and 4-aminopyridine (3-AP, 4-AP) potentiate vasoconstrictor responses of the isolated ear artery of the rabbit to noradrenaline and histamine. 2. 3- and 4-AP have no effect on the responses of potassium-depolarized arteries to noradrenaline, histamine or calcium. 3. The results suggest that the aminopyridines have no direct effect on the contractile machinery or on pharmacomechanical coupling, but cause potentiation by influencing electrical events at the cell membrane. 4. 4-AP causes a greater potentiation of the response te electrical stimulation than of the response to noradrenaline. This suggests that the aminopyridines may also cause an increase in the amount of noradrenaline released in response to sympathetic nerve stimulation.