A morpholinocatechol compound (UK42620) with clonidine‐ and tyramine‐like actions

1 The actions of a morpholinocatechol (UK42620) were studied in rat isolated atria preparations consisting of spontaneously beating left and right atrial pairs. 2 UK42620 produced positive inotropic and chronotropic responses and, in atria that were incubated with [³H]‐noradrenaline, it also produced a massive increase in the release of radioactivity. 3 These actions of UK42620 were similar to those of tyramine and were blocked by the β‐adrenoceptor antagonist propranolol (0.3 μM ) and by the neuronal uptake blocker desipramine (1 μM ). 4 In the presence of desipramine, UK42620 but not tyramine produced a decrease in the stimulation‐induced efflux of radioactivity that was antagonized by idazoxan. 5 Thus, UK42620 had prejunctional α₂‐adrenoceptor activity like that of clonidine‐ and tyramine‐like activity releasing large amounts of noradrenaline.     

Prejunctional effects of cromakalim,nicorandil and pinacidil on noradrenergic transmission in rat isolated mesenteric artery

1 The present study investigated the effects of cromakalim, nicorandil and pinacidil on resting and stimulation-induced (S-I) effluxes of radioactivity from rat isolated mesenteric artery preparations in which the noradrenergic transmitter stores had been radiolabelled with [³H]-noradrenaline. The efflux of radioactivity evoked by field stimulation of peri-arterial sympathetic nerves (pulses at 2 Hz frequency in trains of 60 s duration) was taken as an index of transmitter noradrenaline release. 2 Cromakalim (1–100 μm ) and nicroandil (1–1000 μm ) produced minor effects on resting and S-I effluxes of radioactivity, but these did not exhibit concentration-dependency. 3 Pinacidil (1–1000 μm ) produced concentration-dependent increases, in both resting and S-I effluxes of radioactivity. With 1000 μm pinacidil, resting and S-I effluxes were increased to approximately 348% and 358% of their respective control values. 4 The effects of pinacidil on resting and S-I effluxes were unaltered when the neuronal amine transport system was inhibited by desipramine (1 μm ). 5 Inhibition of monoamine oxidase with pargyline (100μm ) treatment markedly reduced the enhancement of resting efflux by 1000 μm pinacidil but did not alter its effect on S-I efflux. It is proposed that the enhanced resting efflux produced by pinacidil without pargyline treatment consists of deaminated [³H]-noradrenaline metabolites formed from [³H]-noradrenaline displaced from transmitter storage vesicles by pinacidil. 6 The enhancement of S-I efflux by pinacidil does not appear to involve disruption of α₂-adrenoceptor auto-inhibition of transmitter release since equi-effective concentrations of phentolamine (1 μm ) and pinacidil (1000 μm ) produced additive effects on S-I efflux, whereas increasing the concentration of phentolamine from 1 to 2m produced no further increases in S-I efflux. 7 In conclusion this, study has provided no evidence of a prejunctional inhibitory effect of the potassium channel openers cromakalim, nicorandil and pinacidil on transmitter noradrenaline release. However, the findings with pinacidil suggest that, in high concentrations, pinacidil displaces noradrenaline from transmitter stores, such that deaminated noradrenaline metabolites are released from the nerve terminals. Furthermore, pinacidil enhances S-I transmitter noradrenaline release, possibly by blocking neuronal potassium channels.     

Tachyphylaxis to ethacrynic acid in the isolated atrium of guinea-pig and its relation to noradrenaline stores.

1 The isolated electrically-paced atrium of the guinea-pig developed a dose-dependent increase in the force of contraction in response to ethacrynic acid (12-100 microgram/ml) which was blocked by pretreatment of the animals with reserpine but was unaffected by desipramine or colchicine added to the bathing medium. 2 There was a rapidly developing tachyphylaxis to repeated doses of ethacrynic acid which was not reversed by rest or incubation of the tissue with noradrenaline. 3 There was no cross tachyphylaxis between ethacrynic acid and tyramine, amphetamine or nicotine. 4 Ethacrynic acid (200 microgram/ml) decreased the noradrenaline content of the atria by 32%. 5 It is concluded that ethacrynic acid exerts its effects indirectly through the release of endogenous noradrenaline and that the mechanism of release seems to be different from that of other known indirect sympathomimetic drugs.     

PREJUNCTIONAL ACTIONS OF TACRINE ON AUTONOMIC NEUROEFFECTOR TRANSMISSION IN RABBIT ISOLATED PULMONARY ARTERY AND RAT ISOLATED ATRIA

1. This study investigated the effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) on the resting and stimulation-induced (SI) release of radioactive substances from isolated preparations of rat atria and rabbit pulmonary artery in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline, and from rat atrial preparations in which cholinergic transmitter stores had been labelled with [3H]-acetylcholine. In addition, the effect of tacrine on the uptake of [3H]-noradrenaline by noradrenergic nerves in rat atria was determined. 2. Tacrine produced concentration-dependent increases in the resting efflux of radioactivity from both the [3H]-noradrenaline-loaded artery and atrial preparations. Blockade of neuronal amine transport with desipramine reduced the release of radioactivity evoked by tacrine from atria but not that evoked from artery preparations. Inhibition of monoamine oxidase by pargyline pretreatment markedly reduced the tacrine-evoked release of radioactivity in both atrial and artery preparations. 3. The radioactivity released from [3H]-noradrenaline-labelled rat atrial preparations by 30 mumol/L tacrine consisted entirely of the deaminated metabolite [3H]-DOPEG. The evoked release of [3H]-DOPEG from atria was reduced by approximately 50% by desipramine (1 mumol/L). When atrial monoamine oxidase had been inhibited by pargyline treatment in vivo and in vitro, 30 mumol/L tacrine evoked the release of [3H]-noradrenaline instead of [3H]-DOPEG. However, the amounts of [3H]-noradrenaline released by tacrine when monoamine oxidase was inhibited were only about 25% of the amounts of [3H]-DOPEG released in untreated atria. 4. Tacrine, in concentrations of 1 and 10 mumol/L, enhanced the release of radioactivity evoked by field stimulation of [3H]-noradrenaline-loaded rabbit pulmonary artery preparations. This effect was unaltered by desipramine or pretreatment with pargyline. However, in artery preparations pretreated with pargyline, a high concentration of tacrine (100 mumol/L) markedly reduced SI efflux. In contrast to the findings with artery preparations, tacrine (1-30 mumol/L) did not alter SI efflux in rat atrial preparations. 5. It is concluded that tacrine displaces noradrenaline from intraneuronal transmitter stores of sympathetically-innervated tissues, and that the displaced amine is totally metabolized by monoamine oxidase before leaving the nerve terminals. When deamination of neuronal cytoplasmic noradrenaline is prevented, only a portion of the noradrenaline displaced from storage vesicles passes to the extracellular space. It is likely that the transfer of cytoplasmic noradrenaline out of the terminals is limited by the activity of the amine transport mechanism.     

Effects of methylenedioxymethamphetamine on the release of monoamines from rat brain slices

The effects of 3,4-methylenedioxymethamphetamine (MDMA) on monoamine release were investigated in superfused slices of rat striatum and hippocampus. MDMA (10 microM) increased the resting release of radioactivity from slices incubated in [3H]dopamine, [3H]5-hydroxytryptamine or [3H]noradrenaline. These effects of MDMA (10 microM) were blocked by the neuronal uptake inhibitors, cocaine (10 microM), fluoxetine (1 microM) and desmethylimipramine (1 microM), respectively. MDMA (10 microM) enhanced the stimulation-induced efflux of radioactivity from slices incubated in [3H]noradrenaline but not from slices incubated in [3H]5-hydroxytryptamine or [3H]dopamine. These results demonstrate for the first time a direct noradrenaline-releasing action of MDMA and differential effects of MDMA on the stimulation-induced release of noradrenaline, dopamine and 5-hydroxytryptamine from rat superfused brain slices.     

A morpholinocatechol compound (UK42620) with clonidine- and tyramine-like actions

1. The actions of a morpholinocatechol (UK42620) were studied in rat isolated atria preparations consisting of spontaneously beating left and right atrial pairs. 2. UK42620 produced positive inotropic and chronotropic responses and, in atria that were incubated with [(3)H]-noradrenaline, it also produced a massive increase in the release of radioactivity. 3. These actions of UK42620 were similar to those of tyramine and were blocked by the beta-adrenoceptor antagonist propranolol (0.3 microM) and by the neuronal uptake blocker desipramine (1 microM). 4. In the presence of desipramine, UK42620 but not tyramine produced a decrease in the stimulation-induced efflux of radioactivity that was antagonized by idazoxan. 5. Thus, UK42620 had prejunctional alpha(2)-adrenoceptor activity like that of clonidine- and tyramine-like activity releasing large amounts of noradrenaline.     

Interactions of methylenedioxymethamphetamine with monoamine transmitter release mechanisms in rat brain slices

Summary This study investigates the effects of methylenedioxymethamphetamine (MDMA) and amphetamine on monoamine release from rat superfused brain slices in both the presence and absence of vesicular stores of transmitter. MDMA caused the release of radioactivity from slices incubated with [³H]5-hydroxytryptamine, [³H]noradrenaline or [³H]dopamine with EC₅₀ values of 1.9 mol/l (95% confidence limits 1.5–2.3 mol/l), 4.5 mol/l (2.3–8.7 mol/l), and greater than 30 mol/l, respectively. In contrast, amphetamine (0.1–300 mol/l) was more effective in releasing radioactivity from slices incubated with [³H]dopamine than [³H]noradrenaline or [³H]5-hydroxytryptamine. When Ca²⁺ was excluded from the superfusion fluid, the MDMA induced release of radioactivity from slices incubated with [³H]dopamine was unaltered, but that from slices incubated with [³H]noradrenaline or [³H]5-hydroxytryptamine was enhanced. MDMA (10 mol/l) facilitated the stimulation-induced (5 Hz, 1 min) outflow of radioactivity from slices incubated with [³H]noradrenaline or [³H]5-hydroxytryptamine to 7.5-fold and 2.1-fold of control values, respectively, but had no effect on that from slices incubated with [³H]dopamine. Amphetamine (1 mol/l) increased the stimulation-induced outflow from slices incubated with [³H]noradrenaline, but not that from slices incubated with [³H]5-hydroxytryptamine or [³H]dopamine.Inhibition of monoamine oxidase by a 30-min incubation with pargyline (100 mol/l) enhanced the releasing action of MDMA on all three monoamines. Pargyline (100 mol/l) also enhanced the facilitation caused by MDMA, of the stimulation-induced outflow of radioactivity from slices incubated with [³H]noradrenaline, [³H]5-hydroxytryptamine or [³H]dopamine.In some experiments, slices were obtained from reserpinised rats (2.5 mg/kg s.c. 24 h prior) and pre-exposed for 30 min to the monoamine oxidase inhibitor pargyline (100 mol/l). Under these conditions, electrical stimulation evoked a small residual stimulation-induced outflow of radioactivity from slices incubated with [³H]noradrenaline, and failed to evoke an outflow of radioactivity from slices incubated with [³H]5-hydroxytryptamine or [³H]dopamine. However, a Ca²⁺-dependent stimulation-induced outflow of radioactivity was evoked in the presence of either MDMA (10 mol/l) or amphetamine (1 mol/l) from slices incubated with either [³H]dopamine or [³H]noradrenaline, but not from slices incubated with [³H]5-hydroxytryptamine. The stimulation-induced outflow of radioactivity from slices incubated with [³H]noradrenaline was enhanced in the presence of desipramine (1 mol/l), however this enhancement was less than that caused by 10 mol/l MDMA or 1 mol/l amphetamine. The Ca²⁺-dependent response to electrical stimulation in the presence of MDMA from slices incubated with [³H]noradrenaline was greatly reduced when rats were pretreated with a higher dose of reserpine (10 mg/kg s.c.).This study demonstrates that MDMA and amphetamine release radioactivity from brain slices incubated with [³H]noradrenaline, [³H]dopamine and [³H]5-hydroxytryptamine, but their order or potency as releasers of brain monoamines differs. The results also suggest that MDMA and amphetamine have significant effects on the exocytotic release of monoamines and may interact with both vesicular and cytoplasmic monoamines. Correspondence to J. J. Reid at the above address     

EFFECTS OF TRICYCLIC ANTIDEPRESSANTS ON UPTAKE AND RELEASE OF 3H-NORADRENALINE IN ISOLATED GUINEA-PIG ATRIA

SUMMARY 1. Desipramine, nortriptyline, amitriptyline, imipramine and cocaine inhibit the uptake of ³H-noradrenaline in isolated guinea-pig atria. The order of potency is as listed; the concentrations producing 50% inhibition are 0.026, 0.036, 0.193, 0.288 and 3.47 μM, respectively.
2. The effects of desipramine and amitriptyline in a range of concentrations (0.01–100 μM) were tested on the resting and stimulation-induced efflux of radioactivity from isolated guinea-pig atria in which adrenergic transmitter stores had been labelled with ³H-(-)-noradrenaline.
3. Stimulation-induced efflux is slightly enhanced by a low concentration (0.01 μM) of desipramine, but is markedly reduced by concentrations of 30 and 100 μM. With 100 μM, there is a marked increase in resting efflux.
4. Amitriptyline enhances stimulation-induced efflux to a greater extent than does desipramine, and the maximal effect is produced with a higher concentration (30 μM). With 100 μM, stimulation-induced efflux is not significantly altered from control and resting efflux is increased, but to a lesser extent than with 100 μM desipramine.
5. It is concluded that desipramine and amitriptyline exert more than one action on adrenergic transmission: in addition to inhibiting neuronal uptake of noradrenaline, they also release noradrenaline, and desipramine interferes with the release of transmitter in response to nerve stimulation.     

The influence of activation or inhibition of protein kinase C on the release of radioactivity from rat isolated atria labelled with [3H]-noradrenaline.

1. The release of radioactivity from rat isolated atria preloaded with [3H]-noradrenaline ([3H]-NA) evoked by electrical field stimulation (2 Hz, 1 ms, 60 s) of intraneuronal sympathetic nerves, high potassium (64.7 mM) or tyramine (0.3 micron) was used as an index of noradrenaline release. 2. Activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) produced a concentration-dependent enhancement of field stimulation-induced outflow of radioactivity, whereas polymyxin B, an inhibitor of protein kinase C, reduced [3H]-NA release evoked by field stimulation. The enhancement observed in the presence of PMA was attenuated by polymyxin B (10 and 70 microns). 3. Release of noradrenaline evoked by membrane depolarization in a high potassium medium was similarly affected by PMA and polymyxin B. 4. In contrast, the release of noradrenaline evoked by the indirectly acting sympathomimetic amine, tyramine, was not altered by PMA. Polymyxin B in a concentration of 70 microns, but not 10 microns caused a slight reduction in tyramine-induced outflow of radioactivity. 5. The spontaneous outflow of radioactive compounds was not affected by either PMA or polymyxin B in the bathing medium. 6. The findings suggest that protein kinase C may play a role in the exocytotic release of noradrenaline but not in the displacement of noradrenaline by indirectly acting sympathomimetic amines.     

Effect of [D-Ala2,Met5]enkephalinamide and [D-Ala2,D-Leu5]enkephalin on cholinergic and noradrenergic neurotransmission in isolated atria

In rabbit isolated atria, [D-Ala2,Met5]enkephalinamide and [D-Ala2,D-Leu5]enkephalin (0.1-3 microM) inhibited responses to cholinergic nerve stimulation in a concentration-dependent manner without affecting responses to exogenous acetylcholine. The inhibitory effect was blocked by the opiate receptor antagonist naloxone (1 microM). In rabbit atria in which the transmitter acetylcholine stores had been radioactively labelled by preincubating the tissue in [3H]choline, tetrodotoxin (100 ng/ml) significantly (P less than 0.001) blocked the stimulation-induced (2 Hz for 3 min) release of radioactivity. Both [D-Ala2,Met5]enkephalinamide and [D-Ala2,D-Leu5]enkephalin (0.3 and 1 microM) significantly decreased stimulation-induced radioactivity release and their effects were blocked by naloxone (1 microM). In rat isolated atria, [D-Ala2,Met5]enkephalinamide and [D-Ala2,D-Leu5]enkephalin (0.3-3 microM) inhibited responses to cholinergic nerve stimulation without affecting responses to exogenous acetylcholine. The inhibitory effect was blocked by naloxone (1 microM). In guinea-pig isolated atria, responses to cholinergic nerve stimulation were unaffected by the enkephalin analogues. In rabbit, rat and guinea-pig isolated atria, responses to noradrenergic nerve stimulation and exogenous noradrenaline were unaffected by the enkephalin analogues.     

PRE-AND POSTJUNCTIONAL EFFECTS OF NEUROPEPTIDE Y ON THE RABBIT ISOLATED EAR ARTERY

b
1. In the isolated perfused and superfused rabbit ear artery, neuropeptide Y (NPY, 0.3–100 nmol/l) had no direct vasoconstrictor action, but produced a concentration-dependent and reversible enhancement of vasoconstrictor responses to both sympathetic nerve stimulation and exogenous noradrenaline.
2. In arteries in which the noradrenergic transmitter stores had been radiolabelled with [³H]-noradrenaline, 100 nmol/l NPY inhibited the stimulation-induced (1 Hz for 30 s) release of radioactivity, but the lower concentrations tested (10 and 30 nmol/l) had no effect. NPY (10, 30 and 100 nmol/l) had no effect on the resting release of radioactivity.
3. Thus, NPY in low concentrations enhances vasoconstrictor responses in the rabbit ear artery by a postjunctional action; prejunctionally, NPY inhibits stimulation-induced transmitter release when it is present in high concentrations.     

PARADOXICAL EFFECTS OF ENDOTHELIN ON CARDIOVASCULAR NORADRENERGIC NEUROTRANSMISSION

1. Pre- and postjunctional effects of endothelin (1-10 nmol/L) have been studied in the rabbit isolated ear artery and in rat isolated atria. 2. Endothelin produced concentration-dependent increases in arterial perfusion pressure, and had positive chronotropic and inotropic effects in rat atria. 3. Vasoconstrictor responses of the arteries to sympathetic nerve stimulation were reduced by 1 nmol/L endothelin and abolished by 10 nmol/L endothelin. In rat atria, the chronotropic responses to nerve stimulation were markedly reduced by 10 nmol/L endothelin. 4. In rabbit ear artery, vasoconstrictor responses to noradrenaline were enhanced by 1 nmol/L endothelin, but were reduced by 10 nmol/L endothelin. In rat atria, endothelin reduced the chronotropic response to isoprenaline. 5. Endothelin (10 nmol/L) increased the stimulation-induced release of radioactivity in arteries and atria labelled with [3H]-noradrenaline by 91% and 23%, respectively. 6. The pre- and postjunctional effects of endothelin persisted in both arterial and atrial preparations for at least 30 min after its removal.     

FACILITATION OF NORADRENALINE RELEASE BY ISOPRENALINE IN RAT ISOLATED ATRIA DOES NOT INVOLVE ANGIOTENSIN II FORMATION

1. Rat isolated atria were incubated with 3H-noradrenaline and the intramural sympathetic nerves were stimulated at 2 Hz for 60 s. The stimulation-induced (SI) efflux of radioactivity was used as an index of release of transmitter noradrenaline. 2. Isoprenaline (0.1 mumol/L) alone did not increase noradrenaline release. Cocaine (30 mumol/L) produced a 73% increase in the stimulation-induced release of noradrenaline. In the presence of cocaine, isoprenaline enhanced noradrenaline release by 22%. 3. In the presence of cocaine, both angiotensin I (0.3 mumol/L) and angiotensin II (0.3 mumol/L) produced almost two-fold enhancements in the SI release of noradrenaline. 4. Captopril (5 mumol/L) blocked the facilitatory effect of angiotensin I on noradrenaline release but did not alter that of isoprenaline. 5. Saralasin (0.1 mumol/L) reduced the facilitatory effect of angiotensin II on noradrenaline release but did not alter that of isoprenaline. 6. The findings indicate that the facilitation of noradrenaline release by isoprenaline in rat atria is not mediated by local formation of angiotensin II.     

Coronary and aortic vasoconstriction by cathinone, the active constituent of khat

1. The psychostimulant constituent of khat leaves, S-(-)-cathinone, was examined for vascular activity on the coronary vasculature of guinea-pig-isolated perfused hearts and aortic ring preparations. 2. Cathinone caused coronary vasoconstriction, negative inotropy and negative chronotropy in isolated hearts. The major metabolite of cathinone after its ingestion, 1R.2S-(-)-norephedrine (norephedrine), also caused coronary vasoconstriction comparable with that by cathinone. Norephedrine, however, had no effect on force or rate of cardiac contractions. 3. Cocaine (10 microm) potentiated the coronary vasoconstriction and positive inotropy by noradrenaline indicating inhibition of neuronal uptake. The vasoconstriction and negative inotropy by cathinone, however, were not affected, indicating that its action was not via release of noradrenaline from sympathetic neurones. 4. The alpha(1)-adrenoceptor antagonist, prazosin, blocked the vasoconstriction by noradrenaline, but not that produced by cathinone in the presence of cocaine. This indicates that the coronary vasoconstriction by cathinone was not due to an action on alpha(1)-adrenoceptors either directly or indirectly through noradrenaline release. 5. Three repeated doses of cathinone displayed the same coronary vasoconstrictor responses, indicating a lack of tachyphylaxis and therefore confirming that the response was unlikely to be due to indirect sympathomimetic activity through release of noradrenaline. 6. In guinea-pig aortic rings, the order of vasoconstrictor activity was: noradrenaline > norephedrine > cathinone, with each causing approximately equivalent maximum responses. The time to reach plateau contractions was shortest for noradrenaline (5.1 +/- 0.5 min), then norephedrine (9.3 +/- 1.5 min) and cathinone the longest (25.4 +/- 3.2 min, 335 microm dose). 7 These results indicate that cathinone has vasoconstrictor activity which is not due to indirect or direct sympathomimetic activity. The precise mechanism for this vasoconstriction remains to be determined. The coronary vasoconstriction may explain the increased incidence of myocardial infarction in khat chewers, which may arise from coronary vasospasm.     

FACILITATION OF NORADRENERGIC TRANSMISSION BY LOCALLY GENERATED ANGIOTENSIN II IN GUINEA-PIG ISOLATED ATRIA AND IN THE PERFUSED CAUDAL ARTERY OF THE RAT

In guinea-pig isolated atria, angiotensin I and angiotensin II produced concentration dependent increases in the rate of spontaneous beating and in the release of noradrenaline produced by field stimulation of sympathetic nerves. In rat isolated caudal artery preparations, both angiotensin I and angiotensin II had direct vasoconstrictor actions and also produced concentration dependent increases in constrictor responses to periarterial sympathetic stimulation. All the above effects of angiotensin I and angiotensin II were blocked by the receptor antagonist saralasin, but only those of angiotensin I were blocked by the converting enzyme inhibitor enalaprilat (MK-422). The findings confirm that angiotensin II generated locally from precursor angiotensin I within cardiac and vascular tissues may modulate noradrenergic transmitter release.     

Correlation between desipramine levels and (-)-noradrenaline uptake and chronotropic effect in isolated atria of rats

1. The uptake of unlabelled and [(14)C]-desipramine was studied in rat isolated atria incubated in a medium containing concentrations of desipramine ranging from 200 pg/ml to 2 mug/ml. The uptake was found to be dose- and time-dependent. Equilibrium was not reached after 2-3 h of incubation unless concentrations of desipramine higher than 1 mug/ml were used.2. The washout curves of atria previously loaded with desipramine showed that the drug is slowly released and that this release is not influenced by its initial tissue concentration.3. The binding appeared to be at non-specific sites and not at sites where noradrenaline is stored since atria taken from rats treated with 6-hydroxydopamine accumulated the drug at the same rate as control atria.4. The inhibition of (-)-noradrenaline uptake and the potentiation of the chronotropic response to (-)-noradrenaline is correlated with the concentration of desipramine in atria for tissue levels of the drug ranging from 0.01 to 1 mug/g. Higher tissue levels show less potentiation of the effect of (-)-noradrenaline or even inhibition of the maximal response to (-)-noradrenaline. These concentrations of desipramine (> 7 mug/g) markedly depressed the atrial rate.5. The results show that despite the accumulation of desipramine by unspecific sites, concentrations of desipramine in the tissue are correlated with the pharmacological response. Furthermore a gradual shift from potentiation to inhibition of noradrenaline response can be obtained with the same bath concentrations of desipramine by increasing the time of incubation.     

Release of prostaglandins from the rabbit perfused kidney: Effects of vasoconstrictors

1 The rat stomach strip was used to assay prostaglandin E(2)-like material released by a rabbit isolated kidney perfused with Krebs solution.2 Doses of noradrenaline and angiotensin II producing similar vasoconstrictor effects released equivalent amounts of prostaglandins from the kidney.3 8-Leu-angiotensin II, a specific inhibitor of the natural octapeptide, blocked the action of angiotensin II on perfusion pressure and the release of prostaglandins, while the action of noradrenaline on both parameters was unaffected.4 Indomethacin, a specific inhibitor of prostaglandin biosynthesis, blocked the effects of both vasoconstrictors on prostaglandin release while their action on perfusion pressure was significantly enhanced.5 In the kidney effluent, amounts of prostaglandin E(2)-like material increased linearly with the rise in perfusion pressure induced by increasing doses of angiotensin II. These results indicate that prostaglandin output from the isolated kidney follows the rise in perfusion pressure.     

The effects of changes in ionic environment and modification of adrenergic function on the vascular responses to sympathomimetic amines

The responses to each of four sympathomimetic amines: noradrenaline 200 ng, octopamine 50 μg, metaraminol 20 μg and tyramine 100 μg were studied in the perfused rat mesentery preparation. Perfusion with Ca²⁺- and Mg²⁺-free solutions potentiated the responses to all four amines compared with control responses obtained during normal Krebs perfusion. Under perfusion conditions using either normal or Ca²⁺- and Mg²⁺-free Krebs solution, nialamide and reserpine retained their characteristic effects on the responses to each amine. Cocaine and desipramine abolished the responses to tyramine but potentiated those to noradrenaline and metaraminol under all perfusion conditions. The responses to each of the amines were only antagonized by ouabain when Ca²⁺ ions were present in the perfusion solution. It is concluded that perfusion with Ca²⁺- and Mg²⁺-free solution interferes with the normal uptake mechanisms occurring in the adrenergic neuron.     

DISADVANTAGES OF COCAINE AS A NEURONAL UPTAKE BLOCKING AGENT: COMPARISON WITH DESIPRAMINE IN GUINEA-PIG RIGHT ATRIUM

• 1 Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria.
• 2 Right atria were equilibrated with the irreversible α-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic α-adrenoreceptors. Cumulative (–) noradrenaline concentration-response curves were shifted to the left by DMI (0.01–1 μM) without significant change in the resting or maximum rates. Cocaine (1–100 μM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 μM.
• 3 Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 μM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 μM respectively.
• 4 The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. 5. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discorage its use in experiments on sympathetic transmission.

     

Nonexocytotic noradrenaline release and ventricular fibrillation in ischemic rat hearts

In myocardial ischemia, nonexocytotic noradrenaline release has been identified as underlying mechanism of ischemia-evoked noradrenaline release. Nonexocytotic noradrenaline release can be suppressed by inhibitors of the neuronal noradrenaline carrier (uptake), such as desipramine. Utilizing this pharmacological intervention the role of local noradrenaline release in the genesis of ischemia-induced ventricular arrhythmias was studied. Regional ischemia was induced in rat isolated perfused hearts by ligature of the left anterior descending coronary artery, and the venous effluent obtained during the first 2 min of reperfusion was used to measure the release of endogenous noradrenaline by high-performance liquid chromatography methods. Coronary occlusion caused ventricular fibrilation in a well reproducible manner with an incidence of 70 to 80% during a 30 min observation period. Blockade of uptake, by desipramine decreased the occurrence of ischemia-induced ventricular fibrillation to 60% (0.01 mol/1) or 20% (0.1 mol/l), and ventricular fibrillation was completely suppressed by 1 mol/l desipramine. Likewise, desipramine (0.01–1 mol/l) concentration-dependently reduced endogenous noradrenaline release during 30 min of regional myocardial ischemia. Nisoxetine, a structurally unrelated inhibitor of uptake,, also suppressed ischemia-evoked ventricular fibrillation.In contrast to its antifibrillatory effect during regional myocardial ischemia, desipramine precipitated arrhythmias when ventricular fibrillation was induced by perfusing normoxic hearts with exogenous noradrenaline. Combination of desipramine (0.1 mol/1) with exogenous noradrenaline (0.01 to 1 mol/l) increased the incidence of ventricular fibrillation compared to noradrenaline perfusion alone. Under these conditions, uptake₁-blockade is known to increase the extracellular concentration of the perfused noradrenaline. Finally, in the isolated, spontaneously beating papillary muscle of the left rat heart, desipramine (0.1 and 1.0 mol/l) had no effect on the upstroke velocity of action potentials, the action potential duration and the effective refractory period.In conclusion, the findings demonstrate that nonexocytotic noradrenaline release is an important mediator of ischemia-induced ventricular fibrillation in isolated hearts of the rat. It is also documented that uptake, inhibitors such as desipramine reveal their effects on ventricular fibrillation secondary to their action on transmembrane noradrenaline transport.