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α2-Adrenoceptor subtypes-mediated physiological, pharmacological actions

alpha(2)-Adrenoceptors are involved in various physiological functions, particularly in the cardiovascular system and the central nervous system. Different adrenoceptor subtypes (alpha(2A), alpha(2B) and alpha(2C)) have been recognised and the different subtypes may have role in activation of distinct physiological and pharmacological pathways. Some of the actions of alpha(2)-adrenoceptor stimulants are likely to be mediated exclusively by alpha(2A)-adrenoceptor subtype, like antihypertensive and bradycardic effects. alpha(2A)-Adrenoceptor may have dominant role in sedative and hypothermic actions, in inhibition of gastric acid secretion and gastric motor activity, as well as in stabilisation of thrombus. Besides alpha(2A)-adrenergic receptors alpha(2B) and alpha(2C)-adrenoceptor subtypes may also be involved in some of the classical effects of alpha(2)-adrenoceptor stimulants, like in presynaptic regulation of transmitter release and antinociceptive action. alpha(2B)-Adrenoceptor has dominant role in the vasoconstrictor effect of alpha(2)-adrenoceptor agonists, and its activation induces contraction of rat uterus in late pregnancy. alpha(2B)-Adrenoceptor mediates gastric mucosal protective action initiated centrally in the rat, as well as it may have role also in developmental or reproductive processes. alpha(2C)-Adrenoceptor subtypes may be involved in stress-dependent depression and other psychiatric illnesses like attention deficit hyperactivity disorder-together with alpha(2A)-adrenoceptor. alpha(2C)-Adrenergic receptors seem to mediate peripheral actions as well, like venous vasoconstriction. Identification of separate physiological roles for different alpha(2)-adrenergic receptor subtypes could improve design of novel compounds for specific therapeutic goals.

Elevated plasma nociceptin level in patients with Wilson disease

Plasma level of nociceptin, the endogenous agonist of orphanin FQ/ORL1 receptor was found to be significantly elevated in Wilson disease patients (13.98+/-2.44pg/ml, p<0.001, n=20) compared to age-matched healthy controls (9.18+/-1.63pg/ml, n=25). Wilson disease is an autosomal recessive disorder of copper metabolism caused by mutation of the gene ATP7B leading to toxic copper accumulation in the liver and other organs such as brain, kidney and cornea. Measurements were performed by 125I-radioimmunoassay. Neither sex differences nor correlation between plasma nociceptin levels and liver function test results were found. It is suggested that elevated plasma nociceptin level found in Wilson disease patients is due to inhibition of nociceptin-inactivating Zn-metallopeptidases (aminopeptidase N (APN) and endopeptidase 24.15) by the toxic copper deposits in liver and/or brain.

Neurochemical transmission and the sodium-pump

Calcmm-dependent neurotransmltter release-calcmm-hypothesis 2 I Sqmd giant axon 2 2 Neuromuscular Junction 2.3. Sympathetic nerve terminals 2.4 Adrenal medulla Sodmm-pump, general aspects 3 1 Sensltivlty to cardiac glycosldes 3 2. Operation modes of the sodmm-pump 3 3 Sodmm-pump of giant axons, nerve fibres, nerve cell bodies and ganghon cells 3 4 Sodium-pump of heart-, skeletaland smooth-muscle cells 3.5. Stoichlometry of the sodium-pump-uncoupled positive charge 3 6. Number of sodmm Ions transported by the sodium-pump 3.7. Current carried by the sodium-pump Sodmm gradient created by the sodmm-pump, sodmm-calcmm exchange SodlUm-DUmD and neurotransmltter release 5.1 5 2. 5.3 54

Depolarization promotes caffeine induced [3H]-noradrenaline release in calcium-free solution from peripheral sympathetic nerves

The transmitter releasing action of caffeine was studied in the absence of extracellular Ca2+ from the peripheral sympathetic nerves of the rabbit main pulmonary artery. Caffeine (10 mM) increased the release of [3H]-noradrenaline moderately, but not significantly in Ca2(+)-free (+1 mM EGTA) Krebs solution. When peripheral nerve endings/varicosities were depolarized by elevating extracellular K+ to 47.2 mM and 70.8 mM in Ca2(+)-free solution, the transmitter releasing effect of 10 mM caffeine became significant. Ca2+ removal itself transiently increased the [3H]-noradrenaline outflow. In the individual experiments the amount of the caffeine evoked transmitter release at 47.2 mM and 70.8 mM K(+)-depolarization was inversely correlated to the release evoked by Ca2(+)-removal. Our results suggest that caffeine-sensitive calcium stores are present in peripheral nerve terminals of rabbit pulmonary artery, and part of the caffeine sensitive calcium stores may discharge during Ca2(+)-removal from the extracellular solution.

Effect of potassium on the release of [3H]noradrenaline from rabbit and human pulmonary artery

Abstract: The release of [3H]noradrenaline ([3H]NA) from rabbit and human isolated pulmonary artery has been measured. Removal of external potassium ions enhanced both the resting and stimulated release of [3H]NA from the strips. On adding K+ to tissues which had been suspended in K+ ‐free Krebs solution, the release of [3H]NA was reduced in both stimulated and unstimulated tissues. Selective inhibition of presynaptic α2‐‐adrenoceptors by yohimbine significantly potentiated the release of [3H]NA evoked by stimulation in K+ ‐free solution. The presynaptic inhibitory effect of NA was much less pronounced when the release was enhanced by the removal of external K+. Since the activity of Na,K‐ATPase may be affected by removing K+ or by adding it to tissue previously kept in K+‐free solution, the results may indicate involvement of the sodium pump in NA release.

Effect of prostaglandin E1 and indomethacin on responses of longitudinal muscle of guinea-pig ileum to cholecystokinin

Abstract The effect of prostaglandin E 1 (PGE 1 ) and indomethacin (IND) cholecystokinin (CCK)-induced contractions of guinea-pig isolated ileum longitudinal muscle were studied. PGE 1 (2.8–28 nM) consistently and dose dependently increased the contractions evoked by CCK (indirect muscle stimulation) or by ACh. IND (2.7μM) decreased the contractions to both compounds and this was reversed with 2.8–7 nM PGE 1 . Pretreatment of the preparations with phentolamine (2.6μM) or pretreatment of the animals with reserpine (2 mg/kg i.p. 24 h before killing) did not affect PGE 1 potentiation or IND inhibition of CCK-induced contractions. The results indicated that PGE 1 potentiated CCK-induced contractions of the longitudinal muscle of guinea-pig ileum by increasing the response to released ACh. Experiments with IND suggested that endogenous PGs may modulate the effect of CCK or related gastrointestinal hormones.

[3H]Noradrenaline-releasing action of vinpocetine in the isolated main pulmonary artery of the rabbit

Vinpocetine (10−6 − 3 × 10−5 M) increased both the resting and the nerve stimulation‐evoked release of [3H]noradrenaline from the isolated main pulmonary artery of the rabbit in the presence of uptake blockers (cocaine, 3 × 10−5 M; corticosterone, 5 × 10−5 M), and inhibited the nerve stimulation‐evoked postsynaptic response. The resting transmitter releasing action of vinpocetine increased in the absence of cocaine. Exogenously applied (−)noradrenaline [(−)NA] (10−6 M) or clonidine (10−6 M) inhibited the vinpocetine (3 × 10−5 M)‐potentiated [3H]NA release and contracted the circular muscle. The clonidine‐induced contraction was abolished by 10−7 M prazosin. The inhibitory action of (−)‐NA on vinpocetine‐potentiated [3H]NA release was partly antagonized by 3 × 10−7 M yohimbine, a preferential α2‐adrenoceptor blocker. In Ca‐free Krebs solution containing 1 mM EGTA the neurotransmitter releasing action of vinpocetine was abolished, however, its stimulating action on the resting [3H]NA outflow was not changed. In Na‐pump‐inhibited arteries (K‐free solution), where both the resting and the nerve stimulation‐evoked release of neurotransmitter had already been increased, vinpocetine further enhanced the nerve stimulation‐evoked release of [3H]NA. It is concluded that vinpocetine may have α2‐ and α1‐adrenoceptor blocking action, as well as a tyramine‐like effect. The presynaptic neurotransmitter releasing action of vinpocetine is presumably the consequence of its inhibitory action on the Ca‐pump which is suggested by the finding that in K‐free solution vinpocetine was able to enhance further the release of neurotransmitter.

The action of excess potassium and calcium on ouabain‐evoked [3H]‐noradrenaline release from the rabbit pulmonary artery

1 [3H]‐noradrenaline ([3H]‐NA) release from the main pulmonary artery of the rabbit has been measured in the presence of neuronal (cocaine, 3 × 10−5 M) and extraneuronal (corticosterone, 5 × 10−5 M) uptake blockers. 2 Removal of K from the external medium increased the [3H]‐NA release. In the absence of external K, ouabain (10−4 M) further enhanced the neurotransmitter release. The ‘K‐free’ stimulated [3H]‐NA release was inhibited by an increase of external Ca (7.5 mM), an action antagonized by ouabain. 3 After preperfusion of the preparations for 30 min with either excess K (23.6 mM) or excess Ca (7.5 mM), the ouabain‐stimulated [3H]‐NA release was inhibited by about 50%; the rates of inhibition did not differ significantly from each other. However, the characteristic initial delay before ouabain‐evoked neurotransmitter release was shortened in excess K, and prolonged in excess Ca‐containing solution. 4 When both excess K and Ca were applied together 30 min before ouabain perfusion, the action of ouabain in releasing neurotransmitter was also inhibited but the rate of inhibition did not differ significantly from that seen when K or Ca were applied separately. The action of K in shortening the initial delay was partly antagonized by Ca. 5 Excess Ca antagonized the inhibition of ouabain‐stimulated [3H]‐NA release caused by excess K when Ca and ouabain were applied together after 30 min preperfusion with excess K‐containing solution. Again excess Ca failed to inhibit the ouabain‐evoked neurotransmitter release if ouabain and excess K were applied together after excess Ca preperfusion (30 min). In both cases the initial delay of ouabain action was greatly shortened. 6 The results suggest a Na‐Ca competition at the external activation site of the nerve terminal sodium‐pump similar to that of Na‐K competition. Furthermore it seems that there is a sort of K‐Ca competition as well, suggested by the finding that excess Ca prevented the inhibition caused by excess K of ouabain‐evoked noradrenaline release and vice versa.

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

A high concentration of selegiline ((—)‐deprenyl; 10−4M) 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 times 10−5M) and extraneuronal (corticosterone, 5 times 10−5M) uptake blockers, and inhibited the postsynaptic response. The transmitter‐releasing action of 10−4M selegiline was inhibited by a moderate increase of external K+ (23ṁ6mM). Excess K+ by itself abolished the nerve‐evoked release of [3H]noradrenaline but did not increase the resting outflow of radioactivity. Excess Ca2+ (7ṁ mM) increased the stimulation‐evoked transmitter release. In the presence of excess Ca2+, selegiline (10−4M) 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‐ami‐nopyridine (10−5M), 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.

The inhibitory action of pgf2α on release of [3H]noradrenaline enhanced by α2-adrenoceptor blockade, sodium-pump inhibition and 4-aminopyridine in the main pulmonary artery of the rabbit

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