Plasma protein binding of perazine and amitriptyline in psychiatric patients

Summary The free fraction of amitriptyline (AT), measured by equilibrium dialysis in plasma from 29 AT-treated depressed patients, was 5.4–9.8% (mean 7.7%), which was the same as the values in 26 healthy controls (4.9–9.6%, mean 7.6%). The plasma levels of lipoproteins, as reflected by total cholesterol, and of ₁-acid glycoprotein (₁-AGP) did not differ between the two groups. The free fraction of AT in both exhibited a significant negative correlation with the concentrations of those two proteins. The unbound fraction of perazine (PER) was the same (3.1–5.9%, mean 4.4%) in plasma from 22 schizophrenic patients and from 24 healthy volunteers (2.9–6.0%, mean 4.5%). However, in patient plasma ₁-AGP was significantly higher (mean 1.07 vs 0.81 mg/ml) and total cholesterol tended to be lower (mean 173 vs 201 mg/100 ml) than in plasma from normals. In consequence, the free fraction of PER was negatively correlated with the ₁-AGP concentration in plasma from patients and with the cholesterol level in plasma from control subjects; the other correlations were not significant. In 7 patients, the ₁-AGP level was normal prior to PER treatment. Serial blood samples from 6 patients revealed a consistent elevation of ₁-AGP above its pretreatment level during 4 weeks of PER administration in 5 of the subjects and a transient increase in one other. While low lipoprotein levels in schizophrenics seem to be a disease-related trait, the increase of ₁-AGP may be a drug effect.     

Alpha2-adrenoceptor-mediated responses to so-called selective alpha1-adrenoceptor agonists after partial blockade of alpha1-adrenoceptors

Summary In dog saphenous vein — a tissue possessing both postsynaptic ₁- and ₂-adrenoceptors — the effects of two selective ₁-adrenoceptor agonists (phenylephrine and methoxamine) were compared with that of the selective ₂-adrenoceptor agonist, UK-14,304, before and after phenoxybenzamine. Furthermore, the influence exerted by prazosin, yohimbine and verapamil on the effects of these agonists was also studied before and after phenoxybenzamine. In the absence of phenoxybenzamine, prazosin (56 nmol/l) caused a parallel shift of the concentration-response curves of both phenylephrine and methoxamine to the right (by 0.94 and 1.1 log units, respectively) and had no effect on the concentration-response curve of UK-14,304, while 20 nmol/l yohimbine caused a marked parallel shift of the concentration-response curve of UK-14,304 to the right (by 1.18 log units) and caused only minor displacements of those of phenylephrine and methoxamine (by 0.2 and 0.33 log units, respectively). After exposure of the strips to 30 nmol/l phenoxybenzamine, prazosin (56 nmol/l) caused small shifts of the concentration-response curves of both phenylephrine (by 0.36 log units) and methoxamine (by 0.31 log units) and did not change that of UK-14,304, while yohimbine (20 nmol/l) caused pronounced parallel shifts of the concentration-response curves (to the right) of all the agonists: phenylephrine (by 1.0 log units), methoxamine (by 0.93 log units) and UK-14,304 (by 1.28 tog units). When UK-14,304 was added to the bath during a sub-maximal contraction to phenylephrine it caused a further contraction almost up to the maximum; if this procedure was repeated after phenoxybenzamine (30 nmol/1), there was no further contraction to UK-14,304.In the absence of phenoxybenzamine, verapamil (5 mol/l) caused a parallel shift of the concentration-response curve of phenylephrine (or methoxamine) to the right and a non-parallel shift (with marked depression of the maximal effect) of that of UK-14,304. However, after phenoxybenzamine (30 nmol/l), the same concentration of verapamil caused non-parallel shifts of the concentration-response curves of the three agonists to the right with about equal depression of the maximal effects. We conclude that, after removal of ₁-adrenoceptor reserve by phenoxybenzamine, the responses to selective ₁-adrenoceptor agonists are predominantly ₂-adrenoceptor-mediated. This may explain why under these conditions, the selective ₁-and ₂-adrenoceptor agonists are equally antagonized by calcium entry blockers.This work was supported by a grant from the University of Porto (Subsidio para acção de investigação no. 36/85) Send offprint requests to S. Guimarães at the above address     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Influence of the renin-angiotensin system on sympathetic neurotransmission in canine skeletal muscle in vivo

Summary The physiological importance of interactions between angiotensin II and sympathetic neurotransmission was studied in an in vivo model with constant flow blood perfused gracilis muscle in situ in dogs pretreated with desipramine and atropine. Sympathetic nerve stimulation- (2 and 8 Hz, 480 pulses) evoked overflow of endogenous noradrenaline and vasoconstriction, and vasoconstrictor responses to exogenous noradrenaline (0.5 nmol, locally i. a.) were evaluated.Angiotensin converting enzyme inhibition by benazeprilat (10 mg i. v.; n = 8) reduced arterial angiotensin II levels from 26 ± 8 to 2 +- 1 pM and reduced mean arterial and basal muscle perfusion pressures. Subsequent resubstitution of angiotensin II (3, 30 and 90 ng kg^–1 min^–1 i.v.) elevated arterial angiotensin II dose-dependently (to 67 ± 14, 622 ± 63 and 1940 ± 251 pM, respectively), as well as mean arterial and muscle perfusion pressures. Nerve stimulation-evoked noradrenaline overflow was unchanged following benazeprilat (–4 ± 4 and + 1 ± 8% at 2 and 8 Hz, respectively) and during subsequent infusions of angiotensin II. Vasoconstrictor responses to nerve stimulation and exogenous noradrenaline were also uninfluenced by these treatments. Thus, angiotensin II did not enhance sympathetic neurotransmission at the postjunctional level.Another group of animals was pretreated with noncompetitive -adrenoceptor blockade locally by phenoxybenzamine and benextramine (0.5 mg kg^–1 i. a. of each; n = 7), which abolished vasoconstrictor responses to nerve stimulation. The effects of benazeprilat and subsequent angiotensin II infusions (3 and 30 ng kg^–1 min^–1 i.v.) on circulating angiotensin II levels, mean arterial and muscle perfusion pressures were similar in this group. Following -adrenoceptor blockade, however, inhibition of angiotensin converting enzyme reduced sympathetic nerve stimulation-evoked noradrenaline overflow by 23 ± 4 and 21 ± 5% at 2 and 8 Hz, respectively (P < 0.01 for both). Angiotensin II infusions failed to enhance evoked noradrenaline overflow (–5 ± 10 and –18 ± 10% at 2 Hz; +6 ± 13% and –3 ± 14% at 8 Hz) also under these conditions.It is concluded that circulating angiotensin II does not influence sympathetic vascular control in canine skeletal muscle even at very high levels in arterial plasma. Angiotensin converting enzyme inhibition reduces nerve stimulation-evoked noradrenaline overflow only in the presence of -adrenoceptor blockade, suggesting that prejunctional -adrenoceptors have an overriding importance over prejunctional angiotensin II-receptors in the modulation of noradrenaline release in this model. The effect of converting enzyme inhibition may be related to merely local changes in angiotensin II concentration or — unrelated to the renin-angiotensin system — to other consequences of the blockade of this unspecific enzyme. Send offprint requests to J. Schwieler at the above address     

Effects of suramin and α,β-methylene ATP indicate noradrenaline-ATP co-transmission in the response of the mouse vas deferens to single and low frequency pulses

Summary Vasa deferentia from mice were field-stimulated by trains of 10 pulses delivered at 0.5 Hz. The pulses elicited separate twitches, the first of which (corresponding to a single pulse) exceeded the following ones in height and width and often was clearly biphasic. , -Methylene-ATP 1 mol/1 and suramin 100 mol/1 caused almost identical changes. They reduced the height of the first twitch in the train by about one half and also reduced its width in a manner indicating that only the second, slow phase remained, but reduced much more markedly the following twitches in which now a small second, slow phase also became detectable. Idazoxan 0.1 mol/1 or yohimbine 0.1 mol/l, when added in the presence of a, -methylene-ATP or suramin, further decreased the first twitch but enhanced twitches No. 2 to 10. These responses were then almost abolished by prazosin 0.1 mol/l. Successive addition of prazosin 0.1 mol/l and idazoxan 0.1 mol/1 to previously untreated vasa deferentia depressed the response to the first pulse by about one half in a manner indicating that only the first, rapid phase remained, but had comparatively little effect on the responses to the subsequent pulses. Suramin 100 mol/l almost abolished the contractions remaining in the presence of prazosin and idazoxan. The results indicate that the first, rapid phase of the neurogenic contractions elicited by single or low frequency pulses is mediated by ATP which substantially contributes to all responses in a train. The second, slow phase is mediated by noradrenaline which substantially contributes to the response to the first pulse only. The adrenergic contribution seems to be mediated by postjunctional ₁- as well as a ₂-adrenoceptors. Prejunctional ₂-adrenergic autoinhibition depresses the release of both ATP and noradrenaline. Send offprint request to I. v. Kügelgen at the above address     

Protein binding of the analgesics alfentanil and sufentanil in maternal and neonatal plasma

Summary Maternal and umbilical venous plasma was obtained at delivery from 8 mothers and their neonates after an i.v. bolus injection of alfentanil, and from 6 mothers and their neonates after epidural administration of sufentanil. Plasma levels of total (free + bound) alfentanil in neonates were about 3.4-times lower than in their mothers. At 33–55 min after 30 µg sufentanil, total drug levels in mothers were around the limit of detection of the radioimmunoassay (0.05 ng/ml); in one mother who had received 250 µg, the plasma level of total sufentanil was 2.6-times higher than in her neonate. Plasma protein binding of alfentanil was 88.2% in mothers and 67.2% in neonates. Plasma protein binding of sufentanil was 90.7% in mothers and 79.3% in neonates. For both drugs, plasma protein binding was significantly related to the ₁-acid glycoprotein (₁-AGP) level, which was about 2.5-times lower in the infants. Free alfentanil levels in mothers and neonates were similar. Free levels of sufentanil in mothers and neonates differed less from each other than did the total drug levels.     

The effect of yohimbine on sympathetic responsiveness in essential hypertension

Summary We have studied the sympathetic response to blockade of presynaptic ₂-adrenoceptors in essential hypertension by measuring plasma concentrations of noradrenaline after a single oral dose of yohimbine, an ₂-adrenoceptor antagonist.Mean baseline plasma noradrenaline and adrenaline concentrations were similar in the hypertensive and normotensive groups. Yohimbine (0.2 mg×kg^–1 orally) caused a lesser increase in the plasma concentrations of noradrenaline in hypertensive patients (+67%) than in normotensive subjects (+178%) and a pressor response in hypertensive (but not in normotensive) patients.These results are consistent with an alteration in the balance of -adrenoceptors (for example presynaptic ₂-adrenoceptor desensitization and post-synaptic ₁-adrenoceptor hyper-responsiveness) which would help to develop and/or maintain arterial hypertension.     

Depression of exploratory activity by clonidine in rats as a model for the detection of relative pre- and postsynaptic central noradrenergic receptor selectivity of α-adrenolytic drugs

Summary The effects of various -adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic -receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic -receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated ³H-noradrenaline-overflow and the concentrations required to shift the EC₅₀ for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA₂, ratio <1): In contrast, phentolamine and phenoxybenzamine which showed preferential postsynaptic -receptor blocking activity (ratio>1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic -receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic -adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic -receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with -adrenergic receptors in the central nervous system.These results were presented in part at the Spring Meeting of German Pharmacological Society, Mainz, 16–18 March, 1977     

Mianserin protein binding in serum and plasma from healthy subjects and patients with depression and rheumatoid arthritis

Mianserin protein binding was measured in serum from 43 healthy subjects and plasma from 12 elderly depressed patients and 23 patients with rheumatoid arthritis. Free fraction (mean±SD) was 5.5±0.7% in the healthy subjects, 5.0±0.8% in the elderly subjects and 6.0±1.0 in the patients with rheumatoid arthritis. In the group of elderly patients treated with mianserin, a high correlation (r=0.83, P<0.001) between total and free concentrations of mianserin was found. In both groups a high linear correlation (r=+0.90, P<0.001) between the free fraction of mianserin and that of imipramine was found, the latter being about twice as high as for mianserin.In both healthy subjects and arthritis patients the degree of protein binding was positively correlated to the concentration of ₁-acid-glycoprotein and complement C3_c, and somewhat more weakly to haptoglobin. In the healthy subjects protein binding was also highly positively correlated to the concentration of apolipoprotein B, whereas no such correlation was found in the rheumatoid arthritis patients. In the rheumatoid arthritis patients protein binding was highly correlated to the concentration of hemopexin and somewhat more weakly to ceruloplasmin and fibrinogen; a weak negative correlation to the concentration of albumin was also found. Since significant intercorrelations between the concentrations of these proteins were found, the correlation to the degree of binding of mianserin may not necessarily represent binding of the drug to the protein.     

Effect of benzo(α)pyrene treatment on the benzo(α)pyrene hydroxylase activity in maternal liver,placenta, and fetus of the rat during day 13 to day 18 of gestation

Summary Pregnant rats were treated with an oral dose of 40.0 mg benzo() pyrene (BP) per kg body weight and were sacrificed 24 h later. Maternal liver, placenta, and fetus were assayed for BP-hydroxylase activity. The enzyme activity was measured from day 13 to day 18 of gestation. Pretreatment of rats with BP results in an approximately 15-fold increase of BP-hydroxylase activity in the maternal liver compared to the enzyme activity in the liver of control animals. These enzyme levels were maintained throughout the duration of gestation.No BP-hydroxylase activity could be measured in the fetus and the placenta of control animals. After treatment with BP maximal induction of BP-hydroxylase activity in the placenta was not achieved before day 15 of gestation. BP treatment resulted in a detectable enzyme activity in the fetus. However, the degree of induction increased with the age of the fetus. Electron microscopic pictures demonstrate a swelling and enlargement of the endoplasmic reticulum (ER) in the trophoblast cell of the placenta after BP treatment. In the fetal liver the ER is characterized by a moderate enlargement while there is no effect in the maternal liver after treatment of the animals with BP.This work was supported by a grant from the Deutsche Forschungsgemeinschaft given to the Sonderforschungsbereich 29, Embryonale Entwicklung und Differenzierung (Embryonal-Pharmakologie).Parts of this work were presented at the Spring Meeting of the German Pharmacological Society held in Mainz, March 1971.     

Smooth muscle of rabbit aorta contains α1- but not α2-Adrenoceptors

Summary We have investigated the residual contractile response to noradrenaline remaining after phenoxybenzamine (3×10^–7 mol/l) in rabbit aorta, since it has been reported that phenoxybenzamine at low doses completely and irreversibly blocks ₁- but not ₂-adrenoceptors. The contraction elicited by noradrenaline slowly recovered with time after it had been almost abolished by phenoxybenzamine. This residual response was blocked by the ₁-selective antagonist prazosin (3×10^–8 mol/l) but not by the ₂-selective antagonist rauwolscine (3×10^–7 mol/l). The results confirm that the smooth muscle of rabbit aorta contains ₁- not ₂-adrenoceptors.     

Postjunctional α-adrenoceptors

Summary The postsynaptic -adrenoceptors in rat aorta and in pithed rat were investigated according to their sensitivity to nine -adrenergic agonists and to the selective antagonists yohimbine (₂) and prazosin (₁) and the non-selective one, phentolamine. In addition, in radioligand binding studies, the affinity and selectivity of the drugs were determined on rat cerebral cortex using [³H] yohimbine and [³H] prazosin.On rat aorta, prazosin is 1,000 times more potent than yohimbine against each -adrenoceptor agonist, whether ₁- or ₂-selective. Rat aorta probably contains only ₁-adrenoceptors.Pressor effects in pithed rats are mediated by post-junctional ₁- and ₂-adrenoceptors. The dose-response curve for -methylnorepinephrine in the presence of prazosin, using Hofstee's plots, revealed ₁- and ₂-adrenoceptors, respective proportions being 80.5 and 19.5%     

Evidence in favour of a selective α1-adrenoceptor blocking action of WB 4101 in vivo

Summary The -adrenoceptor blocking potency of WB 4101 at ₁- and ₂-adrenoceptors has been investigated in pithed rats.WB 4101 was approximately 97 times more potent at antagonizing the vasopressor responses produced by the selective ₁-adrenoceptor agonist phenylephrine, than those produced by the selective ₂-adrenoceptor agonist M-7.A dose of WB 4101 (3 mg/kg) that caused extensive blockade of vascular ₁-adrenoceptors, but little or no blockade of vascular ₂-adrenoceptors, exerted no significant blockade of the presynaptic ₂-adrenoceptors in the rat heart.The results support the view that WB 4101 is a highly selective antagonist at ₁-adrenoceptors in vivo.     

H3 receptor-mediated inhibition of noradrenaline release: an investigation into the involvement of Ca2+ and K+ ions,G protein and adenylate cyclase

The effects of ₂-adrenoceptor agonists (dexmedetomidine, oxymetazoline), alone or in combination with various -adrenoceptor subtype-selective antagonists (CH-38083, idazoxan, WB4101, BRL44408, ARC-239, prazosin), on noradrenaline- and isoprenaline-induced lipolysis were investigated in human isolated abdominal subcutaneous fat cells. The rank order of potency of antagonists in preventing dexmedetomidine- and oxymetazoline-evoked suppression of isoprenaline-induced lipolysis was (pA₂-values): CH-38083 (7.69 and 7.48) idazoxan (7.5 and 7.41) > BRL 44408 (7.23 and 7.19) WB 4101 (7.13 and 7.12) > prazosin (5.18 and 5.17) > ARC-239 (4.72, 4.9). While CH-38083 and idazoxan, non-subtype selective ₂-adrenoceptor antagonists and BRL44408, a selective a_2A-adrenoceptor antagonist as well as WB4101 potentiated the lipolytic effect of noradrenaline, ARC-239, the selective _2B-adrenoceptor antagonist failed to affect it. In addition since the _2A-adrenoceptor selective agonist, oxymetazoline concentration dependently inhibited the lipolytic effect of isoprenaline, and WB4101 and BRL44408 (a_2A-adrenoceptor antagonists) antagonised the effect of oxymetazoline in a competitive manner, it is concluded that the a_2A-adrenoceptor subtype is involved in antilipolysis. In addition, functional evidence was obtained that there is an interaction between _2A- and -adrenoceptors located on the cell surface of adipocytes, through which locally released noradrenaline and/or circulating circulating adrenaline influence lipolysis.On leave from Institute of Medical Pharmacology, University of Pisa, Via Roma 55, I-5626 Pisa, Italy Correspondence to: E. S. Vizi at the above address     

The effects of some α-adrenoceptor antagonists on the responses of the canine saphenous vein to B-HT 933, UK-14304 and methoxamine

Summary The benzoquinolizines Wy 25309, Wy 26703 and Wy 27127, previously reported as potent antagonists at presynaptic ₂-adrenoceptors were also potent antagonists of B-HT 933 in isolated saphenous veins of the dog confirming their activity at post synaptic ₂-adrenoceptors. Yohimbine was a more potent antagonist of B-HT 933 in isolated saphenous vein than were the Wy compounds or idazoxan contrasting with the reported potencies of these compounds at presynaptic sites in rat vas deferens and raising the possibility of differences between pre- and postsynaptic ₂adrenoceptors. Contractions of the saphenous vein were observed with high concentrations of idazoxan. Send offprint requests to K. F. Rhodes at the above address     

Possible involvement of presynaptic α1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR

Summary The effects of several -adrenoceptor antagonists have been examined on tritium release elicited by electrical stimulation from isolated perfused SHR tail artery preparations prelabelled with ³H-noradrenaline (³H-NA). Phentolamine and yohimbine potently facilitated the stimulation evoked release of tritium at low frequencies of stimulation, but the ₂-adrenoceptor antagonist idazoxan was only weakly active at 1 mol/l, despite antagonising the clonidine-evoked inhibition of ³H-release at a lower concentration of 0.1 mol/l. The ₁-adrenoceptor antagonists prazosin and corynanthine also increased stimulation evoked tritium release in this preparation, suggesting the presence of prejunctional ₁-adrenoceptors. Furthermore, the ₁-adrenoceptor agonist methoxamine (3 mol/l) caused a significant inhibition of tritium-evoked release, an effect which was blocked by prazosin (10 nmol/l).When ₁-adrenoceptors were blocked in the presence of prazosin, idazoxan (0.1 mol/l) produced a significant facilitatory effect on the electrically-evoked release of ³H-transmitter. On the other hand, when ₂-adrenoceptors were blocked in the presence of yohimbine, exposure to idazoxan (0.1 mol/l) reduced significantly the stimulation-evoked release of tritium elicited by electrical stimulation.The results indicate that in the SHR tail arteries, idazoxan has a partial agonist inhibitory activity on transmitter release, which can mask the facilitatory effects due to blockade of presynaptic ₂-adrenoceptors. The inhibitory effects of idazoxan appear to involve presynaptic ₂-adrenoceptors, which when stimulated, reduce ³H-NA release in SHR tail arteries.     

Serum concentration and protein binding of thioridazine and its metabolites in patients with chronic alcoholism

Summary The blood chemistry and clinical pharmacokinetics of thioridazine and its metabolites, side-chain sulphoxide, side-chain sulphone and ring sulphoxide, were studied in 31 alcoholics and were compared with values in 17 thioridazine-treated controls without alcoholism. Pathological blood chemistry values, including abnormal liver function and protein concentrations, were common among the alcoholics. In relation to dosage, the majority had a low serum concentration of thioridazine and at a given concentration of thioridazine they had high serum concentrations of its metabolites. Positive intercorrelations were found between pathological liver function tests, prolonged serum half-life and increased serum concentration of thioridazine. The free fractions of thioridazine, side-chain sulphoxide and ring sulphoxide were significantly higher and those of the side-chain sulphone lower in the alcoholics than in the controls. The free fractions of side-chain and ring sulphoxide were significantly increased in patients with a low concentration of ₁-acid glycoprotein.     

Subclassification of presynaptic α2-adrenoceptors: α2D-autoreceptors and α2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of _2A, _2B, _2C and _2D the presynaptic ₂-autoreceptors, as well as the ₂-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with ³H-noradrenaline or ³H-choline and then superfused and stimulated electrically.The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; ³H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to ₂-autoinhibition or inhibition of ³H-acetylcholine release by endogenous noradrenaline. The ₂-selective agonist 5-bromo6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both ³H-noradrenaline and ³H-choline experiments. Most (³H-noradrenaline) or all (³H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pK_d values of the antagonists were calculated from the shifts. pK_d values from ³H-noradrenaline experiments correlated with pK_d values from ³H-choline experiments (r = 0.981).It is concluded that ₂-autoreceptors and ₂-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native ₂ binding sites, binding sites in cells transfected with ₂ subtype genes, and previously classified presynaptic ₂-adrenoceptors — all taken from the literature — indicates that both are _2D. The results are consonant with the hypothesis that at least the majority of ₂-autoreceptors belong to the _2A/D branch of the ₂-adrenoceptor tree, across mammalian or at least across rodent and lagomorph species. The same may hold true for ₂-adrenoceptors on non-noradrenergic neurones.     

Effect of chronic lithium administration on adrenoceptor binding and adrenoceptor regulation in rat cerebral cortex

Summary Lithium (Li) at a concentration, which exerts prophylactic effects in affective disorders is known to alter noradrenaline turnover and the -adrenoceptor-dependent cAMP accumulation. In the present study the action of chronic Li administration (at least 5 weeks) on agonist and antagonist binding to adrenoceptors and on the regulation of adrenoceptors was investigated in rat cerebral cortex. Li treatment caused a small but significant decrease in the number of -adrenoceptor binding sites by 10% (³H-dihydroalprenolol binding) leaving the number of ₁- and ₂-adrenoceptor binding sites (³H-prazosin and ³H-rauwolscine, respectively) unchanged. The affinity of the radioligands as well as the affinity of agonists to these binding sites were not altered. The up-regulation of -adrenoceptor binding sites produced by repeated reserpine injections was inhibited by 32% in rats treated concomitantly with Li, although the noradrenaline depleting effect of reserpine was not impaired. In contrast, Li treatment had no effect on the up-regulation of -adrenoceptor binding induced by 6-OH-dopamine, nor did it alter the -adrenoceptor down-regulation following chronic administration of desipramine. The up-regulation of ₁-adrenoceptor binding sites caused by reserpine or 6-OH-dopamine also remained unaffected by Li. It is concluded that chronic Li has limited effects on cortical adrenoceptors and their regulation. The inhibition of -adrenoceptor up-regulation caused by reserpine may reflect an action of Li on non-adrenergic systems rather than a general stabilizing effect on adrenoceptors proposed previously.Send offprint requests to: G. Gross     

The effect of selective α1- and α2-adrenoceptor stimulation on intraocular pressure in the conscious rabbit

Summary The influence of topically applied selective ₁- and ₂-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the ₁-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the ₂-subtype caused a marked and dose-dependent ocular hypotensive response, which was blocked by the selective ₂-adrenoceptor antagonist yohimbine. ₂-Agonists induced neither macroscopic ocular side effects, nor an effect on the pupil size. Possibly, the subclass of ₂-adrenoceptor stimulating drugs represent a group of new antiglaucomatous agents.