Curves For Noradrenaline Research Articles

Cooling and response to adrenoceptor agonists of rabbit ear and femoral artery: role of the endothelium.

1. The effects of cooling on the response of the rabbit central ear (cutaneous) and femoral (non-cutaneous) arteries to stimulation of adrenoceptors and the role of the endothelium in these effects, were studied in 2 mm long cylindrical segments. 2. Concentration-response curves for noradrenaline (10(-9)-3 x 10(-4) M), phenylephrine (alpha 1-adrenoceptor agonist, 10(-9)-3 x 10(-4) M) and B-HT 920 (alpha 2-adrenoceptor agonist, 10(-7)-10(-3) M) were recorded isometrically in arteries with and without endothelium at 37 degrees C and at 24 degrees C (cooling). To analyze further the endothelial mechanisms in the responses to adrenoceptor stimulation during cooling, the effects of the adrenoceptor agonists on ear arteries in the presence of NG-nitro-L-arginine methyl esther (L-NAME) (10(-5) M) were also determined. 3. In every condition tested, the three adrenoceptor agonists produced a concentration-dependent arterial contraction and the order of potency in ear and femoral arteries was noradrenaline greater than or equal to phenylephrine greater than B-HT 920. The response of ear and femoral arteries to phenylephrine or B-HT 920 was blocked by prazosin (10(-6) M). Yohimbine (10(-6) M) decreased slightly the response of ear arteries and increased that of femoral arteries to B-HT 920. 4. The sensitivity of both ear and femoral arteries to the three adrenoceptor agonists was significantly lower at 24 degrees C than at 37 degrees C. 5. In ear arteries, endothelium removal or treatment with L-NAME did not influence the response at 37 degrees C, but did increase it during cooling to adrenoceptor stimulation.In femoral arteries, endothelium removal increased the sensitivity to noradrenaline and, especially, to B-HT 920 at 37 degrees C, but did not affect the response at 24 degrees C.6. The results suggest that: (a) rabbit ear and femoral arteries are equipped mainly with alpha 1-adrenoceptors;(b) at 37 degrees C, the contraction of the ear artery to adrenoceptor agonists is mostly endothelium-independent, and in the femoral artery the contraction to alpha 2-adrenoceptor activation is endothelium-dependent; (c) cooling inhibits the contraction to adrenoceptor agonists in both ear and femoral arteries: in the ear artery probably by increasing the availability of endothelial nitric oxide, but in the femoral artery by depressing the sensitivity of alpha-adrenoceptors in the smooth musculature.7. The results suggest that the endothelium may modulate the adrenoceptor response of cutaneous arteries during changes in temperature.

Alpha-adrenoceptor modulation of the efferent function of capsaicin-sensitive sensory neurones in guinea-pig isolated atria.

1. Transmural nerve stimulation of guinea-pig atria, obtained from animals pretreated with reserpine (5 mg kg-1, i.p.), in the presence of atropine 1 microM and of the beta-adrenoceptor blocker CGP 20712A 1 microM, induced a positive inotropic effect which was reduced by the calcitonin gene-related peptide (CGRP) antagonist hCGRP-(8-37) and abolished by pretreatment with capsaicin 1 microM. 2. Noradrenaline concentration-dependently (0.01-10 microM) reduced the increase in cardiac contractility induced by transmural nerve stimulation. The inhibitory effect of noradrenaline was antagonized by yohimbine (0.5-1 microM), in a dose-dependent manner. Prazosin (0.5-1 microM) antagonized the effect of noradrenaline and this effect was independent of concentration. 3. In the presence of yohimbine, the lower part of the inhibitory-response curve for noradrenaline was slightly but significantly shifted by prazosin. A similar degree of antagonism was observed in the presence of 1 microM phenoxybenzamine. 4. The selective alpha 2 agonists BHT 920 and clonidine reduced, in the same concentration-range (0.01-1 microM), the cardiac response to transmural nerve stimulation in a yohimbine-sensitive fashion. 5. Phenylephrine (0.1-100 microM) and methoxamine (1-300 microM) also induced an inhibitory effect on transmural nerve stimulation. The effect of phenylephrine was antagonized by yohimbine (1 microM) more efficiently than by prazosin (0.5 microM). 6. These results are in keeping with the presence of inhibitory prejunctional alpha 2-adrenoceptors on cardiac sensory nerve endings which modulate the efferent function of capsaicin-sensitive neurones.

Interactions between endothelin-1 and other chronotropic agents in rat isolated atria

In isolated spontaneously beating right and left atria and in electrically driven left atrium from rat, endothelin-1 increased the rate and force of contraction, but significantly decreased the positive chronotropic and inotropic responses to sympathetic nerve stimulation. The decrease may be partly dependent on the positive chronotropic and inotropic effects of endothelin-1, since other agents with chronotropic activity (noradrenaline, isoprenaline, serotonin and Bay k 8644) also decreased stimulation-induced chronotropic responses. Endothelin-1 caused a significant rightward shift of the linear portion of the log concentration-response curve for the chronotropic actions of noradrenaline and isoprenaline. The changes in the log concentration-response curve were not a consequence of the direct chronotropic effect of endothelin-1, since they were still evident when the chronotropic action of endothelin-1 was offset by carbachol. Furthermore, the chronotropic agent, Bay k 8644, did not shift the linear portion of the log concentration-response curves for noradrenaline and isoprenaline. The mechanism of the effects of endothelin-1 in rat atria is not known, but they were not changed by blockade of α-adrenoceptors or of L-type voltage-sensitive Ca 2+ channels.

Alpha 1- and alpha 2-adrenergic response in human isolated skin arteries during cooling

1. 1. Dose-response curves for noradrenaline, phenylephrine and clonidine were determined isometrically in 2-mm cylindrical segments from human skin arteries at 24°C and compared to those previously reported at 37°C. 2. 2. Noradrenaline (3 × 10 −10−3 × 10 −4 M) induced dose-dependent contraction and the sensitivity was increased during cooling. Phentolamine (10 −6 M), prazosin (10 −6 M) or yohimbine (10 −6 M) produced a higher rightward shift of the control curve for noradrenaline during cooling. 3. 3. Phenylephrine (10 −11−3 × 10 −4 M) and clonidine (10 −11–10 −6 M) caused dose-dependent contraction and the sensitivity of the arteries was augmented at 24°C. 4. 4. The arteries also showed a lower maximal contraction to the adrenergic agonists used and KC1 (50 mM) during cooling. 5. 5. The results suggest that cooling: (a) increases sensitivity of postjunctional α 1- and α 2-adrenoceptors in human skin arteries and (b) depresses contractility of these arteries to α-adrenergic stimulation and direct activation of vascular smooth muscle.

Further characterization of toxins T 1IV (TsTX-III) and T 2IV from Tityus serrulatus scorpion venom

S. V. Sampaio, E. C. Arantes, W. A. Prado, F. Riccioppo Neto and J. R. Giglio. Further characterization of toxins T 1IV (TsTX-III) and T 2IV from Tityus serrulatus scorpion venom. Toxicon 29, 663–672, 1991.—Toxins T 1IV (TsTX-III) and T 2IV have been purified to homogeneity from Tityus serrulatus scorpion venom and further characterized. Their amino acid composition and SDS-PAGE reveal an approximate mol. wt of 7000. Their intracisternal ld 50 (μg/kg) in mice were 12.9±1.6 and 3.0±0.5, while their N-terminal amino acid sequences were K-E-G-Y-A-M-D-H-E-G-C-K-F-S- and K-E-G-Y-L-M-D-H-E-G-C-K-L-S-C-F-I-R-P-S-G-Y-C-G-R-E-, respectively. This sequence of T 2IV, its amino acid composition and its chromatographic and electrophoretic behaviour identify it as toxin γ (TsTX-I), which is the major toxin from this venom. TsTX-III (13 to 102 μg/kg) produced a long lasting enhancement of the hypertensive effect of noradrenaline and a slight decrease of the hypotensive effect of acetylcholine, while T 2IV (115 μg/kg) induced a prolonged hypotensive effect on the anesthetized rat. On the isolated guinea-pig vas deferens, TsTX-III (2.1 and 3.0 μg/ml) produced a horizontal shift of the dose-response curve for noradrenaline to the left with no change of the maximal response. At a concentration of 1.43 μM, it induced a prolongation of the duration of the B component of the compound action potential. This prolongation was strongly reduced after addition of tetrodotoxin.

Effects of cromakalim (BRL 34915) on mechanical responses of rat vas deferens to noradrenaline and naphazoline

The contractile responses produced by noradrenaline were compared to those produced by naphazoline in the rat isolated vas deferens, a preparation which is usually quiescent. The responses to noradrenaline were biphasic: prominent spontaneous contractile activity was superimposed on contractions produced by naphazoline. After removal of naphazoline the rat vas deferens displayed periodic spontaneous activity. The effects of cromakalim and verapamil were compared on contractions induced by noradrenaline and naphazoline and on spontaneous activity induced by exposure to naphazoline. Cromakalim (1–10 × 10 −7 M) shifted to the right the cumulative concentration-response curve of naphazoline but not that of noradrenaline; at the same range of concentrations, cromakalim inhibited naphazoline-induced periodic spontaneous activity. Verapamil (3–30 × 10 −8 M) flattened in a dose-dependent manner the concentration-response curves of noradrenaline and naphazoline; higher concentrations of verapamil (3–30 × 10 −6 M) were required to modify periodic spontaneous activity. It was concluded that naphazoline but not noradrenaline can block K + channel(s) and that this property can represent an important difference imidazolines and phenethylamines.

Pharmacological characterization of the postjunctional alpha-adrenoceptors of the rat isolated seminal vesicle

In an attempt to define the pharmacological characteristics of the postjunctional alpha-adrenoceptors of the rat seminal vesicle, responses to certain phenylethanolamine and imidazoline agonists were investigated, in vitro, under experimental conditions outlined by Furchgott (1972), using alpha 1-selective, non-selective and alpha 2-selective adrenoceptor antagonists. Adrenaline (ADR), noradrenaline (NA) and phenylephrine (PE) produced concentration-dependent contractions. In many experiments the concentration-response (C-R) curves had a distinct "shoulder" at the level of 60-80% of the maximum response (Emax), a situation reminiscent of the rat anococcygeus muscle and the rat basilar artery. The relative potencies of ADR:NA:PE, derived from their EC50 values, were 4.07:1:0.26. In contrast clonidine, oxymetazoline and naphazoline failed to contract the tissue even in concentrations up to 1 X 10(-3) M. In fact the imidazoline derivatives prevented responses to the phenylethanolamines. The antagonist action of clonidine, against phenylephrine, was studied in detail. Prazosin, phentolamine, yohimbine, corynanthine and clonidine all caused a rightward displacement of the C-R curves for NA without depressing Emax. The Arunlakshana and Schild plots of the data were linear and had slopes not significantly different from unity. The pA2 estimates obtained were 9.17 (9.13-9.21) for prazosin, 8.58 (8.07-9.09) for phentolamine, 6.70 (6.44-6.98) for yohimbine and 7.05 (6.81-7.30) for corynanthine. Clonidine had a pA2 value of 6.60 (6.55-6.67) against phenylephrine. On the basis of results obtained with antagonists, the postjunctional alpha-adrenoceptors of the rat seminal vesicle could be firmly placed in the gross category of "alpha 1".(ABSTRACT TRUNCATED AT 250 WORDS)

α2‐adrenoceptor antisecretory responses in the rat jejunum

1. Addition of noradrenaline (5 microM), UK-14,304 (0.1 microM) or phenylephrine (100 microM) to the serosal surface of sheets of rat jejunum mounted in Ussing type chambers decreased resting transepithelial short circuit current (SSC). These responses were inhibited by the selective alpha 2-adrenoceptor antagonist idazoxan (0.5-5.0 microM) but were not significantly affected by the alpha 1-adrenoceptor antagonist corynanthine (100 microM). 2. All three agonists caused a dose-related reduction of SSC which had been elevated by prior addition of theophylline (4 mM). UK-14,304 was 8.1 times more potent than noradrenaline and 76.2 times more potent than phenylephrine. Idazoxan (10 nM-1 microM) caused rightward, parallel displacements of the concentration-response curves for noradrenaline and UK-14,304; PA2 values were 7.87 and 8.02 respectively. Dose-response curves to noradrenaline were unaffected by prazosin (1 microM). 3. The reduction by noradrenaline (10 microM) of SCC which had been elevated by theophylline (4 mM) was not significantly affected by tetrodotoxin (5 microM), suggesting that noradrenaline reduces SCC in rat jejunum via a direct mucosal mechanism. 4. The resting SCC was markedly reduced in chloride-free PSS and under these conditions, noradrenaline did not elicit any further fall in SCC. In bicarbonate-free physiological salt solution (PSS) the resting SCC and the noradrenaline-induced fall in SCC were not significantly different from those in normal PSS. It is concluded that the decrease in SCC observed with noradrenaline, UK-14,304 and phenylephrine is mediated via stimulation of postjunctional alpha 2-adrenoceptors.

α-Adrenoceptor reserve in the canine cephalic vein

In the canine cephalic vein, the pD 2 for the selective α 1-agonist, phenylephrine, was 6.08 ± 0.08 (n = 14) and that for the selective α 2-agonist, UK-14,304, was 8.32 ± 0.06 (n = 10). The pA 2 values for the antagonism exerted by the selective α 1-antagonist, prazosin, against phenylephrine and UK-14,304 were 7.74 ± 0.05 (n = 14) and 6.28 ± 0.03 (n = 8), respectively, while those for the antagonism exerted by the selective α 2-antagonist, yohimbine, against phenylephrine and UK-14,304 were 7.40 ± 0.02 (n = 14) and 8.93 ± 0.05 (n = 14), respectively. Furthermore, the concentration-response curve for UK-14,304 was typically biphasic, the first phase being antagonized by yohimbine and the second phase by prazosin and phenoxybenzamine. These results show that there are postsynaptic α 1- and α 2-adrenoceptors in the canine cephalic vein. In such a preparation, only one concentration of phenoxybenzamine (1 nM) shifted the concentration-response curves for noradrenaline, adrenaline and isoprenaline to the right without reducing the maximum. However, at the concentrations tested, phenoxybenzamine did not shift the concentration-response curve for phenylephrine to the right without depressing its maximum. It is concluded that: (1) the canine cephalic vein is a suitable preparation to study postsynaptic α 1- and α 2-adrenoceptors; (2) according to the original definition of ‘spare receptors’, there is no α 1-adrenoceptor reserve in canine cephalic vein; (3) UK-14,304 is a partial agonist at α 1-adrenoceptors.

Do both adrenaline and noradrenaline stimulate cardiac α‐adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

Investigation into the age-dependence of release of serotonin and noradrenaline in the rat brain cortex and of autoreceptor-mediated modulation of release

Slices of cerebral cortex from young adult and aged rats, preincubated with [ 3H]serotonin or [ 3H]noradrenaline were superfused and the electrically (3 Hz) evoked overflow of tritium and its modulation by the respective agonists and antagonists were studied. The electrically evoked overflow in slices preincubated with [ 3H]serotonin from 2.5-year-old rats was smaller (by about 35%) than in slices from 3-month-old animals. No difference was found for the inhibition by serotonin and the facilitation by metitepine of the evoked overflow and for the metitepine-induced shift of the concentration-response curve for serotonin to the right. There was no difference between slices preincubated with [ 3H]noradrenaline from 3 months, 1, 2 and 2.5-year-old rats, with respect to the electrically evoked overflow, its inhibition and facilitation by noradrenaline and phentolamine, respectively, and the phentolamine-induced shift of the concentration-response curve for noradrenaline to the right. The present results suggest that the release of serotonin is reduced in the cerebral cortex of aged rats whereas the release of noradrenaline and the autoreceptor-mediated modulation of the release of either monoamine is not altered.

Influence of cooling on the responsiveness of rat isolated anococcygeus muscle to noradrenaline

The isolated anococcygeus muscle of the rat was used to study the effect of temperature on noradrenaline-induced contraction. The preparation was suspended in an organ bath containing Krebs bicarbonate solution for isometric tension recording. A decrease of the bath temperature from 37 degrees C to 20 degrees C (cooling) produced an increase in tissue sensitivity to noradrenaline, as reflected in a 5.37-fold leftward shift in the concentration-response curve, and increased the maximum contractile response to this agonist (14.3%). Cooling had no effect on tissue sensitivity to a selective alpha 1-adrenoceptor agonist, methoxamine, but increased (12.4%) the maximum contraction to a similar extent to that to noradrenaline. 6-Hydroxydopamine pretreatment or nortriptyline (1 mumol/l) induced a leftward shift of the noradrenaline concentration-response curve at 37 degrees C, and profoundly inhibited the potentiating effect of cooling on tissue sensitivity to the catecholamine; the effect of cooling on the maximum response was unaffected. The affinity of noradrenaline or methoxamine for alpha 1-adrenoceptors at 37 degrees C, determined from its dissociation constant (KA), was not significantly different from that at 20 degrees C. KA values were determined by use of irreversible antagonism with phenoxybenzamine. On the other hand, diltiazem at concentration of 3 mumol/l, which almost completely abolished the calcium ion-induced contraction of the potassium ion-depolarized muscle, caused only slight inhibition in the concentration-response curve for noradrenaline. The contractile responses to Ca2+ of the K+-depolarized muscle and of the tissue incubated in Ca2+ -free (EGTA 0.1 mmol/l) Krebs solution containing diltiazem and noradrenaline were both depressed by cooling.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a Novel α1-Adrenoceptor Antagonist, SGB-1534, in Contractile Response of Isolated Canine Arterial and Venous Smooth Muscle to Exogenous Noradrenaline: Comparison with Prazosin, Phentolamine and Yohimbine

The pharmacological properties of SGB-1534, 3-[2-[4-[(o-methoxy-phenyl)-1 -piperazinyl]ethyl]-2,4(1 H,3H)-quinazolinedione monohydrochloride, a selective a,-adrenoceptor antagonist, compared with prazosin, phentolamine and yohimbine, were examined in contractile responses of isolated canine mesenteric arteries and veins and femoral arteries and veins to exogenous noradrenaline. The arteries and veins concentration-dependently contracted when exposed to noradrenaline. The sensitivity to noradrenaline, when compared in terms of pD2 values, was significantly higher in the veins than in the arteries. Phentolamine and yohimbine were competitive antagonists against noradrenaline in the arteries and the veins. SGB-1534 and prazosin caused a parallel shift to the right of the concentration-response curves for noradrenaline only in the arteries: the two antagonists were less effective in the veins than in the arteries when low,concentrations of noradrenaline were applied. The pharmacological characteristics of SGB-1534 resemble those of prazosin. The pA2 values for SGB-1534 against noradrenaline in the arteries were much higher than those for prazosin, phentolamine and yohimbine. The result indicates that SGB-1534 may predominantly act upon arterial resistance vessels rather than the venous side, resulting in potent hypotension.

Effect of aging on presynaptic α 2-adrenoceptor mechanisms in guinea pig ileum

1. Presynaptic α 2-adrenoceptor mechanisms in electrically stimulated longitudinal muscles of ilea isolated from 3, 10, 20 and 47 week-old guinea pigs were studied by analysis of the concentration-response curves of noradrenaline, a full agonist, and clonidine, a partial agonist, and the Scatchard plot of specific binding of [ 3H]- p-aminoelonidine to synaptosomal fractions from the longitudinal muscle of guinea pig ileum. 2. The pD 2 value of noradrenaline and the maximum contraction induced by clonidine increased with age from 3 to 20 weeks and there after decreased to 47 weeks, while the pA 2 value of yohimbine against noradrenaline did not alter with age. 3. The capacity of the maximum binding sites of [ 3H]- p-aminoclonidine increased with increasing age (3–20 weeks), while the dissociation constant ( K d) of [ 3H]- p-aminoclonidine did not change during the same period. 4. The changes in the presynaptic α 2-adrenoeeptor mechanisms with age are considered to be due to the change in the total concentration of presynaptie α 2-adrenoceptors.

Decrease in neuronal uptake of noradrenaline simply explains the supersensitivity after sympathectomy in the rat iris dilator.

The effect of superior cervical ganglionectomy or long term treatment with guanethidine on the mechanical response of dilator muscle of the rat iris to several stimulants was examined. The dose-response curve for noradrenaline (NA) was significantly shifted to the left 7 days after denervation (ca. x 10) or by treatment with guanethidine (ca. x 10), but those for methoxamine (Meth) and acetylcholine (ACh) were not. Cocaine did not further sensitize the denervated muscle to NA, but it sensitized the normal muscle to an extent similar to that caused by denervation. Cocaine did not affect the sensitivity to Meth in both control and denervated muscles. The maximum response to NA decreased by about 30% or 20% after surgical or chemical denervation, respectively, while those to Meth, ACh and Ca2+ did not alter significantly. These results indicate that in the dilator muscle, the sympathetic denervation caused a purely specific supersensitivity to NA which was simply explained by a presynaptic mechanism: an impairment of the neuronal uptake process.

Characterization of a novel .ALPHA.1-adrenoceptor antagonist, SGB-1534, in contractile response of isolated canine arterial and venous smooth muscle to exogenous noradrenaline: Comparison with prazosin, phentolamine and yohimbine.

The pharmacological properties of SGB-1534, 3-[2-[4-[(o-methoxyphenyl)-1-piperazinyl]ethyl]-2,4(1H,3H)- quinazolinedione monohydrochloride, a selective alpha 1-adrenoceptor antagonist, compared with prazosin, phentolamine and yohimbine, were examined in contractile responses of isolated canine mesenteric arteries and veins and femoral arteries and veins to exogenous noradrenaline. The arteries and veins concentration-dependently contracted when exposed to noradrenaline. The sensitivity to noradrenaline, when compared in terms of pD2 values, was significantly higher in the veins than in the arteries. Phentolamine and yohimbine were competitive antagonists against noradrenaline in the arteries and the veins. SGB-1534 and prazosin caused a parallel shift to the right of the concentration-response curves for noradrenaline only in the arteries: the two antagonists were less effective in the veins than in the arteries when low concentrations of noradrenaline were applied. The pharmacological characteristics of SGB-1534 resemble those of prazosin. The pA2 values for SGB-1534 against noradrenaline in the arteries were much higher than those for prazosin, phentolamine and yohimbine. The result indicates that SGB-1534 may predominantly act upon arterial resistance vessels rather than the venous side, resulting in potent hypotension.

Decrease in Neuronal Uptake of Noradrenaline Simply Explains the Supersensitivity after Sympathectomy in the Rat Iris Dilator

The effect of superior cervical ganglionectomy or long term treatment with guanethidine on the mechanical response of dilator muscle of the rat iris to several stimulants was examined. The dose-response curve for noradrenaline (NA) was significantly shifted to the left 7 days after denervation (ca. × 10) or by treatment with guanethidine (ca. ×10), but those for methoxamine (Meth) and acetylcholine (ACh) were not. Cocaine did not further sensitize the denervated muscle to NA, but it sensitized the normal muscle to an extent similar to that caused by denervation. Cocaine did not affect the sensitivity to Meth in both control and denervated muscles. The maximum response to NA decreased by about 30% or 20% after surgical or chemical denervation, respectively, while those to Meth, ACh and Ca2+ did not alter significantly. These results indicate that in the dilator muscle, the sympathetic denervation caused a purely specific supersensitivity to NA which was simply explained by a presynaptic mechanism: an impairment of the neuronal uptake process.

The modulatory role of vascular endothelium in the interaction of agonists and antagonists with alpha-adrenoceptors in the rat aorta.

1. We have examined the effect of endothelium on the antagonistic action of prazosin, doxazosin, yohimbine and phentolamine against phenylephrine, clonidine and noradrenaline. 2. The action of prazosin against phenylephrine was similar to that earlier reported against noradrenaline, acting as a non-competitive antagonist in the presence of endothelium and as a competitive antagonist in the absence of endothelium. Prazosin also acted as a non-competitive antagonist against clonidine in the absence of endothelium. 3. Doxazosin behaved in a similar way to prazosin against noradrenaline, phenylephrine and clonidine acting as a non-competitive antagonist in the presence of endothelium and as competitive antagonist after removal of endothelium. In contrast, yohimbine and phentolamine acted as competitive antagonists both in the presence and in the absence of endothelium. 4. Analysis of the concentration-response curves for noradrenaline, phenylephrine and clonidine in the presence and in the absence of endothelium showed that the affinity for all three agonists was the same but not the efficacy and the receptor reserve, both of which were lower in the presence than in the absence of endothelium. 5. The rank order of agonist potency in the absence of endothelium was noradrenaline greater than phenylephrine greater than clonidine. The rank order of antagonist potency was prazosin greater than or equal to doxazosin greater than phentolamine greater than yohimbine. 6. The results show that vascular endothelium modulates the contractile response to alpha-adrenoceptor agonists and also modifies the action of the antagonists prazosin and doxazosin but not that of yohimbine and phentolamine. This effect of endothelium was related to a change in agonist efficacy and receptor reserve. These results also suggest that the alpha-adrenoceptors of the isolated aorta of the rat are predominantly, if not exclusively of the alpha 1-subtype.

Antagonistic properties of RU 24969, a preferential 5-HT1 receptor agonist, at presynaptic alpha 2-adrenoceptors of central and peripheral neurones.

The effect of RU 24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1 H-indole) on the electrically evoked 3H overflow was studied in superfused rat brain cortex slices preincubated with 3H-noradrenaline or 3H-serotonin and in superfused segments of the rat vena cava preincubated with 3H-noradrenaline. In cortex slices preincubated with 3H-noradrenaline, RU 24969 facilitated the electrically (3 Hz) evoked 3H overflow. This effect was abolished by phentolamine but was not affected by desipramine or the 5-HT3 receptor antagonist ICS 205-930. The concentration-response curve of noradrenaline for its inhibitory effect on the evoked overflow (determined in the presence of desipramine) was shifted to the right by RU 24969 32 and 100 mumol/l. In this respect, RU 24969 was about 500 times less potent than phentolamine. In cortex slices preincubated with 3H-serotonin, the inhibitory effect of 3.2 mumol/l RU 24969 on the electrically evoked 3H overflow was increased by phentolamine. In segments of the vena cava, RU 24969 inhibited the electrically (0.66 Hz) evoked 3H overflow. The concentration-response curve of RU 24969 was U-shaped, since at concentrations higher than 0.1 mumol/l the extent of inhibition decreased with increasing concentrations of RU 24969. In the presence of phentolamine, the concentration-dependent attenuation of the RU 24969-induced inhibition of overflow was no longer detectable. The present results suggest that RU 24969 is a weak antagonist at presynaptic alpha 2-adrenoceptors (by more than 2.5 log units less potent than as an agonist at presynaptic 5-HT1B auto- and heteroreceptors).

Mechanism of Inhibition of Rat Caudal Artery Contraction by Amiloride

Amiloride (N-amidino-3,5-diamino-6-chloropyarzine carboxamide), a potassium sparing diuretic, decreases blood pressure and inhibits vascular smooth muscle contractility in rats. The purpose of these experiments was to investigate the mechanisms by which amiloride inhibits contraction of rat caudal artery. Caudal artery rings were mounted in a muscle chamber that was continuously flushed with circulated bathing medium and the contractions were recorded through a force transducer. The results demonstrate that amiloride competitively inhibited the norepinephrine-stimulated contraction of arterial rings (pA2 = 5.2). This suggests that amiloride and norepinephrine (NE) interact at the same site. The concentration required to produce half-maximum inhibition (IC50) of contraction produced by 10(-6) M norepinephrine was 48 +/- 6 microM. Amiloride (100 microM) shifted the concentration-response curves of norepinephrine and methoxamine to the right without significantly affecting the maximum contraction produced by these agonists. The EC50 value and the threshold concentration of both norepinephrine and methoxamine were increased by about 10-fold in the presence of 100 microM amiloride. Amiloride-induced inhibition of norepinephrine-stimulated contraction was reversible following removal of the drug. Amiloride-induced inhibition of vascular smooth muscle contraction was not blocked by ouabain or monensin. Amiloride at concentrations up to 100 microM did not inhibit K+ stimulated contractions. From these results we conclude that amiloride reversibly inhibits vascular smooth muscle contraction by competing with alpha-adrenoceptor agonists for binding to alpha 1-adrenoceptors.