Inhibition Of Alpha 2-adrenoceptors Research Articles

Central noradrenergic control of penile erection

Penile erection is completely dependent on commands from the central nervous system. Spinal centers controlling penile erection are located in the thoracolumbar and lumbosacral spinal cord. These centers are activated by information from the periphery and supraspinal nuclei so as to elicit penile erection in a variety of physiological contexts. A small number of nuclei including the locus coeruleus located in the pons sends noradrenergic fibers to the forebrain and spinal cord, including those areas controlling penile erection. Recent morphological techniques such as in situ hybridization and autoradiography using radioligand binding permit investigation of the brain and spinal pathways utilizing alpha adrenoceptor subtypes. Furthermore, pharmacological experiments suggest a modulatory role for noradrenaline in the control of penile erection either in the brain or in the spinal cord. The most robust evidence is that central inhibition of alpha-2 adrenoceptors facilitates sexual function. Taken together, the data propose new directions in the physiological exploration of penile erection and the therapeutic approach of erectile dysfunction. International Journal of Impotence Research (2000) 12, Suppl 1, S13-S19

Betagamma dimers derived from Go and Gi proteins contribute different components of adrenergic inhibition of Ca2+ channels in rat sympathetic neurones.

1. Using perforated-patch recordings, we have examined the part played by endogenous G-protein subunits in the alpha2-adrenoceptor-mediated inhibition of N-type Ca2+ currents in sympathetic neurones. 2. Two components of ICa inhibition by noradrenaline were recorded: a prominent, high affinity and voltage-dependent pertussis toxin (PTX)-sensitive pathway and a minor, low affinity and mostly voltage-insensitive PTX-resistant pathway. 3. PTX-sensitive inhibition was reduced by microinjection of antibodies against either GalphaoA,B or Galphai1,2. The voltage-dependent fraction of inhibition was reduced by anti-Galphao but not by anti-Galphai antibody. 4. Antisense depletion of GalphaoA led to a marked reduction of noradrenaline-induced inhibition and voltage dependence. By contrast, Galphai depletion attenuated noradrenergic modulation without affecting the voltage dependence. 5. Expression of the betagamma-binding agents beta-adrenergic receptor kinase 1 (C-terminus, betaARK1C-ter) or Galphai1 with a Cys3 to Ser mutation partially prevented noradrenergic inhibition while alpha-transducin abolished it. Residual inhibition was mostly voltage independent in cells expressing betaARK1C-ter but was strongly reversed by depolarization in Galphai1 Cys3Ser-expressing cells. 6. Expression of the PTX-resistant Galphai1 Cys351Ile mutant in cells treated with PTX restored alpha2-adrenoceptor inhibition. This restored inhibition was weakly reversed by depolarization. Both the degree and voltage dependence of inhibition were correlated with the level of expression of the Galphai1 Cys351Ile subunit. 7. Our findings identify betagamma dimers associated with GalphaoA and Galphai as mediators of the PTX-sensitive alpha2-adrenoceptor-mediated inhibition of N-type Ca2+ channels. Different betagamma combinations may account for the differential voltage-dependent effects of Go and Gi on ICa.

Examination of the role of inhibition of cyclic AMP in α2‐adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

1. We have examined the effect of elevation of cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on alpha 1- and alpha 2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2. Noradrenaline (1 nM-10 microM) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 +/- 0.07, n = 10). The selective alpha 1-adrenoceptor antagonist, prazosin (0.1 microM) and the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective alpha 2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 microM) failed to affect responses to UK-14304 but rauwolscine, 1 microM, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha 2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of alpha 1- and alpha 2-adrenoceptors capable of producing a contraction. 3. Rolipram, 10 micro M, and forskolin, 1 micro M, caused a 2-3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 micro M, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline induced contractions. The combination of forskolin (1 micro M) and rolipram (10 micro M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC.4. Responses to noradrenaline were similarly affected by a combination of rolipram (10 micro M) and prazosin (0.1 micro M) (isolation of alpha 2-adrenoceptors) and the combination of rolipram (10 micro M) and rauwolscine(1 micro M) (isolation of alpha l-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both alpha l- and alpha 2-adrenoceptor-mediated contractions,the effects produced were not similar. In particular, noradrenaline, 0.3-3 micro M, produced a significant contraction in the presence of forskolin (1 micro M) and prazosin (0.1 micro M) (an alpha 2-adrenoceptor-mediated response) but not in the presence of forskolin (1 micro M) and rauwolscine (1 micro M) (an alpha l-adrenoceptor mediated response).5. Five minute exposure to either rolipram (10 micro M) or forskolin (1 micro M) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150-200%, respectively. Neither noradrenaline (1 nM- 100 micro M) nor UK-14304 (1 nM- 100 micro M) affected basal levels of [3H]-cyclic AMP,but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine(1 micro M) but not by prazosin (0.1 micro M). In contrast, alpha 2-adrenoceptor activation did not affect rolipram induced elevation of [3H]-cyclic AMP.6. These findings indicate that M2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha 2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.

Field stimulation-induced noradrenaline release from guinea-pig atria is modulated by prejunctional alpha 2-adrenoceptors and protein kinase C.

Guinea-pig left atria were loaded with 10 microCi 7-[3H]noradrenaline, and noradrenaline release from sympathetic nerve endings was then elicited by refractory period field stimulation. When one pulse of 0.2 ms duration was applied during each refractory period, the resulting transmitter release was halved by 3 x 10(-7) mol/l of the alpha 2-adrenoceptor agonist clonidine and increased about 2.5-fold by either 3 x 10(-7) mol/l of the alpha 2-adrenoceptor antagonist idazoxan, 5 x 10(-3) mol/l of the potassium channel blocker tetraethylammonium chloride (TEA) or 3 x 10(-7) mol/l phorbol-12-myristate-13-acetate (PMA), an activator of protein kinase C (PKC). Phorbol-12-myristate-13-acetate-4-O-methylether, a compound which does not stimulate PKC, was ineffective. The stimulatory effect of PMA was antagonized by 7 x 10(-5) mol/l of the PKC inhibitor polymyxin B. No significant transmitter release was observed when either PMA or TEA was applied together with 10(-7) mol/l of the sodium channel blocker tetrodotoxin. Combinations of either idazoxan and TEA or PMA and TEA caused greater increases of the noradrenaline release than any individual drug given alone. Thus, different mechanisms of action seem to mediate the increase of noradrenaline release by action potential prolongation on the hand and activation of PKC or inhibition of alpha 2-adrenoceptors on the other hand. In contrast, the effects of idazoxan and PMA were not additive, which suggests a common mechanism of action. In atria pretreated for 10 min with 10(-4) mol/l N-ethylmaleimide, an alkylating agent which inactivates Gi-proteins neither idazoxan nor PMA caused a significant increase of the stimulation-induced transmitter release, while TEA was still effective. When a train of four pulses, lasting 0.05 ms each, was applied during each refractory period, the resulting transmitter release was not modified by idazoxan or PMA, but was significantly increased by TEA. From these results, a scheme is proposed which links the regulation of noradrenaline release by prejunctional alpha 2-adrenoceptors and protein kinase C via an influence on a common inhibitory Gi-protein.

Effect of alpha 2-adrenoceptor agonists on gastric pepsin and acid secretion in the rat.

1. The purpose of the present study was to analyze the effects of the alpha 2-adrenoceptor agonists clonidine, guanabenz, detomidine and medetomidine on pepsin secretion in conscious rats provided with gastric chronic fistula and to compare this with acid secretion. 2. Basal interdigestive gastric secretion, which is mainly neurally driven in the rat, and the secretion directly stimulated by the two main stimulants of chief cells, cholecystokinin octapeptide (CCK8) and methacholine, were studied. 3. Basal secretion of pepsin and acid was inhibited by all four drugs with comparable EC50S. 4. CCK-stimulated pepsin and acid secretion was less sensitive than basal pepsin and acid secretion to alpha 2-adrenoceptor inhibition. 5. Methacholine-stimulated pepsin and acid secretion was not changed by clonidine and guanabenz; methacholine-stimulated acid was even marginally increased by clonidine. 6. These results do not favour the presence of alpha 2-receptors on chief cells in the rat stomach. They rather suggest that pepsin inhibition by alpha 2-adrenoceptor agonists is indirect and due to central or peripheral inhibition of the discharge of nerve fibres activating pepsin secretion.

Alpha2-Adrenoceptor Inhibition in Patients with Vibration White Fingers

In healthy subjects the physiological vasoconstriction to local cooling has been linked to sympathetic adrenoceptors of the alpha2-subtype. The present study was designed to determine if the vasoconstriction in response to local cooling in patients suffering from vibration white fingers can be diminished by alpha2-adrenoceptor inhibition. Six men with vibration white fingers, verified in a cold provocation test, were examined. To study the effect of local skin cooling blood flow was measured with laser doppler technique on the dorsum of dig. II or III. The temperature of the laser doppler probe was regulated by Peltier elements. Temperature could be lowered from 30 degrees C to 20 degrees C within 30s. Measurements were performed before and after local inhibition of the alpha2-adrenoceptors. This was achieved by the introduction of the selective alpha2-adrenoceptor inhibitor, rauwolscine, into the finger skin by iontophoresis. During control conditions local cooling consistently resulted in a marked vasoconstriction. Inhibition of the alpha2-adrenoceptors by rauwolscine completely abolished this effect. The present data indicate that substances inhibiting alpha2-adrenoceptors may be of therapeutic value in patients with vibration white fingers.

Lack of inhibition by atrial natriuretic factor on cyclic AMP levels in single nephron segments and the glomerulus

Recently an inhibitory effect of atrial natriuretic factor (ANF) on the adenylate cyclase system has been reported in vascular tissue. In seeking similar affects in renal tissue, we studied the effect of ANF on cyclic AMP levels in single nephron segments and in glomeruli from the rat. Individual nephron segments or glomeruli were incubated in the presence of a phosphodiesterase inhibitor, with or without parathyroid hormone (PTH) or arginine vasopressin (AVP) and varying concentrations of ANF at 37 degrees C for 2 min. The capacity for alpha 2-adrenoceptor inhibition of adenylate cyclase was demonstrated in the proximal convoluted tubule, cortical collecting tubule and in glomeruli. Nevertheless, ANF could not inhibit cAMP formation in any of these nephron segments nor in the glomerulus. Thus, unlike the vasculature, ANF has no inhibitory effect on cAMP formation in these renal tissues.