Activation of α1- and α2-adrenoceptors inhibits noradrenaline release in rabbit renal arteries: effects of pertussis toxin and N-ethylmaleimide

Participation of cAMP in the facilitatory action of beta,gamma-methylene ATP on the noradrenaline release from rabbit ear artery

beta, gamma-Methylene ATP (betagamma-mATP) significantly facilitated the electrically (4 Hz) evoked release of noradrenaline (NA) from the rabbit ear artery by activation of prejunctional purinoceptors on the sympathetic nerve terminals. In the present study, we investigated whether intracellular cAMP is involved in the purinoceptor mediated facilitatory mechanisms. Forskolin, an adenylate cyclase activator, and 8-bromo cAMP, a cAMP analogue, significantly enhanced the NA-release. The enhancement of NA-release by betagamma-mATP was significantly potentiated by Ro20-1724, a phosphodiesterase inhibitor, but abolished by SQ22536, an adenylate cyclase inhibitor. Both drugs alone had no effect on the NA-release. N-ethylmaleimide and pertussis toxin, inhibitors of Gi-proteins, did not affect the NA-release, or the enhancement of NA-release by betagamma-mATP. Alone Cholera toxin (CTX), an activator of Gs-proteins, significantly increased the NA-release, but in the presence of CTX, betagamma-mATP could not produce further enhancement of the NA-release. These results suggest that cAMP is closely associated with the facilitatory action of betagamma-mATP on NA-release in the rabbit ear artery.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Prejunctional neuropeptide Y receptors in human kidney and atrium

The aim of our study was to characterize functionally prejunctional neuropeptide Y (NPY) receptors in human and rabbit renal cortex, as well as in human right atrium. Segments of human atrial appendages and of human and rabbit renal cortex were preincubated with [3H]noradrenaline, superfused with Krebs-Henseleit solution and stimulated electrically in superfusion chambers. The stimulation-induced outflow of radioactivity was taken as an index of endogenous noradrenaline release. The effects of subtype-selective NPY analogs on the stimulation-induced noradrenaline release were studied. NPY, its endogenous analog, peptide YY, and its C-terminal fragment, NPY13-36, but not its analog, [Leu31,Pro34]NPY, concentration dependently (1-100 nM) inhibited [3H]noradrenaline release in all tissues studied. NPY-induced inhibition of [3H]noradrenaline release in human and rabbit kidney was abolished by pretreatment with pertussis toxin. We conclude that prejunctional inhibition of noradrenaline release in human heart and human and rabbit kidney occurs through NPY receptors of the Y2 subtype, which appear to couple to a pertussis toxin-sensitive G protein.

Cellular mechanisms underlying adenosine actions on cholinergic transmission in enteric neurons

Whole cell recordings were used to investigate the effects of adenosine and several of its analogues on voltage-activated calcium currents (VACC) of myenteric and submucosal neurons. Electrophysiological and pharmacological properties of the soma VACC recorded in myenteric neurons indicate that they are carried through N-type calcium channels, similar to those of the submucosal neurons and to those of the calcium conductance that mediates acetylcholine release at the submucosal ganglia. Adenosinergic compounds inhibited, in a concentration-response and in a voltage-dependent manner, VACC in neurons from both enteric plexuses. The pharmacological profile of the receptors that mediate this effect was similar to that of the receptors involved in presynaptic inhibition in enteric neurons and likely of the A1 subtype. The effects of 2-chloroadenosine (CADO) on VACC were prevented by pretreatment with pertussis toxin (PTX), became irreversible with guanosine 5'-O-(3-thiotriphosphate) (inside the pipette), and were abolished with N-ethylmaleimide (NEM; known to uncouple receptors from G protein complexes). Intracellular recordings were used to further evaluate presynaptic effects of adenosine at the submucosal plexus. Adenosinergic compounds reduced the amplitude of fast excitatory postsynaptic potentials (EPSPs) by acting at nerve terminals. This effect was insensitive to PTX and staurosporine (a protein kinase inhibitor) but was abolished by NEM. CADO effects on EPSPs were not reversed by increasing the extracellular calcium concentration. In conclusion, activation of A1 adenosine receptors inhibits VACC via PTX-sensitive G proteins in myenteric and submucosal neurons. Reduction of cholinergic transmission also involves A1 adenosine receptors and appears to involve the activation of PTX-insensitive G proteins.

Prejunctional actions of N-ethyl-maleimide and phenoxybenzamine in rat vas deferens

In studies of electrically evoked isometric contractions of rat vas deferens, N-ethyl-maleimide (30 microM) pretreatment significantly reduced the prejunctional inhibitory potencies of xylazine and 5-hydroxytryptamine but failed to affect the potency of the alpha 1-adrenoceptor agonist amidephrine. Phenoxybenzamine (1 microM) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (10 microM) produced significant shifts in the potency of xylazine and significantly reduced the maximum inhibition, but the combination of phenoxybenzamine or EEDQ and N-ethyl-maleimide (30 microM) produced no further alteration in the effects of xylazine. In displacement studies, N-ethyl-maleimide displaced the binding of [3H]MK 912 ((2S,12bS)1',3'-dimethylspiro- (1,3,4,5',6,6',7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinazoline)- 2,4'- pyrimidin-2'one) to rat renal cortex membranes with a Ki of 466 +/- 133 microM (n = 5), and so does not bind to alpha 2-adrenoceptors in the concentration range in which it affects prejunctional receptor mediated responses. This may suggest that N-ethyl-maleimide has actions other than inactivation of G-proteins or that the irreversible alpha 2-adrenoceptor antagonists phenoxybenzamine and EEDQ inactivate G-proteins sensitive to N-ethyl-maleimide in concentrations at which they bind to alpha 2-adrenoceptors.

Involvement of pertussis toxin-sensitive and -insensitive mechanisms in alpha-adrenoceptor modulation of noradrenaline release from rat sympathetic neurones in tissue culture

1. Sympathetic neurones derived from superior cervical ganglia of neonatal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Three week old cultures of neurones were loaded with [3H]-noradrenaline to label endogenous neurotransmitter stores. Release of noradrenaline was evoked by depolarization with raised extracellular K+ in the presence of desipramine and corticosterone to prevent uptake of released catecholamine. 2. Potassium (55 mmol l-1) depolarization for 30 s caused more than a four fold increase in 3H overflow from basal levels but this increase was reduced by up to 40% in the presence of exogenous noradrenaline (1 mumol l-1). The inhibition by noradrenaline of depolarization-evoked overflow was blocked by the alpha 1/alpha 2-adrenoceptor antagonist, phentolamine. Phentolamine alone did not increase K(+)-evoked 3H overflow. 3. The alpha 2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K(+)-evoked overflow, while the alpha 1-adrenoceptor antagonist, prazosin, was without effect at concentrations up to 0.1 mumol l-1. 4. The beta-adrenoceptor antagonist, propranolol, neither reduced K(+)-evoked overflow nor increased the degree of inhibition caused by the addition of 1 mumol l-1 noradrenaline. 5. The alpha 2-adrenoceptor agonist, clonidine (1 mumol l-1) was less effective than noradrenaline at inhibiting K(+)-evoked overflow, while the alpha 1-adrenoceptor agonist, phenylephrine (1 mumol l-1) had no significant effect. 6. The L-channel calcium blocker, nicardipine (1 mumol l-1) significantly inhibited 3H overflow evoked by K+. In the presence of L-channel block, however, noradrenaline still inhibited residual evoked overflow.7. In the presence or absence of nicardipine, pertussis toxin pretreatment (1 nmol 1-1) reduced, but did not prevent, the effect of noradrenaline (1 micromol 1-1). Pertussis toxin alone caused a significant enhancement of K+-evoked 3H overflow.8. The data indicate that on postganglionic neurones of cultured rat sympathetic ganglia there are alpha 2-adrenoceptors that modulate neurotransmitter release, but no functional beta-adrenoceptors that mediate an enhancement of transmitter release. The data suggest further that in this preparation the mechanism of alpha2-adrenoceptor modulation may involve pertussis toxin sensitive and insensitive G-proteins and effects on calcium channels other than L-type.

Methoxamine inhibits noradrenaline release through activation of alpha 1- and alpha 2-adrenoceptors in rat isolated kidney: involvement of purines and prostaglandins

The effects of the alpha 1-adrenoceptor agonist methoxamine and the alpha 2-adrenoceptor agonist bromoxidine (UK 14034) on the stimulation induced (S-I) outflow of radioactivity at 100 Hz/6 pulses from rat isolated kidney preincubated with 3H-noradrenaline were investigated. Methoxamine (0.3-30 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 83% with a pEC50 of 5.85 (5.71-5.94). UK 14304 (0.0003-0.3 mumol/l) inhibited S-I outflow of radioactivity to a maximum of 99% with a pEC50 of 8.35 (8.26-8.47). alpha-Adrenoceptor antagonist affinities (pKD) against methoxamine and UK 14304 at prejunctional alpha-adrenoceptors were determined. The concentration response curve of methoxamine was shifted to the right by the alpha 1/alpha 2B-adrenoceptor antagonist prazosin (0.1 mumol/l) with a pKD of 7.41 and that of UK 14304 by prazosin (0.3 mumol/l) with a pKD of 6.24. The alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) shifted the concentration response curve of UK 14304 potently to the right with a pKD of 8.34. The concentration response curve of methoxamine was shifted also to the right by rauwolscine (0.1 mumol/l) and the alpha 2-adrenoceptor antagonist idazoxan (0.1 mumol/l), however, both antagonists suppressed the maximum response of methoxamine to 46% and 56%, respectively. A ten times lower concentration of rauwolscine (0.01 mumol/l) did not shift the concentration response curve of methoxamine but the inhibitory effect of methoxamine still reached only a maximum of 59%.(ABSTRACT TRUNCATED AT 250 WORDS)