Cholinesterase activity and exposure time to acetylcholine as factors influencing the muscarinic inhibition of [3H]-noradrenaline overflow from guinea-pig isolated atria

Presynaptic muscarinic receptors inhibiting endogenous noradrenaline release in the portal vein of the freely moving rat

1. In the portal vein of the freely moving unanaesthetized rat, the existence of presynaptically located inhibitory muscarinic receptors was investigated by use of the muscarinic agonist methacholine (MCh) 2. Infusion of MCh (0.3 micrograms min-1) did not significantly inhibit the endogenous noradrenaline (NA) overflow in portal plasma. However, after inducing high intra-synaptic concentrations of NA by blocking the presynaptic alpha 2-adrenoceptors with yohimbine (1 mg kg-1), MCh (0.3 microgram min-1) was able to reduce the yohimbine-induced enhanced NA overflow by 38%. 3. The MCh-induced inhibition was almost completely abolished after blockade of the presynaptic muscarinic receptors with atropine (0.6 mg kg-1). 4. During electrical stimulation of the portal vein nervous plexus the evoked NA overflow was strongly inhibited (95%) during MCh-infusion (0.3 microgram min-1). Again atropine (0.6 mg kg-1) was able to reverse this inhibition. 5. These results show the existence of presynaptic muscarinic receptors inhibiting endogenous NA overflow from the portal vein nervous plexus under conditions of enhanced sympathetic activity in the freely moving rat.

Muscarinic inhibition of [3H]-noradrenaline release on rabbit iris in vitro: effects of stimulation conditions on intrinsic activity of methacholine and pilocarpine

1. Rabbit isolated irides were loaded with [3H]-noradrenaline and superfused with Tyrode solution. The inhibition by the muscarinic agonists (+/-)-methacholine and pilocarpine of the [3H]-noradrenaline overflow into the superfusate evoked by field stimulation (pulses of 1 ms duration, 75 mA) was measured as an index of activation of presynaptic muscarinic receptors. 2. The fractional rate of release per pulse during the first stimulation period (S1) was low with 360 pulses at 3 Hz, intermediate with 360 pulses at 10 Hz and high with 1200 pulses at 10 Hz. Upon repetitive stimulation (7 periods at 20 min intervals), the fractional rates of release per pulse during S7 no longer differed, suggesting a 'long-term' regulation of [3H]-noradrenaline release depending on the stimulation conditions. 3. The evoked [3H]-noradrenaline overflow was depressed by (+/-)-methacholine in a concentration-dependent manner. The EC50 ranged from 0.29 to 0.42 microM. Methacholine nearly abolished the transmitter release evoked at 3 Hz but reduced that induced at 10 Hz by only 50%. Under the latter condition the methacholine concentration-inhibition curve was bell-shaped and no muscarinic inhibition was observed in the presence of methacholine 30 microM. After washout of methacholine the evoked [3H]-noradrenaline release was temporarily enhanced. 4. Atropine 0.1 microM enhanced the [3H]-noradrenaline overflow (evoked by stimulation with 360 or 1200 pulses at 10 Hz), probably antagonizing a presynaptic inhibition by endogenous acetylcholine. The inhibition by methacholine was competitively antagonized by atropine 0.1 microM (apparent -log KB = 8.5-9.0). 5. Depending on the concentration, pilocarpine reduced the [3H]-noradrenaline overflow evoked by 360 pulses at 3 Hz up to 63%. However, at 10 Hz stimulation frequency the compound was inactive as an agonist but competitively antagonized the presynaptic inhibition induced by methacholine. The KB under the latter condition (0.95 microM) was very close to the EC50 value determined at 3 Hz (0.85 microM). 6. The results demonstrate a muscarinic inhibition of noradrenaline release from the rabbit isolated iris. The activation by pilocarpine of the presynaptic receptors provides an alternative explanation for the miosis induced in the rabbit in vivo, which might be the result of a decreased sympathetic tone in the iris dilator muscle.

On the opioid receptor subtype inhibiting the evoked release of 3H-noradrenaline from guinea-pig atria in vitro

Guinea-pig isolated atria were incubated and loaded with 3H-(-)-noradrenaline. The intrinsic nerves were stimulated with trains of 5 or 35 field pulses (4 Hz), and the evoked efflux of 3H-noradrenaline and of total tritium was determined in the presence of atropine, corticosterone, desipramine, and phentolamine by liquid scintillation spectrometry. Ethylketocyclazocine (1.4 nmol/l, IC50), MR 2033 (9.1 nmol/l), dynorphin A (1-13) (25 nmol/l, peptidase inhibitors present), etorphine (71 nmol/l), and [D-Ala2, D-Leu5]-enkephalin (greater than 10 mumol/l, peptidase inhibitors present) inhibited the stimulation-evoked efflux of 3H-noradrenaline in a concentration-dependent manner, but not morphine up to 10 mumol/l. The inhibition by ethylketocyclazocine, MR 2033, and etorphine was antagonized by naloxone 1 mumol/l. Similarly, the MR 2033 effect was antagonized by SKF 10047 1 mumol/l. All antagonists investigated failed to affect the evoked 3H-noradrenaline efflux when present in the absence of exogenous agonists. Arunlakshana-Schild plots were calculated for the antagonism between ethylketocyclazocine and a pair of stereoisomers, (-)-MR 2266 (20 nmol/l-5 mumol/l) and (+)-MR 2267 (0.3-10 mumol/l) at the presynaptic opioid receptor, and pA2 values were estimated. The isomeric affinity ratio was 60, with pA2 values of (-)-MR 2266, 9.06, and (+)-MR 2267, 7.28, respectively. The results show that the 3H-noradrenaline release can be inhibited via activation of presynaptic opioid receptors. Under the conditions presently investigated endogenous opioids do not modulate the evoked transmitter release. The results favour the idea that a single population (presumably of the kappa-subtype) of opioid receptors is present at guinea-pig atrial noradrenergic nerves.