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Muscarinic inhibition of [ 3 H]‐noradrenaline release on rabbit iris in vitro : effects of stimulation conditions on intrinsic activity of methacholine and pilocarpine
Author(s) -
Bognar Irene T.,
Pallas Sylvia,
Fuder Hermann,
Muscholl Erich
Publication year - 1988
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1988.tb11601.x
Subject(s) - methacholine , muscarinic acetylcholine receptor , atropine , stimulation , pilocarpine , acetylcholine , endocrinology , chemistry , medicine , physostigmine , biology , receptor , neuroscience , respiratory disease , lung , epilepsy
1 Rabbit isolated irides were loaded with [ 3 H]‐noradrenaline and superfused with Tyrode solution. The inhibition by the muscarinic agonists (±)‐methacholine and pilocarpine of the [ 3 H]‐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 [ 3 H]‐noradrenaline release depending on the stimulation conditions. 3 The evoked [ 3 H]‐noradrenaline overflow was depressed by (±)‐methacholine in a concentration‐dependent manner. The EC 50 ranged from 0.29 to 0.42 μ m . 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 μ m . After washout of methacholine the evoked [ 3 H]‐noradrenaline release was temporarily enhanced. 4 Atropine 0.1 μ m enhanced the [ 3 H]‐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 μ m (apparent ‐log K B = 8.5–9.0). 5 Depending on the concentration, pilocarpine reduced the [ 3 H]‐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 K B under the latter condition (0.95 μ m ) was very close to the EC 50 value determined at 3 Hz (0.85 μ m ). 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.