M 3-Type Muscarinic Receptors Predominantly Mediate Neurogenic Quick Contraction of Bovine Ciliary Muscle

Abstract

1. The present experiments were designed to investigate which subtypes of muscarinic receptors are involved in the neurogenic quick contraction of bovine ciliary muscle in connection to quick eye focal accommodation. 2. Transmural electrical stimulation (TES) produced a transient contraction, which was abolished in the presence of 3×10 −7 M tetrodotoxin and 10 −6 M atropine, but greatly augmented by 3×10 −7 M physostigmine. 3. The exogenously applied acetylcholine (ACh: 10 −9 to 3×10 −6 M) produced a concentration-dependent contraction, which was competitively antagonized by 10 −6 M atropine and augmented by 3×10 −7 M physostigmine, but unaffected by 3×10 −7 M tetrodotoxin. 4. The magnitude and time to peak of the maximal contraction produced by TES were significantly greater (1267.5±86.0 mg, P<0.005) and shorter (9.0±0.2 sec, P<0.005) than corresponding values (97.0±9.9 mg and 20.3±2.1 sec, respectively) of the phasic contraction caused by exogenously applied 10 −5 M ACh, at which concentration the agonist caused the maximal contraction. The velocity (140.6±7.8 mg/sec) of the transient contraction caused by TES was approximately 28-fold greater than that of the phasic contraction caused by ACh (5.1±0.9 mg/sec). 5. The contractions produced by TES were greatly attenuated by 4-diphenylacetoxy- N-methylpiperidine (4-DAMP) as an M 3 antagonist and slightly by pirenzepine as an M 1 antagonist (20.2±7.9% inhibition at the highest concentration), but not by methoctramine (MET) as an M 2 antagonist. The IC 50 value (−log M) for 4-DAMP was determined to be 7.17±0.14. 6. Scatchard plot analysis of [ 3H]-quinuclidinylbenzilate (QNB) binding revealed that the binding sites constituted a single population with a K d of 31.2±0.8 pM and a B max of 895.5±93.2 fmol/mg protein. The activity in inhibiting [ 3H]-QNB binding was most potent with 4-DAMP (-log K i=7.98±0.02), but less potent with pirenzepine (−log K i=6.43±0.04) and MET (−log K i=7.32±0.16). 4-DAMP was approximately 35- and 5-fold more potent than pirenzepine and MET in terms of −log K i values, respectively, suggesting the predominant localization of M 3 receptor subtypes in the bovine ciliary muscle membrane. 7. These results suggest that TES produces a neurogenic quick contraction of the bovine ciliary muscle, which would be mediated mainly by ACh released from the intramural nerve terminals and subsequent excitation of M 3 receptor subtypes localized on the ciliary muscle cells, and that neurogenic quick contraction of the ciliary muscle is possibly involved in part in eye focal accommodation.