The affinity of atropine for muscarine receptors in human sphincter pupillae

Reactivity of human iris-sphincter to muscarinic drugs in vitro

From tissue to tissue the contractile response of human irides to carbachol varied by 40 fold. The mean EC50 value of carbachol in tissues remaining in an in vitro environment for 24-37 h was equal, however, to that obtained from tissues examined during 79-161 h. The maximum response of the tissue to the highest concentration of carbachol increased up to 24 h, then a gradual decline in the maximum occurred. In 38 observations, the average decline after 72 h was approximately 30%. A plot of negative log EC50 values (n = 38) of carbachol exhibited normal Gaussian curve. The geometric mean EC50 value of carbachol was 0.38 mumol/l (0.28-0.51 mumol/l, 95% C.L.). Based on EC50 values, the rank order of potency of cholinergic agonists is as follows: Muscarine = carbachol, 1 > pilocarpine, 1/5 > methacholine, 1/23 > bethanechol, 1/29 > acetylcholine, 1/1310. The percent maximum contraction of irides to muscarine, carbachol, pilocarpine, methacholine and bethanechol were 100, 100, 80, 76 and 95, respectively. Acetylcholine at the highest concentration tested produced 71% of the maximum produced by carbachol. Within a concentration range of 1 to 100 mumol/l, physostigmine consistently contracted isolated irides. The mean EC50 value was 6.73 mumol/l. The effect was sensitive to blockade by atropine. When the temperature of the bathing medium was lowered from 37.5 degrees C to 27.5 degrees C or 17.5 degrees C the magnitude and the duration of the response of the iris to carbachol was increased, the EC50 value, however, was not changed significantly. The response to pilocarpine was similarly altered by the lower temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Complex dose-response curves of atropine in man explained by different functions of M1- and M2-cholinoceptors

In the present study we set out to explain the complex atropine dose-response curves in man in relation to M-cholinoceptor subtype occupancy. In healthy volunteers the effects of atropine on heart rate and salivary flow were quantified. M-cholinoceptor subtype occupancy by antagonist present in plasma samples was detected in an in vitro radioreceptor assay. Atropine effects were studied without and after propranolol (240 mg oral dose) and without and after pirenzepine (1.1 mg i.v.) to differentiate beta-adrenoceptor and M-cholinoceptor subtype mediated effects. 1. In receptor binding studies, M-cholinoceptors in bovine cerebral cortex membranes were labelled with 3H-pirenzepine (pKd = 8.05), M-cholinoceptors in rat salivary gland membranes with 3H-N-methylscopolamine (pKd = 9.02). Atropine competed for binding of these ligands with a small (2.1-fold) preferential selectivity via the cerebral in comparison to the glandular receptors (pKi = 9.18 versus 8.86). Pirenzepine showed a marked selectivity (40-fold) in this respect with pKi-values of 8.05 (M1: cerebral cortex) and 6.45 (M2: salivary glands). 2. At heart rate and at salivary flow, bivalent dose-response curves of atropine were observed with opposite effect vectors. The typical antagonist effects at M-cholinoceptors (i.e. an increase of heart rate and an inhibition of salivary flow) were observed at doses greater than 1 microgram/kg, whereas "paradoxical" cholinomimetic effects of atropine became apparent at lower doses. From a superposition of two isotherms with opposite effect vectors ED50-values were calculated, which were in the range of half-maximal M-cholinoceptor occupancy in the in vitro radioreceptor assay of plasma samples.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Receptor-responses in fresh human ciliary muscle

The physiological and pharmacological properties of ciliary muscle isolated from fresh human eyes were investigated. The muscle exhibited no spontaneous activity. Concentration-dependent contractions in response to carbachol were competitively antagonized by atropine (pA2 = 8.95). The muscle, precontracted by carbachol (2.7 X 10(-4)M), responded to the application of isoprenaline by concentration-dependent relaxation blocked by propranolol (3.5 X 10(-9)M to 3.5 X 10(-8)M; pA2 = 9.15). Angiotensin-evoked contractions were antagonized by 8-Ala-angiotensin II (4.5 X 10(-8)M) in a competitive manner, but were not inhibited by phentolamine or propranolol. Contractions generated by electrical stimulation of the muscle (30 ms, 20 Hz, 60 pulses) were antagonized by atropine (10(-7) M) and tetrodotoxin (6.3 X 10(-7) M). Phentolamine and propranolol did not influence these responses. An increase of the external potassium concentration ([K+]o) from 5.4 to 158.8 mM produced a mechanical response, antagonized by atropine, but not influenced by tetrodotoxin, phentolamine or propranolol. The human ciliary muscle appears to carry muscarinic and angiotensin receptors and beta 2-adrenoceptors. The estimate of Katropine for muscarinic receptors mediating carbachol-induced contractions agrees with estimates of Katropine reported for human and rabbit iris.