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 ∼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 μmol/l (0.28–0.51 μmol/1, 95% C.L.). Based on EC50 values, the rank order of potency of cholinergic agonists is as follows: Muscarine = carbachol, 1 > pilocarpine, 115 > 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 μmol/1, physostigmine consistently contracted isolated irides. The mean EC50 value was 6.73 μmol/1. The effect was sensitive to blockade by atropine.
When the temperature of the bathing medium was lowered from 37.5°C to 27.5°C or 17.5°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.
At 37.5°C, muscarinic receptor related dissociation constants (KB) of atropine, atropine methylbromide and cyclopentolate were 2.2, 0.37 and 8 nmol/l, respectively. Thus, in spite of many variables (such as hypoxia, temperature, iridic pigment, age, and drug diffusion) which could modify the quantitation of the response, the muscarinic receptor related dissociation constants of atropine in human iris appear to be similar to those reported in other mammalian species. Therefore, the species specific sensitivities of the iris-sphincter muscle to cholinergic stimulants may be attributed to post-receptor molecular events.