Pharmacological characterisation of muscarinic receptors coupled to calcium mobilisation in human lens cells

Purpose: To identify the muscarinic receptor subtype coupled to calcium mobilisation in human lens cells. Methods: The immortalised human tens epithelial cell line, B3, was used in this study. Cells plated on a glass coverslip were loaded with the calcium-sensitive dye Fura2 and fluorescence from a patch of 10 confluent cells captured with a photomultiplier. Computation of the 340 nm / 375 nm ratio permitted the intracellular level of calcium to be monitored in real time. Results: Acetylcholine induced a biphasic increase in intracellular calcium with a Km of ≈ 1 μM the peak amplitude. Muscarinic receptor antagonists inhibited the response to ACh (1 μM) and inhibition curves for a range of muscarinic receptor antagonists were constructed. A pK_b value for each antagonist was calculated from the observed IC₅₀. For 4-DAMP, zemifenacin and pirenzepine the pK_b values were 8.9, 7.9 and 6.7 respectively. Conclusions: The rank order of potency 4-DAMP > zamifenacin > pirenzepine is consistent with the functional coupling of M3 receptois to calcium mobilisation in human lens cells. This receptor sub-type is expressed in other eye tissues and is a potential target for anti-glaucoma therapy, for example. The above data show that possible lens complications should be taken into account when designing such therapy.