Comparative adrenocholinergic control of intracellular Ca2+ in the layers of the ciliary body epithelium

Both nonpigmented epithelia (NPE) and pigmented epithelia (PE) of the ciliary body are thought to participate in the formation of aqueous humor and its pharmacologic regulation. The aim of this study was to identify the similarities and differences in intracellular Ca2+ ([Ca2+]i) changes in each of the two layers in response to adrenergic and cholinergic inputs.

Incubation of the Dutch belted rabbit ciliary body (CB) in low-[Ca2+] solution was used to induce the functional dissociation of the NPE and PE layers from each other. These layers or the intact undissociated CB were loaded with the fluorescent Ca2+ indicator fura-2 and mounted in superfusion chambers for fluorometric measurement of [Ca2+]i.

In the NPE of the intact CB epithelium or in the isolated NPE, 10 microM acetylcholine (ACh), 1 microM brimonidine (UK 14304), or 1 microM epinephrine each elicited minimal rises in [Ca2+]i. On the other hand, the combination of either adrenergic drug with ACh resulted in large mobilizations of this cation. The alpha 1-adrenergic agonist phenylephrine was unable to induce Ca2+ mobilization in the isolated NPE and failed to do so in 10 of 12 intact CB specimens. In the isolated PE, both epinephrine and phenylephrine elicited substantial similar [Ca2+]i increases, ACh induced a smaller and slower rise, and the response to its combination with either adrenergic drug was essentially additive. UK 14304 (+/- ACh) had no measurable effect in these cells.

Each layer of the CBE exhibits distinct alpha-adrenergic control mechanisms. The NPE contains an alpha 2-adrenergic mechanism highly dependent on the cholinergic tone. The PE, in contrast, contains an alpha 1-adrenergic pathway that operates independently of cholinergic input. This segregation of mechanisms provides a basis for highly complex regulatory responses in the intact, syncytially organized ciliary body epithelium.