Adrenergic stimulation of ciliary process epithelium causes surface membrane internalization

An ultrastructural change induced in the nonpigmented epithelium (NPE) of the ciliary processes by adrenergic stimulation in the albino rabbit was studied. Thirty min after topical treatment with 2% isoproterenol, an extensive intracellular membranous network, previously reported to be smooth endoplasmic reticulum, was revealed by electron microscopy. It was postulated that this network originated from the plasma membrane. Using cationized ferritin (CF) as an ultrastructural tracer, freshly isolated anterior segments were incubated in buffer containing 10^-5 M isoproterenol and 0.2% CF. As early as 10 min, and for at least 30 min, the isoproterenol-treated NPE cells contained a membranous network that was morphologically similar to that which occurs in vivo. CF particles were present within the network, indicating that the membranous network had originated at the cell surface. This labeling was prevented by pretreatment with the β-adrenergic antagonist timolol maleate. In both treated and control ciliary processes, CF was present in the ciliary canals between the NPE and the underlying pigmented epithelium after 10 min incubation. This suggests that the NPE is able to transport CF from its basilar to apical surface. These experiments imply that the NPE is able to internalize rapidly large amounts of plasma membrane in response to adrenergic stimulation. This response may be part of the mechanism of adrenergic receptor desensitization, alteration of aqueous humor production, or another adrenergic response.