Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 92

The rate of aqueous formation by the ciliary epithelium is influenced by sympathetic and parasympathetic innervation as well as humoral mechanisms to maintain a steady‐state IOP. Adrenergic regulation of AH formation is complex and the role of some receptor subtypes remains unclear. The β‐adrenergic antagonists, such as timolol, lower IOP by decreasing AH production. During sleep, AH formation decreases ~50% via modulation of the β‐arrestin/cAMP signaling pathway by β‐adrenergic receptors in humans and reducing the ocular hypotensive effects of timolol when instilled at night. Thus, IOP exhibits a circadian rhythm, which varies depending on whether animals are nocturnal or diurnal. For example, diurnal species such as dogs and nonhuman primates exhibit the greatest IOP during the early day, while in nocturnal species such as cats and rats IOP peaks at night.

Cholinergic regulation of AH formation and composition is similarly ambiguous. For example, parasympathomimetic nerve stimulation or drugs have been demonstrated to increase, decrease, or not change the rate of AH production; these differences are likely due to species and technique‐related effects. Cholinergic agents may regulate amino acid transport from the blood to the AH as well as modulate inorganic ion concentrations within the AH. In aggregate, these studies suggest that the influence of parasympathetic drugs such as pilocarpine is relatively minor in AH formation and that their efficacy in decreasing IOP is likely due to increased AH outflow.