Abstract
The purpose of these experiments was to demonstrate the presence of somatostatin receptors on the nonpigmented epithelial cells of the rabbit ciliary body and their link with intracellular Ca2+homeostasis. Freshly excised rabbit ciliary processes and nonpigmented cell layer explants were loaded with the fluorescent dye fura-2, and free-Ca2+concentration ([Ca2+]i) in the nonpigmented cells was measured with fluorescence ratio imaging. The cells were continuously perfused, and drugs were added to the perfusate. Somatostatin-14 (SS14, 0.1–1.0 μm) or acetylcholine (ACh, 10 μm) applied alone produced small increases in [Ca2+]i. However, SS14 (0.1 μm) in combination with ACh (10 μm) induced a massive increase in [Ca2+]i(25.7±3.3 times the baseline level,n=28). The dose-response curve for SS14 (in the presence of 10 μmACh) was sigmoidal with an EC50of 3.9 nmand Hill coefficient of 2.5, indicating the requirement for multiple SS receptor activation. Somatostatin-28 could mimic the effect of SS14, although a much higher concentration was required. Shifting the SS14 dose–response curve to the right by about two-orders of magnitude resulted in a fit to the SS28 data. The response to ACh+SS14 could not be blocked by the α2-adrenergic blocker yohimbine (Yoh, 10 μm) or the A1-specific adenosinergic antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 μm). Incubation of the tissue with pertussis toxin (PTx, 1 μg ml−1) did not alter the response to ACh alone but eliminated the synergistic effect of somatostatin. We conclude that nonpigmented epithelial cells of the rabbit ciliary body possess a novel somatostatin receptor whose activation can synergistically potentiate the rise in [Ca2+]iproduced by ACh. This potentiation appears to occur via a pertussis-toxin-sensitive pathway, perhaps through Gi.
Published Version
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