SYNERGISTIC RISE IN CA2-CELLS( PRODUCED BY SOMATOSTATIN AND ACETYLCHOLINE IN CILIARY BODY EPITHELIAL)

The purpose of these experiments was to demonstrate the presence of so matostatin receptors on the nonpigmented epithelial cells of the rabbi t ciliary body and their link with intracellular Ca2+ homeostasis. Fre shly excised rabbit ciliary processes and nonpigmented cell layer expl ants were loaded with the fluorescent dye fura-2, and free-Ca2+ concen tration ([Ca2+](1)) in the nonpigmented cells was measured with fluore scence ratio imaging. The cells were continuously perfused, and drugs were added to the perfusate. Somatostatin-14 (SS14, 0.1-1.0 mu M) or a cetylcholine (ACh, 10 mu M) applied alone produced small increases in [Ca2+](1). However, SS14 (0.1 mu M) in combination with ACh (10 mu M) induced a massive increase in [Ca2+](i) (25.7 +/- 3.3 times the baseli ne level, n = 28). The dose-response curve for SS14 (in the presence o f 10 mu M. ACh) was sigmoidal with an EC50 of 3.9 nM and Hill coeffici ent of 2.5, indicating the requirement for multiple SS receptor activa tion. Somatostatin-28 could mimic the effect of SS14, although a much higher concentration was required. Shifting the SS14 dose-response cur ve 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 alpha(2)-adrenergic blocker yohimbine (Yoh, 10 mu M) or the A(1)-speci fic adenosinergic antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX , 1 mu M). Incubation of the tissue with pertussis toxin (PTx, 1 mu g ml(-1)) did not alter the response to ACh alone but eliminated the syn ergistic effect of somatostatin. We conclude that nonpigmented epithel ial cells of the rabbit ciliary body possess a novel somatostatin rece ptor whose activation can synergistically potentiate the rise in [Ca2](i) produced by ACh. This potentiation appears to occur via a pertuss is-toxin-sensitive pathway, perhaps through G(i). (C) 1997 Academic Pr ess Limited.