In Vivo Electrical Stimulation of the Sympathetic Nerve of the Eye Increases Inositol Phosphate Production and Prostaglandin Release in the Rabbit Iris Muscle

The effects of in vivo electrical stimulation of the sympathetic nerve of the eye on phosphatidylinositol 4,5‐bisphosphate (PIP2) hydrolysis in rabbit iris and release of arachidonate and prostaglandin (PG) E2 into aqueous humor were investigated. myo‐[3H]Inositol or [l‐14C]ara‐chidonate was injected intracamerally into each eye 3 h before electrical stimulation of one of the sympathetic trunks. Tissue phosphoinositides were determined by TLC, and 3H‐Iabeled inositol phosphates were analyzed by either ion‐exchange chromatography or HPLC. The aqueous humor was analyzed for 14C‐labeled arachidonate and PGE2 by radiochromatography and for unlabeled PGE2 by radioimmunoassay. The results obtained from this study can be summarized as follows: (a) The rates of in vivo incorporation of myo‐[3H]inositol into phosphoinositides and accumulation of 3H‐labeled inositol phosphates in the iris muscle increased with time and then leveled off between 3 and 5 h. (b) Distribution of 3H radioactivity in inositol phosphates, as determined by HPLC, showed that of the total radioactivity in inositol phosphates, 53.6% was recovered in myo‐inositol 1‐phosphate. 36% in myo‐inosi‐tol bisphosphate, 0.95% in myo‐inositol 1,3,4‐trisphosphate (1,3,4‐IP3), and 2.6% in 1,4,5‐IP3. (c) Electrical stimulation of the sympathetic nerve resulted in a significant loss of 3H radioactivity from PIP2 and a concomitant increase of that in IP3, an observation indicating that PIP2 is the physiological substrate for α1‐adrenergic receptors in this tissue, (d) Release of IP3 and liberation of arachidonate for PGE2 synthesis are dependent on the duration of stimulation and the intensity (voltage) of stimulus. We conclude that IP3 accumulation and arachidonate release are produced in the iris muscle under in vivo conditions that produce contraction, i.e., electrical stimulation of the sympathetic nerve. These observations add further support to the concept that PIP2 and its derived second messengers are involved in excitation‐contraction coupling in the smooth muscle cells of the iris.