SYNERGISTIC EFFECT OF ADRENERGIC AND MUSCARINIC RECEPTOR ACTIVATION ON [CA2+](I) IN RABBIT CILIARY BODY EPITHELIUM

1. Changes in cytosolic free calcium concentration ([Ca2+](i)) in response to cholinergic and adrenergic agents alone and in combination were investigated using fura-2 fluorescence imaging in intact non-pigmented epithelial cells of rabbit ciliary body. 2. Resting ('baseline') [Ca2+](i) was 147 +/- 6 nM (mean +/- S.E.M.). Acetylcholine (ACh, 10 mu M) doubled [Ca2+](i), and adrenaline (1 mu M) increased it by about 36 %. When ACh (10 mu M) and adrenaline (1 mu M) were applied together [Ca2+](i) was transiently increased to 1160 +/- 160 nM, about 7 times the response induced by ACh alone. 3. Noradrenaline and 5-bromo-6-(2-imidazolin-2-yl-amino)-quinoxaline (UK 14304) had effects similar to adrenaline in enhancing the response to ACh. Phenylephrine (Phe) had a relatively smaller effect and none was observed for methoxamine and isoprenaline (Iso). 4. The response to ACh and adrenaline could be blocked by atropine (1 mu M, 87 +/- 5 %), yohimbine (1 mu M, 73 +/- 8%), and to a lesser degree by prazosin (1 mu M). Propranolol had no effect. 5. Lowering the extracellular calcium concentration to 3 nM dropped the baseline [Ca2+](i) by half and reduced the response to ACh and adrenaline to a small and transient rise in [Ca2+](i). Addition of La3+ to Ca2+-containing solution also lowered [Ca2+](i) and largely reduced the response. 6. We conclude that simultaneous activation of muscarinic and alpha(2)-adrenergic receptors induces a large increase in [Ca2+](i), which is the result of both Ca2+ release and influx.