INTRACELLULAR CALCIUM-RELEASE MEDIATED BY NORADRENALINE AND ACETYLCHOLINE IN MAMMALIAN PINEAL CELLS

The effects of noradrenergic and cholinergic receptor agonists on intr acellular Ca2+ concentration ([Ca2+](i)) in single dissociated rat pin eal cells were investigated by microfluorimetric measurements in Fura- 2 acetoxymethyl ester (Fura-2/AM) loaded cells. Noradrenaline (NA) evo ked characteristic biphasic increments of intracellular Ca2+ consistin g of one or more leading spikes followed by a plateau, resulting from the release of Ca2+ from intracellular stores and from the influx of C a2+ from the external medium, respectively. This response was reproduc ed by the alpha(1)-adrenoceptor agonist, phenylephrine (PE), in the pr esence of the beta-adrenoceptor antagonist, propranolol, and was aboli shed when NA or PE was applied in conjunction with the alpha(1)-adreno ceptor antagonist, prazosin. The curve relating the peak amplitude of the Ca2+ increments to different PE concentrations (0.5-10 mu M) showe d a half-maximum response at 0.6 mu M PE, and saturation at concentrat ions greater than 2 mu M. Acetylcholine (ACh) also elicited transient Ca2+ increments consisting of an abrupt rise to a maximum value which decayed exponentially to the basal Ca2+ level. A half-maximum response was achieved at 59 mu M ACh. The muscarinic cholinergic receptor agon ist, carbachol (CCh), similarly activated Ca2+ increments while the mu scarinic antagonist, atropine, abolished them. In the absence of extra cellular Ca2+, repetitive stimuli with either alpha(1)-adrenergic and muscarinic agonists produced a progressive decrement in the amplitude of the Ca2+ signals because of the depletion of intracellular stores. However, extinction of the response to muscarinic agonists did not pre clude a response to adrenergic agonists, while the contrary was not tr ue. These results suggest that these agonists liberate Ca2+ from two f unctionally distinct, caffeine-insensitive, Ca2+ intracellular stores.