Intracellular calcium release mediated by noradrenaline and acetylcholine in mammalian pineal cells

Abstract: The effects of noradrenergic and cholinergic receptor agonists on intracellular Ca²⁺ concentration ([Ca²⁺]_i) in single dissociated rat pineal cells were investigated by microfluorimetric measurements in Fura-2 acetoxymethyl ester (Fura-2/AM) loaded cells. Noradrenaline (NA) evoked characteristic biphasic increments of intracellular Ca²⁺ consisting of one or more leading spikes followed by a plateau, resulting from the release of Ca²⁺ from intracellular stores and from the influx of Ca²⁺ from the external medium, respectively. This response was reproduced by the α₁ adrenoceptor agonist, phenylephrine (PE), in the presence of the β-adrenoceptor antagonist, propranolol, and was abolished when NA or PE was applied in conjunction with the α₁-adrenoceptor antagonist, prazosin. The curve relating the peak amplitude of the Ca²⁺ increments to different PE concentrations (0.5–10 μM) showed a half-maximum response at 0.6 μM PE, and saturation at concentrations greater than 2 μM. 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 μM ACh. The muscarinic cholinergic receptor agonist, carbachol (CCh), similarly activated Ca²⁺ increments while the muscarinic antagonist, atropine, abolished them. In the absence of extracellular Ca²⁺, repetitive stimuli with either α₁-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 preclude a response to adrenergic agonists, while the contrary was not true. These results suggest that these agonists liberate Ca²⁺ from two functionally distinct, caffeine-insensitive, Ca²⁺ intracellular stores.