Dimethylphenylpiperazinium iodide (a nicotinic agonist) evokes noradrenaline release from human neuroblastoma SH-SY5Y cells that have been pretreated with 12-O-tetradecanoylphorbol 13-acetate for 8 min. This effect of dimethylphenylpiperazinium iodide was inhibited by 1 microM mecamylamine but not by 1 microM atropine, which suggests that SH-SY5Y cells express nicotinic receptors coupled to the release of noradrenaline. Dimethylphenylpiperazinium iodide-evoked release was enhanced by 5 microM Bay K 8644 (an L-type calcium agonist) and inhibited by 1 microM nifedipine. Dimethylphenylpiperazinium iodide depolarised SH-SY5Y cells and enhanced the level of intracellular calcium in cells loaded with fura 2. The effects of dimethylphenylpiperazinium iodide on noradrenaline release, depolarisation, and intracellular calcium levels were all inhibited by 1 microM desmethylimipramine. The results of this study show that nicotinic receptors in SH-SY5Y cells stimulate noradrenaline release by activation of L-type calcium channels.

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SUMMARY1. To determine whether stimulation of atrial receptors reflexly affects coronary blood flow, experiments were performed in seven dogs anaesthetized with achloralose. The left upper and middle pulmonary vein-atrial junctions and atrial appendage were stretched by distension of small balloons. Coronary blood flow was measured using an electromagnetic flowmeter positioned around the origin of the left anterior descending or circumflex coronary artery.2. Following prevention of the reflex increase in heart rate by atrial pacing or propranolol, distension of the small balloons was shown to result in a decrease in mean coronary blood flow.3. Cooling of the cervical vagosympathetic nerve trunks to 9°C abolished the response of a decrease in mean coronary blood flow to distension of the balloons.4. The response of a decrease in mean coronary blood flow was abolished by bretylium tosylate.5. It was shown that a decrease in mean coronary blood flow occurred in response to stimulation of left atrial receptors, and that this decrease was reflex in nature. This reflex response is likely to involve atrial receptors discharging into afferent myelinated vagal fibres, and the efferent limb involves cardiac sympathetic pathways.

Replacement of Ca2+ with Ba2+ in HEPES-buffered saline stimulated [3H]noradrenalin release in the human neuroblastoma clone SH-SY5Y by up to 20% of the cell content in the absence of other secretory stimuli. The Ba(2+)-evoked release was inhibited by 85% by 3 microM tetrodotoxin and 95% by 5 microM nifedipine. Ba2+ also increased the potency of K(+)-evoked release of [3H]noradrenalin, as maximal release was observed with 60 mM K+ compared with the 100 mM K+ necessary to achieve maximal release in the presence of Ca2+. In contrast, replacing Ca2+ with Ba2+ had little effect on carbachol- and bradykinin-evoked release of [3H]noradrenalin. No evidence was obtained from studies on changes in [Ca2+]i (in response to 100 microM carbachol) using fura-2 that Ba2+ could enter intracellular stores in SH-SY5Y cells. Whole-cell patch-clamp studies showed that Ba2+ depolarizes SH-SY5Y cells as well as enhancing inward Ca2+ channel currents and shifting their voltage dependence to more negative values. These results are discussed in terms of the hypothesis that Ba2+ blocks K+ channels, leading to depolarization followed by opening of voltage-sensitive Na+ channels. This in turn opens voltage-sensitive L-type Ca2+ channels, which are coupled to the release of [3H]noradrenalin in SH-SY5Y cells.

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