Temperature sensitivity of cyclic AMP production and catecholamine-induced refractoriness in a rat astrocytoma cell line

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Abstract
Intracellular cyclic[c]AMP was increased more than 100-fold when rat C6-2B astrocytoma cells were treated with isoproterenol in the cold (4.degree. C). When the cells were treated with the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, and isoproterenol, cAMP levels rose more than 150-fold. The levels achieved compared favorably with the 250-fold increase in cAMP produced by (-)isoproterenol at 37.degree. C. (-)Isoproterenol at 5 nM stimulated half-maximal cAMP production at 4.degree. C and at 37.degree. C and was blocked by (-)propranolol at both temperatures. The concentrations of cAMP attained by these cells after (-)isoproterenol stimulation in the cold may be accounted for, in part, by alterations in the efflux of the nucleotide from the cells since extracellular cAMP, an indicator of cAMP efflux, was found to be dramatically reduced in the cold. The cells, when exposed to (-)isoproterenol for up to 6 h at low temperature, maintained normal responsiveness to this agent when rechallenged at 4.degree. or 37.degree. C. They did not display agonist-induced refractoriness during that period of exposure at 4.degree. C, although refractoriness is always seen within 90 min at 37.degree. C. Refractoriness, once established by (-)isoproterenol treatment at 37.degree. C, was not reversed by exposure of the cells to cold. These data suggest that the development of catecholamine refractoriness requires a temperature-sensitive step that lies distal to the hormone-receptor interaction and cAMP generation.

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