Desensitization and recovery of muscarinic and histaminergic Ca2+ mobilization in 1321N1 astrocytoma cells.
Intracellular free Ca2+ was monitored in suspensions of 1321N1 astrocytoma cells by using the Ca2+ indicator fura-2. The cytoplasmic Ca2+ concentration increased from 237 +/- 6 nM to 1580 +/- 170 nM within 3-5 s of addition of 300 microM-carbachol. After the peak in response, the Ca2+ concentration diminished, establishing a new steady state in about 1 min that was approx. 150 nM above the previous baseline. Histamine increased cytoplasmic Ca2+ to about 40% of the maximal value seen with carbachol. In Ca2+-free buffer each agonist elicited a normal initial increase in cytoplasmic Ca2+, but the sustained portion of the response was abolished. The increase in Ca2+ in response to either carbachol or histamine could be completely inhibited by pretreating the cells with carbachol; the response to carbachol could be partially inhibited by pretreating the cells with histamine. The Ca2+ responses did not recover in the continued presence of carbachol. However, if the carbachol was washed out or if atropine was added after carbachol, the responses to agonist recovered in a time-dependent manner (half-time 3-4 min), and recovery depended on the presence of extracellular calcium. The results indicate that carbachol and histamine stimulate release of Ca2+ from the same intracellular Ca2+ store, that depletion of this store is responsible for heterologous desensitization between these two agonists, and that repletion of the agonist-sensitive Ca2+ pool does not occur in the continued presence of agonist or in the absence of extracellular Ca2+.
McDonough
P M
PM
Division of Pharmacology, University of California, San Diego, La Jolla 92093.
Eubanks
J H
JH
Brown
J H
JH
D001254:Astrocytoma / Q000378:metabolism*; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology*; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives*; D006632:Histamine / Q000494:pharmacology*; D007424:Intracellular Fluid / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
D001572:Benzofurans; D005456:Fluorescent Dyes; C049925:fura-2-am; D006632:Histamine; D002217:Carbachol; D002118:Calcium; D016257:Fura-2