title: Cyclic nucleotide-dependent regulation of agonist-induced calcium increases in mouse megakaryocytes.
abstract: 1. The regulatory effects of cyclic AMP and cyclic GMP on ADP- and thrombin-induced increases in [Ca2+]i were studied in mouse bone marrow megakaryocytes. Changes in [Ca2+]i were continuously monitored in single Fura-2-loaded cells using microspectrofluorometry, and cyclic nucleotides were directly introduced into the single cells using the whole-cell patch-clamp technique. 2. ADP increased [Ca2+]i in a concentration-dependent fashion, and its threshold concentration was in the order of 0.01 microM. A low dose of ADP (below 0.1 microM) induced a transient response of [Ca2+]i which recovered to original levels during the stimulation. A high dose of ADP (0.3-10 microM) induced a biphasic response of [Ca2+]i with an initial peak and a plateau lasting until the end of the stimulation. Repeated stimulation with the same dose of ADP induced a reduced response, probably as a result of desensitization. 3. Thrombin increased [Ca2+]i in a concentration-dependent manner. The time courses of the responses were different from those caused by ADP. Thrombin-induced responses lacked the initial sharp peak observed in ADP-induced responses, and caused a sustained response. 4. The ADP-induced increase in [Ca2+]i was antagonized by the presence of prostaglandin E1 (PGE1, 100-1000 nM), in the medium, and by direct injection of cyclic AMP (100-500 microM) or cyclic GMP (500 microM) into the megakaryocyte. When 500 microM-cyclic AMP was injected into the cells, the rise of [Ca2+]i induced by ADP was reduced by 85%. Effects of these antagonists were inhibited by treatment with a protein kinase inhibitor, H-8. Thrombin-induced increases in [Ca2+]i were reduced by direct injection of cyclic AMP or cyclic GMP. 5. ADP could induce an increase in [Ca2+]i in the absence of external Ca2+. The time course of the response was essentially similar to that observed in the normal condition (1 mM-CaCl2), but the size of the response was reduced by 33%. Thus, 67% of the rise in [Ca2+]i induced by ADP could be accounted for by calcium mobilization from internal storage pools. The presence of NiCl2 (5 mM) duplicated the effects of external Ca2+ removal, suggesting the involvement of a Ca2+ influx pathway, which could be inhibited by Ni2+ in ADP stimulation. 6. Injection of cyclic AMP or cyclic GMP reduced ADP-induced increases in [Ca2+]i under conditions of inhibited Ca2+ influx by NiCl2 (5 mM).(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Physiology, Jichi Medical School, Tochigi-ken, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Ikeda
authors.forename: K
authors.initials: K
authors.lastname: Kurokawa
authors.forename: Y
authors.initials: Y
authors.lastname: Maruyama
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D001853:Bone Marrow / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism*; D006152:Cyclic GMP / Q000378:metabolism*; D015232:Dinoprostone / Q000378:metabolism; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins; D007546:Isoquinolines / Q000378:metabolism; D008533:Megakaryocytes / Q000378:metabolism*; D051379:Mice; D047428:Protein Kinase Inhibitors
chemical_list: D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; D007546:Isoquinolines; D047428:Protein Kinase Inhibitors; C015482:protein kinase modulator; D000244:Adenosine Diphosphate; C053177:N-(2-(methylamino)ethyl)-5-isoquinolinesulfonamide; D000242:Cyclic AMP; D006152:Cyclic GMP; D015232:Dinoprostone; D002118:Calcium