Gluconeogenesis stimulated by extracellular ATP is triggered by the initial increase in the intracellular Ca2+ concentration of the periphery of hepatocytes.
Extracellular ATP, ADP and GTP increased the intracellular free Ca2+ concentration ([Ca2+]i) in a suspension of isolated rat hepatocytes. The [Ca2+]i was determined by measuring fura-2 fluorescence, and its increase was biphasic. The initial transient rise was followed by a longer-lasting plateau. The peak of the early component preceded the plateau level of the second component. A time course of change in [Ca2+]i in single cells at 100 microM-ATP was very similar to that observed in the suspension system. Preincubation of hepatocytes with 40 mM-caffeine, 2 mM-oxalate or 60 microM-dantrolene sodium inhibited the P2 purinergic response. The plateau phase was not observed when measured in the presence of extracellular 100 microM-LaCl3 or in the absence of extracellular Ca2+. The distribution of [Ca2+]i in single hepatocytes was also determined by fluorescence image analysis. In the initial phase, the increase in [Ca2+]i is greater in the peripheral region than the central region of the cell. Degradation of extracellular ATP by ecto-ATPase in the hepatocyte suspension was measured; the amount of ATP degradation was less than 10-15% of the initial amount (100 microM) during the measurement of the intracellular [Ca2+]i in the cell suspension. Extracellular ATP stimulated glucose synthesis. The rate of glucose production also showed two components, the initial fast component within 1 min and the subsequent slower component. The rate of the initial fast component did not depend on the presence or absence of extracellular Ca2+, whereas the rate of the subsequent component depended on it. The present study shows that the initial transient rise in [Ca2+]i plays an important role in triggering the gluconeogenesis.
Koike
M
M
Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Japan.
Kashiwagura
T
T
Takeguchi
N
N
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D005110:Extracellular Space / Q000378:metabolism; D005943:Gluconeogenesis / Q000187:drug effects* / Q000502:physiology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D013268:Stimulation, Chemical
D002121:Calcium Channel Blockers; D000244:Adenosine Diphosphate; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; C027201:ectoATPase; D002118:Calcium