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| Titel: |
Circadian variation on the regulation of hepatic pyruvate kinase activity and the effects of epidermal growth factor in fed and fasted mice |
| Auteur: |
Mittelstaedt, Roberta A. Feuers, Ritchie J. Tsai, T. H. Scheving, Lawrence E. |
| Verschenen in: |
Biological rhythm research |
| Paginering: | Jaargang 19 (1988) nr. 2 pagina's 97-109 |
| Jaar: |
1988-05 |
| Inhoud: |
The activity of hepatic pyruvate kinase, from mice synchronized to either a standard light-dark cycle (LD12:12) or a reversed cycle (DL12:12) and killed at six different times of day, was measured at subsaturating (v) and saturating (V) levels of its substrate, phosphoenolpyruvate. Mice were either fed or fasted for longer than 30 hours and injected two hours into the light (LD mice) or dark (DL mice) with either saline or epidermal growth factor (EGF). In fed mice injected with saline in either the light or the dark, both v and V reached maximum values (540 or 486 and 1295 or 1197 U/L, respectively) during the dark span. In each of these cases, activities were found to have significant differences across the times of day at which tissues were taken (P < 0.01, by ANOVA). When EGF was administered two hours into the light to fed animals, no change in either v or V was noted after 6 hours. After 10 hours, v (153%) and V (80%) were increased, and after 14 hours v (41%) and V (25%) were significantly decreased. The ratio of v/V was increased after 10 hours and decreased after 16 hours by EGF treatment (P < 0.01), indicating activation followed by an inactivation of pyruvate kinase catalytic activity. When EGF was given two hours into the dark, different effects were observed. After 6 hours, v decreased 20% (P < 0.05) and after 14 hours, V decreased 28% (P < 0.05). Fasting caused 57-66% decreases (P < 0.01) in overall mean values for v and V from saline or EGF-treated mice. No change in V across time could be detected in fasted mice treated two hours into the dark and for both treatment times, most of the effects of EGF were diminished by fasting. These data suggest that fasting dramatically alters circadian variation in catalytic activity and enzyme concentration while it reduces the apparent pyruvate kinase concentration and inhibits catalytic activity across the day, even in the presence of EGF. In fed mice, EGF increased enzyme concentration as an early response when given during the early-light span, but decreased concentration as an early response when given during the early-dark span. EGF also altered the catalytic activity of the enzyme, but the timing of these effects varied with the time of treatment. It is suggested that EGF may play a role in the regulation of phosphorylation and dephosphorylation and synthesis of hepatic pyruvate kinase. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
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