Circadian rhythms of lipogenic and hypoglycaemic responses to insulin in the golden hamster (Mesocricetus auratus).

Lipogenesis and blood glucose concentrations were determined at 4-hourly intervals in control and insulin-treated golden hamsters maintained on 14-h daily photoperiods (08.00-22.00 h). Lipogenesis was studied by measuring the incorporation of label into liver and fat pad lipids in animals killed 30 min after i.p. [3H]acetate injection and 2 h after insulin or saline (control) injections. Circadian rhythms of lipogenesis and plasma glucose concentration were present in both control and insulin-treated hamsters. In control animals most lipogenic activity occurred during the dark period and early during the daily photoperiod (14 h light: 10 h darkness). There were dramatic differences in the lipogenic (fat pad) and hypoglycaemic responses to insulin which varied as a function of the time of day at which insulin was injected. Insulin stimulated fivefold increases in lipid deposition (fat pad incorporation) when injected late during the dark period but had little or no effect 4-8 h after the onset of light. Daily injections for 8 days also produced variable cumulative effects on body fat stores as a function of the time of day. Insulin injected late during the dark period stimulated a 40% increase in abdominal fat weight over controls, whereas insulin injected at 4 and 12 h after the onset of light had no effect on abdominal fat weight. Insulin decreased plasma glucose concentrations markedly at 8 and 20-24 h after the onset of light but had no apparent hypoglycaemic activity (120 min after its injection) at 4 h after the onset of light.(ABSTRACT TRUNCATED AT 250 WORDS)