Adenosine and the control of lipolysis

1. The rate of noradrenaline-stimulated lipolysis is lower in fat-cells from lactating than from pregnant rats; this difference is eliminated by the addition of adenosine deaminase [Aitchison, Clegg & Vernon (1982) Biochem. J. 202, 243-247]. 2. The activity of 5'-nucleotidase, and hence the capacity of the cells to synthesize adenosine, was the same in fat-cells and also stromal cells of adipose tissue from pregnant, lactating and male rats. 3. The response and sensitivity of fat-cells to the anti-lipolytic effects of adenosine were measured by incubating cells in the presence of noradrenaline, adenosine deaminase (to remove endogenous adenosine) and various concentrations of the adenosine analogue N6-phenylisopropyladenosine (PIA). 4. PIA caused a greater inhibition of the rate of noradrenaline-stimulated lipolysis in adipocytes from lactating than from pregnant rats. 5. The concentration of PIA required to inhibit by 50% the rate of noradrenaline-stimulated lipolysis fell from over l0OnM for fat-cells from pregnant rats to 30nm for fat-cells from lactating rats. 6. The decreased rate of noradrenaline-stimulated lipolysis during lactation was not due to the smaller mean cell volume of adipocytes during this state.