[Stimulation of embryonic rat cells in culture by calf serum. II. Changes of ATP- and cAMP levels; activation of a cAMP specific phosphodiesterase].

Embryonic rat cells cultured in a synthetic medium are stopped in G t phase if serum is omitted from the culture medium. On addition of calf serum these cells resume DNA synthesis after a lag phase of 15 — 20 hours. In the present paper the events concerning the metabolism of adenosine phosphates after stimulation by serum have been examined. The results are the following: 1. The concentration of cAMP in cells which have been incubated for 24 hours in serum-free medium is 32 times higher than that in cells growing in medium containing 10 per cent calf serum; the accumulated cAMP is metabolized within 10 min after the addition of 10 per cent calf serum. This has been confirmed by a double labelling technique after preincubation with radioactive adenosine as well as by estimation of cAMP using a protein-binding assay. 2. A cAMP-specific phosphodiesterase is activated shortly after stimulation; the enzyme activity in dialysed cell homogenates is increased within 30 min by a factor of 2. 3. Cells incubated in serum-free and those in serum-containing medium have equal amounts of ATP. On addition of serum the ATP concentration in cells preincubated in serum-free medium decreases by 27%; the original level is restored 20 — 30 min later.