Effects of adrenaline and methylisobutylxanthine on adenosine 3':5'-monophosphate levels in cultures of beating heart cells of the newborn rat.

(-)-Adrenaline caused concentration-dependent increases in cAMP levels and the rate of beating in eight-day-old heart cell cultures of newborn rats. Half-maximal increases in both parameters (5- and 0.2 -fold, respectively) occurred at about 10(-6)M. Following the addition of 3 X 10(-7) M adrenaline, the cellular cAMP level rose to a max imum in 30 sec. The rise was abolished by 5 X 10(-8) M (-)-propranolol and was greatly magnified by 10(-4) M 1-methyl-3-isobutylxanthine. In the presence of the latter compound, the average rate of accumulation of cAMP in thecultures during the first 10 seconds of exposure to 3 X 10(-7) M adrenaline was 8.78 pmol/mg of protein-sec, which is 230 times more rapid than the basal accumulation rate. These findings may be taken as evidence in support of the view that cAMP is involved in the positive chronotropic action of adrenaline on cardiac pacemaker cells.