Myocardial cyclic AMP determinations: the importance of rapidity of sample freezing

Using specially modified automatic freeze clamps studies were undertaken to determine the importance of rapid freeze clamping in the determination of myocardial cAMP content. Modification of the freeze clamps allowed the separation of the physical processes of clamping and freezing. When the tissue was clamped and maintained at 37 degrees C for periods in excess of 5 s before freezing cyclic AMP content increased by up to 100%. When clamped and held at room temperature (20 degrees C) increase were again observed if the freezing delay exceeded 5 s, however, the increases were not as great as those observed with 37 degrees C clamping. The biphasic time-dependency curve for tissue cAMP content v. clamping delay bears a close similarity to that observed following the onset of tissue ischemia suggesting perhaps that ischemia, secondary to mechanical clamping, rather than sympathetic stimulation, is the cause of the rise. It can be concluded that for reliable measurements of cAMP content tissue should be frozen within less than 5 s of sampling.