Cyclic Adenosine 3′,5′-Monophosphate in Guinea Pig Cerebral Cortical Slices: I. FORMATION OF CYCLIC ADENOSINE 3′,5′-MONOPHOSPHATE FROM ENDOGENOUS ADENOSINE TRIPHOSPHATE AND FROM RADIOACTIVE ADENOSINE TRIPHOSPHATE FORMED DURING A PRIOR INCUBATION WITH RADIOACTIVE ADENINE

Abstract

Abstract Cyclic adenosine 3',5'-monophosphate (cyclic AMP) accumulates in guinea pig cerebral cortical slices during incubations with histamine, histamine-norepinephrine, adenosine, adenosine-histamine, adenosine-norepinephrine, adenosine-histamine-norepinephrine, veratridine, and veratridine-histamine. The rate of increase of cyclic AMP levels in response to various agents appears to be limited to some extent by the rate of diffusion of small molecules into the extracellular space of the slices. The diffusion of [3H]sorbitol occurred with a t½ of l3 min. Increases in total levels of cyclic AMP and increases in levels of radioactive cyclic AMP derived from intracellular adenine nucleotides, labeled by a prior incubation with radioactive adenine, occurred on similar time courses and to similar extents. Radioactive cyclic AMP represents a small but relatively constant fraction (7 to 13%) of the total amount of cyclic AMP. During stimulation with histamine, however, the initial accumulation of radioactive cyclic AMP represents a significantly smaller fraction of the total amount of cyclic AMP. Except in this instance, the data provide no evidence for the presence of more than one major compartment of adenine nucleotides in brain slices that can serve as a source of nucleotide precursor for cyclic AMP. The nucleotides of this compartment or compartments appear to be uniformly labeled by incubation with radioactive adenine and to represent about 30% of the total adenine nucleotides of the slice. Conversion of radioactive adenine to cyclic AMP is proportional to the steady state levels of the cyclic nucleotide and occurs at rates of up to 10 pmoles per mg of protein per min in the continued presence of stimulatory agents.