Correlation of cell-free brain cyclic nucleotide phosphodiesterase activities to cyclic AMP decay in intact brain...

Differential and gradient centrifugation of rat brain cerebral cortical homogenates show three cyclic nucleotide phosphodiesterase (CN PDE) activities localized to different subcellular fractions with varying relative specific activities and responsiveness to pharmacologic agents. Type I (calcium/calmodulin-activatable) CN PDE is found primarily in the cytosolic fraction, Type II (cGMP-activatable) CN PDE is predominately membrane associated, and Type IV (cGMP-insensitive) cAMP PDE is distributed equally between soluble and particulate fractions. Fractionation of cerebral cortical membranes shows that Type II and Type IV CN PDE activities reside in synaptosomes. Type II CN PDE is the predominate hydrolytic activity in synaptosomes whereas Type IV cAMP PDE contributes only a small percentage of the total cAMP hydrolysis and Type I CN PDE is not detected in this fraction. The contribution of CN PDE isozymes to the regulation of intracellular cAMP levels was studied using rat brain cortical slices. The rate of cAMP decay in the absence and presence of selective CN PDE inhibitors after adenosine or beta-adrenergic agonist stimulation was determined using an adenine prelabeling technique. These studies show that a rolipram-sensitive, high affinity cAMP PDE (Type IV) is principally responsible for cyclic AMP decay in intact cortical tissue following elevation of cyclic AMP levels by either adenosine or beta-adrenergic receptor agonists. However, this isozyme, which is sensitive to inhibition by rolipram, RO 20-1724 and SQ 65442 contributes only a small percentage of the total cAMP hydrolytic activity in cell-free preparations of cortex.