Phosphodiesterase Activities for Cyclic Nucleotides in Nerve Endings from the Bovine Posterior Pituitary Gland

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Abstract
The cyclic nucleotide phosphodiesterase (PDE) activities were studied in a nerve ending fraction from bovine neural lobes. Most of the activity was particulate and unaffected by calcium. Lineweaver‐Burk plots for this fraction showed negative cooperativity with apparent Km values for cyclic AMP of 11 μM and for cyclic GMP of 4 μM. The soluble activities for both cyclic nucleotides were activated by calcium and inhibited by calmodulin‐binding drugs (trifluoperazine and calmidazolium). The apparent Km values were 50 μM for cyclic AMP and 20 μM for cyclic GMP for the soluble activities. Sucrose density gradients resolved the soluble activities into two peaks. The activity with the higher sedimentation rate (MW 122,000 daltons) hydrolysed both cyclic nucleotides and was calcium‐calmodulin‐dependent. The other peak (MW 47,000 daltons) had a higher affinity for cyclic AMP than for cyclic GMP and was calcium‐independent. Solubilized particulate activities gave two main peaks on the density gradient, both calcium‐independent. One was mainly for cyclic AMP (MW 47,000 daltons) and the other mainly for cyclic GMP (MW 133,000 daltons). The function of PDEs in relation to secretion was discussed.