Significance of phosphodiesterase in the brain

Age-related changes of cyclic AMP phosphodiesterase activity in rat brain regions and a new phosphodiesterase inhibitor--nootropic agent adafenoxate

1. The low- and high-KM cyclic AMP phosphodiesterase (cAMP PDE) activity in cerebral cortex, striatum, hypothalamus and hippocampus of young (4-5-month-old) and aged (22-month-old) rats has been studied. 2. A significant rise in the high-KM cAMP PDE activity in the cerebral cortex, hypothalamus and hippocampus in aged rats has been found. 3. The activity of the low-KM cAMP PDE does not change during senescence in all the brain structures studied. 4. In a series of increased concentrations (from 5 x 10(-4) to 1 x 10(-5) M) adafenoxate inhibits low- and high-KM cAMP PDE in most of the brain structures studied in both age groups. 5. The present results provide evidence for realization of the CNS effects of adafenoxate through inhibition of cAMP PDE activity and regulation of the intracellular level of cAMP.

Ontogeny of cyclic AMP and cyclic GMP phosphodiesterase activities and of calmodulin levels in guinea pig ureter

Cyclic AMP and cyclic GMP phosphodiesterase (PDE) activities and calmodulin levels were determined in ureters from guinea pigs of the following ages: 50 and 56 days fetuses, three, 10, 21, 50, and 90 days, and three years old. While there is little change in ureteral cyclic AMP-PDE with age, cyclic GMP-PDE increases with age. Activity of cyclic GMP-PDE in supernatants prepared from three-year-old guinea pig ureter homogenates is 462% and 216% higher than that from 50-day fetus and three-day animals, respectively. Calmodulin levels have a bimodal distribution with age; values are highest in supernatants from 10 and 21 day ureters, but also increase in the three-year ureters when compared to 50 and 90-day values.

Decreased adenosine cyclic 3',5'-monophosphate phosphodiesterase activity in rat brain following triethyltin intoxication

The possible involvement of cerebral cAMP-phosphodiesterase (PDE) in the intoxication and brain edema formation after exposure to triethyltin (TET) has been studied in vitro and in vivo in the rat. In vitro studies showed an irreversible inhibition of the particulate and soluble phosphodiesterase activities. In vivo, both high i.v. single dose and repeated oral administration of low TET doses led to a significant decrease of the particulate activities. Phosphodiesterase inhibition preceded edema formation. The soluble activities were less influenced and their inhibition could be a consequence of the edema formed in the brain tissue rather than of a TET direct action upon the enzyme. Kinetic analysis of the brain particulate enzyme from the TET-treated rats showed a significant decrease in the Vmax of the substrate high affinity enzyme form when compared to controls.

An improved, automated adenylate cyclase assay utilizing preparative HPLC: effects of phosphodiesterase inhibitors

Analysis of adenylate cyclase (ACase) activity in broken cell preparations usually involves conversion of [alpha-32P]ATP to [32P]cyclic AMP (cAMP) followed by purification of cAMP by liquid chromatographic methods. An automated, preparative reverse-phase HPLC procedure was developed that purifies cAMP rapidly and decreases variability and background. It permits the separation procedure to be validated rapidly prior to use with actual samples, and is readily adaptable for assaying guanylate cyclase, phosphodiesterases (PDE), or a variety of other related nucleotide-metabolizing enzymes. For ACase assays, 4.5% ZnSO4-10% Ba(OH)2 is added to the incubation mixture, and following centrifugation, the supernatant is injected on an HPLC apparatus fitted with a Waters Z-Module containing a 10-microns C18 reverse-phase cartridge. Using a mobile phase of 0.15 M sodium acetate-20% methanol (pH 5.0) at a flow rate of 4 ml/min, cAMP is eluted at k' greater than 1.25, whereas k' less than 0.5 for all other adenine nucleotides, permitting collection of the cAMP fraction after running the other nucleotides to waste. The method was validated by characterizing dopamine-sensitive ACase in homogenates of striatum from Sprague-Dawley rats. Basal activity (177 +/- 16 pmol/mg protein/min), the stimulation by dopamine (186 +/- 19 pmol/mg/min), the apparent Km for dopamine (5.0 +/- 1.5 microM), and expected effects of varying magnesium, EGTA, and GTP were similar to available data. However, it was found that isobutylmethylxanthine (IBMX) or theophylline, usually included in the incubation mixture as PDE inhibitors, markedly inhibited the synthesis of cAMP in both the presence and absence of dopamine. A consequence of this inhibition was a marked change in the apparent Km of dopamine calculated from a Lineweaver-Burk plot. The use of IBMX to inhibit PDEs was compared with an alternate strategy, the addition of excess exogenous cAMP. Simultaneous analysis of PDE and ACase activity was accomplished by including [3H]cAMP in the incubation and quantifying the amounts of [3H]cAMP hydrolyzed and [32P]cAMP synthesized. Without IBMX, a concentration of 1 mM exogenous cAMP was sufficient to prevent significant loss of [3H]cAMP. In the absence of exogenous cAMP, 0.5 mM IBMX did not completely prevent the breakdown of [3H]cAMP, whereas 2.5 mM IBMX did. Although there was 25% less [3H]cAMP recovered in the presence of 0.5 mM IBMX than with 2.5 mM IBMX, there was no difference in the amount of [32P]cAMP formed (either with or without dopamine).(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphodiesterase activities for cyclic nucleotides in nerve endings from the bovine posterior pituitary gland

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 microM and for cyclic GMP of 4 microM. 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 microM for cyclic AMP and 20 microM 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.