The identification of a new cyclic nucleotide phosphodiesterase activity in human and guinea-pig cardiac ventricle. Implications for the mechanism of action of selective phosphodiesterase inhibitors

Cyclic GMP, cyclic AMP, glucose at birth, and maturation of rat liver mitochondria

The improvement in mitochondrial functions which normally occurs in newborn rat liver in vivo during the few hours following delivery is inhibited by a glucose injection at birth (Meister, R., Comte, J., Baggetto, L., G., Godinot, C. and Gautheron, D.C. (1983) Biochim. Biophys. Acta 722, 36-42). To test whether this improvement could be correlated to changes in cyclic nucleotides, the levels of cAMP and cGMP have been measured during the 2 h following birth. At birth, a short rise followed by a decrease of cAMP occurs, then a significant increase of cAMP level is observed between 45 min and 2 h. The cAMP level for animals injected at birth with glucose is lower than for control animals at each time studied. The cGMP level is not significantly affected in control animals, while in glucose-treated animals a significant decrease of cGMP is observed in the postnatal 2 h. The present work shows also that the glucose-induced inhibition of mitochondrial maturation is mimicked by injection at birth of either 8-Br-cGMP or nitroprusside. The latter transiently increases intracellular cGMP. In contrast, the glucose-induced inhibition is prevented by the injection at birth of either dbcAMP or alkylxanthines together with glucose (Comte, J., Meister, R., Baggetto, L.G., Godinot, C. and Gautheron, D.C. (1986) Biochem. Pharmacol. 35, 2411-2416). It is concluded that the postnatal improvement of mitochondrial functions is stimulated by cAMP and inhibited by cGMP, and that glucose-induced inhibition of the maturation is at least partly supported by a decrease in cAMP but not correlated to an increase in cGMP.

Tissue distribution and selective inhibition of subtypes of high affinity cAMP phosphodiesterase

High affinity cAMP phosphodiesterase (PDE), also referred to as PDE III, or low Km PDE occurs as two subtypes. One subtype is sensitive to inhibition by cGMP while the other is relatively insensitive. To be consistent with previously recommended nomenclature, these subtypes were designated Types IV and V PDEs respectively. Tissue distribution of these subtypes of high affinity cAMP PDE was investigated using comparative potencies of specific inhibitors. Of the tissues examined, dog heart contained the highest proportion of the cGMP inhibitable form (Type IV PDE), whereas dog kidney cortex and brain were composed almost entirely of the cGMP non-inhibitable form (Type V PDE). Enoximone and other new cardiotonic drugs that inhibit high affinity cAMP PDE were shown to be specific for the cGMP inhibitable form, whereas rolipram was specific for the cGMP non-inhibitable form. The apparently partially competitive kinetics shown by one of these drugs, enoximone, was due to the presence of both subtypes of the enzyme. When the activity of the cGMP non-inhibitable form was suppressed by rolipram, competitive inhibition of the cGMP inhibitable subtype by enoximone was observed. Rat heart high affinity cAMP PDE activity contained a higher proportion of the cGMP non-inhibitable subtype than did the enzyme from dog heart. It is suggested that this may account for the relative insensitivity of rats to the cardiotonic PDE inhibitors.

Ovarian effects of SK&F 86002-A2 in the rat: site of action

In a preliminary 30-day study, oral administration of SK&F 86002-A2, an inhibitor of prostaglandin and leukotriene synthesis, blocked ovulation and altered ovarian structure and hormone production in rats. The purpose of the present study was to determine if the locus of action of SK&F 86002-A2 for these effects was the ovary or some other site in the female reproductive system, using a number of experimental approaches. A single sc or intraovarian injection of SK&F 86002-A2 did not block spontaneous or gonadotropin-induced ovulation in proestrous rats, whereas indomethacin, a positive control, acutely disrupted the ovulatory process. Since neither route of administration blocked ovulation, integrated pituitary and ovarian events were not negatively affected by a single injection of SK&F 86002-A2 at doses which caused ovarian dysfunction when administered repeatedly for 30 days. In contrast to a single dose, oral administration of SK&F 86002-A2 to hypophysectomized rats for 2 weeks suppressed follicular growth and estradiol production in response to sc administration of pregnant mare serum gonadotropin. Although ovarian function was suppressed in hypophysectomized rats, LH surges induced by estradiol in ovariectomized rats were not affected by administration of SK&F 86002-A2 for 2 weeks. Thus, hypothalamic/pituitary dysfunction did not contribute to the ovarian effects of SK&F 86002 that occurred after repeated dosing. In conclusion, these results indicate that disruption of ovarian cycles by SK&F 86002-A2 is related to a direct effect on the ovary, and not to altered hypothalamic/pituitary function and LH release. Specifically, SK&F 86002-A2 may suppress the ovarian response to gonadotrophin, retarding follicular growth and estrogen production. The ovarian effects are consistent with a pharmacological expression of the inhibitory action of SK&F 86002-A2 on prostaglandin and leukotriene synthesis.

Effects of SK&F 94120, an inhibitor of cyclic nucleotide phosphodiesterase type III, on human platelets

Elevation of cyclic AMP concentrations in platelets inhibits agonist-induced responses. Pharmacological interventions which could increase the levels of platelet cyclic AMP include activation of the synthesis of cyclic AMP or inhibition of its breakdown. In this study we have investigated the effects of SK&F 94120 on human platelet phosphodiesterase (PDE) activities separated by ion-exchange chromatography, and studied the effects of this agent on platelet responses caused by the agonists collagen, U44069 and ADP. Four PDE activities were identified from human platelet preparations. The PDE activities found comprised a cyclic GMP selective PDE, a Ca2+/calmodulin stimulated PDE, a cyclic GMP stimulated PDE and a "low Km" PDE activity called PDE III by analogy with activities described in other tissues. SK&F 94120 was found selectively to inhibit the "low Km" PDE III activity with an IC50 of 10.8 microM, which is consistent with the effects of this compound on cardiac ventricle PDE activities. Exposure of human platelets to SK&F 94120 produced concentration dependent increases in cyclic AMP, showing that inhibition of PDE III activity alone can cause an increase in the level of platelet cyclic AMP. SK&F 94120 also caused an inhibition of platelet responses to collagen, U44069 and ADP. However, SK&F 94120 was much less effective as an inhibitor of aggregation induced by ADP (IC50 greater than 100 microM) than by collagen (IC50 = 24.1 microM) or by U44069 (IC50 = 1.7 microM). Isobutylmethylxanthine (IBMX), a non-selective PDE inhibitor, was less effective than SK&F 94120 as an inhibitor of platelet responses for the same measured increase in cyclic AMP levels. M&B 22948 and rolipram, inhibitors of PDE I and PDE IV respectively, had no significant effect on platelet responses. These data suggest that selective inhibition of PDE III is the primary mechanism of action of SK&F 94120 as an inhibitor of agonist-induced platelet responses, and that increased cyclic AMP in the pool controlled by PDE III has important consequences on platelet responses. Moreover, these data suggest that some form of compartmentalization of cyclic AMP and/or PDE activity exists in human platelets.

Influence of rolipram on the cyclic 3',5'-adenosine monophosphate response to histamine and adenosine in slices of guinea-pig cerebral cortex

The effect of the phosphodiesterase (PDE) inhibitor rolipram on the cyclic AMP responses to adenosine, histamine and combinations of these two agonists, was examined in [3H]adenine-labelled slices of guinea-pig cerebral cortex. Constant levels of [3H]-cyclic AMP were achieved within 10 min of agonist addition, both in the presence and absence of rolipram (0.1 mM). Histamine (1 mM) produced an 8-fold increase in [3H]-cyclic AMP (compared with basal) which was increased 7-fold by rolipram. The responses to adenosine (0.1 mM) and adenosine and histamine in combination were larger than that to histamine alone (46-fold or more compared with basal) but the potentiation by rolipram was much smaller (2.5-fold or less). With both agonists the effect of rolipram was dose-dependent, the steady state [3H]-cyclic AMP levels increasing 1-2-fold for a 10-fold increase in rolipram concentration. Removal of the histamine or adenosine stimulus once steady state had been reached resulted in a rapid fall in [3H]-cyclic AMP levels with a half time of less than 5 min. Rolipram (0.1 mM) did not significantly alter the initial rates of fall in [3H]-cyclic AMP levels but increased the time taken for them to return to basal levels. The findings of higher steady state levels of cyclic AMP in the presence of rolipram, together with an almost unaltered rate of cyclic AMP turnover, are consistent with an interaction of rolipram with PDE which is overcome by an increase in cyclic AMP concentration. However, the relatively smaller effects of rolipram on the higher steady levels of cyclic AMP produced by adenosine and the rather shallow dose-dependence of the PDE inhibitor on the responses to both agonists are inconsistent with a simple competitive inhibition of total PDE activity in responding cells. The results can be explained, however, by the involvement of different forms of PDE, with the rolipram-sensitive, calcium-independent form dominating at low cyclic AMP levels and the rolipram-insensitive, calcium-dependent form becoming more important when cyclic AMP levels are higher.

Action of mediators on airway smooth muscle: functional antagonism as a mechanism for bronchodilator drugs

The beta-adrenoceptor agonists have become the cornerstone of bronchodilator therapy. These agents are "functional" or "physiologic" antagonists that actively relax airway smooth muscle through a cyclic-AMP (cAMP)-mediated decrease in myoplasmic Ca2+ content. Hence, unlike specific receptor antagonists, the sympathomimetics should reverse bronchoconstriction regardless of the mediator(s) involved. Indeed, one of the primary beneficial attributes of beta-adrenoceptor agonists is their inhibitory activity against a wide range of bronchoconstrictors. As successful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: 1) cardiovascular and skeletal muscle side effects, 2) an inherent subsensitivity of the patient population to beta-adrenoceptor agonists, 3) the development of tolerance, and 4) loss of efficacy during severe asthmatic episodes. These limitations are not specific for individual agents but are shared by all beta-adrenoceptor agonists. A significant improvement in the pharmacotherapy of asthma would be obtained by identifying novel bronchodilators devoid of one or more of the aforementioned liabilities. The development of isozyme-selective phosphodiesterase (PDE) inhibitors is one promising approach toward this goal. Interest in PDE inhibition as a therapeutic target has been renewed by the realization that PDEs exist in multiple isoforms and that the distribution of these isoforms varies significantly among tissues. This information, coupled with the recent synthesis of PDE inhibitors selective for several of the isozymes, raises the possibility of breeding organ-selectivity into this class of compounds. Results from preliminary experiments with isozyme-selective PDE inhibitors have helped to identify appropriate drug targets in airway smooth muscle. These early studies suggest that the synthesis of novel isozyme-selective PDE inhibitors not only may provide tools with which to understand the biologic function of various PDE isozymes, but may also lead to the development of improved therapeutic agents.

Effects of phosphodiesterase inhibitors on normal and chemically-skinned isolated airway smooth muscle

1. The effects of three phosphodiesterase inhibitors (papaverine, isobutyl methyl xanthine (IBMX) and SKF 94120) were examined on tension responses and cyclic nucleotide content (both cyclic AMP and cyclic GMP) of normal and Triton X-100 skinned isolated trachealis of the guinea-pig. 2. The three inhibitors were approximately equipotent in eliciting concentration-dependent relaxation of histamine-induced contractions of the trachealis. 3. Papaverine-induced relaxation was associated with concentration-related increases in the levels of both cyclic nucleotides. 4. IBMX at low concentrations (1 mumol l-1) produced significant relaxation (36%) of histamine-contracted trachealis without changing cyclic nucleotide levels. At a ten fold higher concentration IBMX-induced relaxation (95%) was associated with a selective increase in tissue cyclic GMP levels. Only at the highest concentration tested (100 mumol l-1) did IBMX increase cyclic AMP levels significantly. 5. SKF 94120 (1 mumol l-1) elicited a 23% relaxation of the contracted trachealis without altering the tissue content of either cyclic nucleotide. At the two higher concentrations tested (10 and 100 mumol l-1), SKF 94120-induced relaxation was accompanied by a selective increase in the levels of cyclic AMP. 6. In the skinned trachealis Ca2+ (10 and 20 mumol l-1)-induced contractions were significantly inhibited by the calmodulin antagonist calmidazolium (10 mumol l-1) and by cyclic AMP (10 mumol l-1), the catalytic subunit of cyclic AMP-dependent protein kinase (0.1 mumol l-1) and cyclic GMP (10 mumol l-1). 7. Papaverine (100 mumol l-1) significantly inhibited (31 +/- 6%) the Ca2+-induced contractions of the skinned trachealis. Both IBMX and SKF 94120 were without effect. It is concluded that cyclic nucleotide-dependent mechanisms have an inhibitory action on the biochemical processes that lead to contraction of the guinea-pig trachealis. The results suggest that a functional sarcoplasmic reticular and/or plasma membrane is essential for the expression of IBMXand SKF 94120-induced relaxation. This is not the case for papaverine. The results also highlight the fact that significant relaxant responses of airway smooth muscle can be produced by phosphodiesterase- inhibiting drugs without concomitant elevations in tissue cyclic nucleotide content.

Effects of forskolin on contractile responses and protein phosphorylation in the isolated perfused rat heart

Continuous perfusion of rat hearts with concentrations of forskolin between 0.1 and 12 microM resulted in transient increases in tension after 45 s, followed by a return to the control value after 5 min. In contrast, the content of cyclic AMP increased linearly with time over this period, reaching values up to 35 times control after 5 min. Increases in contractile force, intracellular cyclic AMP concentration and the proportion of phosphorylase in the a form were dependent on the concentration of forskolin when measured 45 s and 120 s after initiation of perfusion. In hearts perfused for 45 s with various concentrations of forskolin, the measured cyclic AMP-dependent protein kinase activity ratio and phosphorylase a content for a given measured intracellular cyclic AMP concentration were both much less than the corresponding values in hearts perfused for 30 s with various concentrations of isoprenaline. The phosphorylation of the contractile proteins troponin-I and C-protein also showed a concentration-dependent increase in hearts perfused with forskolin. There was a strong correlation between the cyclic AMP-dependent protein kinase activity ratios and the phosphorylation of the contractile proteins under all perfusion conditions. These results suggest that cyclic AMP is compartmented in perfused rat heart, and that much of the cyclic AMP produced in response to forskolin is unavailable to activate cyclic AMP-dependent protein kinase.

Strategic approaches to drug design. II. Modelling studies on phosphodiesterase substrates and inhibitors

Modelling studies have been carried out on the phosphodiesterase (PDE) substrates, adenosine- and guanosine-3'5'-cyclic monophosphates, and on a number of non-specific and type III-specific phosphodiesterase inhibitors. These studies have assisted the understanding of PDE substrate differentiation and the design of potent, selective PDE type III inhibitors.