The effects of inhibitors of adenylate cyclase and phosphodiesterase on D. discoideum aggregation.

The action of two adenine analogues on the aggregation of D. discoideum amebae was examined. SQ22536 and SQ20009 are inhibitors of adenylate cyclase and phosphodiesterase, respectively, in higher eukaryotes. Both compounds are shown here to inhibit the differentiation of cells to aggregation-competence. SQ22536-treated cells exhibited normal accumulation of their adenylate cyclase activity as measured in cell lysates but the amebae did not synthesize cAMP. The ability of this drug to compete for cAMP surface-binding sites, and the observation that the effects of the drug could be reversed by imposed pulses of cAMP, suggest that SQ22536 functions as a cAMP antagonist. The effects of SQ20009 on cell differentiation did not appear to be mediated by an inhibition of phosphodiesterase activity or cAMP binding to the cell surface. Amebae were arrested at a very early stage in development and remained unresponsive to external cAMP.     

Action of forskolin on non-electrolyte permeability across the frog skin as compared to that of vasopressin and isoprenaline

Forskolin, a natural diterpene activating the adenyl cyclase in a receptor-independent manner, increases symmetrically both transepithelial fluxes of urea and erithrytol through the frog skin. The effect is dose-dependent, being 5 X 10(-6) M the dose necessary to obtain the maximal action. Forskolin-induced permeabilization is inversely proportional to the molecular weight of water soluble molecules (urea greater than erythritol greater than mannitol); also the permeability of a mainly lipid soluble molecule, i.e. antipyrine, is slightly increased by the diterpene. The permeability pattern is more similar to that induced by isoprenaline as compared to that elicited by vasopressin. Differently from what occurs in other tissues, small doses of forskolin (10(-8) M) are unable to potentiate the actions of vasopressin and isoprenaline on urea permeability across the frog skin. Moreover, the maximal action of forskolin is not additive with the maximal ones of isoprenaline and vasopressin.     

Participation of cAMP in the facilitatory action of beta,gamma-methylene ATP on the noradrenaline release from rabbit ear artery

beta, gamma-Methylene ATP (betagamma-mATP) significantly facilitated the electrically (4 Hz) evoked release of noradrenaline (NA) from the rabbit ear artery by activation of prejunctional purinoceptors on the sympathetic nerve terminals. In the present study, we investigated whether intracellular cAMP is involved in the purinoceptor mediated facilitatory mechanisms. Forskolin, an adenylate cyclase activator, and 8-bromo cAMP, a cAMP analogue, significantly enhanced the NA-release. The enhancement of NA-release by betagamma-mATP was significantly potentiated by Ro20-1724, a phosphodiesterase inhibitor, but abolished by SQ22536, an adenylate cyclase inhibitor. Both drugs alone had no effect on the NA-release. N-ethylmaleimide and pertussis toxin, inhibitors of Gi-proteins, did not affect the NA-release, or the enhancement of NA-release by betagamma-mATP. Alone Cholera toxin (CTX), an activator of Gs-proteins, significantly increased the NA-release, but in the presence of CTX, betagamma-mATP could not produce further enhancement of the NA-release. These results suggest that cAMP is closely associated with the facilitatory action of betagamma-mATP on NA-release in the rabbit ear artery.     

Papaverine reduces the sodium permeability of the apical membrane and the Potassium permeability of the basolateral membrane in isolated frog skin

Summary The effect of papaverine, an inhibitor of the phosphodiesterase responsible for breakdown of cAMP, on the transepithelial sodium transport across the isolated frog skin was investigated.Serosal addition of papaverine caused initially an increase in the short-circuit current (SCC), a doubling of the cellular cAMP content and a depolarization of the intracellular potential under SCC conditions (V _scc).The initial increase in the SCC was followed by a pronounced decrease both in the SCC and in the natriferic action of antidiuretic hormone (ADH), but papaverine had no inhibitory effect on the ability of ADH to increase the cellular cAMP content. As SCC declines, no hyperpolarization was observed.The I/V relationship across the apical membrane during the inhibitory phase, revealed that papaverine reduces the sodium permeability of the apical membrane (P _Na ^a )as well as intracellular sodium concentration. These observations and the previously noted effect of papaverine on V _scc indicates that papaverine must have an effect on the cellular Cl or K permeability.The basolateral Na,K,2Cl cotransporter was blocked with bumetanide, which should bring the cellular chloride in equilibrium. Bumetanide had no effect on basal SCC and V _scc. When papaverine was added to skins preincubated with bumetanide, the effect of papaverine on SCC and V _scc was unchanged. Therefore, the depolarization of V _scc, observed during the papaverine induced inhibition of the SCC, must be due to a reduction in the cellular K permeability.In conclusion, it is suggested that papaverine reduces the sodium permeability of the apical membrane and the potassium permeability of the basolateral membrane of the frog skin epithelium.     

Adenosine 3'',5''-cyclic monophosphate and morphology in Neurospora crassa: drug-induced alterations.

Grown in liquid culture in the presence of a variety of structurally unrelated drugs, mycelia of wild-type Neurospora assume a colonial or semicolonial growth habit similar to that of known morphological mutants. Drugs that produce these morphological changes include atropine, theophylline, histamine, and several of the quinoline-containing antimalarials. Each of these drugs decrease the endogenous adenosine 3',5'-cyclic monophosphate (cAMP) concentration of mycelia as a result of their effect on the activity of adenyl cyclase, the cAMP-dependent phosphodiesterase, or both. The evidence indicates a relationship between the degree of morphological abnormality, the degree to which intracellular cAMP is reduced, and the action of the drugs on the adenyl cyclase and phosphodiesterase.     

Endothelium-dependent and -independent vasorelaxation by a theophylline derivative MCPT: roles of cyclic nucleotides, potassium channel opening and phosphodiesterase inhibition

The vasorelaxation activities of MCPT, a newly synthesized xanthine derivative, were investigated in this study. In phenylephrine (PE)-precontracted rat aortic rings with intact endothelium, MCPT caused a concentration-dependent relaxation, which was inhibited by endothelium removed. This relaxation was also reduced by the presence of nitric oxide synthase inhibitor Nomega-nitro-L-arginine methylester (L-NAME, 100 microM), soluble guanylyl cyclase (sGC) inhibitors methylene blue (10 microM), 1 H-[1,2,4] oxidazolol [4,3-a] quinoxalin-1-one (ODQ, 1 microM), adenylyl cyclase (AC) blocker SQ 22536 (100 microM), ATP-sensitive K+ channel blocker (KATP) glibenclamide (1 microM), a Ca2+ activated K+ channels blocker tetraethylammonium (TEA, 10 mM) and a voltage-dependent potassium channels blocker 4-aminopyridine (4-AP, 100 microM). The vasorelaxant effects of MCPT together with IBMX (0.5 microM) had an additive action. In PE-preconstricted endothelium-denuded aortic rings, the vasorelaxant effects of MCPT were attenuated by pretreatments with glibenclamide (1 microM), SQ 22536 (100 microM) or ODQ (1 microM), respectively. MCPT enhanced cAMP-dependent vasodilator isoprenaline- and NO donor/cGMP-dependent vasodilator sodium nitroprusside-induced relaxation activities in endothelium-denuded aortic rings. In A-10 cell and washed human platelets, MCPT induced a concentration-dependent increase in intracellular cyclic GMP and cyclic AMP levels. In phosphodiesterase assay, MCPT displayed inhibition effects on PDE 3, PDE 4 and PDE 5. The inhibition % were 52 +/- 3.9, 32 +/- 2.6 and 8 +/- 1.1 respectively. The Western blot analysis on HUVEC indicated that MCPT increased the expression of eNOS. It is concluded that the vasorelaxation by MCPT may be mediated by the inhibition of phosphodiesterase, stimulation of NO/sGC/ cGMP and AC/cAMP pathways, and the opening of K+ channels.     

Mechanism of the inhibition of platelet aggregation produced by prostaglandin F2 alpha

The inhibition of human platelet aggregation produced by PGF2 alpha is not specific for thromboxane A2 mimetics. Aggregation waves induced by PAF and thrombin are also inhibited by PGF2 alpha (8 microM); ADP is unaffected. These effects are still seen in platelets from aspirin-treated donors and platelets desensitized to thromboxane-like agonists (e.g. 11,9-epoxymethano PGH2). In contrast the thromboxane receptor antagonist EP 045 (up to 20 microM) had no effect on primary aggregation induced by PAF, thrombin and ADP. We have previously shown that EP 045 (IC50 = 0.5 microM), but not PGF2 alpha (28 microM), displaces the specific binding of [3H] 9,11-epoxymethano PGH2 to washed human platelets. PGF2 alpha produces small increases in cAMP levels, and both this effect and the anti-aggregation are diminished by the adenyl cyclase inhibitor SQ 22536. The rise in cAMP induced by PGF2 alpha is inhibited to a greater extent by the presence of ADP than by thrombin, PAF or a thromboxane mimetic. The ability of aggregating agents to inhibit this increase correlates inversely with their sensitivity to inhibition by PGF2 alpha. We suggest that the very weak effect of PGF2 alpha on cyclic AMP production is sufficient to account for its inhibitory activity, and it is unlikely to be a competitive antagonist at the platelet thromboxane receptor as suggested by others.     

AQP1 gene expression is upregulated by arginine vasopressin and cyclic AMP agonists in trophoblast cells

Aquaporins (AQPs) are water channels that regulate water flow in many tissues. As AQP1 is a candidate to regulate placental fluid exchange, we sought to investigate the effect of arginine vasopressin (AVP) and cAMP agonists on AQP1 gene expression in first trimester-derived extravillous cytotrophoblasts (HTR-8/Svneo) and two highly proliferative carcinoma trophoblast-like cell lines but with a number of functional features of the syncytiotrophoblast namely; JAR and JEG-3 cells. Our data demonstrated that AVP (0.1 nM) significantly increased the expression of AQP1 mRNA at 10 h in HTR-8/SVneo and JEG-3 cells (P<0.05). Both SP-cAMP, a membrane-permeable and phosphodiesterase resistant cAMP, and forskolin, an adenylate cyclase stimulator significantly increased AQP1 mRNA expression in all cell lines after 2 h in a dose-dependent manner (P<0.05) with a parallel increase in protein expression. In the time course study, 5 microM of either SP-cAMP or forskolin significantly stimulated AQP1 mRNA expression after 2 h in HTR-8/SVneo cells and after 10 h in JAR and JEG-3 cells. AQP1 protein expression was highest after 20 h in both HTR-8/SVneo and JEG-3 cells (P<0.05). AVP-stimulated cAMP elevation was blocked in the presence of 9-(tetrahydro-2'-furyl) adenine (SQ22536) (100 microM), a cell-permeable adenylate cyclase inhibitor (P<0.05). These results indicate that in trophoblasts-like cells AQP1 gene expression is upregulated by both AVP and cAMP agonists. Furthermore, our data demonstrate that a cAMP-dependent pathway is responsible for the AVP effect on AQP1. Thus, modulation of AQP1 expression by maternal hormones may regulate invasion and fetal-placental-amnion water homeostasis during gestation.     

Inhibition of adenylate cyclase of catfish and rat hepatocyte membranes by 9-(tetrahydro-2-furyl)adenine (SQ 22536).

The adenosine analogue 9-(Tetrahydro-2-furyl)adenine, SQ 22536, inhibited adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] activity of crude membrane preparations from catfish (Ictalurus melas) and rat isolated hepatocytes in a non-competitive manner. The IC50s were reduced in the presence of NaF. SQ 22536 reduced the activity of adenylate cyclase also in the presence of increasing concentrations of GTP, as well as Mg++ and Mn++. In the presence of catecholamines (epinephrine, norepinephrine, isoproterenol, phenylephrine) SQ 22536 reduced their activating effect on adenylate cyclase in both catfish and rat membranes. SQ 22536 also inhibited the effect of glucagon (0.1 microM) on rat membrane cyclase activity.     

Inhibition of ADP-ribosyltransferase activity of cholera toxin by MDL 12330A and chlorpromazine

ADP-ribosylation by cholera toxin of the guanine nucleotide binding regulatory protein (Gs) of rat liver membrane adenylate cyclase was inhibited by 0.1-1 mM MDL 12330A or 0.1-1 mM chlorpromazine. Basal as well as cholera toxin activated adenylate cyclase activity in liver membranes was also inhibited by the two drugs. NAD glycohydrolase activity and self-ADP-ribosylation of cholera toxin were also inhibited by MDL 12330A and chlorpromazine. These effects of MDL 12330A and chlorpromazine may be related to their effects on cholera toxin-induced fluid secretion in vivo.     

cAMP Level Modulates Scleral Collagen Remodeling,a Critical Step in the Development of Myopia

The development of myopia is associated with decreased ocular scleral collagen synthesis in humans and animal models. Collagen synthesis is, in part, under the influence of cyclic adenosine monophosphate (cAMP). We investigated the associations between cAMP, myopia development in guinea pigs, and collagen synthesis by human scleral fibroblasts (HSFs). Form-deprived myopia (FDM) was induced by unilateral masking of guinea pig eyes. Scleral cAMP levels increased selectively in the FDM eyes and returned to normal levels after unmasking and recovery. Unilateral subconjunctival treatment with the adenylyl cyclase (AC) activator forskolin resulted in a myopic shift accompanied by reduced collagen mRNA levels, but it did not affect retinal electroretinograms. The AC inhibitor SQ22536 attenuated the progression of FDM. Moreover, forskolin inhibited collagen mRNA levels and collagen secretion by HSFs. The inhibition was reversed by SQ22536. These results demonstrate a critical role of cAMP in control of myopia development. Selective regulation of cAMP to control scleral collagen synthesis may be a novel therapeutic strategy for preventing and treating myopia.     

Intracellular cAMP involvement in the synchronized activity of noradrenaline in response to evoked release of the transmitter quanta in the frog synapses

An analogue of cyclic AMP (db-cAMP) penetrating into the frog neuromuscular junction's cell, as well as the adenylyl cyclase activator forskolin, and inhibitor of nucleotide-depending phosphodiesterase isobutilmethylxantine alter the kinetics of the quanta secretion resulting in synchronizing of the process of the transmitter release. Following a db-cAMP preliminary action, no such synchronizing of the transmitter release occurred. Action of noradrenaline on the time course of the secretion seems to be realised through activation of presynaptic beta-adrenoreceptors, augmentation of the adenylyl cyclase activity, and the rise of the intracellular cAMP.     

Signaling pathways for modulation of mouse sperm motility by adenosine and catecholamine agonists

Capacitation of mammalian sperm, including alterations in flagellar motility, is presumably modulated by chemical signals encountered in the female reproductive tract. This work investigates signaling pathways for adenosine and catecholamine agonists that stimulate sperm kinetic activity. We show that 2-chloro-2'-deoxyadenosine and isoproterenol robustly accelerate flagellar beat frequency with EC50s near 10 and 0.05 microM, respectively. The several-fold acceleration is maximal by 60 sec. Although extracellular Ca2+ is required for agonist action on the flagellar beat, agonist treatment does not elevate sperm cytosolic [Ca2+] but does increase cAMP content. Acceleration does not require the conventional transmembrane adenylyl cyclase ADCY3, since it persists in sperm of ADCY3 knockout mice and in wild-type sperm in the presence of the inhibitors of conventional adenylyl cyclases SQ-22536, MDL-12330A, or 2', 5'-dideoxyadenosine. In contrast, the acceleration by these agents is absent in sperm that lack the predominant atypical adenylyl cyclase, SACY. Responses to these agonists are also absent in sperm from mice lacking the sperm-specific Calpha2 catalytic subunit of protein kinase A (PRKACA). Agonist responses also are strongly suppressed in wild-type sperm by the protein kinase inhibitor H-89. These results show that adenosine and catecholamine analogs activate sperm motility by mechanisms that require extracellular Ca2+, the atypical sperm adenylyl cyclase, cAMP, and protein kinase A.     

A novel biomimetic olfactory-based biosensor for single olfactory sensory neuron monitoring

This paper presents a novel biomimetic olfactory biosensor for the study of olfactory transduction mechanisms on the basis of light addressable potentiometric sensor (LAPS), in which rat olfactory sensory neurons (OSNs) are used as sensing elements. Rat OSNs are cultured on the surface of LAPS chip. To validate the origin of the electrical signals recorded by LAPS, the inhibitory effect of MDL12330A to the olfactory signals of OSNs is tested, which is the specific inhibitor of adenylyl cyclase. The enhancive effect of LY294002 to the responses of OSNs is also investigated, which is the specific inhibitor of phosphatidylinositol 3-kinase (PI3K). The results show that this hybrid biosensor can record the responses of OSNs to odours efficiently in a non-invasive way for a long term, and the responses can be inhibited by MDL12330A and enhanced by LY294002. All these results demonstrate that this hybrid biosensor can be used to monitor electrophysiology of OSNs in a non-invasive way and suggest it could be a promising tool for the study of olfactory transduction mechanisms.     

Effects of sodium fluoride on the mechanical activity in mouse gastric preparations

The aim of the present study was to investigate the responses induced by sodium fluoride (NaF) on gastric mechanical activity, using mouse whole-stomach preparations. The mechanical activity was recorded in vitro as changes of intraluminal pressure. In most of the preparations, NaF induced a tetrodotoxin-insensitive biphasic effect characterized by early relaxation followed by slowly developing contractile response. The contraction was dependent on the concentration of NaF, whereas the relaxation was observed at only 10-30 mmol/L NaF. The contractile effect was significantly reduced by nifedipine (an L-type Ca(2+) channel blocker), ryanodine or ruthenium red (inhibitors of Ca(2+) release from sarcoplasmic reticulum), and GF109203X (a protein kinase C inhibitor). Moreover, it was abolished by neomycin (an inhibitor of phospholipase C) and potentiated by SQ22536 (an inhibitor of adenylyl cyclase). All the drugs significantly increased the relaxation, except SQ22536, which abolished it. The present results suggest that NaF causes a complex mechanical response in the whole-stomach, which might explain gastric discomfort after fluoride ingestion. The relaxation appears owing to production of cAMP, while the contractile effects imply activation of phospholipase C, protein kinase C, influx of Ca(2+), and release of Ca(2+) from ryanodine-sensitive intracellular store.     

Cellular mechanism of adrenalin stimulated chloride secretion via beta-adrenoceptor in T84 cells

In the present study, the intracellular regulatory pathways involved in the adrenalin-stimulated chloride secretion across T84 cells were investigated. Biphasic characteristics were observed in the Isc response to the basolateral addition of adrenalin (0.25 nM-100 microM). The biphasic response was almost abolished by removing ambient Cl(-). Chloride secretion was found to depend on the activities of basolaterally located Na+-K+-2Cl(-) cotransporters and K+ channels. The alpha-adrenoceptor antagonist phentolamine did not have any effect on either phase of adrenalin-induced Isc, while after pretreatment of the beta-adrenoceptor antagonist propranolol, the adrenalin-induced Isc was substantially abolished, suggesting the biphasic response may be mediated by the beta-adrenoceptor. Under whole cell patch-clamp conditions, T84 cells responded to adrenalin with a rise in inward current. The current, which exhibited a linear I-V relationship and time- and voltage-independent characteristics, was inhibited by the chloride channel blocker DPC and the reverse potential was close to the equilibrium potential for Cl(-) (0 mV), implying that the current was Cl(-) selective. When preloaded with a Ca2+-chelating agent, BAPTA/AM did not affect the Isc response to adrenalin, whereas the Isc was destroyed by pretreating the cells with an adenyl cyclase inhibitor, MDL12330A. These observations were further supported by the intracellular [cAMP] measurement experiment, indicating that adrenalin induced chloride secretion could be mediated by a beta-adrenoceptor only involving cAMP as an intracellular second messenger.     

Inhibition of adenylate cyclase by adenosine analogues in preparations of broken and intact human platelets. Evidence for the unidirectional control of platelet function by cyclic AMP.

Whereas adenosine itself exerted independent stimulatory and inhibitory effects on the adenylate cyclase activity of a platelet particulate fraction at low and high concentrations respectively, 2-substituted and N6-monosubstituted adenosines had stimulatory but greatly decreased inhibitory effects. Deoxyadenosines, on the other hand, had enhanced inhibitory but no stimulatory effects. The most potent inhibitors found were, in order of increasing activity, 9-(tetrahydro-2-furyl)adenine (SQ 22536), 2',5'-dideoxyadenosine and 2'-deoxyadenosine 3'-monophosphate. Kinetic studies on prostaglandin E1-activated adenylate cyclase showed that the inhibition caused by either 2',5'-dideoxyadenosine or compound SQ 22536 was non-competitive with MgATP and that the former compound, at least, showed negative co-operativity; 50% inhibition was observed with 4 micron-2',5'-dideoxyadenosine or 13 micron-SQ 22536. These two compounds also inhibited both the basal and prostaglandin E1-activated adenylate cyclase activities of intact platelets, when these were measured as the increases in cyclic [3H]AMP in platelets that had been labelled with [3H]adenine and were then incubated briefly with papaverine or papaverine and prostaglandin E1. Both compounds, but particularly 2',5'-dideoxyadenosine, markedly decreased the inhibition by prostaglandin E1 of platelet aggregation induced by ADP or [arginine]vasopressin as well as the associated increases in platelet cyclic AMP, so providing further evidence that the effects of prostaglandin E1 on platelet aggregation are mediated by cyclic AMP. 2'-Deoxyadenosine 3'-monophosphate did not affect the inhibition of aggregation by prostaglandin E1, suggesting that the site of action of deoxyadenosine derivatives on adenylate cyclase is intracellular. Neither 2',5'-dideoxyadenosine nor compound SQ 22536 alone induced platelet aggregation. Moreover, neither compound potentiated platelet aggregation or the platelet release reaction when suboptimal concentrations of ADP, [arginine]vasopressin, collagen or arachidonate were added to heparinized or citrated platelet-rich plasma in the absence of prostaglandin E1. These results show that cyclic AMP plays no significant role in the responses of platelets to aggregating agents in the absence of compounds that increase the platelet cyclic AMP concentration above the resting value.     

Developmental pattern of cAMP, adenyl cyclase, and cAMP phosphodiesterase in the palate, lung, and liver of the fetal mouse: alterations resulting from exposure to methylmercury at levels inhibiting palate closure

Exposure to methylmercury (MeHg: 10 mg Hg/kg maternal body weight) on 12(6) (days hours) of gestation significantly delays palate closure in the Swiss Webster CFW mouse. The cAMP content and activity of adenyl cyclase and phosphodiesterase (PDE) were measured in the tissues of control and MeHg-induced cleft palates between 13(6) and 17(6) of gestation. Lung and liver were investigated similarly to determine if MeHg affected the adenyl cyclase system of the palate in a unique manner. In control palatal tissue, cAMP levels increased sharply from 13(22) (undetectable) to 14(6) (maximum). PDE activity increased similarly up to 14(2), but decreased 50% between 14(2) and 14(6). Since it has been reported that cAMP induces the synthesis of PDE, the difference in cAMP/PDE from 13(22) to 14(2) and from 14(2) to 14(6) suggests the localization of relatively high levels of cAMP in at least two separate compartments. Between 14(6) and 14(10), the adenyl cyclase activity of control palates decreased significantly. This rapid decrease suggests relatively high adenyl cyclase activity in the medial edge epithelial cells which undergo autolysis prior to shelf fusion (centered at 14(15). Maternal MeHg administration at 12(6) delayed the median time of palatal shelf rotation (14(13)) by 5 hours, and significantly altered the developmental pattern of the adenyl cyclase system. Thus, the increase in cAMP between 14(2) and 14(6) was abolished and the decrease in adenyl cyclase activity between 14(6) and 14(10) was delayed by almost 20 hours. These changes may be manifestions of a MeHg-induced delay in medial edge epithelial cell differentiation. In a previous study, we observed that the fetal liver exhibits the highest MeHg concentration of all tissues. Since MeHg only slightly altered the adenyl cyclase system of the fetal liver compared to the lung and palate (in which MeHg uptake is considerably less), it may be that the effects of MeHg on palatal tissue are not due to a direct effect of MeHg on components of the adenyl cyclase system.     

Cyclic AMP modulation of ion transport across frog retinal pigment epithelium. Measurements in the short-circuit state

In the frog retinal pigment epithelium (RPE), the cellular levels of cyclic AMP (cAMP) were measured in control conditions and after treatment with substances that are known to inhibit phosphodiesterase (PDE) activity (isobutyl-1-methylxanthine, SQ65442) or stimulate adenylate cyclase activity (forskolin). The cAMP levels were elevated by a factor of 5-7 compared with the controls in PDE-treated tissues and by a factor of 18 in forskolin-treated tissues. The exogenous application of cAMP (1 mM), PDE inhibitors (0.5 mM), or forskolin (0.1 mM) all produced similar changes in epithelial electrical parameters, such as transepithelial potential (TEP) and resistance (Rt), as well as changes in active ion transport. Adding 1 mM cAMP to the solution bathing the apical membrane transiently increased the short-circuit current (SCC) and the TEP (apical side positive) and decreased Rt. Microelectrode experiments showed that the elevation in TEP is due mainly to a depolarization of the basal membrane followed by, and perhaps also accompanied by, a smaller hyperpolarization of the apical membrane. The ratio of the apical to the basolateral membrane resistance increased in the presence of cAMP, and this increase, coupled with the decrease in Rt and the basolateral membrane depolarization, is consistent with a conductance increase at the basolateral membrane. Radioactive tracer experiments showed that cAMP increased the active secretion of Na (choroid to retina) and the active absorption of K (retina to choroid). Cyclic AMP also abolished the active absorption of Cl across the RPE. In sum, elevated cellular levels of cAMP affect active and passive transport mechanisms at the apical and basolateral membranes of the bullfrog RPE.