Inhibition of cyclic AMP efflux by insect pheromones and fatty acids

New perspectives in signaling mediated by receptors coupled to stimulatory G protein: the emerging significance of cAMP efflux and extracellular cAMP-adenosine pathway

G protein-coupled receptors (GPCRs) linked to stimulatory G (Gs) proteins (GsPCRs) mediate increases in intracellular cyclic AMP as consequence of activation of nine adenylyl cyclases , which differ considerably in their cellular distribution and activation mechanisms. Once produced, cyclic AMP may act via distinct intracellular signaling effectors such as protein kinase A and the exchange proteins activated by cAMP (Epacs). More recently, attention has been focused on the efflux of cAMP through a specific transport system named multidrug resistance proteins that belongs to the ATP-binding cassette transporter superfamily. Outside the cell, cAMP is metabolized into adenosine, which is able to activate four distinct subtypes of adenosine receptors, members of the GPCR family: A₁, A_2A, A_2B, and A₃. Taking into account that this phenomenon occurs in numerous cell types, as consequence of GsPCR activation and increment in intracellular cAMP levels, in this review, we will discuss the impact of cAMP efflux and the extracellular cAMP-adenosine pathway on the regulation of GsPCR-induced cell response.

Cyclic AMP export from lymphocytes in hypertension

While the importance of receptor-mediated intracellular cyclic AMP in blood pressure regulation is well documented, few studies have evaluated the physiologic relevance of cyclic AMP exported from cells. We report evidence of a relationship between blood pressure and the transport of intracellular cyclic AMP from lymphocytes. Twenty-eight hypertensive and 56 normotensive white and black volunteers (mean age 40 years) were studied. Both intra- and extracellular concentrations of cyclic AMP were determined in lymphocytes following incubation with 10(-5) M isoproterenol. Compared to normotensives, hypertensives (p = 0.001), particularly white hypertensives (p = 0.023) had higher levels of exported cyclic AMP. These values were independent of intracellular concentrations of cyclic AMP, which were similar across the groups. Exported cyclic AMP was independent of both sodium excretion and beta-adrenergic receptor sensitivity, the latter being lower in white hypertensives (p = 0.024). Across all subjects, exported cyclic AMP was correlated with MAP (r = .39, p < 0.001). These findings indicate that the active transport of cyclic AMP may be enhanced in hypertension and suggest a possible pathway which might explain existing data of increased cyclic AMP levels in hypertension.

Differential effects of omega-3 fish oils on protein kinase activities in vitro

We studied the in vitro effects of omega-3 fish oils and other fatty acids on the activity of crude protein kinase C from S49 lymphoma cells, on partially purified enzyme from rat cerebrum, on homogeneous protein kinase C from bovine brain, and, for comparison, on type I adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase. In the absence of exogenous phospholipid, the fish oils cis-5,8,11,14,17-eicosapentaenoic acid (EPA) and acid (DCHA) enhance the catalytic cis-4,7,10,13,16,19-docosahexaenoic activity of protein kinase C and support the binding of [3H]phorbol 12,13-dibutyrate, both to approximately 50% of the level supported by phosphatidylserine. In the presence of phosphatidylserine, the omega-3 fatty acids reduce catalytic activity and [3H]phorbol 12,13-dibutyrate binding by about one-half. The effects of the omega-3 fatty acids on enzyme activity suggest that fish oils act as partial agonists competitively with phosphatidylserine. EPA, DCHA, and arachidonate (but not a variety of saturated fatty acids) inhibit the cAMP-dependent protein kinase. Thus dietary fish oils and cellular fatty acids mobilized by the action of phospholipase A2 may differentially modulate the activities of protein kinase C and cAMP-dependent protein kinase. These data suggest means by which unsaturated fatty acids mobilized within cells may act as second messengers.

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucleotides, eicosanoids

Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent Ki values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. Ki,PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. Ki,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: Km = 1.5 mmol/l, Jmax = 0.34 pmol S-1 cm-1, r (extracellular/intracellular amount at steady state) = 0.91; for cGMP: Km = 0.29 mmol/l, Jmax = 0.31 pmol S-1 cm-1, r = 0.55. Comparison of app. Ki,cGMP with app. Ki,PAH of ten substrates gave a linear relation with a ratio of 1.83 +/- 0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1 alpha, A1, B1, E2, F2 alpha, D2, A2 and B2 with an app. Ki,PAH between 0.08 and 0.18 mmol/l. The app. Ki of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1 alpha (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. Ki,PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1 alpha (0.77 mmol/l) and 2,3-dinor-6-keto-PGF1 alpha (0.57 mmol/l) as well as that of thromboxane B2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. Ki,SO4(2-) 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed Km = 0.61 mmol/l and Jmax of 4.26 pmol S-1 cm-1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct regulation of ion channels by fatty acids

A variety of fatty acids regulate the activity of specific ion channels by mechanisms not involving the enzymatic pathways that convert arachidonic acid to oxygenated metabolites. Furthermore, these actions of fatty acids occur in patches of membrane excised from the cell and are not mediated by cellular signal transduction pathways that require soluble factors such as nucleotides and calcium. Thus, fatty acids themselves appear to regulate the action of channels directly, much as they regulate the action of several purified enzymes, and might constitute a new class of first or second messengers acting on ion channels.