Forskolin activation of serotonin-stimulated adenylate cyclase in the liver fluke Fasciola hepatica

Second messengers mediating mechanical responses to the FARP GYIRFamide in the fluke Fasciola hepatica

Spontaneous phasic contractions recorded from isolated body strips of Fasciola hepatica were increased in frequency and amplitude by GYIRFamide, an FMRFamide-related peptide (FaRP). Superfusion with guanosine 5'-O-(2-thiodiphosphate) (100 microM, n = 5) reduced the effects of GYIRFamide on both frequency (by 82%) and amplitude (by 75%). The adenylate cyclase inhibitor MDL-12330A (25 microM) increased spontaneous activity. MDL-12330A completely inhibited the frequency response to GYIRFamide and reduced the amplitude response by 66% as measured relative to this elevated basal activity (n = 6). Inhibition of phospholipase C (PLC) with neomycin sulfate (1 mM) had no direct effect on activity but reduced the frequency response to GYIRFamide by 64% and the amplitude increase by 95% (n = 9). The protein kinase C (PKC) inhibitor chelerythrine chloride (10 microM) also reduced frequency and amplitude responses by 98 and 99%, respectively, without affecting basal contractility (n = 5). Phorbol 12-myristate 13-acetate, an activator of PKC, increased contraction frequency and amplitude (n = 6). It was concluded that GYIRFamide stimulates mechanical activity in F. hepatica through a G protein, via a PLC- and PKC-dependent second messenger pathway.

Ionic mechanisms underlying spontaneous muscle contractions in the liver fluke, Fasciola hepatica

Spontaneous contractions of liver fluke muscle were abolished in Ca(2+)-free saline and by 100 microM nifedipine and reduced by 5 mM cadmium chloride, suggesting that they are dependent on extracellular Ca(2+). Caffeine (5 mM) significantly increased contraction amplitude and frequency. Ryanodine (100 microM) failed to block the caffeine response but significantly reduced spontaneous contraction frequency, suggesting that intracellular stores have a functional role. Cyclopiazonic acid (5 microM) had no effect on the caffeine response or spontaneous activity. 3-Isobutyl-1-methylxanthine (IBMX), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate significantly increased spontaneous contractions, which implies that cAMP has a regulatory function in motility. Caffeine, however, produced no measurable increase in cAMP. The caffeine effect was inhibited by cadmium chloride and nifedipine, whereas IBMX-induced increases in amplitude were reduced by cadmium chloride. Thus caffeine and cAMP appear capable of opening plasma membrane Ca(2+) channels, but the involvement of cAMP in caffeine responses has not been proved.

Effects of adenosine analogues on basal, prostaglandin E1- and forskolin-stimulated cyclic AMP formation in intact neuroblastoma cells

We have examined the effects of R-phenylisopropyladenosine (R-PIA) and other adenosine analogues on basal, prostaglandin E1 (PGE1)- and forskolin-stimulated cyclic AMP (cAMP) formation in intact N1E-115 neuroblastoma cells, to determine whether the cells contain A1 adenosine receptors that are negatively coupled with adenylate cyclase. Basal levels of cAMP (68 +/- 7 pmol/mg protein; mean +/- SE, N = 15) were not altered by low concentrations of R-PIA. The apparent lack of inhibition was not due to increases in cAMP due to activation of a stimulatory A2 receptor by endogenously-synthesized adenosine. By comparison, low levels of R-PIA did reduce significantly (P less than 0.05) PGE1-dependent increases in cAMP formation (maximum response to PGE1, 972 +/- 77 pmol cAMP/mg protein; EC50 for PGE1, 0.2 microM). Inhibition was dose dependent, and resulted in a 30-50% maximum reduction in production stimulated by PGE1. Nanomolar concentrations of R-PIA elicited half-maximal inhibition; the inhibitory response was blocked by 8-phenyltheophylline (8-PT). The order of potencies of several adenosine analogues in eliciting this response suggested that inhibition was mediated by an A1 adenosine receptor. Examination of the effects of R-PIA on forskolin-stimulated cAMP formation yielded several interesting findings. First, stimulation by the diterpene by itself was blocked by both adenosine deaminase (ADA) and 8-PT (40 and 25% inhibition respectively). Low concentrations of R-PIA (less than 10(-6) M) had no effect on forskolin-stimulated cAMP production. At higher levels (greater than or equal to 10(-6) M) the analogues acted synergistically with the diterpene, to yield cAMP levels that were up to 3-fold higher than the additive effect of the two agents. Potentiation was stereospecific, Ca2+ dependent, and was blocked by 8-PT. The results of this study suggest that, in N1E-115 neuroblastoma cells, inhibitory A1 receptors are not stimulated in response to non-specific elevations in cAMP, but are associated with specific stimulatory receptors such as those activated by PGE1.

Regulation by forskolin of octopamine-stimulated adenylate cyclase from brain of the dipterous Ceratitis capitata

Forskolin, a diterpene that exerts several pharmacological effects, activates adenylate cyclase in brain and in some other mammalian tissues. Properties of forskolin activation of adenylate cyclase from central nervous system of the dipterous Ceratitis capitata are described. The interaction of forskolin with the insect adenylate cyclase system was studied by evaluating its effect on metal-ATP kinetics, protection against thermal inactivation, membrane fluidity and enzyme modulation by fluoride, guanine nucleotides, octopamine, and ADP-ribosylation by cholera toxin. The diterpene stimulated basal enzyme activity both in membranes and Triton X-100-solubilized preparations, apparently devoid of functional regulatory unit, this effect being rapidly reversed by washing the membranes. An increase of Vmax accounts for the activation of soluble and membrane adenylate cyclase preparations by forskolin, whereas the affinity of the enzyme for the substrate was not affected. Forskolin apparently protects the membrane enzyme from thermal inactivation, and at concentrations that promote the enzyme activity the diterpene does not alter membrane microviscosity. Forskolin does not appear to alter the sensitivity of insect adenylate cyclase to sodium fluoride, guanine nucleotide, or regulatory subunit ADP ribosylated by cholera toxin, the combined effect of these factors with the diterpene resulting in a nearly additive enzymatic activation. However, forskolin blocks the octopamine stimulatory input. Results obtained with the insect adenylate cyclase system are discussed and compared to what is known about mammalian systems to propose a mechanism of enzyme activation by forskolin.

Potentiation of prostaglandin E1-stimulated cAMP formation by 12-O-tetradecanoylphorbol-13-acetate in BALB/c mouse 3T3 cells

Prostaglandin E1 (PGE1: 0.1-100 microM), forskolin (0.1-100 microM), and cholera toxin (20 ng/ml) stimulated cAMP formation of BALB/c 3T3 cells. The pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) enhanced PGE1 (10 microM)-stimulated cAMP formation in a concentration- and a time-dependent manner. If the cells were pretreated with TPA (0.1 microM) for only 1 hr, the above augmentation was not observed. Maximal enhancement was observed by pretreatment of the cells for 5 hr with 0.1 microM TPA. Basal cAMP formation was not affected by TPA pretreatment. Other tumor promoters, such as teleocidin and mezerein, showed a potentiating effect similar to that of TPA on the PGE1-stimulated cAMP formation. However, phorbol which is not a tumor promoter, failed to potentiate PGE1 action significantly. These results suggest that the above TPA action may share some common mechanisms with the tumor-promoting action of this agent. On the other hand, the forskolin- and cholera toxin-stimulated cAMP formations were not changed by pretreatment of the cells with TPA. Therefore, our results indicate that the potentiating action of TPA on PGE1-stimulated cAMP formation in 3T3 cells is not due to the activation of the catalytic unit or the stimulatory guanine nucleotide binding protein (Ns) of adenylate cyclase (AC) system by this agent. It is highly likely that TPA induces some alterations on PGE1 receptors or on PGE1 receptor-Ns coupling systems and consequently induces an augmentation of PGE1-stimulated cellular cAMP response.