Intra-adrenal interactions in fish: catecholamine stimulated cortisol release in sea bass (Dicentrarchus labrax L.)

The effect of the catecholamines, adrenaline and noradrenaline, on sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus) interrenal cortisol production was studied in vitro using a dynamic superfusion system technique. Increasing concentrations of catecholamines (10(-6), 10(-8) and 10(-10) M) stimulated cortisol production in a dose-dependent manner, in sea bass only. The increase in cortisol production stimulated by adrenaline (10(-6) M) and noradrenaline (10(-6) M) was inhibited by sotalol (2 x 10(-5) M), but not by prazosin suggesting that catecholamines stimulate cortisol release through the beta-receptor subtype. To evaluate catecholamine-induced signal transduction in head kidney cells, measurements of cAMP production and [H3]myo-inositol incorporation were determined in head kidney cell suspensions. Adrenaline and noradrenaline (10(-6) M) increased cAMP production, but had no effect on total inositol phosphate accumulation. These results indicate that catecholamines released from the chromaffin cells within the interrenal tissue may act as a paracrine factor to stimulate interrenal steroidogenesis in the sea bass.     

Adrenergic response of splanchnic arteries from cirrhotic patients: role of nitric oxide, prostanoids, and reactive oxygen species

Peripheral and splanchnic vasodilatation in cirrhotic patients has been related to hyporesponsiveness to vasoconstrictors, but studies to examine the vascular adrenergic response provide contradictory results. Hepatic arteries from cirrhotic patients undergoing liver transplantation and mesenteric arteries from liver donors were obtained. Segments 3 mm long from these arteries were mounted in organ baths for testing isometric adrenergic response. The concentration-dependent contraction to noradrenaline (10(-8) to 10(-4) M) was similar in hepatic and mesenteric arteries, and prazosin (alpha 1-adrenergic antagonist, 10(-6) M), but not yohimbine (alpha 2-adrenergic antagonist, 10(-6) M), produced a rightward parallel displacement of this contraction in both types of arteries. Phenylephrine (alpha 1-adrenergic agonist, 10(-8) to 10(-4) M) and clonidine (alpha 2-adrenergic agonist, 10(-8) to 10(-4) M) also produced concentration-dependent contractions that were comparable in hepatic and mesenteric arteries. The inhibitor of cyclooxygenase meclofenamate (10(-5) M), but not the inhibitor of nitric oxide synthesis N(w)-nitro-l-arginine methyl ester (l-NAME, 10(-4) M), potentiated the response to noradrenaline in hepatic arteries; neither inhibitor affected the response to noradrenaline in mesenteric arteries. Diphenyleneiodonium (DPI; 5 x 10(-6) M), but neither catalase (1000 U/ml) nor tiron (10(-4) M), decreased the maximal contraction for noradrenaline similarly in hepatic and mesenteric arteries. Therefore, it is suggested that, in splanchnic arteries from cirrhotic patients, the adrenergic response and the relative contribution of alpha 1- and alpha 2-adrenoceptors in this response is preserved, and prostanoids, but not nitric oxide, may blunt that response. Products dependent on NAD(P)H oxidase might contribute to the adrenergic response in splanchnic arteries from control and cirrhotic patients.     

Histamine interaction on surface recognition sites of H2-type in parietal and non-parietal cells isolated from the guinea pig stomach

In gastric cells isolated by pronase digestion from the guinea pig, histamine stimulated cAMP production in 3 fundic cell fractions (EC50 = 1.6--2 x 10(-4) M) enriched in parietal (94%), peptic (63%) and mucous cells (87%) as well as in antral cells (EC50 = 4 x 10(-4) M) that are devoid of parietal cells. Histamine stimulations were completely inhibited by the H2 antagonist cimetidine (Ki = 0.27--0.57 x 10(-6) M) or by the H1 antagonist diphenhydramine, but at 100-times lower potency (Ki = 22--45.7 x 10(-6) M), indicating the presence of histamine H2 receptors in parietal and nonparietal cells of the guinea pig gastric mucosa.     

Altered physiological responsiveness and decreased cyclic AMP levels in rat atria after dietary cod liver oil supplementation and its possible association with an increased membrane phospholipid n-3/n-6 fatty acid ratio

Rats were given a cod liver oil supplemented diet and a standard diet for 4 months. The cod liver oil supplementation resulted in a marked increase in the 20:5(n-3) and 22:6(n-3) fatty acids and a marked decrease in the 20:4(n-6) fatty acid in phosphatidylcholine and ethanolamine of the atrial membrane. Atria from the cod liver oil treated rats showed a marked decrease in contractile force, heart rate and cyclic AMP (cAMP) levels under basal conditions. Stimulation with noradrenaline (1 X 10(-6) M) during high oxygen saturation and reoxygenation resulted in an equal increase in the mechanical responses of the two groups in spite of the significantly different levels of cAMP, whereas in hypoxia, both the cAMP level and the contractile force were significantly lower in the cod liver oil treated group. These results indicate that changes in the fatty acid composition of heart membrane phospholipids is associated with changes in adenylate cyclase activity and physiological function of the rat heart and that an increase in the n-3/n-6 fatty acid ratio in membrane phospholipids of the heart results, when oxygen is abundant in enhanced cAMP-independent contractile activity.     

Cyclid 3':5'-nucleotide phosphodiesterase. Interconvertible multiple forms and their effects on enzyme activity and kinetics.

An extract of rat liver or human platelet displayed three cyclic 3':5'-nucleotide phosphodiesterase activity peaks (I, II, and III) in a continuous sucrose density gradient when assayed with millimolar adenosine 3':5'-monophosphate (cAMP) or guanosine 3':5'-monophosphate (cGMP). The three fractions obtained from each nucleotide were not superimposable. The molecular weights corresponding to the three activity peaks of cAMP phosphodiesterase in rat liver were approximately: I, 22,000; II, 75,000; and III, 140,000. In both tissues, fraction I was barely detectable when assayed with micromolar concentrations of either nucleotide, presumably because fraction I has low affinity for cAMP and cGMP. Any one of the three forms upon recentrifugation on the gradient generated the others, indicating that they were interconvertible. The multiple forms appear to represent different aggregated states of the enzyme. The ratio of the three forms of cAMP phosphodiesterase in the platelet was shifted by dibutyryl cAMP (B2cAMP) and by the enzyme concentration. B2cAMP enhanced the formation of fraction I. Low enzyme concentration favored the equilibrium towards fraction I, while high enzyme concentration favored fraction III. When phosphodiesterase activities in the extract of rat liver, human platelets, or bovine brain were examined as a function of enzyme concentration, rectilinear rates were observed with micromolar, but not with millimolar cAMP or cGMP. The specific activity with millimolar cAMP was higher with low than with high protein concentrations, suggesting that the dissociated form catalyzed the hydrolysis of cAMP faster than that of the associated form. In contrast, the specific activity with millimolar cGMP was lower with low than with high protein concentrations. Supplementing the reaction mixture with bovine serum albumin to a final constant protein concentration did not affect the activity, suggesting that the concentration of the enzyme rather than that of extraneous proteins affected the enzyme activity. A change in enzyme concentration affected the kinetic properties of phosphodiesterase. A low enzyme concentration of cAMP phosphodiesterase yielded a linear Lineweaver-Burk plot, and a Km of 1.2 X 10(-4) M (bovine), 3 X 10(-5) M (platelet), or 5 X 10(-4) M (liver), while a high enzyme concentration yielded a nonlinear plot, and apparent Km values of 1.4 X 10(-4) M and 2 X 10(-5) M (brain), 4 X 10(-5) M and 3 X 10(-6) M (platelet), or 4 X 10(-5) M and 3 X 10(-6) (liver). Since a low enzyme concentration favored fraction I, the dissociated form, whereas a high enzyme concentration favored fraction III, the associated form, these kinetic constants suggest that the dissociated form exhibits a high Km and the associated form exhibits a low Km. In contrast, a high enzyme concentration gave a linear kinetic plot for cGMP phosphodiesterase, while a low enzyme concentration gave a nonlinear plot...     

Cyclic AMP activation of respiration of the liver mitochondria in different metabolic states]

cAMP 10(-6) activates the liver mitochondria respiration in all the metabolic states and failed to change or increased the phosphorylation rate in the oxidation of saturating concentration of succinate and isocitrate. Preincubation of mitochondria or homogenate of the liver with cAMP is obligatory for this effect. The fraction V of serum albumin and EDTA did not prevent the effect. Noradrenaline enhanced the mitochondrial respiration only in incubation with the homogenate. The effect of noradrenaline and cAMP was not summed up. Probably the noradrenaline effect was mediated through cAMP. The data obtained are against the decisive role of the respiration and phosphorylation uncoupling or the oxidation substrate accumulation and lead to the assumption on the mitochondria enzymes activation.     

The role of cyclic nucleotides and prostaglandins in heart function.

A short review of the role of cyclic nucleotides and prostaglandins (PGs) in normal and pathological functions of the heart is given. Possible interrelationships of these two regulatory systems have been studied by using spontaneously beating rat atria preparations. Addition of noradrenaline (NA) to the incubate (1 . 10(-6) M) caused an increase in amplitude and frequency which was preceded and parallelled by an elevation of the tissue cAMP level. A transient increase in cGMP and PGE values was also seen. Propranolol (5 . 10(-6) M) abolished the increase in amplitude and frequency as well as in cAMP and PGE concentrations. Indomethacin (1 . 10(-5) M) inhibited the formation of PGE. The increase in cGMP was blocked by phenoxybenzamine. Interchange between beta- and alpha-receptors according as the temperature is lowered has been described earlier. Hypothermia (20 degrees C) had a positive inotropic effect on the atria and increased the tissue cAMP concentration. Loading of the atria caused an increase in cAMP without any effects on cGMP or PGs. Slight hypoxia did not change the cAMP or PG levels, but elevated the cGMP values. Arrhythmias induced by hypo- or hyperpotassemia did not modify the biochemical parameters measured. PGF2alpha (1. 10(-5) M) normalized the atrial rhythm and increased the amplitude without changing cyclic nucleotide or PG levels. PGE1 (1 . 10(-4) M) increased the amplitude of normorhythmic atria and the tissue concentration of cAMP. PGE2 was the only PG tested which stimulated the heart adenylate cyclase in vitro. There seems to be close but complicated relationships between cyclic nucleotides and PGs in the heart.     

Effect of biologically active agents on the feeding response of Dileptus anser]

A study was made of the effects of pharmacological agents--adrenaline, noradrenaline, serotonine, dibenamine, theophylline, and emidazole--on the phagocytar activity of Dileptus anser. These effects were estimated in terms of food response changes towards chemical food models (CFM) made in cysteine, lecithine or tween-40 solutions. The above pharmacological agents were also tested as phagocytosis inductors. Of these, only adrenalin appeared to be an effective inductor of the food response. The CFM, made in adrenaline, was stimulated by 10(-6) M theophylline, and inhibited with 10(-4)--10(-8) M imidazole. The addition of 10(-3)--10(-12) M adrenaline or 10(-8)--10(-10) M serotonine to the Dileptus-containing medium stimulated phagocytosis of CFM. 5.10(-6) M dibenamin decreased phagocytotic intensity of CFM. 10(-6) M theophylline stimulated, while 10(-4) M inhibited the food response. It is proposed that protozoans have receptors capable of accepting hormones. A possible role of the system of cyclic AMP in transporting hormonal and food stimules in protozoans is discussed.     

Cyclic 3', 5'-AMP relay dictyostelium discoideum. V. Adaptation of the cAMP signaling response during cAMP stimulation

In dictyostelium discoideum, extracellular cAMP activates adenylate cyclase, which leads to an increase in intracellular cAMP and the rate of cAMP secretion. The signaling response to a constant cAMP stimulus is terminated after several minutes by an adaptation mechanism. The time- course of adaptation stimuli of 10(-6) or 10(-7) M cAMP was assessed. We used a perfusion technique to deliver defined cAMP stimuli to [(3)H]adenosine-labeled amoebae and monitored their secretion of [(3)H]cAMP. Amoebae were pretreated with 10(-6) or 10(-7) M cAMP to periods of 0.33-12 minutes, and then immediately given test stimuli of 10(-8) M to 2.5 x 10(-7) M cAMP. The response to a given test stimulus was progressively attenuated and finally extinguished as the duration of the pretreatment stimulus increased. During concentration of the test stimulus. The responses to test stimuli of 10(-8), 5 x 10(-8), 10(-7), or 2.5 x 10(-7) M cAMP were extinguished after approximately 1, 2.25,2.5, and 10 min, respectively. 1.5 min of stimulation with 10(-7) M cAMP was necessary to extinguish the response of a test stimulus of 10(-8) M cAMP. Our data suggest that adaptation begins within 20 s of stimulation, rises rapidly for approximately 2.5 min, and reaches a plateau after approximately 10 min. The absolute rate of rise was faster during pretreatment with 10(-6) than with 10(-7) M cAMP. These results support a working hypothesis in which the occupancy of surface cAMP receptors leads to changes in two opposing cellular processes, excitation and adaptation, that control the activity of D. discoideum adenylate cyclase.     

Activators of Protein Kinase C Act at a Postreceptor Site to Amplify Cyclic AMP Production in Rat Pinealocytes

Activation of alpha 1-adrenoceptors appears to amplify beta-adrenergic stimulation of cyclic AMP (cAMP) accumulation in rat pinealocytes severalfold by a mechanism involving activation of a Ca2+-, phospholipid-dependent protein kinase (protein kinase C). The mechanism of action of protein kinase C was investigated in this report using intact cells. Activation of protein kinase C with 4 beta-phorbol 12-myristate 13-acetate (PMA; 10(-7) M) or the alpha 1-adrenergic agonist phenylephrine (PE; 10(-6) M) did not inhibit cAMP efflux in beta-adrenergically stimulated cells. The amplification of the beta-adrenergic cAMP response by these agents also occurred in the presence of isobutylmethylxanthine (10(-3) M) and Ro 20-1724 (10(-4) M), an observation suggesting that inhibition of cAMP phosphodiesterase activity is not the mechanism of action. Furthermore, although PMA (10(-7) M) caused a sixfold increase in the magnitude of the cAMP response to isoproterenol, it did not alter the EC50 of the response (1.7 X 10(-8) M), a result indicating that protein kinase C activation does not alter beta-adrenoceptor sensitivity. The cAMP response following cholera toxin pretreatment (60-120 min) was rapidly and markedly enhanced by alpha 1-adrenergic agonists (cirazoline greater than PE greater than methoxamine), by phorbol esters (PMA greater than 4 beta-phorbol 12,13,-dibutyrate much greater than 4 alpha-phorbol 12,13-didecanoate), and by synthetic diacylglycerols (1,2-dioctanoylglycerol greater than 1-oleoyl 2-acetylglycerol much greater than diolein). The cAMP response to forskolin (10(-5)-10(-3) M) was also increased by PE (3 X 10(-6) M) and PMA (10(-7) M).(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the alpha-adrenergic activation of hepatic glucose output. I. Studies on the alpha-adrenergic activation of phosphorylase and gluconeogenesis and inactivation of glycogen synthase in isolated rat liver parenchymal cells.

Epinephrine and the alpha-adrenergic agonist phenylephrine activated phosphorylase, glycogenolysis, and gluconeogenesis from lactate in a dose-dependent manner in isolated rat liver parenchymal cells. The half-maximally active dose of epinephrine was 10-7 M and of phenylephrine was 10(-6) M. These effects were blocked by alpha-adrenergic antagonists including phenoxybenzamine, but were largely unaffected by beta-adrenergic antagonists including propranolol. Epinephrine caused a transient 2-fold elevation of adenosine 3':5'-monophosphate (cAMP) which was abolished by propranolol and other beta blockers, but was unaffected by phenoxybenzamine and other alpha blockers. Phenoxybenzamine and propranolol were shown to be specific for their respective adrenergic receptors and to not affect the actions of glucagon or exogenous cAMP. Neither epinephrine (10-7 M), phenylephrine (10-5 M), nor glucagon (10-7 M) inactivated glycogen synthase in liver cells from fed rats. When the glycogen synthase activity ratio (-glucose 6-phosphate/+ glucose 6-phosphate) was increased from 0.09 to 0.66 by preincubation of such cells with 40 mM glucose, these agents substantially inactivated the enzyme. Incubation of hepatocytes from fed rats resulted in glycogen depletion which was correlated with an increase in the glycogen synthase activity ratio and a decrease in phosphorylase alpha activity. In hepatocytes from fasted animals, the glycogen synthase activity ratio was 0.32 +/- 0.03, and epinephrine, glucagon, and phenylephrine were able to lower this significantly. The effects of epinephrine and phenylephrine on the enzyme were blocked by phenoxybenzamine, but were largely unaffected by propranolol. Maximal phosphorylase activation in hepatocytes from fasted rats incubated with 10(-5) M phenylephrine preceded the maximal inactivation of glycogen synthase. Addition of glucose rapidly reduced, in a dose-dependent manner, both basal and phenylephrine-elevated phosphorylase alpha activity in hepatocytes prepared from fasted rats. Glucose also increased the glycogen synthase activity ratio, but this effect lagged behind the change in phosphorylase. Phenylephrine (10-5 M) and glucagon (5 x 10(-10) M) decreased by one-half the fall in phosphoryalse alpha activity seen with 10 mM glucose and markedly suppressed the elevation of glycogen synthase activity. The following conclusions are drawn from these findings. (a) The effects of epinephrine and phenylephrine on carbohydrate metabolism in rat liver parenchymal cells are mediated predominantly by alpha-adrenergic receptors. (b) Stimulation of these receptors by epinephrine or phenylephrine results in activation of phosphorylase and gluconeogenesis and inactivation of glycogen synthase by mechanisms not involving an increase in cellular cAMP. (c) Activation of beta-adrenergic receptors by epinephrine leads to the accumulation of cAMP, but this is associated with minimal activation of phosphorylase or inactivation of glycogen synthase...     

Some features of cyclic adenosine monophosphate metabolism in mouse liver and hepatoma 22]

The levels of cyclic adenosine monophosphate (cAMP) and two forms of cAMP phosphodiesterase with low (PDE1) and high (PDE2) affinity for the substrate were determined in homogenates from mouse liver and transplanted hepatoma 22. The level of cAMP in the tumour is 3 times lower than that in liver. By te kinetic parameters (Vmax, Km, pH optimum) adenylate cyclase from tumour does not show any significant differences as compared to the liver enzyme; the enzyme from hepatoma is, however, more sensitive to activation by F- ions. The activities of adenylate cyclase in liver and tumour cells are the same. Phosphodiesterases of cAMP from tumour and liver cells are similar in their Km values (3,3-10(-4) M for PDE1 and 2-10(-6) M for PDE2); however, the maximal and real rates of cAMP hydrolysis in hepatoma are much higher than in liver. The fact that both cAMP phosphodiesterase activities have similar dependence on Mg2+ and Ca2+ concentrations, suggests that PDE1 is a latent form of PDE2. In tumour cells the equilibrium between these two forms is probably shifted towards the enzyme with high affinity for the substrate. The results suggest that a decreased cAMP level in hepatoma cells (as compared to the liver) is due to the activation of PDE2.     

Effect of synthetic cyclic nucleotides on immunologic histamine release from calf granulocytes.

Sensitized bovine granulocytes release histamine when exposed to specific antigens. A unique modulation of histamine release by adrenergic agents has been shown in the bovine; beta-adrenergic agonists enhance and alpha-adrenergic agonists inhibit histamine release. This is an opposite response to that reported in other species. The present study was undertaken to determine the possible relationship between cyclic nucleotides and adrenergic agents in this species. Dibutyryl cAMP enhanced antigen-induced histamine release over the complete concentration range tested (10(-6)--10(-3)M); it also overcame, in a dose-dependent manner, the inhibition of antigen-induced histamine release produced by 10(-4) M phenylephrine. The 8-bromo cGMP AND 0-MONOBUTYRYL CGMP had no significant effect on antigen-induced histamine release nor did 8-bromo-cGMP have any significant effect on the enhancement of histamine release produced by 10(-4) M dibutyryl cAMP. These findings suggest that only cAMP has a role in the modulation of antigen-induced histamine release from bovine granulocytes.     

Effect of cyclic adenosine-3,5-monophosphate and its dibutyryl analog on macromolecule biosynthesis in actively proliferating cells

The effect of cAMP and its dibutyryl analogue on the biosynthesis of nucleic acids and protein in active proliferating cells was studied. It was shown that cAMP (10(-3)--10(-4)M) caused stimulation of the biosynthesis of DNA and RNA in Ehrlich's ascites carcinoma (EAC) cells and intensification of collagen biosynthesis in the chick embryo cartilage tissue in vitro. Dibutyryl -- cAMP (10(-3)--10(-4)M) has an inhibitory action on the biosynthesis of macromolecules both in EAC cells and embryonic cartilage tissue. Addition of cAMP phosphodiesterase inhibitors together with cAMP to the incubation media prevents the stimulation of macromolecular biosynthesis observed under the influence of cAMP. Studies on cAMP metabolism revealed that this compound is rapidly catabolized to AMP and adenosine. The latter enters the cells and incorporates into the adenyl nucleotide intracellular pool. The stimulant action of exogenous cAMP is related to its extracellular metabolism rather than to the intracellular effects of the nucleotide.     

Influence of cyclic AMP on DNA synthesis in slices of regenerating rat liver.

DNA synthesis in slices of regenerating rat liver is inhibited by adenosine cyclic 3',5'-monophosphate [cAMP]. The number of cells synthesizing DNA as assayed by 2-14C-thymidine incorporation is reduced by 65% in the presence of 10(-3) M cAMP. The inhibition of cAMP is not specific; other adenosine compounds, N6,O2,-dibutyryl adenosine 3',5'-monophosphate, 5'AMP and adenosine have the same effect. Moreover, the concentration of cAMP in the cell required for this inhibition is much higher than the normal levels of cAMP in liver cells.     

Effect of adenosine 3',5'-cyclic monophosphate on the RNA and DNA synthesis and cell proliferation of rat hepatocytes in primary culture: a radioautographic study.

The effect of a 24-h treatment with various doses (from 1.5-10-minus 8 to 3.0-10-minus 3 M) of adenosine 3',5'-cyclic monophospahte (cAMP) on morphometric parameters, [5--3H]uridine radioactivity concentration (URC), [methyl--3H]thymidine [Me--3H]-Tr) labelling index per hour (L.I./h) and per cent mitotic index (M.I.%) of young rat differentiated hepatocytes in primary tissue culture were investigated by morphometric and radioautographic methods. In such cells cAMP was found to induce: (1) a reduction of the apparent surface area (ASA) of total nucleoli, karyoplasm and cytoplasm; (2) significant increases in URC of all the subcellular compartments at all the dosages employed (only cAMP at 1.5-10-minus 8 M did not change karyoplasmic and cytoplasmic URC values); (3) marked increments in [Me--3H]Tdr L.I./h and M.I.% from the lowest dose up to 1.5-10-minus 4 M; at higher doses the L.I./h and M.I.% were less stimulated or approached control values. In cultured rat hepatocytes, adenosine-5'-phosphate (5'-AMP) (1.5-10-minus 4 M per 24 h) increased the karyoplasmic and total cell ASA, the lone total nucleolar URC and both the L.I./h and M.I.%. However, these metabolic effects were significantly less intense than those elicited by isomolar cAMP. Theophylline (Theo) (5.5-10-minus 5 M per 24 h) reduced the in vitro rat hepatocyte total nucleolar ASA but affected neither other morphometric nor any of the URC values. The same dose of Theo plus cAMP (1.5-10-minus M) had no morphometric effect but significantly increased the URC values of all primary rat hepatocyte compartments. Actinomycin D (DAct) (0.1 mug/ml per 24 h) plus cAMP (1.5-10-minus 4 M) decreased the cultured rat hepatocyte total nucleolar ASA but enlarged that of karyoplasm and cytoplasm and, further, markedly curtailed all the compartmental URC values. These data support the hypothesis that cAMP amplified the template activity of the liver chromatin and accelerates the flow of differentiated primary young rat hepatocytes into the various stages of the mitotic cell cycle.     

3',5'-cyclic adenosine monophosphate as an intracellular second messenger of luteinizing hormone: application of the forskolin criteria

The role of adenosine 3',5'-cyclic monophosphate (cAMP) as an intracellular second messenger of luteinizing hormone (LH) was reinvestigated in vitro with diterpene forskolin, a highly specific activator of adenylate cyclase. Treatment of cultured testicular cells from adult hypophysectomized rats with increasing concentrations (10(7)-10(-4) M) of forskolin produced dose-dependent increments in cAMP and testosterone accumulation. Concomitant blockade of cAMP-phosphodiesterase activity with 3-isobutyl-1-methyl-xanthine (10(-4) M) resulted in significant (P less than 0.05) enhancement of the forskolin effect for all but the 10(-4) M forskolin dose. Potency evaluation as judged by half-maximal stimulation of testosterone accumulation revealed median effective doses (mean +/- SE) of 1.25 +/- 0.2 x 10(-5), 1.7 +/- 0.5 x 10(-5), and 2.5 +/- 0.4 x 10(-10) M for forskolin, N6, O2'-dibutyryl cAMP (Bt2cAMP), and human chorionic gonadotropin (hCG), respectively. Examination of the time requirements of forskolin disclosed time-dependent increments in the accumulation of extracellular cAMP and testosterone, the earliest significant (P less than 0.05) increases being noted by 6 hr of treatment. In comparison, a minimal time requirement of less than or equal to 12 hr was noted for hCG- and choleragen-stimulated androgen biosynthesis, whereas the apparent onset of action of Bt2cAMP was delayed to the 24-hr time point. Although 10(-7) M of forskolin by itself did not alter the accumulation of testosterone, its addition resulted in substantial amplification of the hCG effect, producing a 4.6-fold reduction in the median effective dose (ED50) of hCG. Moreover, concurrent treatment with this functionally inert dose of forskolin rendered steroidogenically inert doses of hCG (eg, 10(-11) or 3 x 10(-11) M) steroidogenically potent. However, combined treatment with maximally stimulatory doses of Bt2cAMP (10(-4) M) and one of several testicular cell agonists [forskolin (10(-4) M), choleragen (10(-9) M) or hCG (10(-9) M)] did not prove additive. Taken together, our findings indicate that forskolin, like LH, is capable of stimulating testicular cAMP generation as well as androgen biosynthesis and that a functionally inert low dose of forskolin can significantly amplify LH hormonal action. Inasmuch as forskolin-stimulated and forskolin-amplified hormonal action are acceptable as novel criteria of cAMP dependence, our observations provide new evidence in keeping with the notion that cAMP may be in intracellular second messenger of LH.     

Noradrenaline inhibition of transmitter efflux in a renal artery preparation and the presynaptic receptor theory.

The effect of noradrenaline on the stimulation-induced efflux of tritium in cattle renal arteries preincubated with [3H]noradrenaline was determined. Preparations were stimulated transmurally, with 300 shocks over a range of frequencies (1--15 Hz) in the presence and absence of noradrenaline (3 X 10(-6) M). The agonist inhibited the efflux most at 1 Hz but the extent of the inhibition did not vary with frequency between 2, 5, and 15 Hz. It is concluded that a negative feedback system, modulating neurotransmitter release, and increasingly activated by endogenously released noradrenaline as the frequency of stimulation rises, cannot account for the pattern of efflux inhibition induced by exogenous noradrenaline.     

MCH-induced pigment aggregation in teleost melanophores is associated with a cAMP reduction.

It has previously been shown that alpha 2-adrenoceptors are involved in noradrenaline-induced pigment aggregation within fish melanophores. In the present investigation, melanin concentrating hormone (MCH) elicited pigment aggregation (EC50 approximately 1 x 10(-7) M) that was associated with a significant reduction in the cAMP content; 1 x 10(-7) M MCH reduced the cAMP content from a basal level of 50.4 +/- 2.8 pmol/mg protein to 36.9 +/- 3.8 pmol/mg protein. Like the alpha 2-adrenoceptor-induced pigment aggregation, the MCH response was effectively blocked by the adenylate cyclase stimulator forskolin. These findings suggest that attenuation of cAMP may serve as an intracellular signal transduction mechanism for both MCH and noradrenaline.     

Both somatostatin and the caudal neuropeptide, urotensin II, stimulate lipid mobilization from coho salmon liver incubated in vitro

The direct effects of somatostatin-14 (SRIF; synthetic ovine) and the fish caudal neuropeptide, urotensin II (UII; synthetic Gillichthys), on fatty acid (FA) release and on lipolytic enzyme (triacylglycerol lipase) activity were determined on coho salmon liver slices incubated in vitro. FA release was continuously measured by pH-stat titration. Additionally, gas chromatographic analysis of the incubation medium was performed to determine the type and relative composition of medium fatty acid constituents. SRIF and UII both stimulated FA release in a dose-dependent manner; the two peptides appeared to stimulate FA release in an equimolar manner. Maximal response was obtained at 1 X 10(-5) M; ED50 was approximately 2 X 10(-7) M. SRIF-stimulated FA release did not result in differential secretion of any particular FA type. Tissue triacylglycerol lipase activity was significantly enhanced by addition of UII or SRIF (2 X 10(-6) M). Dibutyryl cAMP and IBMX both stimulated FA release and lipase activity; dbcAMP stimulated FA release in dose-dependent manner. These results indicate that SRIF and UII directly enhance lipid mobilization from salmon liver slices and suggest that SRIF- and UII-stimulated lipid mobilization from salmon liver slices is mediated through cAMP.