Role of Phospholipids in Coupling of Adenosine and Dopamine Receptors to Striatal Adenylate Cyclase

Treatment of striatal washed particles with phospholipase A(2) or C abolished the activation of adenylate cyclase by dopamine but not by N(16)-phenylisopropyl adenosine (PIA). The inhibition of dopamine-sensitive cyclase was dependent on Ca2+ and increased with time and phospholipase concentration. F(-)-sensitive cyclase was not affected by phospholipase A(2) treatment, but was enhanced by phospholipase C treatment. Phospholipase D did not affect basal, PIA, dopamine, or F(-)-sensitive cyclase activities. The observed effects of phospholipase A(2) were not due to either the detergent effect of lysophospholipids or to contaminating proteases. Dopamine-sensitive cyclase, inactivated by pretreatment with phospholipase A(2), was restored by asolectin (a soybean mixed phospholipid), phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine, but not by phosphatidylinositol. Phosphatidylserine and phosphatidylcholine were equipotent in restoring dopamine-sensitive activity. Lubrol-PX, a nonionic detergent, abolished completely the dopamine-sensitive cyclase activity, whereas PIA-sensitive activity was slightly inhibited. In contrast, digitonin inhibited dopamine- and PIA-sensitive cyclase activity in a parallel fashion. Lubrol-PX released some adenylate cyclase into a 16,000 x g supernatant fraction that was stimulated by PIA but not by dopamine. Removal of most of the free detergent by Bio-bead SM 2 enhanced stimulation by PIA but did not restore sensitive cyclase. The data suggest that the requirement for phospholipids for the coupling of dopamine and adenosine receptors to the striatal adenylate cyclase may be different and that the adenosine receptors may be more tightly coupled to the enzyme than are dopamine receptors.     

Regulation of Calmodulin- and Dopamine-Stimulated Adenylate Cyclase Activities by Light in Bovine Retina

Abstract: Neural retina from most species contains 3,4-dihydroxyphenylethylamine (dopamine) receptors coupled to stimulation of adenylate cyclase activity. It has been demonstrated that release of dopamine from its neurons and subsequent occupation of dopamine receptors is increased by light. In this study, we have shown that adenylate cyclase activity in bovine retina is highly responsive to the endogenous Ca²⁺-binding protein, cal-modulin, and that calmodulin can increase dopamine-sen-sitive adenylate cyclase activity in bovine retina. We further demonstrate that both dopamine- and calmodulin-stimulated adenylate cyclase activities can be regulated by alterations in light. Bovine retinas were dissected from the eye under a low-intensity red safety light, defined as dark conditions, and incubated for 20 min in an oxygenated Krebs Henseleit buffer under either dark or light conditions. The retinas were then homogenized and adenylate cyclase activity measured in a paniculate fraction washed to deplete it of endogenous Ca²⁺ and calmodulin. Activation of adenylate cyclase activity by calmodulin, dopamine, and the nonhydrolyzable GTP analog, gua-nosine-5′-(β,γ-imido)triphosphate (GppNHp), was significantly (60%) greater in paniculate fractions from retinas that had been incubated under dark conditions as compared to those incubated under light conditions. Basal, Mn²⁺-, and GTP-stimulated adenylate cyclase activities were not altered by changes in lighting conditions. Calmodulin could increase the maximum stimulation of adenylate cyclase by dopamine in retinas incubated under either dark or light conditions, but the degree of its effect was greater in retinas incubated under light conditions. Activation of adenylate cyclase by calmodulin, dopamine, and GppNHp in paniculate fractions from retinas incubated under light conditions was indistinguishable from the activation obtained when retinas were incubated in the dark in the presence of exogenous dopamine. These results suggest that an increased release of dopamine occurs in light. The decreased response of adenylate cyclase to exogenous dopamine can then be explained by a subsequent down-regulation of dopamine receptor activity. The down-regulation of dopamine receptor activity can also regulate activation of adenylate cyclase by GppNHp and calmodulin. The results suggest that dopamine, calmodulin, and GppNHp are modulators of a common component of adenylate cyclase activity, and this component is regulated by light.     

Interaction of the Inhibitory GTP Regulatory Component with Soluble Cerebral Cortical Adenylate Cyclase

Functional interaction of the inhibitory GTP regulatory component (Ni) with the adenylate cyclase catalytic subunit has not previously been demonstrated after detergent solubilization. The present report describes a sodium cholate-solubilized preparation of rat cerebral cortical membrane adenylate cyclase that retains guanine nucleotide-mediated inhibition of activity. Methods of membrane preparation, cholate extraction, and assay conditions were manipulated such that guanosine-5'-(beta-gamma-imido)triphosphate [Gpp(NH)p] inhibited basal activity 40-60%. The rank order of potency among various GTP analogs was similar in cholate extracts and in membranes: guanosine-5'-0-(3-thiotriphosphate) greater than Gpp(NH)p greater than GTP. Inclusion of 0.1 mM EGTA reduced basal activity 70-90% and abolished Gpp(NH)p inhibition of basal activity in both membranes and cholate extracts. Forskolin-stimulated activity was also inhibited by Gpp(NH)p. Treatment of either membranes or cholate extracts with N-ethylmaleimide abolished Gpp(NH)p inhibition. Gel filtration of the cholate extract over a Sepharose 6B column in 0.1% Lubrol PX partially resolved the adenylate cyclase components. However, Gpp(NH)p inhibition of basal activity (60% of the control) was maintained in select column fractions. Sucrose gradient centrifugation totally resolved the catalytic subunit from both functional Ni and stimulatory GTP regulatory component (Ns) activities. The sedimentation of functional Ni activity was detected by assaying the ability of sucrose gradient fractions to confer Gpp(NH)p inhibition of the resolved catalytic activity. Labeling of gradient or column fractions with pertussis toxin and [32P]NAD revealed that both the 39,000- and 41,000-dalton substrates comigrated with the functional Ni activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reconstitution of the Gs protein from B16 melanoma clones of high and low experimental metastatic potential into S49 cyc-membranes

The ability of a series of B16 melanoma clones to form experimental lung metastases in syngeneic mice has been shown to correlate positively with adenylate cyclase activity. (Sheppard et al, Int. J. Cancer 37 (1986) 713-722). To begin to identify the components of the adenylate cyclase complex that account for enhanced enzyme activity in highly metastatic tumor populations, cholate extracts containing the GTP-binding protein GS from B16 melanoma clones of different metastatic capacities were reconstituted with membranes prepared from S49 cyc-, a variant lymphoma cell line that lacks GS function. The results revealed that extracts from a highly metastatic B16 clone (F10-C23) reconstituted significantly greater adenylate cyclase activities in S49 cyc- membranes than parallel preparations from a B16 clone (F1-C29) of low metastatic capacity. The data suggest that aberrations in GS function may contribute to the heightened responsiveness of adenylate cyclase observed in B16 melanoma clones of increased metastatic potential.     

Adenylate cyclase inhibition by hormones. The Mg2+ hypothesis

In washed anterior pituitary membranes, there is enough GTP to occupy Ns and therefore to obtain activation of adenylate cyclase by vasointestinal peptide. GTP concentrations needed to obtain adenylate cyclase inhibition by dopamine (above 5 X 10- M) stimulate the adenylate cyclase. The dopamine effect is a blockade of this stimulation. We propose that at least in this system, Ni does not inhibit but stimulates the adenylate cyclase and that inhibitory hormones block this stimulation. We also demonstrate in several adenylate cyclase systems that hormones produced adenylate cyclase inhibition by lowering their Mg affinity A general model for adenylate cyclase activation and inhibition is proposed.     

Adenylate cyclase from sea urchin eggs is positively and negatively regulated by D-1 and D-2 dopamine receptors

The adenylate cyclase present in membranes prepared from sea urchin eggs is sensitive to dopamine stimulation. The receptor sites coupled to sea urchin adenylate cyclase were characterized by means of specific agonists and antagonists. The D-1 dopamine agonist SKF-38393 was able to stimulate enzyme activity, while the two D-1 dopamine antagonists, SCH-23390 and SKF-83566, suppressed the stimulatory effect of dopamine. In addition, the D-2 dopamine agonists, PPHT and metergoline, brought about a dose-dependent inhibition of dopamine-stimulated adenylate cyclase activity. These data show that: (i) in sea urchin eggs adenylate cyclase is regulated by dopamine receptors; (ii) these receptors share characteristics with D-1 and D-2 dopamine receptors present in the mammalian brain.     

Specificity of the dopamine sensitive adenylate cyclase for antipsychotic antagonists

Chlorpromazine, haloperidol and clozapine are approximately equipotent in antagonizing dopamine sensitive adenylate cyclase activity in homogenates of rat brain striatum, in contrast to the differences in clinical antipsychotic potencies reported by others. The antagonism appeared to occur at a structurally specific dopamine site, as inhibition by a series of chlorpromazine analogues of similar hydrophobicity exhibited a structural specificity similar to that found for their neuroleptic and cataleptic activities. Sulpiride, a dopamine antagonist with antipsychotic activity, and metoclopramide, a structurally related central dopamine antagonist, failed to inhibit the dopamine sensitive adenylate cyclase. Pre-treatment of rats with haloperidol (3 mg/kg per day) for 6 or 28 days did not induce a supersensitive response of the adenylate cyclase to stimulation by dopamine or apomorphine or inhibition by clozapine. It was concluded that the dopamine sensitive adenylate cyclase may not be the site of action of all anti-psychotic agents.     

Lack of adenylate and guanylate cyclases responsiveness to hormones in a spontaneous murine thyroid tumor

Animals with tumors were obtained from Dr. ZAJDELA and belong to sublines (XVIInc/Z/E) in which some individuals (TT) developed after 15 months thyroid tumors weighing between 150 and 1200 mg. Hyperplasia affects thyrocytes which do not present a follicular structure. The purpose of our work was to assay the action of various effectors on the adenylate and guanylate cyclase system in vitro. The following results have been obtained: the cyclic-AMP content of tumor tissue is not raised either by TSH or PGE₂. Nevertheless, TSH enhances the phosphatidylinositol phosphate turnover (phospholipid effect) as in normal tissue. This latter observation points at the existence of functional TSH receptors in tumor cells. The study of adenylate cyclase activity of the tumor homogenate shows the presence of this enzyme and its responsiveness to NaF and GppNHp. Unexpectedly, the cyclase is also sensitive to the stimulation by TSH.A tentative interpretation of these facts is that no component of the cyclase is missing, but that they are physically separated. The homogeneization allows the various components to interact productively.A parallel study was devoted to cyclic-GMP. Carbamylcholine fails to increase the cyclic-GMP content of the tumor tissue, whereas it has the described phospholipid effect on phosphatidylinositol. Nevertheless, there is no deficiency in the guanylate cyclase activity, since nitroprusside enhances strongly the cyclic-GMP content of the tumor.To conclude, the murine thyroid tumor presents a genetic alteration that results in the uncoupling of effector binding and catalytic stimulation of adenylate and guanylate cyclase.     

Vasopressin affects adenylate cyclase activity in rat brain: a possible neuromodulator

The effect of vasopressin on adenylate cyclase activity was measured in the homogenates of selected rat brain regions. Adenylate cyclase activity in homogenate of the caudate nucleus did not change significantly with various concentrations of vasopressin. Furthermore, vasopressin did not reliably alter adenylate cyclase activity in various brain regions. Vasopressin in low concentrations significantly enhanced the activation of caudate adenylate cyclase activity by dopamine. This effect of vasopressin was dose dependent. Maximal enhancement by vasopressin occurred at 100 microM vasopressin. These results indicate that vasopressin may not have a direct effect on brain adenylate cyclase activity but appears to modulate the action of dopamine on brain adenylate cyclase.     

Protease inhibitors block hormonal activation of adenylate cyclase

To investigate the mechanism of serine protease stimulation of rat ovarian adenylate cyclase, a variety of synthetic protease inhibitors were used. These inhibitors blocked trypsin, chymotrypsin and hCG stimulation of adenylate cyclase in nearly the same manner. The inhibition of hormone stimulated adnylate cyclase could not be explained by a loss of [¹²⁵I]hCG binding. Cholera toxin and epinephrine stimulation of adenylate cyclase were similarly inhibited, whereas basal and fluoride-stimulated activities were only affected by higher doses of the inhibitors. The results suggest that adenylate cyclase in the ovary may be regulated by membrane protease activity.     

Activation of adenylate cyclase by forskolin in rat brain and testis

Detergent-dispersed adenylate cyclase from rat cerebrum was detected in two components, one sensitive to Ca2+ and calmodulin and another sensitive to fluoride or guanyl-5'-yl imidodiphosphate (Gpp(NH)p). The enzyme activity of both components was markedly augmented by forskolin assayed in the presence or absence of other enzyme activators (e.g., NaF, Gpp(NH)p, calmodulin). The catalytic subunit fraction in which G/F protein was totally lacking was also activated by forskolin. During 1-35 days of postnatal development, the basal adenylate cyclase activities in either cerebrum and cerebellum particulate preparations progressively increased. While the fluoride sensitivity of the cerebrum and cerebellum enzyme increased during postnatal development, the responsiveness to forskolin remained unaltered. There was no enhancement of soluble adenylate cyclase (from rat testis) by forskolin under the assay conditions in which there was a marked stimulatory action on the particulate enzyme. The results seen with the solubilized enzyme, with either Lubrol PX or cholate, indicate that the effects of forskolin on the cyclase do not require either G/F protein or calmodulin and the results of our study of brain enzymes support this view. Data on soluble testis cyclase (a poor or absent response to forskolin by this enzyme) imply that it lacks a protein (other than the catalytic unit) which could confer greater stimulation. The present results do not rule out an alternative explanation that forskolin stimulates adenylate cyclase by a direct interaction with the catalytic subunit, if the catalytic proteins do differ widely in various species of cells and their response to this diterpene.     

The dopamine receptor adenylate cyclase complex: Evidence for post recognition site involvement for the development of supersensitivity

The dopamine receptor adenylate cyclase complex of a rat striatal membrane preparation became more responsive to dopamine following the injection of 6-hydroxydopamine (6-OHDA) into the median forebrain bundle or following the subcutaneous implantation of morphine pellets. Moreover, the membrane cyclase system was more responsive to activation by GTP, guanyl-5-yl-imidodiphosphate and Mn-ATP. These observations suggest that both 6-OHDA and morphine induce similar biochemical changes in striatum and that the increased responsiveness arises, in part, from modification of the nucleotide regulatory and/or catalytic components of adenylate cyclase.     

Pertussis toxin reverses Gpp(NH)p inhibition of basal and forskolin activated adipocyte adenylate cyclase

Inhibition of basal adenylate cyclase by GTP or guanyl-5'-yl imidodiphosphate was abolished in membranes isolated from rat adipocytes previously incubated with pertussis toxin. Forskolin (0.1 microM) stimulated adenylate cyclase about 4-fold and inhibition of cyclase by GTP or guanyl-5'-yl imidodiphosphate was also abolished by pertussis toxin treatment of rat adipocytes. Forskolin (1 microM) increased adenylate cyclase activity at least ten-fold and the inhibitory effect of GppNHp was reduced but not abolished by pertussis toxin. In rabbit adipocytes, pertussis toxin reversed the inhibition of adenylate cyclase activity by GppNHp to the same extent as that by GTP in the presence of 1 microM forskolin. The present results indicate that pertussis toxin can reverse the inhibition of adipocyte adenylate cyclase by nonhydrolyzable GTP analogs as well as that by GTP.     

The role of phospholipids in TSH stimulation of adenylate cyclase in thyroid plasma membranes

Treatment of bovine thyroid plasma membranes with phospholipase A or C inhibited the stimulation of adenylate cyclase activity by thyroid-stimulating hormone (TSH). In general, basal and NaF-stimulated adenylate cyclase activity was not influenced by such treatment. When plasma membranes were incubated with 1–2 units/ml phospholipase A, subsequent addition of phosphatidylcholine or phosphatidylserine but not phosphatidylethanolamine partially restored TSH stimulation. Phosphatidylcholine was more effective than phosphatidylserine in that it caused greater restoration of the TSH response and smaller amounts of phosphatidylcholine were active. However, when the TSH effect was obliterated by treatment of plasma membranes with 10 units/ml phospholipase A, phospholipids were unable to restore any response to TSH. Lubrol PX, a nonionic detergent, inhibited basal, TSH- and NaF-stimulated adenylate cyclase activities in thyroid plasma membranes. Although phosphatidylcholine partially restored TSH stimulation of adenylate cyclase activity in the presence of Lubrol PX, it did not have a similar effect on the stimulation induced by NaF. These results indicate that phospholipids are probably essential components in the system by which TSH stimulates adenylate cyclase activity in thyroid plasma membranes. The effects do not seem to involve the catalytic activity of adenylate cyclase but the data do not permit a distinction between decreased binding of TSH to its receptor or impairment of the signal from the bound hormone to the enzyme activity.     

Stimulation of brain adenylate cyclase activity by the undecapeptide substance P and its modulation by the calcium ion

Synthetic substance P stimulated adenylate cyclase activity in particulate preparations from rat and human brain.The concentration of substance P for half maximal stimulation in rat brain was 1.8 · 10⁻⁷ M.The stimulatory effect of substance P on the rat brain adenylate cyclase activity was 88% compared with 48% by noradrenalin, 163% by prostaglandin E₁ and 184% by prostaglandin E₂.Both the basal and substance P-stimulated adenylate cyclase activity in rat brain were inhibited by concentration of Ca²⁺ above 10⁻⁶ M.The chelating agent ethyleneglycol-bis-(β-aminoethylether)-N,N′-tetraacetic acid at a concentration of 0.1 mM reduced the basal adenylate cyclase activity by 64% and eliminated the substance P-stimulated activity.The inhibition by ethyleneglycol-bis-(β-aminoethylether)-N,N′-tetraacetic acid was completely reversed by increasing concentrations of Ca²⁺.     

Induction of catecholamine-responsive adenylate cyclase in HeLa cells by sodium butyrate. Evidence for a more efficient stimulatory regulatory component

HeLa cells, when exposed to 5 mM sodium butyrate, increased their responsiveness to isoproterenol and their number of beta-receptors. As untreated HeLa cells have a substantial number of receptors but respond poorly to isoproterenol, the effect of butyrate could be due to quantitative or qualitative changes in beta-receptors or other components of the adenylate cyclase system. Receptors were analyzed by membrane/membrane and membrane/cell fusion techniques. HeLa donor membranes, treated to inactivate regulatory and catalytic components of adenylate cyclase, were fused with Fc cells, which lack beta-receptors. Isoproterenol-stimulated adenylate cyclase activity in the fusates was proportional to the number of receptors present. There appeared to be only quantitative but not qualitative differences in beta-receptors from control and butyrate-treated HeLa. Prostaglandin E1 receptors from neuroblastoma cell membranes were similarly coupled to HeLa adenylate cyclase. The hybrid prostaglandin E1-stimulated activity was lower when acceptor membranes were from control HeLa than when they were from butyrate-treated HeLa cells. These results suggested that butyrate was altering the ability of the regulatory component to interact with receptors. HeLa membranes were extracted with sodium cholate and the extracts used to reconstitute effector-stimulated adenylate cyclase activity in S49 cyc- membranes, which lack a functional regulatory component. Whereas extracts from control and butyrate-treated HeLa were equally effective in restoring NaF-stimulated activity in cyc- membranes, extracts from control HeLa were less efficient in reconstituting isoproterenol- and prostaglandin E1-stimulated activities. We conclude that the poor response of control HeLa to beta-agonists is due to a limited activity of the regulatory component but not the receptor. Butyrate induces quantitative changes in the receptor and qualitative changes in the regulatory component that facilitate its ability to couple to receptors but do not alter its ability to interact with the catalytic component of adenylate cyclase.     

Adenylate cyclase system of human skeletal muscle: Characteristics of catecholamine stimulation and nucleotide regulation

The subcellular localization of adenylate cyclase was examined in human skeletal muscle. Three major subcellular membrane fractions, plasmalemma, sarcoplasmic reticulum and mitochondria, were characterized by membrane-marker biochemical studies, by dodecyl sulfate polycrylamide gel electrophoresis and by electron microscopy. About 60% of the adenylate cyclase of the homogenate was found in the plasmalemmal fraction and 10–14% in the sarcoplasmic reticulum and mitochondria. When the plasmalemmal preparation was subjected to discontinuous sucrose gradients, the distribution of adenylate cyclase in different subfractions closely paralleled that of (Na⁺ + K⁺)-ATPase. The highest specific activity was found in a fraction which setteled at the 0.6–0.8 M sucrose interface. The electron microscopic study of this fraction revealed the presence of flattened sacs of variable sizes and was devoid of mitochondrial and myofibrillar material. The electron microscopy of each fraction supported the biochemical studies with enzyme markers. The three major membrane fractions also contained a low K_m phosphodiesterase activity, the highest specific activity being associated with sarcoplasmic reticulum.The plasmalemmal adenylate cyclase was more sensitive to catecholamine stimulation than that associated with sarcoplasmic reticulum or mitochondria. The catecholamine-sensitive, but not the basal, enzyme was further stimulated by GTP. The plasmalemmal adenylate cyclase had typical Michaelis-Menten kinetics with respect to ATP and the apparent K_m for ATP was approx. 0.3. mM. The pH optimum for that enzyme was 7.5. The enzyme required Mg²⁺, and the concentration to achieve half-maximal stimulation was approx. 3 mM. Higher concentrations of Mg²⁺ (about 10 mM) were inhibitory. Solubilization of the plasmalemmal membrane fraction with Lubrol-PX resulted in preferential extraction of 106 000- and 40 000-dalton protein components. The solubilized adenylate cyclase lost its sensitivity for catecholamine stimulation, and the extent of fluoride stimulation was reduced to one-sixth of that of the intact membranes. It is concluded that the catalytically active and hormone-sensitive adenylate cyclase is predominantly localized in the surface membranes of the cells within skeletal muscle. (That “plasmalemmal” fraction is considered likely to contain, in addition to plasmalemma of muscle cells, plasmalemma of bloodvessel cells (endothelium, and perhaps smooth muscle) which may be responsible for a certain amount of the adenylate cyclase activity and other propertiesobserved in that fraction.)The method of preparation used in this study provides a convenient material for evaluating the catecholamine-adenylate cyclase interactions in human skeletal muscle.     

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99) stimulates postjunctional D-1 and D-2 dopamine receptors

6,7-Dihydroxy-2-dimethylaminotetralin (TL-99) mimicks the ability of dopamine either to enhance adenylate cyclase activity in homogenates of goldfish retina or to inhibit adenylate cyclase activity in homogenates of the intermediate lobe (IL) of the rat pituitary gland previously treated with cholera toxin. Both the dopamine stimulated adenylate cyclase activity in the fish retina and the dopamine inhibited adenylate cyclase activity in the rat IL are associated with cells postjunctional to the dopaminergic neurons innervating these tissues. Therefore, the present data do not support the contention that TL-99 is a selective presynaptic dopamine receptor agonist.     

Catecholamine-Sensitive Adenylate Cyclase in the White Perch (Roccus americanus) Retina: Evidence for β-Adrenergic and Dopamine Receptors Linked to Adenylate Cyclase

We have examined the catecholamine-sensitive adenylate cyclase in the retina of the white perch (Roccus americanus). Both dopamine and the beta-adrenergic agonist isoproterenol stimulate cyclic AMP accumulation in this retina, but serotonin, an indoleamine, and phenylephrine, an alpha-adrenergic agonist, had no effect. The stimulation of adenylate cyclase by isoproterenol is more potent and effective than that of dopamine. The effects of dopamine and isoproterenol are mediated via independent dopamine and beta-adrenergic receptors. Haloperidol, a dopamine antagonist, blocks the stimulatory effect of dopamine but not of isoproterenol. Conversely, propranolol, a beta-adrenergic antagonist, blocks the stimulatory effect of isoproterenol but not of dopamine. The effects of dopamine and isoproterenol are not additive. In fractions of purified horizontal cells we found evidence for dopamine receptors linked to adenylate cyclase but did not find evidence for the presence of cyclase coupled beta-adrenergic receptors. The cellular location of the beta-adrenergic receptors is unknown. Our findings demonstrate the existence of both beta-adrenergic and dopamine receptors coupled to adenylate cyclase in the white perch retina. However, we did not find either epinephrine or norepinephrine, endogenous ligands of the beta-receptor, to be present in retinal extracts subjected to HPLC.     

Role of dopamine and indolamine derivatives in the regulation of the sea urchin adenylate cyclase

The adenylate cyclase of the sea urchin egg is stimulated by dopamine in the presence of GTP. The enzyme activity is strongly enhanced when Gpp (NH)p is substituted for GTP, or after cholera toxin treatment. Gramine, an indolamine derivative, brings about non-competitive inhibition of the dopamine-stimulated adenylate cyclase activity. Pertussis toxin causes an attenuation of the gramine-induced inhibition of adenylate cyclase. These results show that dopamine and indolamine derivatives partecipate in the regulation of the adenylate cyclase activity of the sea urchin egg.