Decrease In Adenylate Cyclase Activity Research Articles

Alpha-Adrenergic receptor function in schizophrenia. Receptor number, cyclic adenosine monophosphate production, adenylate cyclase activity, and effect of drugs.

alpha-Adrenergic receptor function was assessed in platelets from drug-free schizophrenic patients and control subjects. The number of alpha-receptors was similar in platelet membranes from schizophrenic patients and control subjects. In intact platelets from schizophrenic male, but not female, patients, prostaglandin E1 (PGE1)-stimulated cyclic adenosine monophosphate (cAMP) level was less than in control subjects. This defect may be due, at least in part, to decreased adenylate cyclase activity. In platelet lysates from schizophrenic patients, but not from normal control subjects, adenylate cyclase activity was diminished and PGE1-stimulated adenylate cyclase activity could be restored partially by the addition of guanosine triphosphate. Treatment with neuroleptic drugs or lithium carbonate did not change alpha-receptor number or cAMP production in platelets from schizophrenic patients, but high doses of propranolol hydrochloride increased cAMP production without affecting the number of alpha-receptors. If the production of cAMP in neurons is similar to that in platelets, diminished cAMP production may be associated with a vulnerability to schizophrenia.

Pertussis toxin inhibits enkephalin stimulation of GTPase of NG108-15 cells.

In neuroblastoma-glioma (NG108-15) hybrid cells, opiates inhibit adenylate cyclase and stimulate a low Km GTPase. It has been postulated that the stimulation of GTPase plays a role in opiate inhibition of adenylate cyclase (Koski, G., and Klee, W. A. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 4185-4189). Treatment of NG108-15 cells with pertussis toxin attenuates receptor-mediated inhibition of adenylate cyclase. The toxin acts by catalyzing the ADP-ribosylation of a 41,000-dalton substrate believed to be a part of the receptor-adenylate cyclase complex. We have found that toxin treatment of NG108-15 results in inhibition of the opiate-stimulated GTPase. The concentration of toxin required for inhibition of this GTPase was similar to that needed for both attenuation of opiate inhibition of adenylate cyclase and ADP ribosylation of the 41,000-dalton substrate. Inhibition of the opiate-induced GTPase by pertussis toxin in isolated membranes required NAD, consistent with the hypothesis that this effect of the toxin resulted from ADP ribosylation of a protein component of the system. Since the opiate-stimulated GTPase is believed to play a role in the receptor-mediated decrease in adenylate cyclase activity, inhibition of this GTPase may be an important part of the mechanism by which the toxin interferes with opiate action on adenylate cyclase.

Attenuation of enkephalin activity in neuroblastoma X glioma NG108-15 hybrid cells by phospholipases.

The role of membrane phospholipids in enkephalin receptor-mediated inhibition of adenylate cyclase (EC 4.6.1.1) activity in neuroblastoma X glioma NG108-15 hybrids was studied by selective hydrolysis of lipids with phospholipases. When NG108-15 cells were treated with phospholipase C from Clostridium welchii at 37 degrees C, an enzyme concentration--dependent decrease in adenylate cyclase activity was observed. The basal and prostaglandin E1 (PGE1)-stimulated adenylate cyclase activities were more sensitive to phospholipase C (EC 3.1.4.3) treatment than were the NaF-5'-guanylylimidodiphosphate (Gpp(NH)p)-sensitive adenylate cyclase activities. Further, Leu5-enkephalin inhibition of basal or PGE1-stimulated adenylate cyclase activity was attenuated by phospholipase C treatment, characterized by a decrease of enkephalin potency and of maximal inhibitory level. [3H]D-Ala2-Met5-enkephalinamide binding revealed a decrease in receptor affinity with no measurable reduction in number of binding sites after phospholipase C treatment. Although opiate receptor was still under the regulation of guanine nucleotide after phospholipase C treatment, adenylate cyclase activity was more sensitive to the stimulation of Gpp(NH)p. Thus, the reduction of opiate agonist affinity was not due to the uncoupling of opiate receptor from N-component. Further, treatment of NG108-15 hybrid cell membrane with phospholipase C at 24 degrees C produced analogous attenuation of enkephalin potency and efficacy without alteration in receptor binding. The reduction in enkephalin potency could be reversed by treating NG108-15 membrane with phosphatidylcholine, but not with phosphatidylserine, phosphatidylinositol, or cerebroside sulfate. The enkephalin activity in NG108-15 cells was not altered by treating the cells with phospholipase A2 o phospholipase C from Bacillus cereus. Hence, apparently, there was a specific lipid dependency in enkephalin inhibition of adenylate cyclase activity.

Guanine nucleotide mediated desensitization of adenylate cyclase in cell free preparations from a Leydig cell tumour.

Cell free desensitization of a tumour Leydig cell plasma membrane adenylate cyclase has been demonstrated in the presence of guanine nucleotides. In experiments in which the membranes were pre-incubated with various nucleotides and LH, it was shown that this decreased adenylate cyclase activity was dependent on the presence of GTP and occurred both in the presence and absence of ATP. While pre-treatment with LH alone appeared to enhance subsequent adenylate cyclase activity, this hormone was able to potentiate the desensitizing effect of GTP. The desensitizing effect of GTP was not inhibited by sodium fluoride. In contrast, the GTP analogue p(NH)ppG (guanosine 5'beta, gamma-imido triphosphate) caused a persistent activation of the adenylate cyclase. GMP and guanosine also initially inhibited the adenylate cyclase activity, but this was entirely reversed by p(NH)ppG plus LH. GDP in addition to GTP caused desensitization but this was only partially reversed by p(NH)ppG plus LH. It is proposed that in similarity with the ovary (Bockaert et al. 1976; Ezra & Salomon 1982a) desensitization of Leydig tumour cell plasma membrane adenylate cyclase may involve a GTP-mediated phosphorylation step.

Deficient activity of receptor-cyclase coupling protein is transformed lymphoblasts of patients with pseudohypoparathyroidism, type I.

Erythrocytes of many patients with pseudohypoparathyroidism, type 1 (PHP-I) exhibit quantitatively reduced activity of the N protein, the guanine nucleotide-binding regulatory component of adenylate cyclase. We have designated this group of patients PHP-Ia to distinguish them from PHP-Ib patients, in whom erythrocyte N activity is quantitatively normal. In virus-transformed lymphoblasts of three normal, three PHP-Ia, and two PHP-Ib subjects, we compared N and adenylate cyclase activities, as well as cAMP accumulation and susceptibility to radiolabeling in the presence of [32P]NAD and cholera toxin. In comparison to normal lymphoblasts, N activities were reduced by approximately 50% in lymphoblasts of the PHP-Ia patients, but were not reduced in lymphoblasts from the PHP-Ib patients. Toxin-catalyzed radiolabeling of the 42,000 molecular weight subunit of the N protein was also reduced in lymphoblasts of the PHP-Ia patients. These results are consistent with the hypothesis that N deficiency is generalized in tissues of PHP-Ia patients. Deficient lymphoblast N activity in PHP-Ia was not associated with decreases in adenylate cyclase activity or cAMP accumulation, probably because these activities involve many potential regulable cellular components in addition to the N protein.

Selective inhibition by epinephrine of parathyroid hormone-stimulated adenylate cyclase in rat renal cortex.

Parathyroid hormone- (PTH) stimulated adenylate cyclase activity in homogenates of rat renal cortex was inhibited by l-epinephrine. The specificity of the inhibition indicated that it was mediated by alpha 2-receptors. The inhibition of PTH-stimulated activity was greater than the inhibition of basal activity. The absolute decrease in adenylate cyclase activity produced by 10-4 M l-epinephrine was from 16.3 +/-0.6 (SE) to 11.2 +/- 0.6 pmol.min-1.mg-1 for activity stimulated by 10 microgram/ml PTH. Basal activity was decreased from 2.3 +/- 0.07 to 1.7 +/- 0.04. A similar inhibition of PTH-stimulated adenylate cyclase by l-epinephrine was demonstrated in preparations of renal cortical tubules. In contrast, the quantitative decrease in vasopressin-or calcitonin-stimulated activity by 10-4 M l-epinephrine was the same as the decrease in basal activity. These results demonstrate that PTH receptors that stimulated adenylate cyclase and alpha 2-adrenergic receptors that inhibit adenylate cyclase are present on the same cells in the renal tubules. Thus, a mechanism exists whereby alpha-adrenergic agonists can oppose the tubular actions of PTH via a direct inhibition of adenylate cyclase activity.

In Vitro Effect of Inhibin on Cyclic AMP‐Phosphodiesterase Activity in Rat Testes

We have previously reported that incubation of testicular slices with inhibin in the presence of 6 × 10−8 M oF SH resulted in a dose‐related reduction in accumulation of cyclic AMP due to a decrease in adenyl cyclase activity. The present study demonstrates that inhibin enhances the cyclic AMP‐phosphodiesterase activity in rat testicular tissue. These results suggest an additional mechanism by which cyclic AMP levels in gonadal tissue could be regulated by inhibin.

Changes in serum LH, progesterone, and specific binding of 125I-hCG to luteal cells during regression and development of bovine corpora lutea.

Changes in concentrations of LII and progesterone in serum and specific binding of125 I-hCG to crude membrane preparations of luteal cells were examined during spontaneous regression (Group 1), prostaglandin-induced regression (Group 2), and growth (Group 3) of bovine corpora lutes (CL). Specific binding of '25IhCG (presumably by the LII receptor) to luteal homogenates was validated using Scatchard analysis and shown to be specific for LH/hCG with high affinity (Ka 2.18 * 0.76 X 10lM�). Al) cattle had estrous cycles synchronized with a synthetic prostaglandin F2OE, doprostenol (PG). Group 1 had ovaries removed on Day 17, 18, 19, on estrus, or 24 h after estrus. Group 2 had ovaries removed at 0, 12, 24, 48, 60, or 72 h after PG. Group 3 had ovaries removed at 5, 7, 9, 11, 13, or 15 days after PG. Blood samples were taken from all animals at 8 h intervals up to time of ovariectomy for measurement of serum progesterone and LH and at 2 h intervals around estrus to determine time of preovulatory LH surge. Data from heifers in Group 3 were grouped for analysis according to days after preovulatory LII surges. During spontaneous regression (Group 1), Day 19 to Day 21 of the estrous cycle, serum proges- terone, specific binding of hCG, and CL weight decreased (P O.05), while binding of hCG to corpora lutes decreased (P<0.05). From these data we suggest that synchronization of estrus using two injections of prostaglandin results in short-term endocrine changes that are similar to changes seen during the late luteal phase of the estrous cycle. Since serum progesterone declined to basal levels prior to binding of hCG to corpora lutes during PG-induced regression, other events such as decreased adenylate cyclase activity may be more closely associated with decreased function of corpora lutea than decreases in LII receptor during regression of corpora lutea. In addition, we conclude that changes in numbers of LII receptors may be more closely associated to progesterone secretion in early luteal devel- opment (<4.5 days post-LH surge) than during late luteal development (4.5 to 12.4 days post-LH surge). This suggests that with an increase in LH binding sites, the early developing corpus luteum may become more responsive to basal levels of serum LII causing an increase in progesterone secretion.

In vivo effect of human growth hormone on hepatic adenylate cyclase activity.

Significant increases in basal, and glucagon and fluoride stimulated adenylate cyclase activity were observed in liver plasma membranes of hypophysectomized rats compared to normal adult and weanling rats. The fluoride stimulated adenylate cyclase activity was 2-3 fold greater in the membranes from hypophysectomized animals while the glucagon stimulated activity was 5-7 fold greater, and the basal activity was approximately double that of membranes from normal adult animals. Administration of growth hormone to hypophysectomized rats by an intramuscular or intravenous route decreased adenylate cyclase activity to levels equivalent to those in normal adult rats. Estradiol and thyroxine replacement did not alter the adenylate cyclase activity of the membranes from hypophysectomized animals. The fluoride or epinephrine stimulated adenylate cyclase activity of rat diaphragm homogenates was not affected by hypophysectomy.

Protein phosphorylation mediates effects of isoproterenol on adenylate cyclase activity in rat cortical membranes

The effects of (-)-isoproterenol on adenylate cyclase activity were studied in rat cerebral cortical membranes prepared and assayed in the presence of calcium ions. In assays carried out in the presence of high Mg2+ concentrations (5-10 mM) and of Ca2+ in the micromolar range, addition of 1-100 micro M (-)-isoproterenol caused over 50% inhibition of adenylate cyclase activity. Since these conditions are optimal for supporting endogenous phosphorylative activity in synaptic membranes, we tested whether the observed effects are mediated by changes in the phosphorylation of specific proteins in these membranes. This was done by preincubation of lysed synaptosomes under phosphorylating conditions in the presence and absence of isoproterenol followed by extensive washes and analysis of cyclic AMP formation in resuspended membranes. Addition of (-)-isoproterenol to the preincubation resulted in a 30% decrease of adenylate cyclase activity in the reincubation. Inclusion of [gamma-32P]ATP in the preincubation and examination of the phosphorylation state of specific proteins in membranes entering the reincubation revealed that (-)-isoproterenol inhibited the phosphorylation of a specific protein band with apparent molecular weight of 47,000 (designated band F). These results support the hypothesis that alterations in membrane protein phosphorylation induced by neurotransmitters play a role in the regulation of adenylate cyclase activity.

Age-associated decrease of adenylate cyclase activity in rat myocardium

Myocardial inotropic and chronotropic responses to beta-adrenergic agonists are diminished with aging. Since myocardial beta-adrenergic receptors are unaltered with age, we tested the hypothesis that this decreased responsiveness is related to a defect in the adenylate cyclase system. Isoproterenol-stimulated adenylate cyclase activity was assessed in myocardial membranes from Fischer 344 rats of 3, 12, and 24 months of age. Basal, as well as F −, GTP −, and hormone-stimulated adenylate cyclase activity was decreased by 20–30% with age. The K m of the enzyme for ATP was not found to be statistically different for any age group studied. These data support the hypothesis that the diminished responsiveness seen in senescence is a result of an alteration in either the catalytic subunit or the coupling protein (N) of the adenylate cyclase complex.

Effect of diesel particulate exposure on adenylate and guanylate cyclase of rat and guinea pig liver and lung.

An examination of the effects of 250 or 1500 micrograms m-3 concentrations of diesel particulate from diesel exhaust on the activity of adenylate or guanylate cyclase was undertaken using liver and lung tissue of rats and guinea pigs. These membrane and cytosolic enzymes were selected to screen for functional or regulatory alterations in these tissues. The studies for adenylate cyclase used the microsomal membrane fraction of each tissue; for guanylate cyclase, the microsomal membrane and supernate fractions were used. Basal and fluoride-stimulated adenylate cyclase activity were measured. Basal and sodium azide-stimulated guanylate cyclase activity were also determined. The basal activity of rat liver adenylate cyclase is generally unchanged throughout 52 weeks of diesel exposure. Stimulated adenylate cyclase shows an age-related decrease for all animal treatments throughout the study. Changes in enzyme activity occurred at 12 weeks and 52 weeks after 1500 micrograms m-3 exposure. Soluble stimulated guinea pig lung guanylate cyclase was first increased (6 weeks) and then decreased (24 weeks) by diesel exposure. At 52 weeks, there was no change. The data suggest the following trends: (1) an increased basal adenylate cyclase in the rat lung; (2) an age-related decrease in adenylate cyclase activity in rat liver, and (3) a biphasic exposure-related response of soluble guanylate cyclase for the guinea pig lung during the first 24 weeks, but no change at 52 weeks. In general, however, these studies suggest that diesel exposure does not substantially alter either of these intracellular regulating enzymes.

Testicular cyclic nucleotide and adrenal catecholamine metabolism following chronic exposure to cadmium.

Cadmium (Cd) produces injurious effects on reproductive function and has been implicated in the pathogeneses of hypertension. The present article summarizes available data on alterations in the cyclic AMP system of testicular and prostatic tissue as well as in catecholamine metabolism in adrenal glands following exposure to Cd and subsequent withdrawal. Daily Cd (1 mg/kg IP) for 45 days decreased prostatic and testicular weights of mature male rats. In prostate, chronic treatment with Cd reduced cyclic AMP levels to 57% of normal values which appeared to be due to the decrease in adenylate cyclase activity since cyclic AMP metabolism by phosphodiesterase was not significantly altered. Cyclic AMP binding to prostatic protein kinase was increased following Cd administration as was the activity of the cyclic AMP-dependent form of protein kinase. In contrast to the prostate, testicular adenylate cyclase was stimulated by Cd treatment. However, the endogenous cyclic AMP levels remained unaffected since the increase in testicular adenylate cyclase was offset by a concomitant increase in the activity of phosphodiesterase. Although the activities of the cyclic AMP-dependent and the independent forms of testicular protein kinase were significantly depressed, the binding of cyclic AMP to protein kinase from testes of Cd-treated rats was not affected. Discontinuation of treatment for 28 days in rats that had previously been given the heavy metal for 45 days resulted in at least a partial reversal of several of the cadmium-induced changes in cyclic AMP metabolism of the rat prostate and testes. However, the weight of the prostate glands remained essentially in the same range as that seen in the "treated group."Data suggest that cyclic AMP metabolism in both the primary and the secondary reproductive organs is altered following chronic Cd treatment and that some changes persist even 28 days following the termination of daily exposure to the heavy metal.Cd treatment also increased adrenal weights and augmented the levels of adrenal norepinephrine and epinephrine as well as the activity of tyrosine hydroxylase. Discontinuation of the heavy metal treatment for 28 days, in rats previously injected with Cd for 45 days, restored the activity of tyrosine hydroxylase as well as the amount of norepinephrine and epinephrine. In contrast, adrenal weights were restored only partially following withdrawal of Cd treatment. Evidence indicates that the changes in adrenal catecholamine metabolism may be the result of stress induced by chronic exposure to this heavy metal. In addition, some of the untoward effects such as hyperglycemia and arterial hypertension seen during Cd toxicity might be related to increased synthesis of epinephrine in adrenal glands.

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.

Specificity for guanine nucleotide activation and stabilization of rabbit cardiac adenylate cyclase

These studies examined the structural specificity for guanine nucleotide-facilitated hormonal activation and guanine nucleotide stabilization of cardiac adenylate cyclase. 1. The phosphonate analogues of GTP, p[CH(2)]ppG (guanosine 5'-[betagamma-methylene]-triphosphate) and pp[CH(2)]pG (guanosine 5'-[alphabeta-methylene]triphosphate), were the most effective activators of adenylate cyclase. Other nucleotides producing significant activation (P<0.01) were, in decreasing order of activation: ITP, GDP, GMP, GTP, XTP, CTP, p[NH]ppG (guanosine 5'-[betagamma-imido]triphosphate), dGTP and 2'-O-methyl-GTP. Guanosine, cyclic GMP, UTP and ppppG (guanosine tetraphosphate) had no effect, and 7-methyl-GTP caused a decrease in the activity. 2. Preincubation of membranes at 37 degrees C for 15min before assay at 24 degrees C produced an 80% decrease in adenylate cyclase activity, and preincubation with p[CH(2)]ppG and pp[CH(2)]pG protected and resulted in a net increase in activity. Other nucleotides that completely or partially preserved activity in decreasing order of effectiveness were p[NH]ppG, GDP, GTP, dGTP, ITP, ppppG, 2'-O-methyl-GTP, GMP, CTP and XTP. Several compounds had no effect, including guanosine, cyclic GMP and UTP, whereas preincubation with 7-methyl-GTP produced a further decrease (P<0.05) in activity. 3. The concentration-dependence for activation and stabilization by the naturally occurring guanine nucleotides was examined in the absence of a regenerating system and revealed GMP to have no stabilizing effect and to be less potent than either GDP or GTP in activating adenylate cyclase. 4. A significant correlation (r=0.90) was found between the properties of activation and stabilization for the compounds examined. These findings are consistent with there being a single nucleotide site through which both the activation and stabilization of adenylate cyclase are mediated.

Impairment of Hormone Dependent Signal Transfer by Chronic SSPE Virus Infection

In a CNS-derived cell line, the cellular response to hormonal stimulation, represented by the rise of intracellular cAMP levels, is impaired under the influence of a persisting neurotropic virus infection. This dysfunction is caused by the decrease in adenylate cyclase activity, most probably due to the virus-induced loss of active catalytic units.

Relationship between changes in the calcium dependent regulatory protein and adenylate cyclase during viral transformation

The levels of the calcium dependent regulatory protein in transformed chicken embryo fibroblasts are higher both in soluble fractions and membrane fractions compared to untransformed cells. The kinetics for changes in the calcium dependent regulatory protein, hexose transport, and adenylate cyclase were compared using a temperature sensitive mutant of Rous sarcoma virus. Decreases in adenylate cyclase activity and increased hexose transport accompanying transformation occurred with half-lives of approximately 7 to 8 hours. Increases in the calcium dependent regulatory protein occurred much slower with a half-life of seventeen hours. It is concluded that the increase in calcium dependent regulatory protein levels is a late event during viral transformation and that the decline in adenylate cyclase activity cannot be due to changes in the amount of calcium dependent regulatory protein.

Effect of metabolic inhibitors on vasopressin‐stimulated transport systems in the toad bladder

Vasopressin increases the permeability of receptor cells to water and, in tissues such as toad bladder, to solutes such as urea. While cyclic AMP appears to play a major role in mediating the effects of vasopressin, there is evidence that activation of the water permeability system and the urea permeability system involves separate pathways. In the present study, we have shown that inhibitors of oxidative metabolism (rotenone, dinitrophenol, and methylene blue) selectively inhibit either vasopressin-stimulated water flow or vasopressin-stimulated urea transport. There was no inhibition, however, when exogenous cyclic AMP was substituted for vasopressin, and little to no inhibition when the potent analogue 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) was employed. Rotenone had no effect on adenylate cyclase activity or cyclic AMP levels within the cell; dinitrophenol decreased adenylate cyclase activity minimally. Additional studies with vinblastine and nocodazole, inhibitors of microtubule assembly, demonstrated an inhibition of vasopressin and cyclic AMP-stimulated water flow but showed no effect on urea transport. We would conclude that water and urea transport, as examples of hormone-stimulated processes, have different links to cell metabolism, and that in addition to cyclic AMP, a non-nucleotide pathway may be involved in the action of vasopressin.

Effect of theophylline on myocardial adenylate cyclase activity.

Theophylline (0.001-10.0 mM) did not increase but rather decrased adenylate cyclase activity (AC) of guinea-pig auricles. Isoprenaline (1-100 microgram) and sodium fluoride (0.3-10.0 mM) stimulated AC in a concentration-dependent manner.

Effect of liposomes containing cholesterol on adenylate cyclase activity of cultured mammalian fibroblasts

Liposomes prepared with cholesterol and dipalmitoyl phosphatidylcholine were incubated with a clone of normal rat kidney fibroblast of cells in culture. The cells took up [ 14C]cholesterol in proportion to the concentration of liposomes in the incubation medium, and the uptake increased with time over the four hours of study. Two cell membrane enzymes, adenylate cyclase and ( Na + + K +)-ATPase , exhibited decreased activity after treatment with cholesterol-containing liposomes. The decrease in adenylate cyclase activity was directly proportional to the uptake of [ 14C]cholesterol. When a variety of subclones of NRK 5W were examined some were found to respond to cholesterol treatment and some did not. These data are consistent with the view that membrane cholesterol content plays a role in controlling the activity of some plasma membrane enzymes.