The present data disagree with earlier suggestions that thrombin's effect on platelets is to cause a decrease in intracellular cyclic3¢,5¢-adenosinemonophosphate. Washed human platelets or platelet-rich plasma were incubated at 37°C with human thrombin. After centrifugation, the supernates were assayed for nucleotides and calcium released. The platelet pellets, and in some experiments the supernates as well, were assayed by
The secretion of calcitonin by slices of porcine thyroid glands has been investigated. Calcitonin in the incubation medium was determined by radioimmunoassay. Secretion of calcitonin was diminished when calcium or magnesium was omitted and was increased stepwise as the concentration of calcium or magnesium in the incubation medium was increased. Calcitonin secretion was augmented substantially when either the quantity of thyroid tissue or volume of incubation medium was increased. Secretion of calcitonin was stimulated by glucagon, theophylline, and dibutyryl cyclic3¢,5¢-adenosinemonophosphate. It is concluded that calcitonin secretion is regulated by the concentration of calcium and magnesium, that secretion may be inhibited by calcitonin or a precursor and that secretion can be stimulated by increasing the concentration of cyclic3¢,5¢-adenosinemonophosphate in the parafollicular cells of the thyroid gland.
The relation of cyclic3¢,5¢-adenosinemonophosphate to platelet function has been studied by investigating the influence of this compound and of its N 6 -2¢-0-dibutyryl derivative on platelet aggregation and other aspects of platelet behavior after demonstration of adenyl cyclase activity in disrupted platelets.
quantitatively similar response. Purified fractions of cells containing 85-90% PMN responded to polystyrene beads with a much smaller increase in cyclic AMP content. Phagocytosis of paraffin oil emulsion in the unfractionated mixed human leukocyte preparation was associated with little or no change in cyclic AMP levels. There was no change in cyclic AMP content of rabbit alveolar macrophages or guinea pig PMN during phagocytosis of polystyrene beads. All of these observations are consistent with the view that particle uptake per se does not increase cyclic AMP levels in phagocytic cells. It seems probable that the increase in cyclic AMP concentration that results when unfractionated human blood leukocytes are incubated with polystyrene beads occurs in cells other than PMN.
The effects, in vivo, of epinephrine, glucagon, and dibutyryl cyclicadenosine3¢,5¢- monophosphate (cyclic AMP) on the glycogen content of rat heart and liver and, in vitro, upon adenylate cyclase activity in homogenates of rat heart and liver were determined during the latter third of gestation and the neonatal period. Hepatic glycogen was depleted by epinephrine, glucagon, and dibutyryl cyclicadenosine3¢,5¢-monophosphate, but
receptors and that only prostaglandin E1 can bring about relaxation of umbilical arteries. (d) 1 muM atropine blocked the effect of 100 muM acetylcholine on cGMP content without altering the responses to histamine, bradykinin, serotonin, or K+ ion. (e) Pyrilamine (an H1 antagonist), but not metiamide (an H2 antagonist), blocked the effect of histamine on cGMP from which it is inferred that histamine causes accumulation of cGMP in umbilical artery via its interaction with H1 receptors. The results are consistent with the view that metabolism of the two cyclic nucleotides is independently controlled in the human umbilical artery and that […]
maximal inhibition was observed at 2 nM. Scatchard analysis indicated two functionally independent classes of receptor sites: 62,000 high affinity sites/cell with a dissociation constant (K d ) of 1.3 nM, and 510,000 low affinity sites/cell with a K d of 16.2 nM. Secretin inhibited tracer binding but with a 1,000 times lower potency than native VIP. VIP strongly stimulated adenosine3¢:5¢ monophosphate (cyclic AMP) production in human gallbladder epithelial cells. At 37°C, 0.1 nM and 10 nM VIP raised cyclic AMP levels 44 and 100 times above the basal level, respectively. Maximal values remained constant between 60 and 90 min at 15°C. The importance of the VIP-induced cyclic AMP rise was related, at least in part, to a low phosphodiesterase activity in human gallbladder epithelial cells. At
Glucagon, infused intravenously into fasting, well-hydrated, normal men in doses of 25-200 ng/kg per min, induced up to 30-fold increases in both plasma and urinary cyclic AMP. Cyclic GMP levels were unaffected by glucagon. Simultaneous cyclic AMP and inulin clearance studies demonstrated that the glucagon-induced increase in urinary cyclic AMP was entirely due to glomerular filtration of the elevated plasma levels of the nucleotide. The cyclic AMP response to glucagon was not mediated by parathyroid hormone or epinephrine, and trypsintreated glucagon was completely inactive.
phosphate, calcium, sodium, and potassium by the kidney. The infusion of 25HCC in a dosage of 60 U/h to moderately saline-expanded animals (2.5% body weight) induced a fall in the excretion of all of the ions under study after 90-120 min similar to that observed in previous experiments from this laboratory. Mean decrements in fractional excretion were: phosphate, 42.0% (P < 0.005); calcium, 25.0% (P < 0.005); sodium, 23.4% (P < 0.001); and potassium, 14.7% (P < 0.005). The superimposition of either porcine or salmon TCT (1-100 MRC U/h for 2 h) resulted in no further alterations in electrolyte excretion. However, the infusion of TCT during steady-state saline expansion, before the administration of 25HCC, obviated the renal transport effects of the vitamin D metabolite. Both in the latter studies, as well as those in which similar doses of TCT were given to hydropenic animals, the hormone itself failed to induce any consistent alteration in electrolyte excretion. Cyclic AMP (50 mg/h) caused an increase in the excretion of phosphate, sodium, and potassium and no change in calcium excretion. Like TCT, the nucleotide blocked the action of 25HCC on the kidney. Raising […]
concentrations of PHA produced 25-300% increases in cyclic AMP levels, alterations being demonstrated within 1-2 min. The early changes in cyclic AMP concentration appear to precede previously reported metabolic changes in PHA-stimulated cells. After 6 hr cyclic AMP levels in PHA-stimulated cells had usually fallen to the levels of control cells. After 24 hr the level in PHA-stimulated cells was characteristically below that of the control cells. Adenyl cyclase, the enzyme which converts ATP to cyclic AMP, was measured in
When thrombin is added to washed human platelets, one of its actions results in activation of a phospholipase that hydrolyzes arachidonic acid from phospholipids. The arachidonate is converted to the cyclic endoperoxides (prostaglandin G2 and prostaglandin H2) by fatty acid cyclo-oxygenase. These compounds are then converted to thromboxane A2, also called rabbit aorta-contracting substance, by thromboxane synthetase. These labile, pharmacologically active compounds then break down to inactive products including thromboxane B2 and malonaldehyde. Incubation of platelets with either dibutyryl cyclicadenosine3',5'-monophosphate (dBcAMP) or prostaglandin E1 (PGE1) before thrombin addition blocks the subsequent formation of oxygenated products of arachidonic acid including thromboxane A2, thromboxane B2, and malonaldehyde. In contrast, when
hypophysectomized dogs by infusion of 2.5% dextrose. Infusion of adenosine3¢,5¢-cyclicmonophosphate (cyclic AMP) in the left renal artery decreased ipsilaterally glomerular filtration rate (GFR), cortical and non-cortical renal plasma flow, and tended to increase urine flow (V) and free-water clearance (C H 2 O ) despite a decrease in mean arterial pressure. Infusion of dibutyryl adenosine3¢,5¢-cyclicmonophosphate (dibutyryl cyclic AMP) in the left renal artery increased V and C H 2 O significantly (P<0.01) bilaterally with essentially no change in GFR, in total renal plasma flow or its cortical and non-cortical components. For each kidney the magnitude of the change in V was similar to the magnitude of the change in C H 2 O and the change in sodium excretion was trivial. Cyclic AMP probably produced its effects on renal hemodynamics and mean arterial pressure wholly or in part through the action of metabolites such as 5¢-AMP and adenosine on the renal and systemic vasculature. The absence of an effect of dibutyryl cyclic AMP on renal hemodynamics and its bilateral effect may be explained by the resistance of this nucleotide derivative to metabolism. Dibutyryl cyclic AMP appears to decrease by direct cellular effect(s) proximal tubular sodium reabsorption but does not prevent virtually complete reabsorption of the increased load of sodium in the distal nephron. This effect on the kidney […]
An analysis of prostaglandin-stimulated adenosine3',5'-cyclicmonophosphate (cyclic AMP) accumulation in cultured human umbilical vein endothelial cells showed prostacyclin (PGI2) to be the most potent agonist followed by prostaglandin (PG)H2, which was more potent than PGE2, while PGD2 was essentially inactive. The endothelial cells studied apparently have a high rate of cyclic AMP phosphodiesterase activity because significant PGI2- mediated increases in cyclic AMP could not be shown in the presence of the
Data from cultured cells have suggested that cyclic AMP and cyclic GMP may be important determinants of cell growth and transformation. However, few studies have examined cyclic nucleotide content and metabolism in naturally occurring tumors of man. Accordingly, in the present study we compared cAMP and cGMP levels and metabolism in carcinomas of the human colon to those of the adjacent uninvolved mucosa after therapeutic resection of these tissues. The cAMP content of the tumors, determined in samples frozen 30 min after excision, was significantly lower than that of the adjacent mucosa, when expressed on the basis of tissue wet weight, protein, or DNA content. By contrast, the cGMP content of the tumors was higher than that of the surrounding mucosa if calculated on the basis of tissue wet weight, but this difference did not persist when correction was made for the higher protein or DNA content of the tumors. Incubation of slices of mucosa or tumor with or without theophylline in vitro increased tissue cAMP and cGMP content above levels observed in frozen samples of the same tissue. However, after such incubations cAMP levels in the tumors remained clearly below that of the mucosa, while cGMP content of the two tissues did not differ. The failure of theophylline to abolish differences in cAMP content and the comparable activities of […]
bringing about a suppression of adrenal cyclic GMP levels. Physiological i.v. pulse doses of ACTH produced a rapid dose related increase in adrenal cyclic GMP. In vitro incubation of quartered adrenal pairs with 500 mU ACTH produced elevated cyclic AMP levels and suppression of cyclic GMP. Whereas adrenal cyclic AMP fell rapidly to 50% of control levels after hypophysectomy and remained at about 1 pmol per mg tissue for 7 days, adrenal cyclic GMP showed a biphasic rhythm in long-term hypophysectomized animals. After an initial peak at 1 […]