Bernd Simon

Interaction of laxatives with enzymes of cyclic AM P metabolism from human colonic mucosa

Abstract. The mechanism by which laxatives such as dioctyl sodium sulfosuccinate and ricinoleic acid evoke colonic fluid secretion has been suggested to involve mucosal cyclic AMP. Ricinoleic acid and dioctyl sodium sulfosuccinate were tested for their capacity to modulate the key enzymes of cAMP‐metabolism‐ adenylate cyclase and cAMP‐phosphodiesterase‐in human colonic mucosa.

Adenylate Cyclase of Human Gastric Mucosa

It has been shown that human gastric mucosa contains a prostaglandin-sensitive adenylate cyclase system. Of the prostaglandins tested the Etype induced an about 3–3.5-fold increase of enzyme activity. The prostaglandin A 2 by causing an about 2.5-fold enhancement of cAMP formation and the F-prostaglandins (1.4–2.0-fold stimulation) were less effective in activating the enzyme system.Maximally effective concentrations of histamine and prostaglandin E 2 were additive with respect to enzyme activity indicating that both hormones act via individual adenylate cyclases.

Hormone-sensitive adenylate cyclase in human colonic mucosa.

Human colonic adenylate cyclase has been shown to be sensitive to vasoactive intestinal polypeptide (VIP) and prostaglandins of the E- and F-type. Maximal activation of enzyme activity averaged 200% for VIP and 300-350% for the E-prostaglandins. Both classes of hormones had an additive effect on enzyme activity indicating the existence of two distinct hormone-sensitive adenylate cyclases in human colonic mucosa.

Catecholamine-sensitive adenylate cyclase of human fat cell ghosts: A comparative study using different beta-adrenergic agents

Some of the effects of beta-adrenergic agonists and antagonists on the adenylate cyclase system of human fat cell ghosts were studied. Isoproterenol, by causing about a fourfold increase of enzyme activity, was more potent than epinephrine and norepinephrine (about 2.5--3.0-fold stimulation). The beta2-adrenergic agonists salbutamol, terbutalin, and fenoterol were considerably less effective than the naturally occurring catecholamines. The stimulatory actions of isoproterenol and beta2-adrenergic agonists were competitively inhibited by the beta-blocking agent propranolol. Isoproterenol stimulation was also inhibited by the selective beta1-adrenergic antagonist practolol. This compound, however, was less potent than propranolol. The results are suggestive for an adenylate cyclase system in human fat cell ghosts coupled to beta1-adrenergic receptor sites. These receptors differ from the cardiac beta receptors with respect to practolol affinity.

Effects of Prostaglandins and Their Methylated Analogues upon Human Adenylate Cyclase in the Upper Gastrointestinal Tract

The effects of prostaglandin E2, prostaglandin A2 and their methylated analogues upon the adenylate cyclase in human gastric mucosa were studied. PG E2 and 16,16-dimethyl-PG E2 stimulated the enzyme activity to a greater degree than PG A2 and 16,16-dimethyl-PG A2. 16,16-dimethyl-PG E2 was less potent in activating the enzyme system than its parent compound. The results suggest that the more pronounced antisecretory activity of 16,16-dimethyl-PG E2 is due to its greater resistance against enzymatic degradation.

Histamine-sensitive adenylate cyclase in fundic gastric mucosa

icant difference between the gastric emptying of equal volume meals of lactose in water (290 mOsm/ kg) or milk (mean mOsm/kg of 283) in individuals with lactose malabsorption (8). The two of four subjects in the Levitt and Donaldson study (7) who did not produce H2 after a 5-g lactose dose were adults. If given in the form of milk, the dose would have been 100 ml (less than half a cup). The adults in our study, those 18 years and older, received between 251 and 575 ml of milk (about one to two-and-one-third cups). Calloway and Murphy have demonstrated that breath H~ is elevated when as little as 2 to 5 g of raffinose and stachyose is fed (8), and these sugars should be partially hydrolyzed and absorbed. Several details make it difficult to compare our study with that of Newcomer et al. Our Native Americans were seven-eighthsto full-blooded and represented 26 tribes whereas those of Newcomer et al were one-eighthto full-blooded and were almost entirely of one tribe. We do not suggest there are tribal differences, but the percentage of Indian blood does seem important, and Newcomer et al demonstrated this. We are reluctant to administer a large lactose load to children who may be lactose malabsorbers, and we believe that this test can be performed with sufficiently small doses so as to detect malabsorption and yet enduce minimal symptoms. The realistic dose also supplies information on the response to lactose as normally consumed.

Stimulation of human colonic adenylate cyclase.

To The Editor: A messenger function of cyclic AMP in hormoneinduced transport processes in different parts of the human gastrointestinal tract has recently been established by several groups. The existence of an histamine-sensitive adenylate cyclase system in human gastric mucosa--restricted to the oxyntic gland area--has been offered as evidence: for an involvement of this cyclic nucleotide in the action of this secretagogue (1-3). In addition, secretory inhibitors of gastric acid secretion such as prostaglandins and the vasoactive intestinal polypeptide (VIP) were also shown to be potent stimulators of gastric mucosal adenylate cyclase in human beings (4, 5), indicating that this nucleotide may be linked to the inhibition of gastric acid secretion in man, too. The importance of the human intestinal adenylate cyclase system in the pathogenesis of small-bowel diarrhea is evidenced (a) by the synchronous changes of intracellular cAMP concentrations and the extent of intestinal secretion induced by the exotoxins of Vibrio cholerae and of some other bacteriae (6, 7) and (b) by the observation that elevated serum concentrations of VIP and prostaglandins (both potent stimulators of human intestinal adenylate cyclase) can induce intestinal secretion of water and electrolytes (8-10). The stimulatory effects of bile acids on colonic adenylate cyclase suggested that this universal effector system might play an important role in colonic function, too (11, 12). We therefore studied the influence of gastrointestinal hormones on the human colonic adenylate cyclase. Biopsy specimens of colonic mucosa were obtained endoscopically (N = 8). The tissue fragments were gently homogenized and assayed for enzyme activity by the method of Salomon et al (13). The protein content of the samples was determined according to the Lowry method (14). Basal enzyme activity in homogenates of human colonic mucosa averaged 350 pmol cAMP/mg protein/15 rain. As shown in Table 1, both VIP (10/xM) as well as prostaglandin E2 (0.3 raM) are capable of activating the human enzyme system by 220% and 300%, respectively. The/3-adrenergic agonists isoproterenol and epinephrine, as well as glucagon, had no stimulatory effects. TABLE 1. ADENYLATE CYCLASE ACTIVITY IN HUMAN COLONIC MUCOSAL HOMOGENATE: ACTION OF HORMONES

Human Fat Cell Adenylate Cyclase

SummaryThe effects of guanine nucleotides on basal and parathyroid hormone-stimulated adenylate cyclase of human fat cell ghosts were studied.GTP (10−7–10−3 M) caused a dose-dependent inhibition of basal enzyme activity, but it had no significant effect on PTH-stimulated rates of cAMP-formation.The guanine nucleotide analogue 5′-guanylyl-imidodiphosphate GMP (PNP) when applied in the same concentration range, stimulated basal as well as PTH-activated adenylate cyclase activity up to 300%. GMP (PNP) activation was non-linear with time.PTH-activated the human fat cell adenylate cyclase via an individual receptor distinct from beta-adrenergic receptor sites.

p-CMB-dextran T 10. A useful tool in evaluating the functional importance of superficial SH-groups in rat adipocytes

AbstractA large SH-blocking reagentp-CMB-dextran T 10 (MW about 10,000) was applied to isolated rat adipocytes. This compound possesses the same reactivity towards SH-groups as uncoupledp-CMB. p-CMB-Dextran T 10 did not influence the basal glucose uptake in contrast to uncoupledp-CMB.The results are discussed with respect to the usefulness ofp-CMB-dextran T 10 in evaluating the role of superficial SH-groups in various membrane functions.

Different effects of dietary chenodeoxycholic acid and ursodeoxycholic acid on colonic adenylate cyclase in the rat.

SummaryThe oral administration of dietary chenodeoxycholic acid (1%), but not of ursodeoxycholic acid (1%), to male Sprague Dawley rats results in a significant increase in the colonic adenylate cyclase activity without any influence on the colonic cyclic-AMP phosphodiesterase activity. No effect of chronic bile acid feeding on the response of colonic adenylate cyclase to prostaglandin E2 and vasoactive intestinal peptide is observed. These data emphasize a dependence of the cyclic-AMP adenylate cyclase activation on the chemical structure of the bile acid. This may be of pathophysiologic relevance with respect to the frequently observed diarrhea as a side effect of oral chenodeoxycholic, but not ursodeoxycholic acid therapy for cholesterol gallstone dissolution in man.