Pharmacodynamics of racemic and S(-)-atenolol in humans

The cardiovascular actions of racemic atenolol (RSATN) have been well characterized in humans, but the actions of S(-)-atenolol (SATN) when administered alone are unknown. In this study, responses of heart rate (HR) and Doppler-derived aortic blood flow profiles to upright treadmill exercise were compared after oral administration of 50 mg SATN and 100 mg RSATN in eight healthy, adult, male volunteers. After a single-blind, placebo run-in period, subjects were randomly allocated in a double-blind, crossover fashion to receive SATN and RSATN. Each study period was separated by a 7-day washout period. Multiple submaximal exercise tests were performed and data were collected over the 24 hours after each treatment. Both SATN and RSATN significantly (P < .05) blunted peak exercise HR by 38 +/- 3 and 37 +/- 3 beats/min, respectively. Aortic blood flow acceleration measured during peak exercise decreased after SATN and RSATN, by 13 +/- 4 and 13 +/- 3 m/sec2, respectively (P < .05). No difference in hemodynamic effect was observed between treatments. Pharmacodynamic parameters derived from plasma S(-)-atenolol concentration-effect (HR) curves after SATN, RSATN, and total atenolol plasma concentrations after RSATN did not differ significantly. Predicted maximum reductions in heart rate (Emax) and EC50 for S(-)-atenolol after SATN were 39.6 +/- 5.8 beats/min and 38.4 +/- 40.9 ng/ml versus 34.5 +/- 8 beats/min and 25.9 +/- 29.9 ng/ml for RSATN, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Single dose pharmacokinetics of (S)-atenolol administered orally as a single enantiomer formulation and as a racemic mixture (Tenormin)

The purpose of this study was to describe the pharmacokinetics of and heart rate and blood pressure responses to (S)-atenolol (SATN) and (R)-atenolol (RATN) after oral administration of (S)-atenolol and (R,S)-atenolol (Tenormin) in man. Eight male subjects were given single oral doses of 50 mg of SATN as a single enantiomer formulation (SEF) and 100 mg of Tenormin (TMN) using a randomized, double-blind, 2-period, complete crossover study design. Subjects performed exercise tolerance tests (Bruce Protocol) before and 2, 4, 6, 8, 12, and 24 h after drug administration. Plasma samples were obtained 2 min before and 30 min, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 16, and 24 h after dosing. Urine was collected for the first 48 h after dosing. Plasma and urine samples were analyzed for SATN and RATN by an enantioselective HPLC method. SEF and Tenormin attenuate exercise-induced increases in heart rate and systolic blood pressure. Mean changes in exercise heart rates 4 h after dosing were -38 +/- 3 bpm and -37 +/- 3 bpm for SEF and TMN, respectively, P = 0.792. Mean changes in exercise systolic blood pressure were -42 +/- 12 mm Hg and -55 +/- 14 mm Hg for SEF and TMN, respectively, P = 0.484. Mean area under the plasma level time curve (AUC0-24) and mean Cmax for SATN for SEF were significantly lower than for SATN after TMN.(ABSTRACT TRUNCATED AT 250 WORDS)

Predicting the clinical response to cisapride in children with chronic intestinal pseudo-obstruction

We assessed upper gastrointestinal anatomy and function with contrast radiology and antroduodenal manometry in 51 children with chronic intestinal pseudo-obstruction (CIP) prior to entering these patients into an open-label outpatient trial of cisapride. The diagnosis of CIP was based on characteristic symptoms requiring special nutritional support (parenteral in 30, tube feeding in 12) or interfering with daily activities (documented by diary in nine). At a time the subjects were not acutely ill, antroduodenal pressures were recorded for > 4 h fasting and > 1 h after a complex liquid meal. Results were categorized by the most prominent manometric abnormality as myopathy (n = 6), absent migrating motor complex (MMC) (n = 27), failure to induce fed pattern (n = 7), MMC plus discrete abnormalities (n = 7), and postprandial duodenal hypomotility (n = 4). Patients in the first two categories did not have effective MMCs, but those in the last three categories did. Compared to children without MMCs, those with MMCs rarely required parenteral nutrition (p < 0.001). All children were treated with oral cisapride 0.2 mg/kg/dose t.i.d., and evaluated every 2 months for up to 1 yr. Of 49 evaluable subjects, the final global assessment was unchanged in 25, fair (improved symptom score) in 17, or excellent (change from TPN to tube feeding or tube feeding to oral feeding) in seven. Children with MMCs (13 of 18) responded more often to cisapride than those without MMCs (11 of 31), p < 0.02. All four subjects with postprandial duodenal hypomotility had excellent responses. Children with normal diameter bowel responded more often than those with dilated bowel, p < 0.004. To summarize, in children with CIP, absence of the MMC was associated with need for greater intensity of nutritional support and decreased response rate to cisapride. The response to cisapride was highly variable within the study group, but often could be predicted by the presence or absence of bowel dilation and MMCs.

Antroduodenal manometry in the evaluation of chronic functional gastrointestinal symptoms

Intraluminal pressure in the gastric antrum and duodenum was studied in 44 children and adolescents referred for evaluation because of functional symptoms, including vomiting, abdominal distension, and abdominal pain. Manometric abnormalities were found in 39 patients (89%). Abnormalities during fasting included absence of the migrating motor complex; retrograde, phase 3-like episodes; increased frequency, decreased duration, and decreased amplitude of phase 3 episodes; tonic duodenal contractions; nonpropagated bursts of duodenal contractions; and consistently low-amplitude or absent contractions. Postprandial abnormalities included a phase 1-like pattern (postprandial hypomotility) and phase 3-like episodes (failure to induce a fed pattern). The presence or absence of the migrating motor complex was a predictor of disability. Parenteral alimentation was needed by only 4 of 28 patients with the migrating motor complex, but by 13 of 16 patients without the migrating motor complex (P less than .001). In 15 of 18 patients studied on consecutive days, oral cisapride was associated with increases in the number and amplitude of duodenal contractions after a complex-liquid meal (P less than .02). It is concluded that antroduodenal manometry is a useful technique that elucidates the underlying gastrointestinal motility disorder present in the majority of children and adolescents with severe functional symptoms.

Inhibitory control of proximal colonic motility by the sympathetic nervous system

The purpose of this study is to determine whether or not the sympathetic nervous system provides a tonic inhibitory input to the colon in chloralose-anesthetized cats. Proximal and midcolonic motility were monitored using extraluminal force transducers. An intravenous bolus injection of 5 mg of phentolamine in 14 animals elicited a pronounced increase in proximal colon contractility. The minute motility index changed from 0 +/- 0 to 26 +/- 4 after phentolamine administration. Midcolonic motility also increased in response to phentolamine. Specific blockade of alpha 2-receptors, but not alpha 1-receptors, caused the same response seen with phentolamine. alpha-Adrenergic blockade increased colon contractility after spinal cord transection but not after ganglionic blockade. Blockade of alpha-adrenergic receptors was also performed before vagal and pelvic nerve stimulation and in both cases increased colonic motility. Vagal stimulation alone had no effect on colonic contractility, while pelvic nerve stimulation increased motility at the midcolon. alpha-Receptor blockade did not alter the ineffectiveness of vagal stimulation but did unmask excitatory effects of pelvic nerve stimulation on the proximal colon. All excitatory colonic responses were prevented by blocking muscarinic cholinergic receptors. These data indicate that tonic sympathetic nervous system activity exerts an inhibitory effect on colonic motility. The inhibitory effect is mediated through alpha 2-adrenergic receptors. Based on these findings, we suggest that alterations in sympathetic nervous system activity may be extremely important for the regulation of circular muscle contractions in the colon.

Effect of ranitidine on gastric acid hypersecretion in an infant with short bowel syndrome

We studied the effect of ranitidine given in graded bolus intravenous doses on gastric acid hypersecretion in an unfed 3-month-old male with short bowel syndrome. We measured gastric volume and H+ serially for 12 h following each bolus and correlated inhibition of H+ secretion with plasma ranitidine concentration. In the first 4 h post drug, doses of 0.3, 1.0, 2.0, and 4.0 mg/kg resulted in 78, 93, 97, and 98% inhibition, respectively. The cumulative 12-h effect of the drug was to inhibit H+ secretion 67, 63, 72, and 87%. The IC50 for H+ secretion was between 50 and 100 ng/ml, and the IC90 between 130 and 150 ng/ml. Volume of gastric secretions was reduced by approximately 50% by all ranitidine doses. Because gastric acid hypersecretion interferes with nutrient absorption, the infant was treated with ranitidine during a 5-week trial of enteral feeding. A decrease in the antisecretory effect of ranitidine apparent at the end of the treatment period temporally related to an increase in oxyntic mucosal function. No adverse drug effects were observed during treatment.

Activation of central nervous system cholecystokinin receptors stimulates respiration in the cat

Cholecystokinin-octapeptide (CCK-OP) was injected into the lateral brain ventricle of chloralose-anesthetized cats while monitoring pulmonary ventilation, mean blood pressure (BP) and heart rate (HR). Initial values for respiratory minute volume, BP and HR were 436 +/- 32 ml/min, 126 +/- 8 mm Hg and 204 +/- 9 beats/min, respectively. CCK-OP in doses of 8.8 x 10(-13) to 8.8 x 10(-10) M (i.e., 1 to 1000 ng) increased respiratory minute volume by +56 +/- 18 ml/min to +370 +/- 73 ml/min, respectively. This response was due to an increase in tidal volume (P less than .05) with no effect on respiratory rate. Most of these doses increased BP and HR (P less than .05). These responses were mimicked when CCK-OP was administered into and restricted to the lateral and 3rd ventricles, whereas no significant responses occurred with administration into the 4th ventricle. The highest dose of CCK-OP, when given i.v., produced no significant effects. Central nervous system injections (lateral ventricle) of CCK-OP in deafferented animals also produced respiratory and cardiovascular stimulation, but higher doses were required. Central nervous system injections of comparable doses of desulfated CCK-OP or pentagastrin had no effect on respiratory minute volume, BP or HR. Proglumide (0.8 and 8.0 mg), a specific CCK receptor antagonist, prevented both respiratory and cardiovascular effects induced by CCK-OP. These doses of proglumide exerted no antagonism of CO2-induced respiratory stimulation. These data suggest that a specific receptor reacting with CCK-OP may be important for the control of respiration.

Respiratory and cardiovascular effects of intraventricular cholecystokinin

Cholecystokinin (1-300 ng) was administered into the lateral brain ventricle of chloralose-anesthetized cats while monitoring tracheal airflow, arterial blood pressure, and heart rate. Dose-related increases in respiratory activity occurred in each animal tested, and were due to an increase in tidal volume. When 300-1000 ng of cholecystokinin was administered intravenously, no respiratory stimulant effect was observed. These results indicate that cholecystokinin acts in the brain to stimulate respiration.

Hindbrain GABA receptors influence parasympathetic outflow to the stomach

Blockade of gamma-aminobutyric acid receptor function by direct microinjection of bicuculline into the nucleus ambiguous in cats produced a marked increase in gastric motility which was mediated by the vagus nerve. This effect was reversed by muscimol. These data indicate that the nucleus ambiguous may be an important brain site influencing gastric function and that the neurotransmitter controlling parasympathetic overflow from this nucleus to the stomach is gamma-aminobutyric acid.

A mathematical model for membrane transport of amino acid and Na+ in vesicles

A model with a carrier having sites for both amino acid and Na+ can account for AIB (alpha-aminoisobutyric acid) transport kinetics observed in membrane vesicles from SV3T3 (simian virus 40-tranformed Balb/c33 cells) and 3T3 (the parent cell line). The main feature of this cotransport model is that Na+ binding to carrier decreases the effective Km for AIB transport. Na+ transport kinetics observed in both vesicle systems can be described by passive (possibly facilitated) diffusion. The lag of Na+ transport across the membrane compared to that for AIB, coupled to the Na+-dependent decrease in the Km for AIB, accounts for the overshoot in intravesicular AIB observed for SV3T3 in the presence of an initial Na+ gradient. Extra-vesicular Na+ maintains a decrease in the Km for AIB influx before intra-vesicular Na+ has accumulated to balance it with a comparable decrease in the Km for AIB efflux. 3T3 vesicles display little overshoot, and this finding can be explained mostly by a lower carrier affinity for Na+.

Amino acid and 22Na+ uptake in membrane vesicles from confluent simian virus 40 transformed Balb/c3T3 and Balb/c3T3

The studies reported here were carried out to characterize further previously described changes in membrane localized amino acid transport associated with simian virus 40 transformation of the mammalian cell line, Balb/c3T3. Membrane vesicles were prepared from confluent cultures of both simian virus 40 transformed Balb/c3T3 (SV3T3) and the untransformed parent line, Balb/c3T3 (3T3). An initial, externally imposed out greater than in, 100 mM a+ gradient produces acceleration of early ingress of alpha-aminoisobutyric acid (AlB) in vesicles from both cell lines, but transient, concentrative uptake (overshooting) only in SV3T3 vesicles. Early ingress of L-leucine is also accelerated in SV3T3 vesicles by a Na+ gradient, and overshooting is also demonstrable. Na+-gradient independent AIB permeability of SV3T3 and 3T3 membranes was estimated using uptake data, a first order rate equation and measurements of vesicle size derived from quasi-elastic light-scattering studies. AIB permeability of SV3T3 membranes is greater than that of 3T3 membranes (113 A/min and 43 A/min, respectively), suggesting that overshooting in 3T3 vesicles is not attenuated by a Na+-independent AIB "leak". Na+ permeability of the two membranes is similar, ruling out the possibility that a slower rate of Na+ equilibration across the SV3T3 membrane allows development of the overshoot. In SV3T3 vesicles the height of a Na+-gradient dependent overshoot varies with the initial [Na+]o/[Na+]i ratio, and ln[Na+]o/[Na+]i is linearly related to ln AIB uptake at overshoot peak/AIB uptake at equilibrium, consistent with the possibility that for [Na+]o/[Na+]i ratios in the range studied, AIB overshoot is energized by a constant proportion of the energy available from the initial electrochemical gradient for Na+. These results are consistent with the possibility that Na+-gradient dependent overshooting in SV3T3 vesicles is produced by Na+-amino acid carrier interactions resulting in either an increase in maximum transport velocity or an increase in carrier affinity for AIB.

[Amino acid transport by membrane vesicles of virally transformed and nontransformed cells: effects of sodium gradient and cell density]

Mixed membrane vesicle populations composed of plasma membrane and endoplasmic reticulum were prepared from Balb/c 3T3 and simian virus 40-transformed Balb/c 3T3 mouse fibroblasts. The initial rates of uptake of L-leucine and alpha-aminoisobutyric acid by these vesicles were stimulated by a NaCl gradient (external greater than internal). Cation specificity for stimulation of L-leucine uptake was Na+ greater than Li+ greater than K+. NaSCN was as effective as NaCl. Stimulation of uptake of both amino acids by a NaCl gradient was twice as great in vesicles from transformed as compared to non-transformed cells. The NaCl gradient produced transient accumulation of both L-leucine and alpha-aminoisobutyric acid to twice the equilibrium level in vesicles from transformed cells. No such "overshoot" was observed in vesicles from nontransformed cells. In vesicles from the contact-inhibitable Balb/c 3T3 cells, transport of alpha-aminoisobutyric acid, but non L-leucine, exhibited a density-dependent decrease in Na+ gradient induced stimulation, from 248% for sub-confluent to 109% with confluent cells. No density-related changes in uptake were noted with vesicles from the transformed cells. These studies suggest that variation in amino acid uptake associated with viral transformation may be related, at least in part, to alterations in Na+ permeability of the surface membrane.

gamma-glutamyl transpeptidase of rat intestine: localization and possible role in amino acid transport

gamma-Glutamyl transpeptidase (gamma-GT), an enzyme possibly involved in amino acid transport, was investigated in rat small intestine using the synthetic substrate L-gamma-glutamyl-p-nitroanilide. Enzyme localization and characteristics were correlated with features of amino acid uptake. gamma-GT activity copurified with sucrase and alkaline phosphatase. Activity was maximal at pH 8.2 and was stimulated by monovalent cations. The relative specificity of the gamma-GT reaction with diglycine and eight essential amino acids as substrates correlated well with the rate of intestinal absorption of this dipeptide and these amino acids as observed by others. gamma-GT activity was 12-fold greater in the jejunum than in the ileum, again in agreement with relative rates of amino acid absorption along the length of rat intestine. The specific activity of gamma-GT in villus tip cells was 10 times greater than in crypt cells, and amino acid uptake was 2 to 6 times greater with villus tip than with crypt cells. Bromosulfophthalein, a noncompetitive inhibitor of gamma-GT, inhibited amino acid uptake. These studies support the concept that membrane gamma-GT may be involved in amino acid and dipeptide uptake, and indicate that further investigation of such involvement may be conveniently pursued using mammalian small bowel.

Sodium-stimulated amino acid uptake into isolated membrane vesicles from Balb/c 3T3 cells transformed by simian virus 40

Mediated uptake of amino acids by membrane vesicles isolated from Balb/c 3T3 cells transformed by simian virus 40 has been demonstrated. Initial rates of transport of radioactively labeled L-leucine and alpha-aminoisobutyric acid were enhanced by the addition of NaCl (100 mM) to the reaction mixture at the start of the uptake process. This enhancement included a prominent "overshoot" during initial uptake. Slight stimulation of alpha-aminoisobutyric acid uptake was seen with K+, but none with Li+. The mediated nature of the uptake event for L-leucine was shown by saturation kinetics and by inhibition with L-valine. The transport assay measured predominantly intravesicular amino acid uptake rather than binding, as shown by the variation of uptake in response to changes in extravesicular osmolarity. Electron microscopy confirmed the presence of closed vesicles. Thus, amino acid transport has been characterized in an in vitro membrane vesicles system which should prove useful for studies of growth control.