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W W Fleming

Publications and source records attributed to W W Fleming.

At least 19 recordsLinked to original sources
PubMed1992

Adenylyl cyclase and guanine nucleotide-binding proteins in supersensitive guinea pig ventricles.

Chronic treatment with reserpine (0.1 mg/kg/day x 7 days) leads to the development of adaptive supersensitivity of ventricular myocardium of guinea pigs. The compensatory increase in sensitivity is associated with a small increase in beta-adrenoreceptor number. However, sensitivity is increased to a number of agonists that do not interact with beta-adrenoceptors. An evaluation of the role of both adenylyl cyclase and guanine nucleotide-binding regulatory proteins in the development of adaptive supersensitivity was carried out using crude membrane fragments from untreated control and chronically reserpine-treated guinea pigs. Quantitative analysis of Gs alpha and Gi protein concentrations was accomplished using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Chronic treatment with reserpine reduced basal levels of adenylyl cyclase activity by nearly 60%. The reduced activity was not the result of a loss of endogenous norepinephrine, because incubation of tissues in the presence of propranolol did not alter the basal level of adenylyl cyclase activity. Incubation in the presence of guanylylimido diphosphate (10(-5) M) also significantly reduced basal adenylyl cyclase activity, by nearly 70%. Chronic treatment with reserpine failed to significantly alter the activation of adenylyl cyclase by isoproterenol, impromidine, NaF, or forskolin. These data suggest that chronic treatment with reserpine does not alter agonist-induced activation of adenylyl cyclase. Furthermore, analysis of Gs alpha and Gi indicated that chronic treatment with reserpine did not affect the levels of these regulatory proteins in ventricular myocardial membranes. The data indicate that the enhanced sensitivity of guinea pig ventricular myocardium is not the result of an alteration in adenylyl cyclase activity or in the concentration of guanine nucleotide regulatory proteins. Therefore, the enhanced responsiveness to widely diverse agonists must be due to an alteration in cellular function beyond the level of adenylyl cyclase.

Adenylyl Cyclase Inhibitors↗
PubMed1992

Membrane potential in myenteric neurons associated with tolerance and dependence to morphine.

Chronic treatment of guinea pigs with morphine produces subsensitivity (tolerance) of the longitudinal smooth muscle-myenteric plexus preparation to a variety of inhibitory agonists (e.g., mu opioid, alpha adrenoceptor and adenosine receptor agonists) and supersensitivity (dependence) to a variety of excitatory agonists (e.g., nicotine, 5-hydroxytryptamine and potassium ions). The present investigation was to determine if these changes in sensitivity could be related to changes in electrical properties of the S and AH neurons in the myenteric plexus. S neurons from morphine-implanted animals were significantly depolarized (7 mV) relative to those from placebo-implanted animals, whereas the membrane potential of AH neurons was unchanged. Approximately 60% of S neurons were hyperpolarized by morphine. In this subset of neurons, membranes were significantly depolarized but the threshold was unchanged in morphine-implanted animals. This means that resting potentials of S neurons from tolerant preparations are closer to threshold. The hyperpolarization produced by morphine (0.1 microM) was similar in preparations from morphine- and placebo-implanted animals. Thus, the partially depolarized state of S neurons in the myenteric plexus is the cause of the subsensitivity and supersensitivity to agonists and can explain both tolerance and dependence. Changes in opioid receptors or their coupling to potassium channels do not appear to contribute to tolerance in the longitudinal smooth muscle-myenteric plexus.

Action Potentials↗
PubMed1991

Mesenteric vascular responses of young spontaneously hypertensive rats.

Frequency-responses curves for nerve stimulation and dose-response curve for norepinephrine, 5-hydroxytryptamine potassium chloride, vasopressin and acetylcholine (ACh) were determined in isolated, perfused mesenteric vascular beds from young (approximately 5 weeks) spontanelouly hypertensive (SHR) and Wistar Kyoto rats. Although mean systolic blood pressure (measured by tail cuff plethysmography) was slightly higher in the SHR, this difference was not significant. Slopes and maximum responses were increased significantly for nerve stimulation and all agonists. The basal perfusion pressure was also significantly elevated in the SHR. These differences are consistent with existing evidence that structural changes occur in blood vessels of SHR at an early stage and probably precede development of hypertension. Such structural changes could therefore contribute to development of the hypertension. Cocaine (1 microM) markedly increased responses to nerve stimulation and bolus injections of norepinephrine in preparations from SHR with little or no effect on such responses in Wistar Kyoto preparations, a result consistent with the known greater density of noradrenergic nerves in SHR vasculature. In the presence of cocaine, there was unmasked a selective super-sensitivity (significantly lower ED50) to norepinephrine in the SHR. Thus SHR mesenteric vessels may possess an alteration in adrenoreceptors or their coupling to other cellular mechanisms. Responses to ACh revealed no indication of a deficient endothelial mediated relaxation. An altered media:lumen ratio of small arteries, hypernoradrenergic innervation and supersensitivity to the transmitter may contribute to development of hypertension.

Animals↗
PubMed1991

Cyclic AMP and adaptive supersensitivity in guinea pig atria.

Supersensitivity was induced by injection of guinea pigs with reserpine, 0.1 mg/kg/day for 7 days. This treatment induced chronotropic supersensitivity of isolated right atria to isoproterenol but not to forskolin. The pretreatment induced supersensitivity to the cyclic AMP-generating effects of isoproterenol or forskolin in left, but not right, atria. These results are discussed in reference to the extensive literature describing supersensitivity in the guinea-pig heart, including the possible role of the adenylyl cyclase system.

Animals↗
PubMed1991

Catecholamine and neuropeptide Y levels in tissues from young Dahl rats following 5 days low- or high-salt diet.

The mesenteric vasculature of Dahl salt-sensitive (DS) rats on high-salt diet is supersensitive to nerve stimulation and to norepinephrine. The current experiments were undertaken to examine whether the enhanced sensitivity to nerve stimulation is due solely to the postsynaptic supersensitivity to norepinephrine, to increased sympathetic innervation, to altered transmitter release or to the presence of another transmitter acting as a potentiator. Catecholamine content and neuropeptide Y (NPY) presence were determined in tissues from young (approximately 5 weeks old) male Dahl rats exposed to 5 days of high (7%) or low (0.45%) salt diet. Catecholamine content from mesenteric artery, renal artery, caudal artery, right atrium, aorta, vas deferens and adrenal gland was quantified by high-pressure liquid chromatography with an electrochemical detector. A strain difference, independent of diet, between young DS and Dahl salt-resistant (DR) rats was seen only in adrenal epinephrine content. DS high-salt (+) rats displayed reduced norepinephrine content relative to DR+ in the mesenteric artery and right atrium. The release of norepinephrine from isolated mesenteric vasculature into the perfusate in response to transmural stimulation showed no significant differences between DS+ and DR+ preparations under basal, or deoxycorticosterone acetate (DOCA; 30 microM) perfusion conditions. The addition of 5 microM cocaine to the DOCA perfusion, while increasing total norepinephrine outflow in all preparations, failed to differentiate between DS+ and DR+. NPY immunofluorescence along mesenteric artery sections of DS+ and DR+ rats was not significantly different. Thus, in the tissues examined, enhanced responsiveness of vascular smooth muscle may not be explained by hypernoradrenergic innervation, elevated NPY innervation or altered release of transmitter.

Animals↗
PubMed1991

Specific supersensitivity of the mesenteric vascular bed of Dahl salt-sensitive rats.

Dahl salt-sensitive (DS) and salt-resistant (DR) rats were maintained on a diet containing normal (0.45%) or high (7%) salt for 5 days. The DS rats had slightly higher systolic blood pressures than DR rats, although a high salt diet failed to significantly elevate pressure in either group when compared with their appropriate (low salt diet) controls. The sensitivity of the isolated, perfused mesenteric vasculature from DS rats fed a high salt diet to nerve stimulation was greater when compared with all other groups in the presence or absence of cocaine (1 microM). A similar difference in sensitivity between high salt DS rats and high salt DR rats to bolus injections of norepinephrine was observed only in the presence of cocaine. The change in sensitivity was characterized by a leftward shift of the dose-response curve without a change in maximum response. No difference in sensitivity between the high salt DS group and any other treatment group was observed in response to the pressor agents KCl, angiotensin II, 5-hydroxytryptamine or the depressor agent acetylcholine. These data indicate that DS rats on a short-term, high salt diet possess a significant and specific elevation in sensitivity to nerve stimulation and norepinephrine in the absence of an increase in blood pressure. Differences in the effectiveness of cocaine among the groups suggest that differences may exist in neuronal uptake (uptake 1).(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1991

Involvement of mu, delta and kappa receptors in morphine-induced tolerance in the guinea pig myenteric plexus.

Tolerance in the longitudinal muscle-myenteric plexus (LM-MP) preparation to opioids appears to represent two separate phenomena. One phenomenon is characterized by marked subsensitivity to mu-selective opioid substances and a very short half-life. The other is characterized by moderate subsensitivity to morphine and to several nonopioid inhibitory substances and has a long half-life. The present investigation was to determine if the long half-life type of tolerance involves cross-tolerance between mu-selective opioids, such as morphine, and opioids selective for delta or kappa receptors. Tolerance was induced by s.c. implantation of morphine pellets for 7 days. Control guinea pigs received placebo pellets. Isolated LM-MP preparations were tolerant to the mu-selective agonists morphine and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin as well as to [D-Ala,2D-Leu5]enkephalin and [D-Ser2]leucine enkephalin-Thr, agonists only slightly selective for delta receptors over mu. The delta-selective agonist, [D-Pen2,D-Pen3]enkephalin, produced a biphasic concentration-response curve. The first phase, which included only very weak effects, presumably represented delta-mediated effects. The second phase reached virtually 100% inhibition of the twitch. Significant subsensitivity (tolerance) was demonstrated only for the second phase, presumably representing mu-mediated effects. There was no significant tolerance to the kappa-selective agonist, U50,488H. Consistent with the literature, the results indicate effective twitch inhibition mediated by mu and kappa receptors, but not delta receptors. Among opioids, tolerance induced by morphine pellets in the LM-MP is limited to mu receptor-mediated inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Sensitivity of cerebellar Purkinje neurons to neurotransmitters in genetically epileptic rats.

Epilepsy is a state of neuronal hyperactivity which may be caused by an altered relationship between inhibitory and excitatory influences on neurons. We have conducted experiments using microiontophoresis with in situ extracellular recording of anesthetized rat cerebellar Purkinje neuron activity to determine the sensitivity of these neurons to neurotransmitters in a genetic model of epilepsy. Quantitative evaluations of agonist-induced changes in activity were carried out by using poststimulus time histograms. Current-response curves were generated and linear regression analysis was performed to evaluate changes in responsiveness and sensitivity between control and genetically epilepsy-prone-9 rats. The current required to produce 50% inhibition of activity by GABA was 2.6-fold higher in the genetically epilepsy-prone-9 rats compared to control. In contrast, the amount of current required to produce 50% inhibition by norepinephrine was not significantly different between groups. There also was no significant change in cerebellar neuron sensitivity to the excitatory transmitter, glutamate. The lack of an alteration in sensitivity and responsiveness to norepinephrine or glutamate suggests that the hyperexcitability of neurons may be associated with a specific subsensitivity of the GABAergic system. Such a specific subsensitivity to gamma-aminobutyric acid would, therefore, yield a more excitable state of the neuron and may contribute to the development of the hyperactivity observed in epilepsy.

Animals↗
PubMed1991

Effects of GABA in the morphine-tolerant longitudinal muscle, myenteric plexus preparation of the guinea pig.

The effect of chronic treatment with morphine via pellet implantation on the sensitivity of the longitudinal smooth muscle-myenteric plexus of the guinea pig ileum to the contractile effects of gamma-aminobutyric acid (GABAA)-receptor agonists was assessed. GABA and muscimol elicited concentration-dependent contractions of the longitudinal smooth muscle which were due to the release of acetylcholine because the contractile effects were markedly attenuated by atropine (10 nM). The contractile action of GABA agonists does not involve an intermediate step mediated by nicotinic receptors because the concentration-response curves for GABA were unaffected by hexamethonium (1 mM). Bicuculline (10 microM) produced nearly equivalent rightward shifts of the concentration-response curves for both GABA and muscimol, indicating mediation of the contractile effects of these agents by GABAA receptors. Chronic exposure to morphine via pellet implantation did not alter the sensitivity of this preparation to either GABA or muscimol. This is in contrast to the development of supersensitivity of the longitudinal smooth muscle-myenteric plexus to other excitatory agonists (nicotine, 5-hydroxytryptamine and potassium), which accompanies the development of tolerance to opioids. GABA induces depolarization of myenteric neurons that is observed most prominently in AH neurons and rarely in S neurons. The stimulatory effects of nicotine and of GABA were inhibited by morphine (a predominantly mu opioid agonist) and by U50,488H (a predominantly kappa opioid agonist). The results are discussed within the context that supersensitivity to neuronal stimulants of the myenteric plexus in morphine-tolerant preparations is limited to substances which depolarize S neurons.

Animals↗
PubMed1991

Antagonism of norepinephrine by clonidine in the isolated rat mesenteric vascular bed.

Experiments were done in isolated, perfused mesenteric vascular beds from Sprague-Dawley rats. Bolus injections of norepinephrine (3-100 nmol) induced dose-dependent increases in perfusion pressure with a maximum increase greater than 100 mm Hg. In the same dose range, clonidine had no effect on perfusion pressure. In the presence of an elevated pressure caused by constant infusions of norepinephrine (6-20 microM), bolus injections of clonidine (0.1-10 nmol) or acetylcholine (0.007-7 nmol) caused dose-related decreases in perfusion pressure. Procedures which damage endothelium (brief exposure to methylene blue or reactive oxygen radicals) abolished the depressor action of acetylcholine but only moderately reduced the depressor action of clonidine. The depressor action of clonidine was not antagonized by the alpha-2 adrenoceptor antagonist, idazoxan. Acetylcholine produced depressor responses in the presence of 5-hydroxy-tryptamine or vasopressin, but clonidine did not. Dose-response curves to bolus doses of norepinephrine were shifted markedly to the right by an alpha-1 selective concentration of prazosin (1 nM) and were shifted to the right with depression of maximum by infusions of clonidine (0.3 and 1.0 microM). It is concluded that, in the mesenteric vasculature of the rat: 1) the role of alpha-2 adrenoceptors, in responses to clonidine, is minimal; 2) endothelial factors play little role, if any, in the depressor effects of clonidine and 3) clonidine has a potent ability to interfere with the alpha-1 adrenoceptor-mediated vasoconstriction induced by norepinephrine. This antagonistic action may be at the level of the receptor but could involve postreceptor steps.

Acetylcholine↗
PubMed1978

Species differences in sodium-potassium adenosine triphosphatase activity in the smooth muscle of the guinea-pig and rat vas deferens.

The acitvities of sodium-potassium-activated adenosine triphosphatase (Na+,K+-activated ATPase) and ouabain-inhibited, sodium-potassium-activated adensoine triphosphatase (Na+,K+-ATPase) in subcellular fractions of guinea-pig and rat vasa deferentia were compared to determine whether the ineffectiveness of ouabain and reduced extracellular potassium in the rat vas deferens observed in the preceding paper occurs because of a relatively low level of Na+,K+-ATPase and/or an insensitivity to ouabain. The results indicate that the specific and total activities of Na+,K+-activated ATPase and Na+,K+-ATPase (i.e., the transport enzyme) in the individual subcellular fractions and in the tissue were higher in the vas deferens of the rat than in the guinea pig. The percentage of inhibition of Na+,K+-activated activity by ouabain (8 x 10(-5) M) varied in the subcellular fractions; it was higher in the guinea-pig (range 31--87%) than in the rat (nonsignificant effect to 40%). A greater percentage of total Na+K+- activated ATPase activity was inhibited in the vas deferens of the guinea pig (56%) than the rat (30%). Differences in the effects of lowered extracellular potassium concentration or ouabain on resting membrane potential (preceding paper) are apparently unrelated to the amount of transport enzyme in the vasa deferentia or the two species, or to its relative sensitivity to ouabain.

Animals↗
PubMed1978

Denervation-induced changes in electrophysiologic parameters of the smooth muscle of the guinea-pig and rat was deferens.

In order to further elucidate the mechanisms by which postganglionic denervation causes changes in the dose-response curves obtained in smooth muscle, microelectrodes have been used to investigate cellular changes in the denervated guinea-pig and rat vas deferens. In the guinea-pig vas deferens, chronic denervation produced a partial depolarization (mean change of 8.5 mV) without any change in threshold for the action potential. In the rat vas deferens there was no change in resting potential but the threshold membrane potential because more negative (6.5 mV). Thus, in both species, but apparently by different mechanisms, the resting and threshold membrane potentials are brought closer together by denervation. Such an effect would clearly contribute to the well documented increase in sensitivity to depolarizing agonists which is produced by chronic denervation. In both species, denervation increased the space constant of the smooth muscle, an indication of increased electrical coupling among the cells. This observation is consistent with morphologic evidence of improved coupling induced by denervation and presented previously from this laboratory. The improved coupling appears to be associated with the increased maximum response of the denervated vasa deferentia of both species. These results are discussed in references to known similarities and differences in electrophysiologic characteristics between normal guinea-pig and rat vasa deferentia.

Action Potentials↗
PubMed1978

The effects of ouabain and alterations in potassium concentration on the sensitivity to drugs and the membrane potential of the smooth muscle of the guinea-pig and rat vas deferens.

Previous work has suggested partial depolarization of the smooth muscles cells to be an important factor in the post junctional supersensitivity induced in the guinea-pig vator in the post junctional supersensitivity induced in the guinea-pig vas deferens by chronic denervation or decentralization. The present experiments were undertaken to explore, under acute in vitro conditions, the relationship between membrane potential changes and sensitivity in the vasa deferentia of guinea pigs and rats. The results indicate that sensitivity was altered whenever resting potential was changed by 7 to 10 mV. The results also indicated that considerable electrophysiologic differences exist between the smooth muscles of the guinea-pig and rat vas deferens. Notably, ouabain or lowered external potassium caused a partial depolarization of the guinea-pig, but not the rat, vas deferens. The results, especially when combined with evidence that the rat vas deferens does possess a considerable amount of (Na+-K+)-adenosine triphosphatase, which is ouabain-sensitive, indicates that under the conditions of these experiments the Na pump is electrogenic in the guinea-pig, but not the rat, vas deferens.

Animals↗
PubMed1977

A comparison of the electrical properties and morphological characteristics of the smooth muscle of the rat and guinea-pig vas deferens.

Microelectrodes were used to compare a variety of electrophysiological parameters of the rat and guinea-pig vas deferens. In comparison to the guinea pig, spontaneous junction potentials in the rat tissue were of shorter duration and occurred with greater frequency and amplitude. Action potentials induced by nerve stimulation could be observed in the smooth muscle of both species. However, in the rat tissue the majority of action potentials were generated in the impaled cell while 60% of the action potentials in the guinea-pig vas deferens were propagated. When current was intracellularly applied, spike potentials could be induced in approximately 90% of the cells of the rat vas deferens but in less than 10% of the cells of the guinea-pig vas deferens. The space constant was 1.48 mm for the guinea-pig vas deferens, but less than 0.5 mm for the rat vas deferens. Electromicroscopic examination of the homologous tissues indicates that the differences in electrical properties can be accounted for in part by differences in morphology. The incidence and intimacy of neuromuscular contacts was greater in the rat vas deferens while the incidence of nexuses between smooth muscle cells was greater in the guinea-pig tissue.

Animals↗
PubMed1977

Effects of denervation and reserpine on nexuses in the rat vas deferens.

Permanganate-fixed vasa deferentia from rats were examined for the presence of nexal-like contacts by electron microscopy. There was a significantly greater incidence of nexuses (2X) in chronically denervated tissues (5-7 days) but not in tissues from reserpine-pretreated animals (1.0 mg/kg/day for 5-7 days). The results suggest that an increase in nexal regions may not be a general feature of postjunctional supersensitivity but rather may contribute to other denervation-induced changes in contractile response.

Animals↗