Biomedical subjects

R J Tallarida

Publications and source records attributed to R J Tallarida.

At least 37 records · Page 2Linked to original sources

PubMed1995

The relationship between vessel wall tension and the magnitude and frequency of oscillation in rat aorta.

Aortic rings from adult normotensive rats display spontaneous rhythmic activity that is enhanced by vasoconstricting agents. Graded doses of norepinephrine as well as combinations of norepinephrine and vasodilators produced levels of tension that were inversely related to the magnitude of oscillation and directly related to the frequency. A similar result occurred with KCl stimulation. Oscillations were only slightly affected by removal of the endothelium. These results, when combined with other reported studies, suggest that the oscillations in rat aorta are a manifestation of feedback control that may involve the cyclic release of one or more agents affecting calcium channels.

Animals↗

PubMed1995

Receptor discrimination and control of agonist-antagonist binding.

The law of mass action is the common model for the interaction of agonist and antagonist compounds with cellular receptors. Parameters of the interaction, obtained from functional and radioligand-binding studies, allow discrimination and subtyping of receptors and aid in understanding specific mechanisms. This article reviews the theory and associated mathematical models and graphical transformations of data that underlie the determination of receptor parameters. The main theory assumes that agonist and antagonist compounds bind to cells that have a fixed number of receptors and provides the framework for obtaining drug-receptor parameters from data and their graphical transformations. Conditions that produce a change in receptor number, a newer concept in pharmacology, can have an important effect on the parameter values derived in the usual way. This review concludes with a discussion of the quantitative study of receptor-mediated feedback control of endogenous ligands, a very new topic with potentially important implications for understanding antagonist effectiveness, loss of control, and chaos in regulated mass action binding.

Animals↗

PubMed1994

A quantitative study of dopamine control in the rat striatum.

The control of dopamine release by negative feedback via dopaminergic autoreceptors has been well established. Using previously derived mathematical equations, we have incorporated this feedback component into a mass-action binding model in which the rate of ligand release is a function of the quantity that is bound. A consequence of this model is that competitive antagonists augment release, whereas competing agonists reduce such release. In order to obtain values of the parameters needed in the feedback equations, in vivo microdialysis was used to deliver drugs locally to the rat striatum and to monitor the resulting changes in extracellular dopamine levels. The dopamine agonists apomorphine and quinpirole and the antagonists haloperidol and sulpiride were individually infused into the rat striatum via the microdialysis probe, and the effect of each on extracellular dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic levels was recorded. The data obtained from the microdialysis experiments were used to construct dopamine's control curve, a new quantitative concept that describes the feedback control of the dopaminergic system. Once known, the ligand's control curve has predictive value that may be useful in the design of efficient drug tests. Alterations of dopamine's control curve, which may occur with aging, drug addiction and pathological states, can now be studied and may provide new insights for potential therapeutic strategies.

Animals↗

PubMed1994

New mechanism of action of the cancer chemotherapeutic agent 5-fluorouracil in human cells.

5-Fluorouracil (5-FlUra), a cancer chemotherapeutic agent used in the treatment of colon, breast, ovarian and prostate cancer, is incorporated into DNA as a result of its utilization as 5-FldUTP during DNA synthesis. This promutagenic DNA lesion is excised by the base excision repair enzyme uracil DNA glycosylase (UDG). In this report we describe for the first time a mechanism by which 5-FlUra as the free base specifically binds in vivo to the UDG in noncycling human cells, thereby inhibiting its activity. By using 5-FlUra concentrations which did not elicit demonstrable cell toxicity, a dose-dependent decrease in UDG activity was detected which approached 30% of that observed in control cells. In contrast, exposure of cells to equivalent concentrations of uracil, 5-fluorodeoxyuridine or 5-bromouracil had no effect on UDG activity. Subsequent studies demonstrated a reversible binding of 5-FlUra to the glycosylase. Kinetic analysis using nonlinear regression analysis demonstrated a competitive mode of inhibition and indicated a tight binding of 5-FlUra to UDG in vivo, although the 5-FlUra-UDG complex was easily dissociated in vitro. These findings describe a potentially new and novel mechanism of action of 5-FlUra in a nonproliferating human cell population. The potential relevance of these findings to the utility of 5-FlUra as a cancer chemotherapeutic agent is considered.

Binding, Competitive↗

PubMed1993

The association of age and current world cancer mortality rates in relation to a century of cancer deaths in the U.S.

Recent data from a variety of countries and from the individual states in the US show that the annual cancer death rate is highly correlated with the percentage of the population aged 60 or more years. Historical data for the United States over the period 1900-1990 reveal that the percentage of its population > or = 60 yrs increased with a corresponding increase in the annual cancer death rate. When the US data over this time period are viewed against the current world data and the current individual state data there is a striking similarity, thereby demonstrating that the current world and state cancer mortality values provide an accurate historical picture of the US cancer death rate over almost a century.

Aged↗

PubMed1993

Determination of the drug-receptor dissociation constant of endothelin-1 using photorelaxation of rabbit isolated thoracic aorta.

An in vitro preparation of rabbit thoracic aorta was tested in order to determine whether isometric contractile force induced by endothelin-1 (ET-1) in this preparation is changed when the tissue is exposed to ultraviolet (UV) light. After constriction of an aortic segment to a steady-state level of tension with ET-1, a flash of UV light induced a transient decrease in tension that returned exponentially to equilibrium. This response was endothelium-independent and sensitive to the extracellular sodium concentration. The rate of return to the baseline tension after photorelaxation increased with increasing concentrations of ET-1. UV light had only a slight photorelaxing effect at very low and very high ET-1 concentrations and a much larger effect at concentrations near the A50 value of ET-1. Since an irreversible antagonist does not yet exist for ET-1, the partial irreversible blockade method for getting the dissociation constant (KD) functionally was not possible. The relaxation method, using UV light as a perturbation of ligand-receptor equilibrium, was used instead to obtain KD as well as the forward (k1) and reverse (k2) rate constants of ET-1 in this tissue.

Animals↗

PubMed1993

Mechanisms of excision of 5-fluorouracil by uracil DNA glycosylase in normal human cells.

Recent evidence indicates that 5-fluorouracil (5-FlUra) is incorporated into DNA and is removed by the DNA repair enzyme uracil DNA glycosylase. Synthetic oligonucleotides containing either a single uracil or 5-FlUra residue were constructed to examine the mechanisms by which human cells remove 5-FlUra from DNA. The human uracil DNA glycosylase excised uracil in a manner similar to that observed for the bacterial enzyme. In contrast, a significant difference was observed in their abilities to remove 5-FlUra. In particular, both the bacterial and normal human enzymes displayed 13-17-fold increases in their apparent Km values but the apparent Vmax values remained virtually constant. These results demonstrate that normal human cells possess a defined capacity to remove 5-FlUra incorporated into DNA. However, specific kinetic differences may exist that affect their capacity to remove 5-FlUra formed in DNA after treatment with this cancer chemotherapeutic agent.

Base Sequence↗

PubMed1993

Isobolographic superadditivity between delta and mu opioid agonists in the rat depends on the ratio of compounds, the mu agonist and the analgesic assay used.

The present study was designed to test rigorously, using isobolographic analysis, whether there was a potentiative interaction between delta and mu agonists administered i.c.v. to rats. Factors such as the specific fixed ratio of compounds, the analgesic assay and the mu agonist were varied to determine the generality of the results. Male rats were implanted with i.c.v. cannulas and were tested in the cold (-3 degrees C) or hot (+50 degrees C) water tail-flick test. Full dose-effect curves were generated for the mu agonists, morphine and [N-MePhe3,D-Pro4]morphiceptin and the delta-selective agonist, DPDPE. Each agonist induced dose-related analgesia in the cold water test (ED50 values were 12, 0.79 and 75 micrograms, respectively). In the hot water test, morphine induced analgesia with an ED50 value of 4.4 micrograms, whereas DPDPE failed to produce a full effect at doses up to 200 micrograms. Full dose-effect curves were also generated for various fixed-ratio combinations of DPDPE and morphine in both analgesic assays. Fixed ratios were chosen such that the amount of DPDPE in each dose of the combinations tested was itself subanalgesic. The combination with 20% DPDPE and 80% morphine (by weight) was significantly superadditive in the cold water test as determined by isobolographic analysis, whereas a second combination of the same drugs (40% DPDPE) was not. However, in the hot water test, the 20% DPDPE combination was not superadditive and neither were two other combinations of DPDPE and morphine. No combination of DPDPE and [N-MePhe3,D-Pro4]morphiceplin differed significantly from additivity when tested in the cold water test.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesics↗

PubMed1992

Receptor regulation, competitive antagonism and pA2.

A growing body of evidence suggests that the number of drug receptors on cell surfaces is not fixed, but is dynamically regulated by circumstances that include exposure to the ligand itself. Because most traditional theories of drug action are based on the assumption of a fixed number of receptors, it is desirable to examine the importance of this regulatory process on the interpretation of dose-effect data. Of special interest is the impact of a variable receptor number on the equation of competitive antagonism and associated pA2 which, in the traditional theory, is a quantitative measure of antagonist-receptor affinity. Using a simple model of drug-induced endocytosis or exocytosis, it is shown that if the rate of either is appreciable, the pA2 is no longer a simple measure of affinity.

Binding, Competitive↗

PubMed1992

The effect of lowered extracellular Na+ concentration on ultraviolet light-induced relaxation of vasoconstricted rabbit isolated thoracic aorta.

The effect of lowering extracellular ion concentration on ultraviolet (UV) light-induced photorelaxation of norepinephrine(NE)-constricted rabbit isolated thoracic aorta was investigated. The magnitude of the photorelaxation response (similar to acetylcholine-induced, but not nitroprusside-induced, relaxation) progressively declined, in the absence of an effect on NE-induced vasoconstriction, as the total extracellular ion concentration was progressively reduced. This diminution in the photorelaxation response was duplicated by isosmotic lowering of the extracellular concentration of Na+, but not other ions, from 145 to 25 mM and was not restored by the replenishment of the Na+ deficiency by equimolar amounts of mannitol or Li+. In contrast, choline fully substituted for Na+. These findings suggest a fundamental difference in the ion dependency (and, hence, the mechanisms) of UV-induced photorelaxation and the vasorelaxations induced by acetylcholine or sodium nitroprusside.

Acetylcholine↗

PubMed1992

Antinociceptive interactions of opioid delta receptor agonists with morphine in mice: supra- and sub-additivity.

In this study, the antinociceptive interactions of fixed ratio combinations of intracerebroventricularly (i.c.v.) given morphine and subantinociceptive doses of the delta agonists, [D-Pen2, D-Pen5]enkephalin (DPDPE), [D-Ala2, Glu4]deltorphin (DELT) or [Met5]enkephalin (MET) were examined using the mouse warm water tail flick test. When morphine was coadministered with DPDPE or DELT in a 4:1 and 9:1 mixture, respectively, a synergistic antinociceptive effect was observed. In contrast, when morphine was coadministered with MET in a 1:2 fixed ratio mixture, a subadditive interaction occurred. These results demonstrate both positive and negative modulatory interactions of delta agonists with morphine in an antinociceptive endpoint and that these interactions can be either supra- or subadditive. The data support the concept of a functional interaction between opioid mu and delta receptors and a potential regulatory role for the endogenous ligands of the opioid delta receptor.

Analgesics↗

PubMed1991

Purification and properties of the uracil DNA glycosylase from Bloom's syndrome.

Bloom's syndrome uracil DNA glycosylase was highly purified from two non-transformed cell strains derived from individuals from different ethnic groups. Their properties were then compared to two different highly purified normal human uracil DNA glycosylases. A molecular mass of 37 kDa was observed for each of the four human enzymes as defined by gel-filtration column chromatography and by SDS-PAGE. Each of the 37 kDa proteins was identified as a uracil DNA glycosylase by electroelution from the SDS polyacrylamide gel, determination of glycosylase activity by in vitro biochemical assay and identification of the reaction product as free uracil by co-chromatography with authentic uracil. Bloom's syndrome enzymes differed substantially in their isoelectric point and were thermolabile as compared to the normal human enzymes. Bloom's syndrome enzymes displayed a different Km, Vmax and were strikingly insensitive to 5-fluorouracil and 5-bromouracil, pyrimidine analogues which drastically decreased the activity of the normal human enzymes. In particular, each Bloom's syndrome enzyme required 10-100-fold higher concentrations of each analogue to achieve comparable inhibition of enzyme activity. Potential mechanisms are considered through which an altered uracil DNA glycosylase characterizing this cancer-prone human genetic disorder may arise.

Black People↗

PubMed1991

Control and stability of ligand receptor interaction in the presence of a competitive compound.

The law of mass action is almost universally applied to both endogenous ligands and drugs that interact with specific cellular receptors. The concentration, and hence the receptor binding, of a foreign drug molecule will depend on its pharmacokinetic properties, whereas an endogenous ligand is subject to intrinsic control since the concentration (z) remains within limits around an equilibrium level. We have previously examined this control for ligand-receptor interactions proceeding according to mass action in which the ligand is produced (rate F), eliminated exponentially (rate constant E) and controlled by a feedback function of receptor occupancy, phi (y), where y is the bound concentration. The current study examines the control of an endogenous ligand in the presence of a second compound (agonist or antagonist) that interacts with the same receptor. From a computer solution of the set of differential equations, illustrated in both time-plots and phase plane (y-z) plots, it is shown that if the second agent is a pure competitive antagonist the bound concentration of the endogenous agonist ligand may not decrease appreciably, even for large doses of the antagonist. Instead the level of binding depends on certain parameters of the control function. Further, the computer simulation shows how these parameters affect the time course of released ligand resulting from administration of an antagonist and the suppression of such release when the second compound is an agonist.

Animals↗

PubMed1991

Determination of the functional dissociation constant of a partial agonist by comparison with a higher efficacy agonist.

A formula derived by Gero and Tallarida (1977) relates the equilibrium dissociation constant of a partial agonist (P) and that of a second agonist (A) of greater efficacy that acts on the same receptor. The second agonist may or may not be a strong agonist. Accordingly, if the dissociation constant (K) of one of the compounds is known, say from the method of partial irreversible receptor blockade, then the dissociation constant for the other may be determined from the complete concentration-effect curves of the compounds and the derived formula: kp = KA (Ap-Ai)Pi/(Ap + KA)Ai, where Pi and Ai are equieffective concentrations of P and A, Ap = the concentration of A that gives an effect = the maximum effect of P. The practical use of this formula is illustrated here for several agonists, and for each, the value of K obtained is compared to that obtained by partial irreversible receptor blockade. In all cases tested, the agreement is quite good, thus suggesting that this method may be a practical alternative.

Adrenergic alpha-Agonists↗

PubMed1991

Pharmacological evidence that captopril possesses an endothelium-mediated component of vasodilation: effect of sulfhydryl groups on endothelium-derived relaxing factor.

Captopril, an angiotensin-converting enzyme inhibitor, reportedly can scavenge superoxide anion (O2-), a property attributed to its sulfhydryl group. The present investigation, using rabbit aortic rings precontracted with either norepinephrine or clonidine, was designed to determine whether captopril possesses an endothelium-dependent component of vasodilation related to its ability to protect endothelium-derived relaxing factor (EDRF) from superoxide-mediated destruction. Also studied were enalaprilat, a nonsulfhydryl angiotensin-converting enzyme-inhibitor, superoxide dismutase, and the sulfhydryl compounds glutathione (GSH), N-2-mercaptopropionylglycine (MPG) and N-acetylcysteine (NAC). Captopril, but not enalaprilat, caused dose-dependent relaxations in preconstricted aortic rings containing an intact endothelium. Rings denuded of endothelium were unresponsive to any dose of captopril. Captopril's vasodilation was not related to prostaglandin influence but was associated with an increase in cyclic GMP. Superoxide dismutase, GSH, MPG and NAC also produced endothelium-dependent relaxations similar to captopril. It was also demonstrated that endothelium-dependent relaxations to acetylcholine were enhanced by captopril, GSH, MPG and NAC but not by enalaprilat. In another set of experiments, the ability of captopril to inhibit superoxide-mediated inactivation of EDRF was examined. Pyrogallol, a potent generator of O2-, and superoxide dismutase, a scavenger of O2-, were used as a basis for comparing a possible scavenging effect of captopril. In preconstricted rings, pyrogallol elicited endothelium-dependent contractions that were attenuated by both captopril and superoxide dismutase. Similar effects were found with GSH, MPG and NAC but not with enalaprilat. These results suggest that captopril's endothelium-dependent vasodilation is due to its sulfhydryl group and the ability of the latter to scavenge O2-, thereby protecting EDRF.

Acetylcholine↗

PubMed1990

Modulation of morphine antinociception by peripheral [Leu5]enkephalin: a synergistic interaction.

This study determined whether the modulation of morphine antinociception by [Leu5]enkephalin, a compound which does not produce detectable antinociception when given i.p., was additive or synergistic. Co-administration of graded i.p. doses of morphine and [Leu5]enkephalin 20 min before testing resulted in a progressive leftward and parallel displacement of the i.p. morphine dose-response line. The increase in potency produced by i.p. [Leu5]enkephalin, but not the antinociception of i.p. morphine alone, was blocked by i.c.v. ICI 174,864. The data demonstrate strong and dose-related synergism between the two compounds, supporting the view that (a) peripheral administration of [Leu5]enkephalin can modulate morphine antinociception, apparently at the level of the brain and (b) that the interaction between mu and delta opioid agonists is not the result of simple additive action at the same (i.e., mu) receptor. Further evidence is thus provided for the existence of functional mu-delta interactions.

Analgesics↗

PubMed1990

Further characterization of a control model for ligand-receptor interaction: phase plane geometry, stability, and oscillation.

The local concentration of an endogenous ligand that binds to a specific cellular receptor according to the mass-action law is controlled. The ligand concentration (zeta) remains bounded in an interval that contains the basal point. The current study is a more detailed examination of a control model that we previously proposed in which the feedback is a function (phi) of the concentration (y) of the receptor-bound ligand. Previously we showed that if phi' (y) less than or equal to 0 at a critical point then that point is stable. Accordingly, at an unstable critical point phi' (y) greater than 0. We now show that if phi (y) is continuous in [y1,y2], where y1 and y2 are adjacent critical points, and if phi' (y1) less than or equal to 0 then the critical point at y2 is unstable. The distance between these adjacent critical points is crucial for maintaining control in response to a dose (or release) of ligand. It is further shown that the difference in the slopes of the binding and control curves at their intersection on the phase-plane portrait is an indicator of stability and of the characteristic frequency of oscillation in the system. Together these findings provide new insights needed in the modelling of specific ligand-receptor interactions.

Algorithms↗

PubMed1990

Control and oscillation in ligand receptor interactions according to the law of mass action.

The law of mass action is almost universally applied to interactions of both endogenous ligands and drugs with their specific receptors and results in the familiar hyperbolic equilibrium binding curve of bound (y) vs free (z) concentrations. Whereas the concentration of a drug molecule is governed by its pharmacokinetic properties and, possibly, by intrinsic control mechanisms, natural ligands are certainly controlled since their concentrations normally remain within specific limits. This paper represents a further study of control of this kinetic process in a model based on ligand production (rate F), first-order elimination (rate constant E) and a feedback function of occupancy, phi(y), that modulates these. In the controlled situation the system equilibrium occurs at states called critical points (yc,zc) at which both dy/dt and dz/dt are simultaneously zero. There are only a finite number of such points along the hyperbolic binding curve and these may be either stable or unstable. The basal state is the normal operating point of the system and is necessarily stable; that is, perturbations producing states away from it will return in time to this point. We have previously shown that phi'(y) less than or equal to 0 is a sufficient condition for stability. Accordingly, for a continuous control curve, an adjacent critical point will be unstable, and have phi'(y) greater than 0. If the system coordinates get sufficiently close to such an unstable point there is propulsion to extreme states and loss of control. The distance between the stable and unstable points determines whether a dose (or release) of the ligand will be controlled or not. The current paper focuses on the geometrical properties of the binding and control curves and how these relate to the stability of critical points and the overall control of ligand doses. In particular we show how the magnitude of the (negative) slope of the control curve at the basal point affects the frequency of oscillation about the basal state. It is further shown that high frequency control results in lower receptor occupancy, a result that may explain desensitization.

Animals↗