Biomedical subjects

D B Dixon

Publications and source records attributed to D B Dixon.

28 records · Page 2Linked to original sources

PubMed1996

Azole endothelin antagonists. 3. Using delta log P as a tool to improve absorption.

The oral absorption profile of a family of azole-based ET(A)-selective antagonists has been improved through a rational series of structural modifications which were suggested by analysis of the physicochemical parameter delta log P. Comparison of urea 2 with a series of well-absorbed compounds using delta log P analysis suggested that 2 has an excess capacity for forming hydrogen bonds with solvent. A series of urea modifications were explored as a means of reducing H-bonding capacity while maintaining affinity for the ET(A)-receptor. The correlation between delta log P values and absorption in an intraduodenal (id) bioavailability model was good; this strategy uncovered replacements for each of the urea NH groups which simultaneously improve both potency and drug absorption. A combination of these optimized modifications produces carbamate 16h, a highly-selective ET(A) antagonist with a potency/bioavailability profile consistent with an oral route of administration.

Administration, Oral↗

PubMed1996

Quinine, intracellular pH and modulation of hemi-gap junctions in catfish horizontal cells.

Quinine increases the conductance of hemi-gap junctions in horizontal cells. We investigated the mechanisms of alkalinization and the hypothesis that quinine-induced alkalinization produced these conductance increases. We found that quinine-induced alkalinizations were not blocked by cobalt, amiloride, or DIDS. Therefore, this suggests that the alkalinization is not likely due to net proton flux through opened hemi-gap channels nor is it likely due to an action on Cl-/HCO3- exchanger or Na+/H+ exchanger, both of which are known to regulate pHi in the horizontal cells. Quinine increased hemi-gap conductance even when cells were recorded with patch pipets containing up to 80 mM HEPES. We conclude that quinine-induced alkalinization cannot account solely for the hemi-gap junctional conductance increases.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed1996

Pharmacological characterization of A-127722: an orally active and highly potent ETA-selective receptor antagonist.

Endothelins (ET) are potent vasoactive peptides implicated in the pathogenesis of a number of vascular diseases. The effects of ET on mammalian organs and cells are initiated by binding to ETA or ETB receptors. In this report, we document the pharmacology of A-127722, a novel ETA-selective receptor antagonist. A-127722 inhibits [125I]ET-1 binding to cloned human ETA and ETB receptors competitively with Ki values of 69 pM and 139 nM, respectively. A-127722 exhibits a dose-dependent inhibition of ET-1-induced arachidonic acid release in human pericardium smooth muscle cells with a pA2 value of 10.5 and inhibits ET-1-induced vasoconstriction in isolated rat aorta with a pA2 value of 9.2. In vivo, A-127722 dose-dependently blocks the pressor response to ET-1 (0.3 nmol/kg i.v.) in conscious rats. Statistically significant (P < .05) antagonism is seen at doses greater than 0.1 mg/kg p.o. Maximal inhibition, at 10 mg/kg, remains constant for at least 8 hr after dosing. No effect is seen on the ETB-mediated transient vasodepressor effect of exogenous ET-1. In conclusion, A-127722 is ETA-selective, orally bioavailable and efficacious for inhibiting the effects of ET in the rat, and A-127722 is the most potent ET receptor antagonist yet reported.

Administration, Oral↗

PubMed1995

Amiloride blocks a keratinocyte nonspecific cation channel and inhibits Ca(++)-induced keratinocyte differentiation.

Proliferation and differentiation in many cells are linked to specific changes in transmembrane ion fluxes. Previously, we have identified a nonspecific cation channel in keratinocytes, which is permeable to and activated by Ca++. To test whether this cation channel might serve as a pathway for Ca++ entry, we examined the effect of blocking this channel on membrane currents, markers of differentiation, and intracellular Ca++. In patch clamp studies, 10(-8) to 10(-6) M amiloride decreased the single-channel open probability. The same concentrations of amiloride inhibited the calcium-induced formation of cornified envelopes and activity of transglutaminase in a dose-dependent fashion. Amiloride inhibited the long-term rise of intracellular Ca++ induced by raised extracellular Ca++, without blocking the initial increase of intracellular Ca++. Amiloride at concentrations of 10(-7) to 10(-3) M did not change the resting intracellular pH of keratinocytes, although concentrations of 10(-6) M or greater inhibited the recovery from NH4(+)-induced acidification. To test whether the effect of amiloride was toxic, we measured DNA synthesis in the presence or absence of amiloride. DNA synthesis was unchanged, suggesting that amiloride's actions were not due to toxic effects. Although the exact mechanisms of amiloride's action remains to be determined, these experiments suggest that this compound may inhibit keratinocyte differentiation by blocking the nonspecific cation channel.

Amiloride↗

PubMed1995

Dissociation characteristics of endothelin ETA receptor agonists and antagonists.

Endothelin (ET) binding to receptors has been shown to be almost irreversible. We studied the dissociation characteristics of ETA receptor agonists and antagonists using membranes prepared from rat pituitary cells (MMQ). Consistent with our previous report, competition studies comparing ET-1, ET-3, BQ123, FR139317, PD142893, and Ro46-2005 show that MMQ cells contained predominantly the ETA receptor. [125I]ET-1 bound to the receptor was difficult to dissociate. In contrast, bound BQ123, FR139317, and Ro46-2005 were easier to dissociate, suggesting that antagonist binding was more reversible. Although BQ123 (5 nM), FR139317 (1 nM), and Ro46-2005 (0.5 microM) inhibited 0.1 nM [125I]ET-1 binding by greater than 80% after 15 min of incubation, the inhibition decreased to less than 20% after 24 h of incubation. This decrease in binding inhibition was not the result of antagonist degradation. These results suggest that, similar to our previous observation made with the ETB receptor in porcine cerebellum, the ability of antagonists to inhibit [125I]ET-1 binding to the ETA receptor is critically dependent on the incubation time because of the difference in the dissociation characteristics between antagonists and ET-1.

Amino Acid Sequence↗

PubMed1993

L-glutamate suppresses HVA calcium current in catfish horizontal cells by raising intracellular proton concentration.

Catfish horizontal cells exhibit a high voltage-activated, slowly inactivating calcium (HVA) current. In isolated cells, this current underlies a sustained action potential that can be evoked by depolarization. L-Glutamate at concentrations as low as 1 microM suppressed HVA current and shortened the duration of action potentials. pH indicator dye measurements revealed that L-glutamate acidified cells by up to 0.3 pH units. pH/HVA current dose-response curves indicated that the EC50 for pH inhibition of HVA current was 7.1 and the Hill coefficient was 1.98. L-Glutamate's suppression of HVA current was eliminated when the patch pipette solution contained a high concentration of proton buffer, but was not affected when pipettes contained GTP gamma S. These results support the hypothesis that L-glutamate can modulate HVA current by changing intracellular pH.

Action Potentials↗

PubMed1993

Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina.

A sustained high voltage-activated (HVA), nifedipine- and cadmium-sensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina. pH indicator dyes showed that superfusion with NH4Cl alkalinized these cells and that washout of NH4Cl or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4Cl but was suppressed upon NH4Cl washout or application of Na-acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4Cl and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extracellular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4Cl and reduced by washout of NH4Cl or superfusion with Na-acetate. The Na-acetate and NH4Cl washout-dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium-dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.

1-Methyl-3-isobutylxanthine↗

PubMed1992

Suppression of a calcium current by CNQX and kynurenate.

During our investigation of a sustained high voltage-activated (HVA) calcium current in retinal horizontal cells, we found that the glutamate receptor antagonists CNQX and kynurenate but not AP7 reversibly reduced the peak amplitude of the HVA current. Changes in the HVA current kinetics or activation voltage were not apparent; there was only a reduction in the peak current. The novel effects of these antagonists on HVA calcium currents reported here could have an impact on many studies involving glutamatergic synaptic transmission.

2-Amino-5-phosphonovalerate↗

PubMed1992

Two types of glutamate receptors differentially excite amacrine cells in the tiger salamander retina.

1. Excitatory inputs to amacrine cells in the salamander retinal slice preparation were examined using whole-cell patch pipette voltage-clamp techniques. In strychnine (500 nM) and bicuculline (100 microM), two types of amacrine cell were easily distinguished by their light-evoked excitatory responses: transient and sustained. 2. In transient amacrine cells the current-voltage (I-V) relation for the peak light-evoked current was non-linear with a negative slope region between -50 and -70 mV. Responses reversed near +10 mV and were prolonged at more positive holding potentials. 3. In DL-2-amino-phosphonoheptanoate (AP7, 30 microM), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, both the negatively sloped region of the light I-V relation and the prolongation of the response at positive potentials were eliminated. In 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2 microM), a selective non-NMDA receptor antagonist, light-evoked currents at the most hyperpolarized holding potentials were eliminated. At potentials positive to -85 mV the light-evoked currents lacked a fast onset. The light I-V relation in CNQX had a negative slope region between -35 and -80 mV. 4. With synaptic transmission blocked, kainate evoked responses in transient cells with a resultant I-V relation that was nearly linear, whereas glutamate and NMDA elicited responses with non-linear I-V relations. 5. Light-evoked currents in sustained amacrine cells had a nearly linear I-V relation and reversed near +10 mV. AP7 at a concentration of 30 microM did not affect the light-evoked currents in sustained cells, but 2 microM-CNQX eliminated all light-evoked currents in these cells. 6. With synaptic transmission blocked, sustained amacrine cells responded only to glutamate and kainate, not NMDA. The resultant I-V relations were linear. 7. We conclude that the light-evoked responses of transient amacrine cells are mediated by concomitant activation of both non-NMDA and NMDA receptors whereas the responses of sustained amacrine cells are mediated only by non-NMDA receptors. Furthermore, these data provide supportive evidence that the primary light-evoked excitatory neurotransmitter activating amacrine cells is glutamate.

2-Amino-5-phosphonovalerate↗

PubMed1979

Pseudo-chondrosarcoma: solitary osseous metastases from atypical bronchogenic carcinoma.

A patient with bronchogenic carcinoma with a solitary metastases to the femur in whom the clinical, radiologic, and pathologic setting suggested that the primary tumor was the bone lesion (chondrosarcoma) and the pulmonary lesion was a solitary metastasis is presented. This case is added to the limited literature indicating that mucin-secreting tumors metastatic to bone may simulate primary bone tumors radiographically.

Adenocarcinoma, Mucinous↗