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D L Martin

Publications and source records attributed to D L Martin.

At least 73 records · Page 4Linked to original sources
PubMed1990

Water-soluble renin inhibitors: design of a subnanomolar inhibitor with a prolonged duration of action.

Incorporation of nonreactive polar functionalities at the C- and N-termini of renin inhibitors led to the development of a subnanomolar compound (21) with millimolar solubility. This inhibitor demonstrated excellent efficacy and a long duration of action upon intravenous administration to monkeys. While activity was also observed intraduodenally, a comparison of the blood pressure responses indicated low bioavailability. Subsequent experiments in rats showed that, although the compound was absorbed from the gastrointestinal tract, extensive liver extraction severely limited bioavailability.

Animals↗
PubMed1990

Stimulation of synaptosomal gamma-aminobutyric acid synthesis by glutamate and glutamine.

gamma-Aminobutyric acid (GABA) synthesis was studied in rat brain synaptosomes by measuring the increase of GABA level in the presence of the GABA-transaminase inhibitor gabaculine. The basal rate of synaptosomal GABA synthesis in glucose-containing medium (25.9 nmol/h/mg of protein) was only 3% of the maximal activity of glutamate decarboxylase (GAD; 804 +/- 83 nmol/h/mg of protein), a result indicating that synaptosomal GAD operates at only a small fraction of its catalytic capacity. Synaptosomal GABA synthesis was stimulated more than threefold by adding 500 microM glutamine. Glutamate also stimulated GABA synthesis, but the effect was smaller (1.5-fold). These results indicate that synaptosomal GAD is not saturated by endogenous levels of its substrate, glutamate, and account for part of the unused catalytic capacity. The greater stimulation of GABA synthesis by glutamine indicates that the GAD-containing compartment is more accessible to extrasynaptosomal glutamine than glutamate. The strong stimulation by glutamine also shows that the rates of uptake of glutamine and its conversion to glutamate can be sufficiently rapid to support GABA synthesis in nerve terminals. Synaptosomes carried out a slow net synthesis of aspartate in glucose-containing medium (7.7 nmol/h/mg of protein). Aspartate synthesis was strongly stimulated by glutamate and glutamine, but in this case the stimulation by glutamate was greater. Thus, the larger part of synaptosomal aspartate synthesis occurs in a different compartment than does GABA synthesis.

4-Aminobutyrate Transaminase↗
PubMed1990

Glutamate-dependent active-site labeling of brain glutamate decarboxylase.

A major regulatory feature of brain glutamate decarboxylase (GAD) is a cyclic reaction that controls the relative amounts of holoenzyme and apoenzyme [active and inactive GAD with and without bound pyridoxal 5'-phosphate (pyridoxal-P, the cofactor), respectively]. Previous studies have indicated that progression of the enzyme around the cycle should be stimulated strongly by the substrate, glutamate. To test this prediction, the effect of glutamate on the incorporation of pyridoxal-P into rat-brain GAD was studied by incubating GAD with [32P]pyridoxal-P, followed by reduction with NaBH4 to link irreversibly the cofactor to the enzyme. Adding glutamate to the reaction mixture strongly stimulated labeling of GAD, as expected. 4-Deoxypyridoxine 5'-phosphate (deoxypyridoxine-P), a close structural analogue of pyridoxal-P, was a competitive inhibitor of the activation of glutamate apodecarboxylase by pyridoxal-P (Ki = 0.27 microM) and strongly inhibited glutamate-dependent labeling of GAD. Analysis of labeled GAD by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed two labeled proteins with apparent molecular masses of 59 and 63 kDa. Both proteins could be purified by immunoaffinity chromatography on a column prepared with a monoclonal antibody to GAD, and both were labeled in a glutamate-dependent, deoxypyridoxine-P-sensitive manner, indicating that both were GAD. Three peaks of GAD activity (termed peaks I, II, and III) were separated by chromatography on phenyl-Sepharose, labeled with [32P]pyridoxal-P, purified by immunoaffinity chromatography, and analyzed by SDS-polyacrylamide gel electrophoresis. Peak I contained only the 59-kDa labeled protein. Peaks II and III contained the both the 59- and 63-kDa proteins, but in differing proportions.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity Labels↗
PubMed1990

The role of osmotic pressure and membrane potential in K(+)-stimulated taurine release from cultured astrocytes and LRM55 cells.

The effects of [K+]o on taurine release from glial cells were studied with primary cultures of cerebellar astrocytes and with LRM55 cells, a continuous glial cell line. The characteristics of K(+)-stimulated taurine release were virtually identical in the 2 cell types. Both cerebellar astrocytes and LRM55 cells released taurine when stimulated with high-K+ medium prepared by isosmotically substituting KCl for NaCl, but neither cell type released taurine when stimulated with hyperosmotic high-K+ medium prepared by adding solid KCl to control medium. The membrane potential of LRM55 cells was measured by intracellular recording and was insensitive to changes in [K+]o below 20 mM. LRM55 cells released taurine when stimulated with nondepolarizing concentrations of K+ (13-22 mM) if the isosmotically prepared high-K+ medium was used, but the cells did not release taurine when treated with a depolarizing concentration of K+ (50 mM) if hyperosmotic high-K+ medium was used. The time course of K(+)-stimulated taurine release was quite slow, having a time to peak of 10-15 min. Small changes (2.5-10%) in the osmolarity of the medium strongly affected taurine release by cerebellar astrocytes and LRM55 cells. K(+)-stimulated taurine release from both cell types was inhibited when the osmolarity was increased with sucrose or NaCl and was enhanced when the osmolarity was reduced. Similarly, baseline taurine release was suppressed by small elevations in osmolarity and increased by reduced osmolarity.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1989

An outbreak of shigellosis associated with the consumption of raw oysters.

We describe an outbreak of Shigella sonnei infection among 24 persons who had eaten raw oysters in restaurants in southeastern Texas within five days before the onset of symptoms. The oysters in all eight restaurants were supplied by the same dealer, but examination of a routine water sample collected six days before the probable time of contamination showed the waters where the oysters were harvested to be free of fecal contamination, making widespread sewage contamination unlikely. The suspect oysters were traced to a single boat. Stool swabs from that boat's oyster harvesters allowed the identification of one asymptomatic carrier who had a strain of S. sonnei (determined by colicin typing, plasmid analysis, and testing for susceptibility to antibiotics) that was similar to or the same as that infecting the patients. Although the source of this man's infection was unknown, he reported having eaten no oysters. Investigation revealed that 5-gallon (19-liter) pails were used as toilets aboard the oyster boats. Sewage collected in these pails was often dumped overboard into the harvesting area. We conclude that this outbreak of S. sonnei resulted from poor sanitary procedures that probably allowed stool from a carrier to contaminate oysters either just before or after they were taken aboard the boat.

Animals↗
PubMed1989

Spontaneous and beta-adrenergic receptor-mediated taurine release from astroglial cells do not require extracellular calcium.

Astroglial cells release taurine when stimulated by beta-adrenergic agonists and other neuroactive agents. The Ca2+-dependency of taurine release by an LRM55 astroglial cell line was investigated by removing Ca2+ from the perfusion medium and by using three inorganic and three organic Ca2+-channel blockers (Mn2+, Co2+, Cd2+, verapamil, nifedipine, and diltiazem). Spontaneous release and release stimulated by the beta-adrenergic agonist isoproterenol were not inhibited when cells were perfused with medium containing no added Ca2+ and 10 microM EGTA. Isoproterenol-stimulated taurine release was not blocked when extracellular Ca2+ was completely replaced by Mn2+, Co2+, or Cd2+, nor was it blocked by verapamil, nifedipine, or diltiazem. In fact isoproterenol-stimulated taurine release was increased by 50 microM diltiazem and when Ca2+ was replaced by Co2+. The rate of spontaneous release increased slowly and continually when Co2+ was substituted for Ca2+ but was almost unaffected by substitution of Mn2+ or Cd2+. Application of diltiazem increased spontaneous release significantly, while verapamil and nifedipine appeared to cause small increases. These results indicate that entry of Ca2+ from the extracellular medium is not required for either receptor-mediated or spontaneous taurine release from astroglial cells. Some other changes in the medium did strongly affect release. Both spontaneous and isoproterenol-stimulated release were inhibited by elevated osmotic pressure, and spontaneous release was greatly increased when Ca2+ was completely removed without substituting another divalent cation. Spontaneous release increased when antagonistic metal ions were replaced with Ca2+ and when organic channel blockers were removed.

Astrocytes↗
PubMed1989

Entropy as a factor in the binding of gamma-aminobutyric acid and nipecotic acid to the gamma-aminobutyric acid transport system.

Nipecotic acid is one of the most potent competitive inhibitors and alternative substrates for the high-affinity gamma-aminobutyric acid transport system in neurons, but the structural basis of this potency is unclear. Because gamma-aminobutyrate is a highly flexible molecule in solution, it would be expected to lose rotational entropy upon binding to the transport system, a change which does not favor binding. Nipecotic acid, in contrast, is a much less flexible molecule, and one would expect the loss of conformational entropy upon binding to be smaller thus favoring the binding of nipecotic acid over gamma-aminobutyric acid. To investigate this possibility, the thermodynamic parameters, delta G degrees, delta H degrees, and delta S degrees, were determined for the binding of gamma-aminobutyrate and nipecotic acid to the high affinity GABA transport system in synaptosomes. In keeping with expectations, the apparent entropy change for nipecotic acid binding (112 +/- 13 J.K-1) was more favorable than the apparent entropy change for gamma-aminobutyric acid binding (61.3 +/- 6.6 J.K-1). The results suggest that restricted conformation per se is an important contributory factor to the affinity of nipecotic acid for the high-affinity transport system for gamma-aminobutyric acid.

Animals↗
PubMed1989

Postnatal development of GABA-mediated synaptic inhibition in rat hippocampus.

Developmental alterations in GABAergic synaptic transmission were examined physiologically and biochemically in hippocampus of rats from 3 days of age to adulthood. Neither antidromic nor orthodromic stimulation could elicit identifiable inhibitory postsynaptic potentials in CA1 neurons in slices from rats 5 or 6 days of age. In contrast, at this age these stimuli result in large inhibitory postsynaptic potentials in CA3 pyramidal cells. In the latter cells orthodromic stimulation produced a brief monosynaptic excitatory postsynaptic potential which was followed by a large prolonged biphasic hyperpolarization. These signals were strikingly similar to those recorded in 1-month-old rats. In addition, large recurrent inhibitory postsynaptic potentials were produced by antidromic stimulation. By postnatal day 9 similar inhibitory postsynaptic potentials could be elicited in a majority of neurons of the CA1 subfield. As in mature pyramidal cells, application of GABA antagonists, such as bicuculline, selectively eliminated the antidromic inhibitory postsynaptic potential and the first component of the biphasic inhibitory postsynaptic potential generated by stimulation of stratum radiatum. In the CA3 subfield, this blockade of GABA receptors resulted in prolonged afterdischarges in slices from immature but not month-old rats. Measurements of the equilibrium potential and the conductance of antidromic inhibitory postsynaptic potentials in CA3 neurons were very similar when made during the first postnatal week and at 1 month of age. While on days 10-11 the equilibrium potential was very similar to measurements made at these other ages, the conductance was 3-4 times greater. The activity of glutamate decarboxylase, the synthetic enzyme for GABA, was very low at 3 days in hippocampus, and increased until 30 days of age at which time adult values were obtained. By comparison, hippocampal GABA levels were high early in postnatal life. Glutamate decarboxylase activities in microdissected CA3 and CA1 subfields were similar in immature hippocampus. These results demonstrate dramatic differences in the ontogenesis of functional GABAergic inhibitory synaptic transmission in the CA1 and CA3 subfields of rat hippocampus. The late development of GABA-mediated synaptic inhibition in the CA1 subfield could play a role in the susceptibility of immature hippocampus to seizures. However, the large GABA-mediated inhibitory postsynaptic potentials present in the CA3 subfield at the same age have a critical role in dampening neuronal excitability.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1989

Controlling the medicinal leech.

A simple method is presented for applying a medicinal leech (Hirudo medicinalis) to the precise area in which the clinician desires the leeching to take place.

Animals↗
PubMed1989

Inactivation of cyclic AMP-dependent taurine release from astroglia.

When astroglial cells are exposed to beta-adrenergic agonists for long periods of time (greater than 20 min), transient increases in taurine release and intracellular cyclic AMP (cAMP) are observed. Three phases of taurine release can be distinguished: activation, inactivation, and an elevated steady state. In this article, we present data describing the relationship between intracellular cAMP levels and inactivation of taurine release. To do this, we compared the apparent first-order rate constants for the inactivation of taurine release (ktau) with the apparent first-order rate constant for the decline of intracellular cAMP (kcAMP). We also measured ktau under experimental conditions that were chosen to provide a wide range of intracellular cAMP concentrations or to stimulate release without the involvement of the beta-adrenergic receptor and adenylate cyclase. When taurine release was stimulated with a saturating concentration of isoproterenol, the inactivation of release was significantly faster than the decline of intracellular cAMP. Furthermore, there were no significant differences in ktau measured under any of the experimental conditions used. Thus, inactivation of taurine release does not involve changes in the activity of the beta-adrenergic receptor and adenylate cyclase, i.e., desensitization, and appears to be independent of the intracellular concentration of cAMP. These results indicate that cAMP-mediated events can be regulated by mechanism(s) in addition to those that control receptor-adenylate cyclase interactions and the synthesis of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone↗
PubMed1989

Effects of mild hyperthyroidism on levels of amino acids in the developing Lurcher cerebellum.

This study examines the question of whether intrinsically defective mutant Lurcher Purkinje cells, which degenerate during postnatal weeks two to five, followed by later loss of granule cells are competent to respond to neonatal hyperthyroidism, which is known to cause premature differentiation of Purkinje cells and an acceleration of the peak of proliferation in granule cells in normal rodent cerebellum. Both total amounts and concentrations (per mg wet weight) of Tau, Glu, Asp and GABA were assayed as markers of cell function in Lurcher and wild-type mice made very mildly hyperthyroid by feeding nursing dams high-thyroxine food. Tau, which is present in relatively high concentrations in Purkinje cells, was affected by hyperthyroid treatment in the Lurcher in a manner that is most consistent with an acceleration of the degenerative process in Purkinje cells. The acidic amino acids Glu and Asp show later changes and response to hormone which seem to be a reaction to the Purkinje cell pattern, probably in the granule cells. We conclude that the Lurcher cerebellum is particularly sensitive to thyroid hormone, and that it responds to low levels of hyperthyroidism in a distinct way.

Aging↗
PubMed1989

Spontaneous and beta-adrenergic receptor-mediated taurine release from astroglial cells are independent of manipulations of intracellular calcium.

Stimulation of beta-adrenergic receptors on LRM55 astroglial cells results in cAMP-dependent release of taurine. We have previously demonstrated that extracellular Ca2+ is not required for either spontaneous or receptor-mediated taurine release (Martin et al., 1988b). In the present series of experiments we investigated the relationship between changes in intracellular free Ca2+ ([Ca2+]i) and taurine release. [Ca2+]i was measured using the fluorescent probe fura-2 and was manipulated by changing the concentration of Ca2+ in the incubation medium and by using the Ca2+ ionophore ionomycin. [Ca2+]i was reduced from 150 +/- 95 nM (n = 46) in control medium (containing 1.1 mM CaCl2) to 46 +/- 10 nM (n = 43) in saline containing no CaCl2 and 10 microM EGTA. [Ca2+]i was rapidly elevated to greater than or equal to 1 microM in medium containing 100 microM CaCl2 and 10 microM ionomycin. Taurine release, either spontaneous or stimulated by isoproterenol, was not significantly affected by these manipulations of [Ca2+]i. [Ca2+]i did not change when cells were stimulated with 100 nM isoproterenol in either control saline containing 1.1 mM CaCl2 or in CaCl2-free saline containing 10 microM EGTA. Other secretogogs (serotonin and ethanol) did not cause changes in [Ca2+]i. These data indicate that neither spontaneous or receptor-mediated taurine release from astroglial cells is Ca2+ dependent. However, when cells were preloaded with Ca2+, allowed to recover briefly, and then stimulated with isoproterenol, it was possible to demonstrate transient increases in Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Astrocytes↗
PubMed1988

Step-advancement island flap for fingertip reconstruction.

An island flap is described for fingertip reconstruction, based on one neurovascular bundle. The design incorporates the stepladder principle, which ensures the provision of ample skin with full tactile sensibility in the required position, without the risk of longitudinal volar scarring and with good nail support when necessary. Fourteen such repairs have been carried out with satisfactory results and no complications.

Amputation, Traumatic↗