Biexponential Kinetics of (R)-α-[3H]Methylhistamine Binding to the Rat Brain H3 Histamine Receptor

The H3 histamine receptor is a high-affinity receptor reported to mediate inhibition of CNS histidine decarboxylase activity and depolarization-induced histamine release. We have used (R)-alpha-[3H]methylhistamine, a specific, high-affinity agonist, to characterize ligand binding to this receptor. Saturation binding studies with rat brain membranes disclosed a single class of sites (KD = 0.68 nM; Bmax = 78 fmol/mg of protein). Competition binding assays also yielded an apparently single class of sites with a rank order of potency for ligands characteristic of an H3 histamine receptor: N alpha-methylhistamine, (R)-alpha-methylhistamine greater than histamine, thioperamide greater than impromidine greater than burimamide greater than dimaprit. In contrast, kinetic studies disclosed two classes of sites, one with fast, the other with slow on-and-off rates. Density of (R)-alpha-[3H]methylhistamine binding followed the order: caudate, midbrain (thalamus and hippocampus), cortex greater than hypothalamus greater than brainstem greater than cerebellum. These data are consistent with an H3 histamine receptor, distinct from H1 and H2 receptors, that occurs in two conformations with respect to agonist association and dissociation or with multiple H3 receptor subtypes that are at present pharmacologically undifferentiated.     

Isoforms of Na,K-ATPase inArtemia salina: II. Tissue distribution and kinetic characterization

Summary To characterize the molecular properties conveyed by the isoforms of the subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the 1 isoform, whereas the intestinal enzyme exhibits both the 1 and the 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mol P_i/mg protein/hr (1u)] in whole homogenates was 32±6 in the salt glands and 12±3 in the intestinal preparations (mean±sem). The apparent half-maximal activation constants (K _1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7±0.6mm vs. 23.5±4mm (P<0.01) for Na⁺, 16.6±2.2mm vs. 8.29±1.5mm for K⁺ (P<0.01), and 0.87±0.8mm vs. 0.79±1.1mm for ATP (NS). The apparentK _i's for ouabain inhibition were 1.1×10^–4 m vs. 2×10^–5 m, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity. The differences inK _1/2 for Na⁺ and K⁺ are more marked than those reported for the mammalian Na,K-ATPase isoforms. These differences may be attributed to the relative abundances of the subunit isoforms; other potential determinants (e.g. differences in membrane lipids), however, have not been investigated.During the tenure of an Educational Commission For Foreign Medical Graduates Visiting Associate Professorship.     

Kinetics of basic fibroblast growth factor binding to its receptor and heparan sulfate proteoglycan: a mechanism for cooperactivity.

Basic fibroblast growth factor (bFGF) binds to cell surface receptor (CSR) proteins and to heparan sulfate proteoglycans (HSPG). On the basis of equilibrium dissociation constants (Kd), the CSR has been considered a "high-affinity" binding site and HSPG a "low-affinity" site. We measured the apparent individual on and off rate constants (kon and koff) for bFGF binding to these two sites on intact cells and to each class of binding site in the absence of the other. While the kon's for CSR and HSPG on intact cells were not statistically different (konC = 2.27 x 10(8) M-1 min-1; konH = 0.90 x 10(8) M-1 min-1), the koff for the HSPG was 22.7-fold greater than that for the CSR (koffC = 0.003 min-1; koffH = 0.68 min-1). Thus, the difference in Kd's appears to result from the faster rate at which bFGF is released from the HSPG sites compared to the CSR. The kon's for isolated CSR and HSPG, and the koff for isolated HSPG, did not differ significantly from those for intact cells konC = 2.50 x 10(8) M-1 min-1; konH = 0.92 x 10(8) M-1 min-1; koffH = 0.095 min-1). However, the off rate for isolated CSR (koffC = 0.048 min-1) was statistically indistinguishable from the off rate for HSPG and 16-fold greater than the off rate for CSR on intact cells. The "high-affinity" binding of bFGF to intact cells probably refers only to a complex of bFGF with both CSR and HSPG, and not to the CSR alone.     

Hydration of 3-cyanopyridine to nicotinamide by crude extract nitrile hydratase

Summary The kinetic and stability characteristics of crude extract nitrile hydratase fromBrevibacterium R-312 were studied for the hydration of 3-cyanopyridine to nicotinamide. The enzyme was substrate and product inhibited and had the following kinetic constants:K _m =28 mM;K _p =36 mM;K _s =155 mM;V _m =5.8 mol/min/mg protein (25°C). Itsmaximum temperature and pH (phosphate buffer) were 35°C and 8.0, respectively and it had half-lives of 50 days, 10 days and 1 day at 4°C, 10°C and 25°C, respectively. The crude extract also exhibited amidase activity on nicotinamide, but it became significant only at nicotinamide concentrations greater than 300 mM. Mathematical models for batch and fed-batch hydrations were developed to account for substrate and product inhibitions and for enzyme decay. They predicted to within 10% experimental results for initial substrate and final product concentrations up to 300 mM; the accuracies decreased at higher concentrations primarily because of the relatively rapid hydrolysis of nicotinamide.     

Effect of naloxone on spectral shifts of the diaphragm EMG during inspiratory loading

Shifts in the power spectrum of the diaphragm EMG to lower frequencies may occur in the presence of fatiguing inspiratory flow-resistive loads (IRL). However, such a shift of the centroid frequency (fc) could follow a reduction in central output through a differential reduction in end-inspiratory high-frequency power (HFP). In unanesthetized goats, we tested the hypothesis that activation of the endogenous opioid system by IRL would differentially reduce central respiratory output, causing a reduction in fc. IRL was imposed for 180 min after which naloxone (0.1 mg/kg, NLX) was given. fc was computed from the power spectral density estimated by the Welch method. IRL reduced fc from 148.0 +/- 9.8 (SE) Hz at base line to 141.1 +/- 8.9 Hz or to 95.5 +/- 1.3% of base line by 180 min (both P less than 0.05). NLX increased fc to 148.9 +/- 9.9 Hz or to 100.6 +/- 1.1% of base line (both P less than 0.05). The decline in fc during IRL was found to be the result of a reduction in HFP, predominantly toward the end of inspiration. The reversibility of this fc shift with NLX suggests a central mechanism consequent to elaboration of endogenous opioids and not a peripheral (muscular) event consequent to muscle fatigue.     

Cervical sympathetic and phrenic nerve responses to progressive brain hypoxia

To determine if depression of central respiratory output during progressive brain hypoxia (PBH) can be generalized to other brain stem outputs, we examined the effect of PBH on the tonic (tSCS) and inspiratory-synchronous (iSCS) components of preganglionic superior cervical sympathetic (SCS) nerve activity. Peak phrenic and SCS activity were measured in nine anesthetized, paralyzed, peripherally chemodenervated, vagotomized cats. PBH was produced by inhalation of 0.5% CO in 40% O2 while blood pressure and end-tidal CO2 were maintained constant. A progressive reduction in arterial O2 content from 14.3 +/- 0.6 to 4.5 +/- 0.3 vol% caused a 79 +/- 7% depression of peak phrenic activity and an 84 +/- 10% reduction of iSCS activity, but tSCS activity increased 42 +/- 21%. During CO2 rebreathing, iSCS activity increased in parallel with peak phrenic activity while tSCS activity was unchanged. The slopes of the CO2 responses of both phrenic (6.3 +/- 1.2%max/mmHg) and iSCS (4.6 +/- 0.8%max/mmHg) activity were unaffected by PBH. In four of nine hypocapnic and three of nine hypoxic studies, inspiratory activity in the SCS nerve was observed even after completely silencing the phrenic neurogram.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endogenous opioid effects on abdominal muscle activity during inspiratory loading

In a previous study in unanesthetized goats, we demonstrated that continuous naloxone (NLX) administration during inspiratory flow-resistive loading (IRL) significantly increased tidal volume (VT) but not diaphragm electromyogram (EMGdi). End-expiratory gastric pressure did increase with NLX, implying that increased abdominal muscle activity may have accounted for the NLX effect. In the current study we directly tested the hypothesis that endogenous opioid elaboration depresses the abdominal muscle response to a continuous inspiratory flow-resistive load. In seven unanesthetized goats, VT, arterial blood gases, EMGdi, and EMG activity of external oblique (EMGeo), transversus abdominis (EMGta), and external intercostal (EMGei) muscles were monitored. IRL (50 cmH2O.l-1.s) was continued for 3 h, after which NLX (0.1 mg/kg) or saline was given. Our results showed that VT decreased from 323 +/- 32 (SE) ml at baseline to 260 +/- 16 ml 5 min after the load was imposed (P less than 0.05) and further decreased to 229 +/- 18 and 217 +/- 15 ml by 120 and 180 min, respectively (180 vs. 5 min, P less than 0.05). EMGdi increased from 62 +/- 5 to 83 +/- 4% max at 5 min (P less than 0.05) but was unchanged thereafter. In contrast, for this same time period EMGeo increased from 35 +/- 5 to 58 +/- 11% max but decreased from 67 +/- 11% max at 120 min to 37 +/- 5% max at 180 min (P less than 0.05). NLX administration resulted in significant increases in EMGeo (91% above 180-min value). In contrast, EMGdi increased minimally after NLX (15% above 180-min value).(ABSTRACT TRUNCATED AT 250 WORDS)