Depletion of brainstem epinephrine stores by alpha-methyldopa: possible relation to attenuated sympathetic outflow

The antihypertensive effect of alpha-methyldopa (MD) is believed to be critically dependent on its ability to deplete endogenous catecholamines or cause the synthesis of false neurotransmitters. We used liquid chromatography with electrochemical detection (LCEC) and negative chemical ionization gas chromatography-mass spectrometry (GC-MS) for quantitation of catecholamines and MD metabolites in rat. MD intraperitoneally (100 mg/kg q12 hr X 12 days), significantly increased alpha-methylnorepinephrine (MNE) in brain (1.02 +/- 0.33 micrograms/g), heart (1.67 +/- 0.57 micrograms/g) and adrenal glands (114.93 +/- 50.47 micrograms/g) Endogenous norepinephrine (NE), epinephrine (E) and dopamine (DA) were reduced. ME levels were 2.19 +/- 0.44 micrograms/g (n = 6) in the adrenal gland but only 99 +/- 26 pg/g (n = 3) in the brainstem. MD-induced endogenous brainstem NE depletion was more than compensated by MNE production, but brainstem E depletion was not compensated for by a stoichiometric production of brainstem ME. We conclude (1) although ME is a metabolite of MD, it is present in extremely low concentrations in brainstem and (2) central epinephrine-containing neurons are depleted of neurotransmitter by MD therapy. If this selective epinephrine depletion occurs in the bulbospinal tract neurons responsible for maintaining sympathetic tone, then this effect could contribute to the antihypertensive effect of MD.     

Heterogeneity of holocarboxylase synthetase in patients with biotin-responsive multiple carboxylase deficiency.

Holocarboxylase synthetase activity has been determined in fibroblasts of seven patients with the neonatal form of biotin-responsive multiple carboxylase deficiency. The normal Km for biotin was 15 +/- 3 nmol/l, while in the patients the values ranged from 48 to 1,062 nmol/l. The mean maximum velocity was 27% of normal. Differences among the values obtained for the Km for biotin and the heat stability of holocarboxylase synthetase suggested that the patients studied represented at least four distinct variants at the holocarboxylase synthetase locus.     

SSCP and segregation analysis of the human type X collagen gene (COL10A1) in heritable forms of chondrodysplasia.

Type X collagen is a homotrimeric, short chain, nonfibrillar collagen that is expressed exclusively by hypertrophic chondrocytes at the sites of endochondral ossification. The distribution and pattern of expression of the type X collagen gene (COL10A1) suggests that mutations altering the structure and synthesis of the protein may be responsible for causing heritable forms of chondrodysplasia. We investigated whether mutations within the human COL10A1 gene were responsible for causing the disorders achondroplasia, hypochondroplasia, pseudoachondroplasia, and thanatophoric dysplasia, by analyzing the coding regions of the gene by using PCR and the single-stranded conformational polymorphism technique. By this approach, seven sequence changes were identified within and flanking the coding regions of the gene of the affected persons. We demonstrated that six of these sequence changes were not responsible for causing these forms of chondrodysplasia but were polymorphic in nature. The sequence changes were used to demonstrate discordant segregation between the COL10A1 locus and achondroplasia and pseudoachondroplasia, in nuclear families. This lack of segregation suggests that mutations within or near the COL10A1 locus are not responsible for these disorders. The seventh sequence change resulted in a valine-to-methionine substitution in the carboxyl-terminal domain of the molecule and was identified in only two hypochondroplasic individuals from a single family. Segregation analysis in this family was inconclusive, and the significance of this substitution remains uncertain.     

Prostaglandin D2, another renal prostaglandin?

Prostaglandin (PG) D₂ was biosynthesized by rabbit renal papillae incubates in vitro. Quantification of the renal prostaglandins by gas chromatography-mass spectroscopy demonstrated that the concentration of PGD₂ generated by renal papillae was to the amount of PGE₂ or about 1 μg/g tissue/30 min. Infusion of the sodium salt of PGD₂ into the renal artery of the dog produced a dose related increase in renal blood flow and urine flow, free water clearance, sodium excretion and potassium excretion without changes in systemic hemodynamics. At low doses PGD₂ increased renal blood flow to all cortical zones. Higher concentrations of PGD₂ produced a shift in the intrarenal distribution of blood flow toward the juxtamedullary nephrons.     

The postsynaptic density protein PSD-95 differentially regulates insulin- and Src-mediated current modulation of mouse NMDA receptors expressed in Xenopus oocytes

The NMDA subtype of glutamate receptor is physically associated with the postsynaptic density protein PSD-95 at glutamatergic synapses. The channel activity of NMDA receptors is regulated by different signaling molecules, including protein tyrosine kinases. Because previous results have suggested a role for protein kinase C (PKC) in insulin potentiation of NMDA currents in oocytes, the effects of coexpression of PSD-95 on insulin and PKC potentiation of NMDA currents from these receptors were compared. Another primary objective was to determine if PSD-95 could enable Src to potentiate currents from NR2A/NR1 and NR2B/NR1 receptors expressed in XENOPUS: oocytes. The results show opposite effects of PSD-95 coexpression on Src and insulin modulation of NR2A/NR1 receptor currents. Src potentiation of mouse NR2A/NR1 currents required PSD-95 coexpression. In contrast, PSD-95 coexpression eliminated insulin-mediated potentiation of NR2A/NR1 receptor currents. PSD-95 coexpression also eliminated PKC potentiation of NR2A/NR1 receptor currents. PSD-95 may therefore play a key role in controlling kinase modulation of NR2A/NR1 receptor currents at glutamatergic synapses.