Multiple coupling of neurohormone receptors with cyclic AMP and inositol phosphate production in anterior pituitary cells

Regulation of adenohypophyseal hormone secretions has been shown to involve cyclic AMP production, modulation of phosphatidyl inositol diphosphate breakdown and Ca2+ mobilization. Various neurohormone receptors are positively or negatively coupled to adenylate cyclase activity in anterior pituitary cells. The effects of these neurohormones on adenylate cyclase activity are consistent with the effect on hormone secretions, suggesting that modulation of the enzyme activity is actually involved in the regulation of adenohypophyseal secretions. Thus DA inhibits, whereas VIP stimulates adenylate cyclase activity of the same cell type, which, according to the effect of these neurohormones on prolactin secretion, appear to be lactotrophs. On the other hand, SRIF inhibits, whereas GRF stimulates the adenylate cyclase activity of another cell type, namely somatotrophs, whereas CRF appears to act on a third cell type, corticotrophs. Peripheral hormones have been shown to modulate the sensitivity of anterior pituitary cells to these neurohormones. Estradiol long-term treatment has an anti-dopaminergic effect on prolactin secretion. The steroid also suppresses the dopamine inhibition of adenylate cyclase. This effect appears selective to the DA inhibition, since AII inhibition of the enzyme is only partially reduced, whereas the somatostatin inhibition is markedly increased. Peripheral hormones seem to affect the sensitivity of adenohypophyseal cells not only by modulating the number of receptors for a given neurohormone but also by interfering with the coupling mechanisms of these receptors. AII and DA inhibit the adenylate cyclase activity of lactotroph cells. The prolactin stimulation induced by angiotensin is not consistent with the effect of the peptide on adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)     

alpha-DNA. VI: Comparative study of alpha- and beta-anomeric oligodeoxyribonucleotides in hybridization to mRNA and in cell free translation inhibition.

alpha and beta-anomeric d(G2T12G2) oligodeoxyribonucleotides were compared for their hybridization to rA12: the observed melting temperatures are 27 degrees C for beta-oligodeoxyribonucleotide/RNA hybrid and 53 degrees C for alpha-oligodeoxyribonucleotide/RNA. alpha-oligonucleotides with the four bases, complementary to natural mRNAs, were synthesized for the first time, labeled at their 5'-end with [32P] and used as probes in Northern blot experiments. In spite of these higher affinities for their target RNA's, they were unable to block translation of natural or synthetic mRNA's in rabbit reticulocyte lysate. We have studied the RNase H activity on model rA12:alpha- or beta-d(G2T12G2) hybrids or on mRNA:alpha- or beta-oligonucleotides hybrids. Specific hybridization protects RNA strech when using alpha-oligonucleotides but not beta-oligonucleotides. Thus, our results show the inability of RNase H to degrade RNA in alpha-oligodeoxyribonucleotides:RNA duplexes.     

Transient expression of a Ca2+-activated Cl- current during development of quail sensory neurons

The expression of a calcium-activated chloride current (ICl(Ca)) was studied during the development of the sensory neurons of quail trigeminal ganglia. This current is expressed in 20% of the neurons by the 5th day of embryonic development; it can be found in nearly all neurons by the 7th day and subsequently disappears in half of them. Similar results were obtained with dorsal root ganglion neurons. The disappearance of ICl(Ca) in part of the sensory neurons during development is not due to a selective death of the neurons possessing this current and our results suggest that it is mediated by an interaction of the sensory neurons with their target tissue.     

EPR study of the redox interactions in cytochrome c3 from Desulfovibrio vulgaris Miyazaki

We present a new examination of the EPR redox titration data for the tetraheme cytochrome c3 from Desulfovibrio vulgaris Miyazaki. Our analysis includes the contribution of the interaction potentials between the four redox sites and is based on the model previously developed for the study of cytochrome c3 from Desulfovibrio desulfuricans Norway. We observed, as for D. desulfuricans Norway cytochrome c3, that the conformation of the heme with the lowest redox potential, heme 4, is sensitive to the redox state of the heme with the highest potential, heme 1. However in D. vulgaris Miyazaki cytochrome c3 spectral simulations show that heme 4 is present in two conformational states which interconvert partially when heme 1 is reduced. The sets of redox parameters which satisfy the fitting procedure of the titration curves are in the following domain: -250 mV less than or equal to e41 less than or equal to -220 mV, -325 mV less than or equal to e2 less than or equal to -320 mV, -335 mV less than or equal to e3 less than or equal to -330 mV, -360 mV less than or equal to e4 less than or equal to -355 mV, -5 mV less than or equal to I12 less than or equal to 20 mV, -10 mV less than or equal to I13 less than or equal to 5 mV, -15 mV less than or equal to I23 less than or equal to -5 mV, -15 mV less than or equal to I24 less than or equal to -10 mV, -25 mV, less than or equal to I34 less than or equal to -15 mV. As in D. desulfuricans Norway cytochrome c3 the interactions are moderate. Simple electrostatic considerations suggest that these moderate values could be related to the large accessibility of the hemes to the solvent. Our work does not confirm the existence of a cooperative interaction between heme 2 and heme 3 which has been proposed on the basis of electrochemical measurements.     

Electrophysiology of a chick neuronal nicotinic acetylcholine receptor expressed in Xenopus oocytes after cDNA injection

Brain nicotinic acetylcholine receptors (nAChRs) are made up of protein subunits that differ from those constituting muscle nAChRs. To characterize the physiological properties of one class of avian brain nicotinic receptor, we injected the nuclei of Xenopus oocytes with full-length cDNAs for the ligand binding (alpha 4) and structural (n alpha) subunits. Injected oocytes had large ACh-induced currents in the microampere range that were insensitive to alpha-bungarotoxin, as expected for neuronal nAChRs. We found that these brain nAChRs incorporate at least two alpha 4 subunits and that their functional properties differ from muscle nAChRs in at least two respects: the elementary conductance is considerably smaller (20 pS), and channels in outside out patches stop functioning within a few minutes.     

Isolation and physiological study of an amylolytic strain of Lactobacillus plantarum

Summary An amylolytic lactic acid bacterium identified as Lactobacillus plantarum was isolated from cassava roots (Manihot esculenta var. Ngansa) during reting. The amylolytic enzyme synthesized was an extracellular -amylase with an optimum pH of 5.0 and an optimum temperature of 55° C. Cultured on starch, the strain displayed a growth rate of 0.43 h^–1, a biomass yield of 0.19 g·g^–1 and a lactate yield of 0.81 g·g^–1. The growth kinetics were similar on starch and glucose. Sufficient enzyme was synthesized and starch hydrolysis was not a limiting factor for growth. Biosynthesis of the enzyme was observed when the glucose concentration was less than 6.7 g·l^–1 and reached up to 4 IU·ml^–1 at the end of the fermentation. Offprint requests to: M. Raimbault     

EMG power spectra of elbow extensors during ramp and step isometric contractions

The goal of the present study was to compare electromyogram (EMG) power spectra obtained from step (constant force level) and ramp (progressive increase in the force level) isometric contractions. Data windows of different durations were also analysed for the step contractions, in order to evaluate the stability of EMG power spectrum statistics. Fourteen normal subjects performed (1) five ramp elbow extensions ranging from 0 to 100% of the maximum voluntary contraction (MVC) and (2) three stepwise elbow extensions maintained at five different levels of MVC. Spectral analysis of surface EMG signals obtained from triceps brachii and anconeus was performed. The mean power frequency (MPF) and the median frequency (MF) of each power spectrum were obtained from 256-ms windows taken at 10, 20, 40, 60 and 80% MVC for each type of contraction and in addition on 512-, 1024- and 2048-ms windows for the step contractions. No significant differences (P greater than 0.05) were found in the values of both spectral statistics between the different window lengths. Even though no significant differences (P greater than 0.05) were found between the ramp and the step contractions, significant interactions (P less than 0.05) between these two types of contraction and the force level were found for both the MPF and the MF data. These interactions point out the existence of different behaviours for both the MPF and the MF across force levels between the two types of contraction.     

Calcitonin and vasopressin affect epithelial properties in a renal cell line

We have used a stable clonal variant (D + Sc), isolated from the LLC-PK1 pig kidney-derived cell line and selected for its extensive capacity to form domes, in order to study the hormonal modulation of epithelial permeability in culture. Calcitonin, vasopressin, and other agents that raise intracellular adenosine 3',5'-cyclic monophosphate levels caused a rapid and dramatic decrease in the size and number of domes. This effect was independent of RNA and protein synthesis, and thus appeared unrelated to the production of urokinase, a proteinase synthesized by the cells in response to these agents. Calcitonin caused a decrease in transepithelial electrical resistance, suggesting that the effect of the hormone on domes was due to an increase in the permeability of a paracellular pathway. Thus, in addition to the wellknown effects of vasopressin on collecting duct permeability, part of the in vivo effect(s) of calcitonin and vasopressin on the renal tubule might also involve alterations of epithelial permeability related to those described here.