Search PubMed⌕ Search

Biomedical subjects

J S Davidson

Publications and source records attributed to J S Davidson.

At least 73 records · Page 4Linked to original sources
PubMed1988

Interaction of endogenous chicken gonadotrophin-releasing hormone-I and -II on chicken pituitary cells.

The presence of two endogenous forms of gonadotrophin-releasing hormone (GnRH) in the chicken hypothalamus (chicken GnRH-I ([Gln8]GnRH) and chicken GnRH-II ([His5, Trp7, Tyr8]GnRH)), and the stimulation of gonadotrophins by both forms, suggests the possible existence of GnRH receptor subtypes and gonadotroph subtypes in the chicken pituitary. This question was investigated by assessing the effects of various combinations of the two known forms of chicken hypothalamic GnRH and antagonist analogues of GnRH on LH release from dispersed chicken anterior pituitary cells in both static and perifused systems. The relative inhibition of chicken GnRH-I-stimulated and chicken GnRH-II-stimulated LH release by 12 GnRH antagonists did not differ significantly, suggesting a single GnRH receptor type. Chicken GnRH-II was approximately sixfold more potent than chicken GnRH-I in releasing LH. Release of LH in response to maximal doses of chicken GnRH-I and chicken GnRH-II and to a mixture of both was similar and the two peptides were not additive in their effects, consistent with the presence of a single type of LH gonadotroph and a GnRH receptor which binds both forms of GnRH. Each form of GnRH desensitized cells to subsequent stimulation with the other form, providing additional evidence for a single type of LH gonadotroph. These findings suggest that chicken GnRH-I and -II stimulate gonadotrophin release through a single GnRH receptor type on a single class of LH gonadotroph in the chicken pituitary.

Animals↗
PubMed1988

Glycyrrhetinic acid derivatives: a novel class of inhibitors of gap-junctional intercellular communication. Structure-activity relationships.

Glycyrrhetinic acid was shown previously to inhibit intercellular gap-junctional communication between human fibroblasts. In the present study 31 derivatives of glycyrrhetinic acid were tested for their ability to inhibit communication. Eight of the compounds inhibited communication with high potency (IC50 less than 3 microM) and showed low toxicity, properties which suggest they may be useful pharmacological probes for studies of gap-junction function.

Cell Communication↗
PubMed1987

The initial phase of GnRH-stimulated LH release from pituitary cells is independent of calcium entry through voltage-gated channels.

Kinetic studies on gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release were undertaken using rat and chicken pituitary cell cultures. In response to continuous GnRH stimulation, a biphasic pattern of LH release was demonstrated. The two phases showed different susceptibility to the voltage-gated Ca2+ channel blockers D600 and nifedipine. The first (transient) phase of LH release was unaffected by the Ca2+ channel blockers whereas the second (sustained) phase was inhibited by both drugs. These results indicate that the initial phase of LH release is independent of Ca2+ entry through voltage-gated Ca2+ channels and may depend on mobilisation of intracellular Ca2+ or entry of extracellular Ca2+ through another mechanism.

Animals↗
PubMed1987

Gonadotrophin-releasing hormone--diversity of functions and clinical applications.

Gonadotrophin-releasing hormone (GnRH) plays a pivotal role in the regulation of the reproductive system. Chemically synthesised GnRH and analogues have found wide application in clinical medicine. Pulsatile administration of GnRH is used in the treatment of infertility and cryptorchidism. GnRH agonists inhibit gonadotrophin by pituitary desensitisation while antagonists compete out the effects of endogenous GnRH. These agents find application in contraception, the treatment of hormone-dependent neoplasms, precocious puberty, endometriosis, porphyria and disturbances of the menstrual cycle. It is now apparent that GnRH has been co-opted during evolution for functions other than the stimulation of gonadotrophins. GnRH has paracrine (local) effects in the gonads and placenta, acts as a neurotransmitter in the central nervous system, and has autocrine regulatory effects in some tumour cells. Evolutionary studies in non-mammalian vertebrates show that the GnRH gene has been duplicated to produce different molecular forms of unknown function. In addition to producing GnRH, the GnRH precursor is cleaved to produce peptides with novel activities, including prolactin inhibition.

Animals↗
PubMed1987

Luteinizing hormone release from chicken pituitary cells: synergism between calcium and protein kinase C and its inhibition by calmodulin antagonists.

Continuous stimulation of cultured chicken pituitary cells with a native chicken hypothalamic GnRH, Gln8-GnRH, caused LH release, followed by rapid desensitization. Continuous exposure to calcium ionophore A23187 also produced a short-lived LH response, followed by desensitization. In contrast, continuous stimulation with a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), caused prolonged release of LH, reaching maximum amplitude after 1 h and slowly decreasing thereafter. Cells desensitized to GnRH remained fully responsive to A23187 and TPA, indicating that GnRH-mediated desensitization occurs at a level before protein kinase C activation and calcium mobilization. Simultaneous addition of A23187 and TPA resulted in a synergistic response which was independent of the order of addition of the two compounds. Synergism was also demonstrated between depolarizing concentrations of potassium and TPA and between veratridine and TPA, indicating that calcium entering via voltage-dependent channels also synergizes with phorbol ester. Despite the prolonged action of TPA, rapid pulsatile changes in LH release could be induced in cells treated with TPA and veratridine by rapidly changing the extracellular calcium concentration. This suggest that protein kinase C could function as an amplifier of the calcium signal generated by GnRH. Synergism between calcium and TPA was blocked by trifluoperazine, chlorpromazine, and W-7 [N-(6-aminohexyl)5-chloro-1-napthalenesulfonamide] at concentrations that had no effect on LH release mediated by TPA alone. This suggests that synergism is mediated via calmodulin and not a direct effect of calcium on protein kinase C.

Animals↗
PubMed1987

Barium-induced LH release from chicken pituitary cells: synergism with phorbol ester.

Barium is known to elicit secretion in a number of cell systems. The mechanism of Ba2+ stimulation of LH release in cultured chicken pituitary cells was investigated in the present study. Barium-stimulated LH release was inhibited by extracellular Ca2+, indicating that Ba2+ does not act by stimulating Ca2+ entry. Simultaneous stimulation of the cells with Ba2+ and phorbol ester produced a synergistic response, similar to the synergism obtained with phorbol ester and treatments which increase cytosolic Ca2+. Both Ba2+-stimulated LH release and the synergism of Ba2+ with phorbol ester were inhibited by calcium channel blockers (Co2+, methoxyverapamil and nifedipine) and by calmodulin antagonists (trifluoperazine and chlorpromazine). These results indicate that the actions of Ba2+ are dependent on its entry through Ca2+ channels, and suggest that calmodulin activation is necessary for the synergism between Ba2+ and phorbol ester. Thus, synergism does not result from a direct effect of divalent cations on C-kinase.

Animals↗
PubMed1986

Reversible inhibition of intercellular junctional communication by glycyrrhetinic acid.

Intercellular gap-junctional communication was measured using metabolic co-operation in co-cultures of argininosuccinate synthetase-deficient and argininosuccinate lyase-deficient human fibroblasts. 18-alpha-glycyrrhetinic acid (AGA) was found to inhibit communication by more than 95% at concentrations as low as 2 microM. Concentrations up to 100 microM were not cytotoxic over a period of 2 hours. Communication inhibition was of rapid onset and was readily reversible. Communication remained continuously yet reversibly blocked in cells cultured in the presence of AGA for 20 days. The related compounds 18-beta-glycyrrhetinic acid and carbenoxolone also caused communication inhibition. The effect is probably not mediated via mineralocorticoid or glucocorticoid receptors since aldosterone and glucocorticoids had no effect on communication. AGA thus has properties of a useful inhibitor in the study of intercellular junctional communication.

Carbenoxolone↗
PubMed1986

Desensitization to gonadotropin-releasing hormone in perifused chicken anterior pituitary cells.

A perifusion method consisting of dispersed chicken anterior pituitary cells suspended in columns of Bio-Gel was developed to monitor the dynamics of LH release. The perifused cells responded to chicken I GnRH (Gln8-GnRH) in a dose-dependent manner. The ED50 was 3 X 10(-10) M, and maximal LH release occurred in response to 4 X 10(-9) M Gln8-GnRH. Continuous administration of 10(-7) M Gln8-GnRH and agonist stimulated an initial 8- to 10-fold increase in LH release within minutes. LH release then declined rapidly, reaching basal levels within 100 min. A biphasic response was noted. Calcium ionophore A23187 was effective in releasing additional LH from cells desensitized to 10(-7) Gln8-GnRH and agonist, indicating that total cellular LH was not depleted. In contrast, delivery of 2-min pulses of 10(-7) M and 10(-9) M Gln8-GnRH at a frequency of one pulse every 30 or 60 min for 3-5 h maintained pituitary responsiveness. Exposure to 10(-7) M Gln8-GnRH for 20 min was sufficient to desensitize pituitary cells to subsequent Gln8-GnRH stimulation. However, 20-min exposure to 10(-7) M GnRH antagonist neither evoked LH release nor had a desensitizing effect on subsequent stimulation by 10(-7) M Gln8-GnRH, indicating that receptor activation, not merely receptor binding, is necessary for Gln8-GnRH-mediated homologous desensitization. Pituitary cells desensitized by 20-min exposure to 10(-8) M Gln8-GnRH maintained responsiveness to a higher dose (10(-6 M) of Gln8-GnRH, suggesting that down-regulation of pituitary GnRH receptors might play a part in desensitization. Calcium ionophore A23187 partially desensitized pituitary cells to subsequent stimulation with Gln8-GnRH, probably due to depletion of releasable LH or desensitization of calcium-coupled secretory mechanisms. In calcium-free medium, 10(-7) M Gln8-GnRH did not evoke LH release, but nevertheless partially desensitized cells to subsequent 10(-7) M Gln8-GnRH stimulation. Thus desensitization is partially calcium-dependent. These findings demonstrate that the GnRH-mediated desensitization of gonadotrophs is a characteristic of chicken pituitary cells as in the mammal. However, chicken pituitary cells differ from mammalian cells in that desensitization is more rapid and partially dependent on extracellular calcium.

Animals↗
PubMed1985

Inhibition of intercellular junctional communication in human fibroblasts by triphenylmethane, triphenylmethylchloride, tetraphenylboron and related compounds.

Intercellular junctional communication was measured using [14C]citrulline incorporation in co-cultures of argininosuccinate synthetase-deficient and argininosuccinate lyase-deficient human fibroblasts. Triphenylmethane, triphenylmethylchloride and tetraphenylboron inhibited communication at concentrations at least 12-fold lower than cytotoxic concentrations. This inhibition was of rapid onset and was rapidly reversible by washing the co-cultures. Refractoriness to inhibition did not develop after prolonged exposure. Several other compounds also induced communication inhibition, but only at concentrations slightly below cytotoxic concentrations. Treatment of co-cultures with calcium ionophore A23187 or cycloheximide did not cause communication inhibition. It is suggested that triphenylmethane, triphenylchloride and tetraphenylboron may be useful inhibitors for studying the roles of intercellular junctional communication in some biological systems.

Argininosuccinate Lyase↗
PubMed1985

Effects of 12-O-tetradecanoylphorbol-13-acetate and retinoids on intercellular junctional communication measured with a citrulline incorporation assay.

Inhibition of intercellular junctional communication by 12-O-tetradecanoylphorbol-13-acetate (TPA) and retinoids was investigated using a citrulline incorporation assay. This new assay uses metabolic co-operation between argininosuccinate lyase-deficient human fibroblasts and arginosuccinate synthetase-deficient cells as a measure of junctional communication. Short-term exposure to TPA resulted in virtually complete inhibition of metabolic co-operation when V79 cells were used as the synthetase-deficient type. When synthetase-deficient human fibroblasts were used, inhibition by TPA was only partial. Exposure to high concentrations of TPA for prolonged periods resulted in partial reversal of communication inhibition and a refractory state in which cells were unresponsive to TPA. Retinoic acid and other retinoids also inhibited metabolic co-operation, but did not cause desensitisation of the type seen with TPA after prolonged exposure. Cultures which had been made refractory to TPA remained sensitive to inhibition by retinoic acid and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, indicating that these latter compounds inhibit junctional communication by a mechanism different from TPA. Simultaneous exposure of cultures to TPA and retinoic acid showed that the inhibitory effects on metabolic co-operation of these compounds were additive. Fluocinolone acetonide did not antagonise the effect of TPA. These results suggest that retinoic acid and fluocinolone acetonide exert their anti-tumor-promoting action by mechanisms which are not mediated by intercellular junctional communication.

Animals↗
PubMed1985

Studies on the mechanism of phorbol ester-induced inhibition of intercellular junctional communication.

Intercellular gap-junctional communication was measured using [14C]citrulline incorporation in co-cultures of argininosuccinate lyase-deficient human fibroblasts and argininosuccinate synthetase-deficient Chinese Hamster V79 cells. As previously shown, in this system junctional communication is completely inhibited by the tumor promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In the absence of extracellular calcium, TPA inhibition was less pronounced. However, synergism with calcium ionophore A23187 could not be demonstrated. Chlorpromazine, trifluoperazine and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester partially antagonised the effect of TPA. No antagonism was demonstrable between calmidazolium and TPA. Treatment of co-cultures with exogenous phospholipase C or 1-oleoyl-2-acetylglycerol (OAG) resulted in communication inhibition, suggesting that protein kinase C activation is involved in the mechanism of phorbol ester-mediated communication inhibition. However co-cultures which had been made refractory to TPA by prolonged exposure to high concentrations remained sensitive to inhibition by phospholipase C and OAG. These results suggest either that diacylglycerol can produce other effects independent of protein kinase C activation, or that refractoriness to phorbol esters is not simply due to a decrease in the amount of protein kinase C.

Animals↗
PubMed1985

Use of a new citrulline incorporation assay to investigate inhibition of intercellular communication by 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane in human fibroblasts.

A citrulline incorporation assay has been developed to measure intercellular communication between argininosuccinate synthetase-deficient and argininosuccinate lyase-deficient human fibroblasts (J. S. Davidson, I. M. Baumgarten, and E. H. Harley, Exp. Cell Res., 150: 367-378, 1984). This method was used to investigate the effects of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) on intercellular junctional communication. DDT at a concentration of 20 micrograms/ml inhibited metabolic cooperation by 90 to 98%. This inhibition was of rapid onset and was rapidly reversed by washing the cells. Inhibition of metabolic cooperation by DDT was not dependent on the presence of extracellular free calcium, indicating that DDT does not act by increasing net calcium influx into cells. This system should prove useful in elucidating the relationship between tumor promotion and intercellular communication.

2,4-Dinitrophenol↗
PubMed1984

Prenatal diagnosis of cystic fibrosis in South Africa.

Prenatal diagnosis of cystic fibrosis (CF) has been made possible by the finding that the activity of various enzymes derived from the microvillar membranes of the fetus is decreased in 2nd trimester amniotic fluid. Gamma-glutamyl transpeptidase, aminopeptidase M and the phenylalanine-inhibitable form of alkaline phosphatase (AP) have been found to be of most diagnostic use in this respect, the odds of the fetus being affected with CF being 28:1 if the AP test is positive. When couples have already had a child with CF, pregnancies are being monitored by these methods at the University of Cape Town.

Alkaline Phosphatase↗
PubMed1984

Studies on cultured fibroblasts in a case of methylmalonic aciduria.

A case of methylmalonic aciduria is described. The clinical course was unusually mild, the child surviving to the age of 8 years. Studies on cultured fibroblasts confirmed a defect in propionate metabolism which was non-responsive to hydroxycobalamin in vitro. Polyethylene-glycol-induced cell fusion with a known methylmalonyl co-enzyme apomutase-deficient cell line showed genetic complementation indicating that in this patient the defect was in one of the enzymes required for 5-deoxyadenosyl cobalamin synthesis.

Apoenzymes↗
PubMed1984

Effects of extracellular calcium and magnesium on junctional intercellular communication in human fibroblasts.

The role of extracellular calcium and magnesium in intercellular communication via permeable junctions was studied by measuring metabolic co-operation between ASS- and ASL- human fibroblasts. Communication through pre-existing junctions was not affected by the removal of extracellular calcium and magnesium from the medium. On the other hand, the establishment of communication through new permeable junctions, when ASS- and ASL- cells were mixed, was dependent on the presence of extracellular calcium and magnesium.

Calcium↗
PubMed1984

Metabolic cooperation between argininosuccinate synthetase and argininosuccinate lyase deficient human fibroblasts.

Human fibroblasts deficient in either argininosuccinate synthetase or argininosuccinate lyase show low levels of incorporation of [14C]citrulline into protein. However, when these two cell types are co-cultured [14C]citrulline incorporation is restored to the levels found in control cultures. This metabolic cooperation is cell-density-dependent and does not occur by diffusion of argininosuccinate into the medium. Our results indicate that argininosuccinate passes between the two cell types via intercellular junctions, and this system provides a simple and accurately quantifiable model for the study of intercellular communication.

Arginine↗
PubMed1984

Acid-base and ventilatory adaptation in conscious dogs during chronic hypercapnia.

Ventilation and cisternal cerebrospinal fluid (CSF) and arterial acid-base balance were measured in awake dogs during air control and from 1 h to 26 days of breathing 5% CO2 in air. Ventilation increased 4-fold during acute hypercapnia and then declined to a minimum at 5-10 days. Between 1-3 days and 16-26 days of hypercapnia ventilation was relatively stable at 2.5 times control. [HCO3-]CSF increased rapidly by 12 h of hypercapnia and in the steady-state [HCO3-]CSF was correlated with PCSFCO2. Between 1 h and 1.5 days of hypercapnia, increase in [HCO3-]CSF was also correlated with increase in [NH3]CSF. Despite increase in [HCO3-]CSF, there was no compensation of [H+]CSF throughout 26 days of hypercapnia. Hydrogen ion may have contributed to the control of ventilation during chronic hypercapnia since ventilation was correlated with [HCO3-]a and [HCO3-]CSF. However, a relationship between ventilation and [H+] of arterial blood and CSF during chronic hypercapnia was relatively poor or absent. Ventilatory adaptation to chronic hypercapnia could not be related to metabolism or to [NH3]CSF. The mechanism(s) by which the increase in PCO2 during chronic respiratory acidosis results in sustained elevation of ventilation remains to be resolved.

Acid-Base Equilibrium↗