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Biomedical subjects

M R Boarder

Publications and source records attributed to M R Boarder.

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

Role of phosphoinositide turnover and cyclic AMP accumulation in prostaglandin-stimulated noradrenaline release from cultured adrenal chromaffin cells.

Prostaglandins (PGs) E1, E2 and F2 alpha stimulated release of noradrenaline from chromaffin cells; the most potent was PGF2 alpha with an EC50 of about 0.1 microM. The rank order of potency for release, and the EC50 for each PG, was the same as that for stimulation of (poly)phosphoinositide turnover. PGE1-stimulated release was dependent on extracellular calcium and sensitive to dihydropyridine calcium channel agonists and antagonists at 1 microM, but unlike release stimulated by 50 mM extracellular potassium was not sensitive to verapamil or diltiazem at 10 microM. The PGs also enhanced the turnover of inositol phospholipids, but the PGE1-stimulated formation of inositol phosphates was small compared to that produced by bradykinin, which stimulates a similar degree of release. Unlike release, the stimulation of inositol phosphate formation by PGs was not dependent on the addition of calcium to the medium. In down-regulation experiments, involving 2 hr preincubation with 30 microM PGE1, release in response to the three PGs was attenuated, whereas the release response to bradykinin and nicotine was unaffected. However, the stimulation of (poly)phosphoinositide turnover by PG was not down-regulated by prior exposure to PGE1. This dissociation of the inositol phosphate response suggests that release in response to PGs is not downstream of stimulation of inositol phospholipid hydrolysis. A further series of experiments is reported which shows that release is not a consequence of increased rate of cyclic AMP synthesis. It seems likely that PG stimulation of noradrenaline release is a result of calcium entry through dihydropyridine-sensitive channels by a mechanism independent of these two second messenger systems.

Adrenal Medulla↗
PubMed1990

Comparison of P2 purinergic receptors of aortic endothelial cells with those of adrenal medulla: evidence for heterogeneity of receptor subtype and of inositol phosphate response.

Vascular endothelial cells from different parts of the circulation are known to show different functional responses, presumably corresponding to physiological roles. Previous studies have shown that ATP acts on P2 purinergic receptors of endothelial cells of major blood vessels, stimulating the formation of inositol phosphates. Here we have compared the action of ATP and congeners acting on endothelial cells of bovine thoracic aorta with cells derived from the microvasculature of bovine adrenal medulla. With measurement of total inositol phosphates, cells from the aorta showed a rank order of agonist potency of 2-methylthio-ATP greater than adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) greater than ADP greater than ATP greater than beta, gamma-imido-ATP greater than beta, gamma-methylene-ATP, consistent with action at receptors of the P2Y subtype. However, with adrenal cells the rank order of potency was ATP gamma S greater than ATP greater than beta, gamma-imido-ATP greater than ADP greater than beta, gamma-methylene-ATP = 2-methylthio-ATP. This profile is not consistent with either P2X or P2Y receptors. When the nature of this inositol phosphate response was analyzed with anion exchange chromatography, it was found that the aortic cells showed an inositol trisphosphate stimulation that peaked within a few seconds and rapidly declined, whereas the response of the adrenal medulla cells continued to rise through 5 min. Analysis of isomers of inositol phosphates revealed a different pattern of metabolism between the two cell types, which may account for the different time course of response. With adrenal cells, ATP at low micromolar concentrations caused a dose-dependent increase in levels of cyclic AMP and had a greater than additive effect on cyclic AMP levels when combined with submaximal stimulation by prostaglandin E2. These results suggest the presence of a P2Y receptor on aortic endothelial cells, with an 'atypical' purinocepter, i.e., neither P2X nor P2Y, on adrenal cells. Furthermore, they show that activation of P2 receptors on the two cell types has different functional consequences.

Adenosine Triphosphate↗
PubMed1989

Characterization of endothelin-1 stimulation of catecholamine release from adrenal chromaffin cells.

Addition of endothelin-1 (ET-1) to primary cultures of bovine adrenal chromaffin cells causes a significant enhancement of norepinephrine and epinephrine efflux, with an EC50 of about 1 nM. A maximally effective concentration of endothelin (10 nM) gives a transient increase in noradrenaline release within 5 min. The amounts of noradrenaline and adrenaline released by ET-1 was smaller than the release elicited by maximally effective concentrations of bradykinin or prostaglandin E2. These results raise the possibility that the vascular endothelium may stimulate release from chromaffin cells by production of ET-1.

Adrenal Medulla↗
PubMed1989

Evidence for a dihydropyridine-sensitive and conotoxin-insensitive release of noradrenaline and uptake of calcium in adrenal chromaffin cells.

1. It has been suggested that neuronal voltage-sensitive calcium channels (VSCC) may be divided into dihydropyridine (DHP)-sensitive (L) and DHP-insensitive (N and T), and that both the L and the N type channels are attenuated by the peptide blocker omega-conotoxin. Here the effects of omega-conotoxin on release of noradrenaline and uptake of calcium in bovine adrenal chromaffin cells were investigated. 2. Release of noradrenaline in response to 25 mM K+, 65 mM K+, 10 nM bradykinin or 10 microM prostaglandin E1 was not affected by omega-conotoxin in the range 10 nM-1 microM. 3. 45Ca2+ uptake stimulated by high K+ and prostaglandin was attenuated by 1 microM nitrendipine and enhanced by 1 microM Bay K 8644; these calcium fluxes were not modified by 20 nM omega-conotoxin. 4. With superfused rat brain striatal slices in the same medium as the above cell studies, release of dopamine in response to 25 mM K+ was attenuated by 20 nM omega-conotoxin. 5. These results show that in these neurone-like cells, release may be effected by calcium influx through DHP-sensitive but omega-conotoxin-insensitive VSCC, a result inconsistent with the suggestion that omega-conotoxin blocks both L-type and N-type neuronal calcium channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1989

Characterization of bradykinin-stimulated release of noradrenaline from cultured bovine adrenal chromaffin cells.

Bovine adrenal chromaffin cells were purified and maintained in culture. On exposure to bradykinin they released noradrenaline. The characteristics of this stimulus secretion were compared with the response to elevated extracellular potassium. Bradykinin released noradrenaline with an EC50 of about 2 nM, with maximum release (2-3 times control incubations) less than that elicited by high potassium or nicotine. Bradykinin analogs showed the response to have characteristics of a B2 receptor. On continuous exposure to 10 nM bradykinin the rate of release was maximal in the first 1 to 3 min, with a further slower sustained rate of release. The response is mostly, but not completely, dependent on extracellular calcium, and was inhibited by cadmium in the micromolar range. Whereas the potassium-stimulated release was highly sensitive to dihydrophyridines, the bradykinin response was not attenuated by dihydropyridine calcium channel antagonists. The results are discussed with respect to stimulus-secretion mechanisms possibly involved in the bradyininin-stimulated release in the light of previous observations characterizing a phosphoinositide response to bradykinin in these cells.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

Angiotensin II potentiates prostaglandin stimulation of cyclic AMP levels in intact bovine adrenal medulla cells but not adenylate cyclase in permeabilized cells.

The level of cyclic AMP in primary cultures of bovine adrenal medulla cells is elevated by prostaglandin E1. Angiotensin II is commonly reported to act on receptors linked to phosphoinositide metabolism or to inhibition of adenylate cyclase. We have investigated the effect of angiotensin II on prostaglandin E1-stimulated cyclic AMP levels in these primary cultures. Rather than reducing cyclic AMP levels, we have found that angiotensin II powerfully potentiates prostaglandin E1-stimulated cyclic AMP accumulation in intact cells, both in the presence and absence of phosphodiesterase inhibitors. The 50% maximal response was similar to that for stimulation of phosphoinositide breakdown by angiotensin II in these cultures. The potentiation of stimulated cyclic AMP levels was seen, although to a smaller maximum, with the protein kinase C (Ca2+/phospholipid-dependent enzyme) activating phorbol ester tetradecanoyl phorbolacetate and with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol; pretreatment (24 h) with active phorbol ester, which would be expected to diminish protein kinase C levels, attenuated the angiotensin II potentiation of cyclic AMP. Using digitonin-permeabilized cells we showed that adenylate cyclase activity was stimulated by prostaglandin E1 with the same dose-response relationship as was cyclic AMP accumulation in intact cells, but the permeabilized cells showed no response to angiotensin II. The results are discussed with respect to the hypothesis that the angiotensin II influence on cyclic AMP levels is mediated, in part, by diacylglycerol stimulation of protein kinase C.

Adenylyl Cyclases↗
PubMed1988

Stimulation of formation of inositol phosphates in primary cultures of bovine adrenal chromaffin cells by angiotensin II, histamine, bradykinin, and carbachol.

Histamine, bradykinin, and angiotensin II stimulate release of catecholamines from adrenal medulla. Here we show, using bovine adrenal chromaffin cells in culture, that these agonists as well as carbachol (with hexamethonium) stimulate production of inositol phosphates. The histamine response was mepyramine sensitive, implicating an H1 receptor, whereas bradykinin had a lower EC50 than Met-Lys-bradykinin, and [Des-Arg9]-bradykinin was relatively inactive, implicating a BK-2 receptor. Total inositol phosphates formed in the presence of lithium were measured, with histamine giving the largest response. The relative contribution of chromaffin cells and nonchromaffin cells in the responses was assessed. In each case chromaffin cells were found to be responding to the agonists; in the case of histamine the response was solely on chromaffin cells. When the inositol phosphates accumulating over 2 or 5 min, with no lithium present, were separated on Dowex anion-exchange columns, bradykinin gave the greatest stimulation in the inositol trisphosphate fraction, whereas histamine gave a larger inositol monophosphate accumulation. On resolution of the isomers of stimulated inositol trisphosphate after 2 min of stimulation, the principal isomer present was inositol 1,3,4-trisphosphate in each case. Two hypotheses for the differential responses to histamine and bradykinin are discussed.

Adrenal Medulla↗
PubMed1988

Effect of forskolin and prostaglandin E1 on stimulus secretion coupling in cultured bovine adrenal chromaffin cells.

Treatment of adrenal chromaffin cells with forskolin (0.1-10 microM) stimulated cyclic AMP levels, reduced the maximal stimulation of release of noradrenaline by nicotine, and increased release in response to elevated external potassium and the calcium ionophore A23187. The presence of the phosphodiesterase inhibitor Ro 20-17-24 with forskolin potentiated both the stimulation of cyclic AMP and the inhibition of nicotine-induced noradrenaline release. Dibutyryl cyclic AMP, and the elevation of cyclic AMP with prostaglandin E1, also attenuated nicotine-stimulated release. However, when the stimulation of intracellular cyclic AMP production by prostaglandin E1 was potentiated by low levels of forskolin, there was not a concomitant potentiation of effect on noradrenaline release. Dideoxyforskolin, an analogue of forskolin which does not stimulate adenylate cyclase, inhibited both potassium- and nicotine-stimulated release, probably by a mechanism unrelated to the action of forskolin in these experiments. Using Fura-2 to estimate free intracellular calcium levels, both forskolin and dideoxyforskolin (at 10 microM) reduced the calcium transient in response to nicotine. These results support a model in which elevation of cyclic AMP inhibits the activation of nicotinic receptors, but augments stimulus secretion coupling downstream of calcium entry. The data, however, do not indicate a simple relationship between total intracellular cyclic AMP levels and the attenuation of nicotinic stimulation of release.

4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone↗
PubMed1987

Stimulus secretion coupling in cultured chromaffin cells. Dependency on external sodium and on dihydropyridine-sensitive calcium channels.

Stimulus secretion in response to acetylcholine (or nicotine) in adrenal chromaffin cells involves calcium influx and is tetrodotoxin insensitive. The mechanism by which activation of the nicotinic acetylcholine receptor (AChR) leads to calcium influx is not clear, and the requirement for external sodium is controversial. We report that when bovine chromaffin cells are continuously perfused in the absence of sodium, secretion of catecholamines in response to 3 X 10(-5) M nicotine is not impaired. Also, secretion stimulated by this concentration of nicotine is not potently and stereospecifically inhibited by the (+)- and (-)-isomers of nicardipine. At a concentration of (+)-nicardipine (10(-7) M) which inhibits most of the release stimulated by 50 mM potassium, the response to 3 X 10(-5) M nicotine remains; however, the dose-response curve to nicotine is shifted slightly to the left. The results are discussed with respect to models of stimulus secretion coupling in bovine adrenal chromaffin cells.

Acetylcholine↗
PubMed1987

Peptide E and its products, BAM 18 and Leu-enkephalin, in bovine adrenal medulla and cultured chromaffin cells: release in response to stimulation.

Peptide E is a 25 amino acid opioid peptide which, if cleaved at the sole double basic (Lys-Arg) typical processing site, would generate two opioid fragments, the amino-terminal fragment BAM 18 and the carboxy-terminal fragment Leu-enkephalin. We have analysed extracts of bovine adrenal medulla in order to quantify these three opioid peptides (peptide E, BAM 18, and Leu-enkephalin). Here we present evidence that BAM 18 and Leu-enkephalin were present in similar amounts, whereas peptide E was present at a higher concentration. This is consistent with previous observations showing a preferential accumulation of larger peptides in the bovine adrenal, and also with the Lys-Arg bond being the principal site of cleavage of peptide E. However, when bovine adrenal chromaffin cells were maintained in culture for several days, Leu-enkephalin was found to be present in much greater amounts than was BAM 18-like immunoreactivity. The molar amounts of peptide E still exceeded the estimated levels of BAM 18 and Leu-enkephalin. We provide evidence that under conditions of basal release BAM 18 and peptide E were released, whereas Leu-enkephalin was released in much smaller amounts, if at all. On stimulation with nicotine results were consistent with an increased release of all three peptides with a preferential stimulation of Leu-enkephalin release. Under all conditions, the molar amounts of peptide E released apparently exceeded that of the other peptides. The results are discussed in terms of the regulation of partial proteolysis and the fate of peptide E.

Adrenal Medulla↗
PubMed1987

Secretion of [Met]enkephalyl-Arg6-Phe7-related peptides and catecholamines from bovine adrenal chromaffin cells: modification by changes in cyclic AMP and by treatment with reserpine.

Investigations into the effects of culturing bovine adrenal chromaffin cells in the presence (72 h) of dibutyryl cyclic AMP, forskolin, and reserpine on the level and release of [Met]enkephalyl-Arg6-Phe7 immunoreactivity, noradrenaline, and adrenaline are reported. The assay for [Met]enkephalyl-Arg6-Phe7 immunoreactivity recognises both peptide B, the 31-amino acid carboxy-terminal segment of proenkephalin, and its heptapeptide fragment, [Met]enkephalyl-Arg6-Phe7. Treatments that elevate cyclic AMP increase the amount of peptide immunoreactivity in these cells; this is predominantly peptide B-like immunoreactivity in both control cells and cyclic AMP-elevated cells. Treatment with reserpine gives no change in total immunoreactivity levels, but does not result in increased accumulation of the heptapeptide [Met]enkephalyl-Arg6-Phe7 at the expense of immunoreactivity that elutes with its immediate precursor, peptide B. Cyclic AMP treatment causes either no change or a decrease in levels of accumulated noradrenaline and adrenaline. However, the release of [Met]enkephalin-Arg6-Phe7 immunoreactivity, noradrenaline, and adrenaline is increased by 72-h pretreatment with forskolin or dibutyryl cyclic AMP, whether release is stimulated by nicotine or elevated potassium. In each case the molecular form of [Met]enkephalyl-Arg6-Phe7 immunoreactivity that is released approximately reflects the cell content. Pretreatment with reserpine has no effect on the total [Met]enkephalyl-Arg6-Phe7 immunoreactivity released, but does result in an increased release of the heptapeptide and a decrease in release of peptide B-like immunoreactivity. The studies suggest that the levels of [Met]enkephalyl-Arg6-Phe7 and peptide B available for release are controlled both at the level of proenkephalin synthesis and at the level of double-basic residue proteolysis.

Adrenal Medulla↗
PubMed1986

Met-enkephalyl-Arg6-Phe7 immunoreactivity in a human neuroblastoma cell line: effect of dibutyryl 3':5'-cyclic AMP and reserpine.

The carboxy terminal part of the proenkephalin A sequence is the 31 amino acid peptide B, which has as its final seven amino acids the sequence of the opioid peptide Met-enkephalyl-Arg6-Phe7. Using a radioimmunoassay which recognises both these peptides we have investigated the relative amounts of peptide B and Met-enkephalyl-Arg6-Phe7 in a human neuroblastoma cell line. We show that these cells contain peptide B-like immunoreactivity but not its heptapeptide fragment. This may be due to lack of proteolytic activity cleaving Met-enkephalyl-Arg6-Phe7 from its precursor, peptide B. On treatment with dibutyryl cyclic AMP the level of immunoreactivity approximately doubles, due to increased amounts of peptide B-like immunoreactivity. Treatment with reserpine, which increases conversion of peptide B to the heptapeptide in bovine chromaffin cells in culture does not stimulate the accumulation of Met-enkephalyl-Arg6-Phe7 in the human neuroblastoma cells. The results are discussed with respect to peptide processing.

1-Methyl-3-isobutylxanthine↗
PubMed1986

Breakdown of small enkephalin derivatives and adrenal peptide E by human plasma.

To provide information concerning the fate of opioid peptides introduced into the circulation of man, we have investigated the breakdown of the following peptides when incubated in human plasma: [Met]enkephalin, [Met]enkephalyl-Arg6-Phe7, [Met]enkephalyl-Arg6-Gly7-Leu8, [Met]enkephalyl-Arg6-Arg7-Val8-NH2 and the complex opioid, peptide E. We used a radioimmunoassay recognizing the amino-terminus of opioid peptides (assay for total opioid peptide-like immunoreactivity). The three small enkephalin derivatives were broken down considerably faster than the enkephalins themselves. The rate of loss of immunoreactivity was considerably reduced by bacitracin. When peptide E was incubated in human plasma, a relatively sustained level of opioid peptide-like immunoreactivity was seen. This was shown to be due, not only to the relatively slow aminopeptidase attack of the larger peptide, but also to the generation during breakdown of peptide E of [Leu]enkephalin and [Met]-enkephalin.

Bacitracin↗
PubMed1986

Release of catecholamines from superfused bovine adrenal chromaffin cells cultured on microcarrier beads.

A procedure is described for the establishment of stable primary cultures of bovine chromaffin cells on microcarrier beads. The cells flatten and send out processes with varicosities over a few days and maintain their catecholamine content for 2 weeks. The beads may be incorporated into a superfusion apparatus with a chamber volume of about 150 microliters, enabling the efficient perfusion of a high density of cells. The response to the introduction of nicotine and high potassium into the perfusing medium is shown to be more rapid and more transient than hitherto described, with each secretagogue producing a different degree of preferential stimulation of noradrenaline-secreting cells.

Adrenal Medulla↗
PubMed1985

Metorphamide, a C-terminally amidated opioid peptide, in human adrenal and human phaeochromocytoma.

There appears to be only one possible site for the production of an amidated peptide in the human proenkephalin sequence; this will give rise to the peptide named metorphamide. Since amidation of peptides is commonly an activation step in the synthesis of regulatory peptides, we have examined the levels and form of immunoreactivity to metorphamide in human post-mortem adrenal and phaeochromocytoma extracts. In three out of four post-mortem adrenal extracts, and in each of the two phaeochromocytoma extracts examined, there was 3-4 times more immunoreactivity to the carboxy-terminus of pro-enkephalin, Met-enkephalin(Arg6,Phe7), than to metorphamide. The metorphamide immunoreactivity was shown in each extract to measure only the amidated octapeptide according to gel exclusion and reverse-phase chromatography data. The implications for processing of proenkephalin in human adrenal are indicated.

Adrenal Gland Neoplasms↗
PubMed1985

Opioid peptides in human adrenal: partial characterization and presence of adrenal peptide E.

Evaluation of hypotheses concerning the role of opioid peptides in the human adrenal is handicapped by the lack of information concerning the nature of these peptides. We studied the content of opioid peptides in whole adrenal tissue using several RIAs, including one which cross-reacts with all opioid peptides tested. Opioid peptides were localized to granules which behaved like chromaffin granules on crude sucrose density separation. beta-Endorphin immunoreactivity was a minor component, which was found principally in the form of beta-endorphin-(1-31). The majority of the remaining peptides probably were products of proenkephalin. The human postmortem tissue differed from bovine tissue in that the major accumulating products of this precursor were the size of the enkephalins and their small congeners, and not the intermediate-sized (mol wt, approximately 1500-4000) peptides that predominated in bovine tissue. We also found evidence of the presence of the 25-amino acid complex opioid, peptide E, in human tissue.

Adolescent↗