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Biomedical subjects

R H Michell

Publications and source records attributed to R H Michell.

At least 163 records · Page 9Linked to original sources
PubMed1976

Effects of calcium-antagonistic drugs on the stimulation by carbamoylcholine and histamine of phosphatidylinositol turnover in longitudinal smooth muscle of guinea-pig ileum.

A number of drugs classed as calcium antagonists, spasmolytics, non-specific receptor antagonists or receptor antagonists with multiple sites of action were tested to determine whether they prevent the stimulation of phosphatidylinositol turnover caused in various tissues by the activation of receptors which increase cell-surface Ca2+ permeability. The experiments were done with fragments of longitudinal smooth muscle from guinea-pig ileum; these were incubated in vitro with 32Pi and either 100 muM-carbamoylcholine or 100 muM-histamine, in the presence of antagonistic drugs at concentrations at least sufficient to cause complete blockade of smooth-muscle contraction. The phosphatidylinositol response to carbamoylcholine was not changed by cinchocaine, papaverine, nifedipine, dibenamine, amethocaine, cinnarizine, lidoflazine, methoxyverapamil, prenylamine or two antimuscarinic alkane-bis-ammonium compounds, and the response to histamine was unaffected by the first four drugs. In contrast, phenoxybenzamine prevented the increase in phosphatidylinositol labelling caused by either carbamoylcholine or histamine. The insensitivity of the phosphatidylinositol response to most of the drugs provides further experimental support for the conclusion that the receptor-stimulated phosphatidylinositol breakdown which initiates the increase in phosphatidylinositol turnover is not caused by an increase in intracellular Ca2+. The simplest interpretation of the available information appears to be that phosphatidylinositol breakdown plays a role in the coupling between the receptor-agonist interaction and the opening of cell-surface Ca2+ gates [Michell, R. H. (1975) Biochim. Biophys. Acta 415, 81-147]. If this is correct, then phenoxybenzamine must exert its inhibitory effects on phosphatidylinositol breakdown early in this sequence of events, but the drugs must act at a stage later than phosphatidylinositol breakdown. The unexpected difference in the effects of dibenamine and phenoxybenzamine, which are chemically very similar, may provide a useful experimental tool with which to explore the way in which activated receptors provoke the opening of cell-surface Ca2+ gates.

Anesthetics, Local↗
PubMed1976

Investigation of the relationship between cell-surface calcium-ion gating and phosphatidylinositol turnover by comparison of the effects of elevated extracellular potassium ion concentration on ileium smooth muscle and pancreas.

Incubation of fragments of guinea-pig ileum smooth muscle in the presence of an elevated extracellular K+ concentration, which causes an increase in cell-surface Ca2+ permeability and thus leads to contraction, caused a marked increase in phosphatidylinositol turnover, as assessed by incorporation of 32Pi. This response was not diminished by atropine or propylbenzilycholine mustard, two muscarinic cholinergic antagonists, and was therefore not caused by the release of endogenous acetylcholine within the tissue. In contrast, exposure of guinea-pig pancreas fragments to high extracellular [K+], which does not increase cell-surface Ca2+ permeability or evoke secretion, did not cause an increase in phosphatidylinositol turnover, even though such an increase was triggered by carbamoylcholine, which is a secretagogue. These observations are consistent with a suggested function for phosphatidylinositol breakdown in the mechanisms of cell-surface Ca2+ gates.

Animals↗
PubMed1976

Cholinergically stimulated phosphatidylinositol breakdown in parotid-gland fragments is independent of the ionic environment.

Carbamoylcholine stimulated phosphatidylinositol breakdown in rat parotid-gland fragments incubated in either a Krebs--Ringer medium or an ion-depleted iso-osmotic sucrose medium. This suggest that phosphatidylinositol breakdown is not initiated by a change in the intracellular concentration of Na+ or of K+, and previous experiments have already indicated that it is independent of cyclic nucleotides and Ca2+. Thus it seems that this reaction may be initiated by a direct interaction at the plasma membrane between the activated muscarinic cholinergic receptor system and the enzyme that catalyses phosphatidylinositol breakdown.

Animals↗
PubMed1976

Production of 1,2-diacylglycerol and phosphatidate in human erythrocytes treated with calcium ions and ionophore A23187.

1. When the ionophore A23187 and Ca2+ were added to normal human erythrocytes, the incorporation of 32P into phosphatidate was enhanced within 1 min, but there was only slight labelling of other phospholipids. 2. Labelling of phosphatidate in these cells did not continue to increase after about 20min at 37 degrees C; by this time, radioactivity in phosphatidate was about ten times higher inionophore A23187-treated cells than in controls. A net synthesis of phosphatidate was measured in response to the increase in intracellular Ca2+ concentration; the content of this phospholipid in the cell was increased by approximately 50%. 3. In the presence of 2.5 mM-Ca2+ a maximum effect was seen with about 0.5 mug of ionophore/ml. 4. The concentration of Ca2+ giving half-maximal labelling of phosphatidate in the presence of 10 mug of ionophore A23187/ml was about 10 muM. 5. A rapid decrease of ATP content in the cell occurred in ionophore-treated cells. 6. Labelling of phosphatidate appeared to be secondary to the production of 1,2-diacylglycerol in the cells; accumulation of 1,2-diacylglycerol was only seen after about 15 min. After 60 min, the 1,2-diacylglycerol content of the cells was five to seven times that of untreated control cells. 7. The change in the shape of erythrocytes treated with Ca2+ and ionophore appeared to be related to accumulation of 1,2-diacylglycerol. 8. The source of 1,2-diacylglycerol has not been definitely identified, but its fatty acid compositon was similar to that of phosphatidylcholine. However, it has an unusually high content of hexadecenoic acid, a fatty acid not common in the major erythrocyte phospholipids. 9. Accumulation of 1,2-diacyglycerol also occurred in energy-starved cells, even in the absence of calcium; in this case it appeared to be produced by phosphatidate breakdown.

Adenosine Triphosphate↗
PubMed1976

Preparation of erythrocyte ghosts by a glycol-induced osmotic lysis under isoionic conditions.

A procedure has been developed for obtaining haemoglobin-free, erythrocyte ghosts under ionic conditions approximating that of the cell cytoplasm. Haemolysis was effected by incorporating glycol into cells suspended in the isoionic medium and then diluting with a large volume of glycol-free medium. The ghosts were of uniform spherical shape throughout the preparative procedure and were impermeable to macromolecules. Analysis of polypeptides by sodium dodecyl sulphate-gel electrophoresis at each stage of preparation and comparison with ghosts prepared under hypo-ionic conditions served to distinguish membrane components from those of cytoplasm.

Cell Fractionation↗
PubMed1976

Muscarinic cholinergic stimulation of phosphatidylinositol turnover in the longitudinal smooth muscle of guinea-pig ileum.

1. The metabolism of phosphatidylinositol and phosphatidate was investigated in fragments of longitudinal smooth muscle from guinea-pig ileum incubated with cholinergic and anticholinergic drugs. 2. Incorporation of Pi into these lipids was enhanced by acetylcholine and carbamoylcholine. 3. The receptor responsible for triggering this response was of the muscarinic type, since (a) the response was also produced by the muscarinic agonists acetyl-beta-methylcholine, carbamoyl-beta-methylcholine and pilocarpine, and (b) the response was prevented by atropine and prophylbenzilylcholine mustard, but not by tubocurarine. 4. Increased phosphatidylinositol labellin was clearly observed within 5 min in tissue treated with a high concentration of carbamoylcholine. 5. Halfmaximal stimulation of phosphatidylinositol labelling occurred at approx. 10 muM-muM-carbamoylcholine. 6. Incubation of muscle fragments with carbamoylcholine provoked a decrease in phosphatidylinositol concentration, as would be expected if phosphatidyl-inositol breakdown is the reaction controlled by agonists. 7. This information all appears consistent with the proposal that phosphatidylinositol breakdown may be a reaction intrinsic to the mechanisms of muscarinic cholinergic receptor systems.

Animals↗
PubMed1975

Changes in lipid metabolism and cell morphology following attack by phospholipase C (Clostridium perfringens) on red cells or lymphocytes.

When intact human erythrocytes were treated with phospholipase C (Clostridium perfringens), up to 30% of the membrane phospholipids were broken down without significant cell lysis. Only phosphatidylcholine and sphingomyelin were attacked. Ceramide (derived from sphingomyelin) accumulated, but 1,2-diacylglycerol (derived from phosphatidylcholine) was largely converted into phosphatidate. Up to 12% of the cell phospholipid could be converted into phosphatidate in this way. Pig erythrocytes and lymphocytes showed a similar but smaller synthesis of phosphatidate after phospholipase C attack. Phospholipase C also caused a marked morphological change in erythrocytes, giving rise to spherical cells containing internal membrane vesicles. This change appeared to be due to ceramide and de and diacylglycerol accumulation rather than to increased phosphatidate content of the cells.

Animals↗