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| 2. |
- Gomez, Maria, et al.
(författare)
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Endogenous polyamines modulate Ca2+ channel activity in guinea-pig intestinal smooth muscle
- 1999
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Ingår i: Pflügers Archiv. - : Springer Science and Business Media LLC. - 0031-6768. ; 438:4, s. 445-451
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Tidskriftsartikel (refereegranskat)abstract
- The polyamines spermine and spermidine inhibit L-type Ca2+ channels in whole-cell recordings from guinea-pig ileum cells (Gomez and Hellstrand, Pflugers Arch, 430:501-507, 1995 [4]). To study whether they modulate channel activity under physiological conditions, we further investigated their actions on Ca2+ channels and the effects of altered cellular polyamine contents. In inside-out patches, spermine (0.1-1 mM) inhibited channel activity without affecting the amplitude of unitary currents. In cell-attached recordings, addition of spermine to the bath did not influence channel activity in the patch, indicating that its extracellular action is direct and not mediated via passage of the polyamine through the cell membrane. Cellular contents of spermidine and spermine were decreased by about 50% by organ culture of ileum strips for 5 days with the adenosylmethionine decarboxylase inhibitor CGP 48664 (10 microM). This caused enhanced channel activity in cell-attached recordings, suggesting a reduced level of channel block by endogenous polyamines compared with control cells. Whole-cell recordings in the perforated patch mode showed increased current in polyamine-depleted cells, while this was not seen when cells were dialysed with the pipette solution. We conclude that polyamines block Ca2+ channels from the inside as well as the outside of the cell membrane, and that endogenous polyamines in smooth muscle modulate Ca2+ channel activity.
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| 3. |
- Gromada, J, et al.
(författare)
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Multisite regulation of insulin secretion by cAMP-increasing agonists : evidence that glucagon-like peptide 1 and glucagon act via distinct receptors.
- 1997
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Ingår i: Pflügers Archiv. - 0031-6768 .- 1432-2013. ; 434:5, s. 515-24
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Tidskriftsartikel (refereegranskat)abstract
- The mechanisms by which glucagon-like peptide 1(7-36)amide (GLP-1[7-36]amide) potentiates insulin secretion were investigated by measurements of whole-cell K+ and Ca2+ currents, membrane potential, the cytoplasmic Ca2+ concentration ([Ca2+]i) and exocytosis in mouse pancreatic B-cells. GLP-1(7-36)amide (10 nM) stimulated glucose-induced (10 mM) electrical activity in intact pancreatic islets. The effect was manifested as a 34% increase in the duration of the bursts of action potentials and a corresponding 28% shortening of the silent intervals. GLP-1(7-36)amide had no effect on the electrical activity at subthreshold glucose concentrations (< or = 6.5 mM). In cultured B-cells, GLP-1(7-36)amide produced a decrease of the whole-cell ATP-sensitive K+ (KATP) conductance remaining at 5 mM glucose by approximately 30%. This effect was associated with membrane depolarization and the initiation of electrical activity. GLP-1(7-36)amide produced a protein-kinase-A-(PKA-) and glucose-dependent fourfold potentiation of Ca(2+)-induced exocytosis whilst only increasing the Ca2+ current marginally. The stimulatory action of GLP-1(7-36)amide on exocytosis was mimicked by the pancreatic hormone glucagon and exendin-4, a GLP-1 receptor agonist. Whereas the stimulatory action of GLP-1(7-36)amide could be antagonized by exendin-(9-39), this peptide did not interfere with the ability of glucagon to stimulate exocytosis. We suggest that GLP-1(7-36)amide and glucagon stimulate insulin secretion by binding to distinct receptors. The GLP-1(7-36)amide-induced stimulation of electrical activity and Ca2+ influx can account for (maximally) a doubling of insulin secretion. The remainder of its stimulatory action results from a cAMP/PKA-dependent potentiation of Ca(2+)-dependent exocytosis exerted at a stage distal to the elevation of [Ca2+]i.
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| 6. |
- Malmqvist, Ulf, et al.
(författare)
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Regulation of force and shortening velocity by calcium and myosin phosphorylation in chemically skinned smooth muscle
- 1996
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Ingår i: Pflügers Archiv. - : Springer Science and Business Media LLC. - 0031-6768. ; 433:1-2, s. 42-48
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Tidskriftsartikel (refereegranskat)abstract
- The phosphatase inhibitor okadaic acid (OA) was used to study the relationship between [Ca2+], rates of phosphorylation/dephosphorylation and the mechanical properties of smooth muscle fibres. Force/velocity relationships were determined with the isotonic quick release technique in chemically skinned guinea-pig taenia coli muscles at 22 degrees C. In the maximally thiophosphorylated muscle neither OA (10 microM) nor Ca2+ (increase from pCa 9.0 to pCa 4.5) influenced the force-velocity relationship. When the degree of activation was altered by varying [Ca2+] in the presence of 0.5 microM calmodulin, both force and the maximal shortening velocity (Vmax) were altered. At pCa 5.75, at which force was about 35% of the maximal at pCa 4.5, Vmax was 55% of the maximal value. When OA was introduced into fibres at pCa 6.0, force was increased from less than 5% to 100% of the maximal force obtained in pCa 4.5. The relationship between the degree of myosin light chain phosphorylation and force was similar in the two types of activation; varied [OA] at constant [Ca2+] and at varied [Ca2+]. The relation between force and Vmax when the degree of activation was altered with OA was almost identical to that obtained with varied [Ca2+]. The results show that Ca2+ and OA do not influence force or Vmax in the maximally phosphorylated state and suggest that the level of myosin light chain phosphorylation is the major factor determining Vmax. The finding that the relationship between force and Vmax was similar when activation was altered with OA and Ca2+ suggests, however, that alterations in the absolute rates of phosphorylation and dephosphorylation at a constant phosphorylation level do not influence the mechanical properties of the skinned smooth muscle fibres.
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| 7. |
- Malmqvist, Ulf, et al.
(författare)
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The effects of caldesmon extraction on mechanical properties of skinned smooth muscle fibre preparations
- 1996
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Ingår i: Pflügers Archiv. - : Springer Science and Business Media LLC. - 0031-6768. ; 432:2, s. 241-247
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Tidskriftsartikel (refereegranskat)abstract
- The role of caldesmon in the regulation of smooth muscle contraction was investigated in chemically skinned smooth muscle fibres from the guinea-pig taenia coli. A 19-kDa C-terminal fragment of caldesmon gave a minor (<5%) reduction of force in fully thiophosphorylated fibres, but reduced force by about 50% at intermediate activation levels without affecting the level of light chain phosphorylation. An extraction procedure was developed using incubation in solutions containing high Mg2+ concentrations. Protein analysis revealed a selective decrease in the amount of caldesmon in the fibres. Maximal active force per cross-sectional area was unaffected. The Ca2+ dependence of active force was shifted towards lower Ca2+ concentrations and became less steep. The effects of extraction of caldesmon could in part be reversed by incubation in a solution containing purified caldesmon. The results are consistent with the hypothesis that caldesmon in smooth muscle thin filaments inhibits force generation and plays a role in regulating cooperative attachment of cross-bridges at sub-maximal levels of activation in smooth muscle.
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| 9. |
- Tonkonogi, Michail, et al.
(författare)
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Mitochondrial function in human skeletal muscle is not impaired by high intensity exercise.
- 1999
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Ingår i: Pflügers Archiv. - : Springer Science and Business Media LLC. - 0031-6768 .- 1432-2013. ; 437:4, s. 562-8
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Tidskriftsartikel (refereegranskat)abstract
- The hypothesis that high-intensity (HI) intermittent exercise impairs mitochondrial function was investigated with different microtechniques in human muscle samples. Ten male students performed three bouts of cycling at 130% of peak O2 consumption (V.O2,peak). Muscle biopsies were taken from the vastus lateralis muscle at rest, at fatigue and after 110 min recovery. Mitochondrial function was measured both in isolated mitochondria and in muscle fibre bundles made permeable with saponin (skinned fibres). In isolated mitochondria there was no change in maximal respiration, rate of adenosine 5'-triphosphate (ATP) production (measured with bioluminescence) and respiratory control index after exercise or after recovery. The ATP production per consumed oxygen (P/O ratio) also remained unchanged at fatigue but decreased by 4% (P<0.05) after recovery. In skinned fibres, maximal adenosine 5'-diphosphate (ADP)-stimulated respiration increased by 23% from rest to exhaustion (P<0.05) and remained elevated after recovery, whereas the respiratory rates in the absence of ADP and at 0.1 mM ADP (submaximal respiration) were unchanged. The ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index) decreased at fatigue (P<0.05) but after the recovery period was not significantly different from that at rest. It is concluded that mitochondrial oxidative potential is maintained or improved during exhaustive HI exercise. The finding that the sensitivity of mitochondrial respiration to ADP is reversibly decreased after strenuous exercise may indicate that the control of mitochondrial respiration is altered.
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