Bo Jensen - Academia.edu (original) (raw)

Papers by Bo Jensen

Research paper thumbnail of Functional expression of the murine D2, D3 and D4 dopamine receptors in Xenopus laevis oocytes

FEBS Letters, 1997

were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficientl... more were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficiently expressed in Xenopus oocytes and were shown to bind the D2 antagonist [ IPS I]sulpride. They were all shown to activate Cl 3 influx upon agonist stimulation. Using the diagnostic inhibitor bumetanide, we were able to separate the Na + /K + /2Cl 3 cotransporter component of the Cl 3 influx from the total unidirectional Cl 3 influx. The D Qv subtype was found to operate exclusively through the bumetanide-insensitive Cl 3 influx whereas the other D2-type receptors acted on the Na + /K + /2Cl 3 cotransporter as well. The pertussis toxin sensitivity of the receptor-activated chloride influx via the Na + /K + /2Cl 3 cotransporter varied between the various D2-type receptors showing that they may couple to different G proteins, and activate different second messenger systems.

Research paper thumbnail of Occlusion of K+ in the Na+/K+/2Cl− cotransporter of Ehrlich ascites tumor cells

Biochimica et Biophysica Acta (BBA) - Biomembranes, 1996

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor ce... more Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb +. K* displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K + occlusion. The following observations indicate that the Na+/K+/2C1-cotransporter is responsible for this occlusion: (1) Na + does not compete for the K ÷ binding site, but rather stimulates 86Rb+ occlusion. (2) K ÷ occlusion saturates with increasing [Na +] and [K+], the respective Ko. 5 values being 50 + 7/xM for Na ÷ and 371 ___ 63 /xM for K ÷. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0. 5 value for K ÷ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na ÷. (4) The K + occlusion is sensitive to [C1-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2C1-cotransporter.

Research paper thumbnail of Na+-K+-2Cl−cotransport in Ehrlich cells: regulation by protein phosphatases and kinases

American Journal of Physiology-Cell Physiology, 1998

To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl−c... more To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl−cotransporter in Ehrlich cells, the effect of various PK and PP inhibitors was examined. The PP-1, PP-2A, and PP-3 inhibitor calyculin A (Cal-A) was a potent activator of Na+-K+-2Cl−cotransport (EC50= 35 nM). Activation by Cal-A was rapid (<1 min) but transient. Inactivation is probably due to a 10% cell swelling and/or the concurrent increase in intracellular Cl−concentration. Cell shrinkage also activates the Na+-K+-2Cl−cotransport system. Combining cell shrinkage with Cal-A treatment prolonged the cotransport activation compared with stimulation with Cal-A alone, suggesting PK stimulation by cell shrinkage. Shrinkage-induced cotransport activation was pH and Ca2+/calmodulin dependent. Inhibition of myosin light chain kinase by ML-7 and ML-9 or of PKA by H-89 and KT-5720 inhibited cotransport activity induced by Cal-A and by cell shrinkage, with IC50values similar to reported inhibit...

Research paper thumbnail of Hypertonicity enhances expression of functional Na+/K+/2Cl− cotransporters in Ehrlich ascites tumour cells

Biochimica et Biophysica Acta (BBA) - Biomembranes, 1997

Ehrlich cells exposed to a hypertonic medium for five hours respond by an increased expression of... more Ehrlich cells exposed to a hypertonic medium for five hours respond by an increased expression of Na q rK q r2Cl y cotransport proteins as estimated from immunoprecipitations using polyclonal anti-cotransporter antibodies. The 3.4-fold increase in cotransport expression is followed by a concomitant 2.6-fold increase in the maximal bumetanide-sensitive K q influx during regulatory volume increase, indicating a 2.6-fold increase in the number of functional cotransporters in the plasma membrane. q 1997 Elsevier Science B.V.

Research paper thumbnail of Activation of protein kinase C during cell volume regulation in Ehrlich mouse ascites tumor cells

We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2C1-cotranspo... more We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2C1-cotransporter is activated during regulatory volume decrease after cell shrinkage (hypertonic conditions) as well as during the late phase of regulatory volume decrease (hypotonic conditions). It is, however, quiescent under isotonic conditions. Using a protein kinase C assay system (Amersham, UK) it is here demonstrated that hypertonic cell shrinkage results in an increase in protein kinase C activity to 174% within the first minute, concomitant with the activation of the Na+,K÷,2C1-cotransporter. Hypotonic cell swelling results in a late activation of protein kinase C concomitant with a late activation of the Na+,K+,2CI-cotransporter. The activation of protein kinase C during hypertonic as well as hypotonic conditions is inhibited by H-7. The more specific protein kinase C inhibitor chelerythrine inhibited protein kinase C as well as the Na÷,K+,2C1-cotransporter to the same extent as did H-7. These results indicate the involvement of protein kinase C in the regulation of the Na +,K+,2C1-cotransporter in Ehrlich ascites tumor cells during cell volume regulation.

Research paper thumbnail of Functional expression of the murine D2, D3 and D4 dopamine receptors in Xenopus laevis oocytes

FEBS Letters, 1997

were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficientl... more were expressed in Xenopus laevis oocytes. The D2-type receptors were all similarly and efficiently expressed in Xenopus oocytes and were shown to bind the D2 antagonist [ IPS I]sulpride. They were all shown to activate Cl 3 influx upon agonist stimulation. Using the diagnostic inhibitor bumetanide, we were able to separate the Na + /K + /2Cl 3 cotransporter component of the Cl 3 influx from the total unidirectional Cl 3 influx. The D Qv subtype was found to operate exclusively through the bumetanide-insensitive Cl 3 influx whereas the other D2-type receptors acted on the Na + /K + /2Cl 3 cotransporter as well. The pertussis toxin sensitivity of the receptor-activated chloride influx via the Na + /K + /2Cl 3 cotransporter varied between the various D2-type receptors showing that they may couple to different G proteins, and activate different second messenger systems.

Research paper thumbnail of Occlusion of K+ in the Na+/K+/2Cl− cotransporter of Ehrlich ascites tumor cells

Biochimica et Biophysica Acta (BBA) - Biomembranes, 1996

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor ce... more Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb +. K* displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K + occlusion. The following observations indicate that the Na+/K+/2C1-cotransporter is responsible for this occlusion: (1) Na + does not compete for the K ÷ binding site, but rather stimulates 86Rb+ occlusion. (2) K ÷ occlusion saturates with increasing [Na +] and [K+], the respective Ko. 5 values being 50 + 7/xM for Na ÷ and 371 ___ 63 /xM for K ÷. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0. 5 value for K ÷ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na ÷. (4) The K + occlusion is sensitive to [C1-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2C1-cotransporter.

Research paper thumbnail of Na+-K+-2Cl−cotransport in Ehrlich cells: regulation by protein phosphatases and kinases

American Journal of Physiology-Cell Physiology, 1998

To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl−c... more To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl−cotransporter in Ehrlich cells, the effect of various PK and PP inhibitors was examined. The PP-1, PP-2A, and PP-3 inhibitor calyculin A (Cal-A) was a potent activator of Na+-K+-2Cl−cotransport (EC50= 35 nM). Activation by Cal-A was rapid (<1 min) but transient. Inactivation is probably due to a 10% cell swelling and/or the concurrent increase in intracellular Cl−concentration. Cell shrinkage also activates the Na+-K+-2Cl−cotransport system. Combining cell shrinkage with Cal-A treatment prolonged the cotransport activation compared with stimulation with Cal-A alone, suggesting PK stimulation by cell shrinkage. Shrinkage-induced cotransport activation was pH and Ca2+/calmodulin dependent. Inhibition of myosin light chain kinase by ML-7 and ML-9 or of PKA by H-89 and KT-5720 inhibited cotransport activity induced by Cal-A and by cell shrinkage, with IC50values similar to reported inhibit...

Research paper thumbnail of Hypertonicity enhances expression of functional Na+/K+/2Cl− cotransporters in Ehrlich ascites tumour cells

Biochimica et Biophysica Acta (BBA) - Biomembranes, 1997

Ehrlich cells exposed to a hypertonic medium for five hours respond by an increased expression of... more Ehrlich cells exposed to a hypertonic medium for five hours respond by an increased expression of Na q rK q r2Cl y cotransport proteins as estimated from immunoprecipitations using polyclonal anti-cotransporter antibodies. The 3.4-fold increase in cotransport expression is followed by a concomitant 2.6-fold increase in the maximal bumetanide-sensitive K q influx during regulatory volume increase, indicating a 2.6-fold increase in the number of functional cotransporters in the plasma membrane. q 1997 Elsevier Science B.V.

Research paper thumbnail of Activation of protein kinase C during cell volume regulation in Ehrlich mouse ascites tumor cells

We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2C1-cotranspo... more We have previously demonstrated that in Ehrlich cells a bumetanide-sensitive Na+,K+,2C1-cotransporter is activated during regulatory volume decrease after cell shrinkage (hypertonic conditions) as well as during the late phase of regulatory volume decrease (hypotonic conditions). It is, however, quiescent under isotonic conditions. Using a protein kinase C assay system (Amersham, UK) it is here demonstrated that hypertonic cell shrinkage results in an increase in protein kinase C activity to 174% within the first minute, concomitant with the activation of the Na+,K÷,2C1-cotransporter. Hypotonic cell swelling results in a late activation of protein kinase C concomitant with a late activation of the Na+,K+,2CI-cotransporter. The activation of protein kinase C during hypertonic as well as hypotonic conditions is inhibited by H-7. The more specific protein kinase C inhibitor chelerythrine inhibited protein kinase C as well as the Na÷,K+,2C1-cotransporter to the same extent as did H-7. These results indicate the involvement of protein kinase C in the regulation of the Na +,K+,2C1-cotransporter in Ehrlich ascites tumor cells during cell volume regulation.