Regulation of plasma membrane permeability to calcium in primary cultures of rat hepatocytes.
Experiments were designed to characterize the hormone sensitive transport of Ca2+ from the external media into rat hepatocytes maintained in culture. In the absence of added vasopressin, hepatocytes were nearly impermeable to Ca2+, whereas a significant and rapid influx of Ca2+ could be detected when external Ca2+ was added to hepatocytes after the addition of 20 nM vasopressin. The transport was measured as the initial rate of increase of free intracellular Ca2+ [( Ca2+]i) after Ca2+ addition to the external media. Most data were obtained from the majority of cells on a coverslip immersed in a spectrophotometric cuvette, but selected data were obtained by measuring Ca2+ changes in single cells. Ca2+ influx measured using a large number of cells was similar to data obtained using single cells. The Vmax of Ca2+ influx was 140 nM/s. Ca2+ transport was competitive with H+ so that the Km was 17.4 mM at pH 6.8, 3.7 mM at pH 7.4 and 1.8 mM at pH 7.8. Ca2+ influx was insensitive to external K+ (1 to 70 mM) and to the presence of 5 nM valinomycin, suggesting that it was independent of the electrical potential gradient across the plasma membrane. Transport also appeared to be insensitive to the activity of protein kinase C, which was varied by addition of the activator, 12-myristate 13-acetate phorbol ester, and by addition of the kinase inhibitor, staurosporine. Stimulation of transport following vasopressin addition exhibited a delay with a t1/2 of approximately 30 s. A vasopressin antagonist blocked the activation of transport, if added prior to vasopressin. However, experiments designed to determine the effect of hormone occupancy per se on transport activity indicated that continued hormone occupancy was not required. When the external medium was nominally Ca2+ free and an antagonist was added 1 min after vasopressin, Ca2+ entry, even 8 min after antagonist addition, was rapid. Conversely, preincubation with vasopressin antagonist in medium containing 0.5 mM Ca2+ dramatically lowered plasma membrane Ca2+ permeability. The ER Ca2+ pool emptied by vasopressin was refilled in the presence of vasopressin antagonist plus 0.5 mM Ca2+, but did not refill when the medium contained no added Ca2+. Under the conditions of these experiments, the Ca2+ levels of the ER hormone-sensitive Ca2+ pool were estimated as well as intracellular concentrations of inositol-1,4,5-trisphosphate. The Ca2+ levels of the endoplasmic reticulum correlated inversely with plasma membrane Ca2+ permeability, whereas cellular concentrations of inositol-1,4,5-trisphosphate did not correlate with Ca2+ permeability.(ABSTRACT TRUNCATED AT 400 WORDS)
Zhang
Y
Y
Department of Cellular and Molecular Physiology, College of Medicine, Pennsylvania State University, Hershey.
Duszynski
J
J
Hreniuk
S
S
Waybill
M M
MM
LaNoue
K F
KF
D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000032:analysis / Q000378:metabolism / Q000493:pharmacokinetics*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002463:Cell Membrane Permeability / Q000502:physiology*; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000502:physiology; D008099:Liver / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D010770:Phosphotransferases / Q000502:physiology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D051381:Rats; D011919:Rats, Inbred Strains; D011945:Receptors, Angiotensin / Q000502:physiology; D017483:Receptors, Vasopressin; D013053:Spectrophotometry; D019311:Staurosporine; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000470:Alkaloids; D011945:Receptors, Angiotensin; D017483:Receptors, Vasopressin; D010770:Phosphotransferases; D011493:Protein Kinase C; D019311:Staurosporine; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium