Initial calcium binding rates of canine cardiac relaxing system (sarcoplasmic reticulum fragments) determined by stopped-flow spectrophotometry

A quench-flow kinetic investigation of calcium ion accumulation by isolated cardiac sarcoplasmic reticulum. Dependence of initial velocity on free calcium ion concentration and influence of preincubation with a protein kinase, MgATP, and cyclic AMP

Ca2+ accumulation at pH 6.8 by isolated rabbit heart microsomes derived chiefly from sarcoplasmic reticulum was investigated by a quench-flow technique. The reaction was terminated at preset times by addition to the reaction mixture of an equal volume of 10 to 50 mM ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid buffered at pH 6.0. The initial velocity of Ca2+ accumulation by microsomal preparations exhibiting a steady state Ca2+ accumulation of 25.6 nmol Ca2+/mg increased from 3.67 to 33.4 nmol Ca2+/mg - s as the free Ca2+ concentration was raised from 0.2 to 18.9 muM. Preincubation of the cardiac microsomes with a partly purified soluble cardiac cyclic AMP-dependent protein kinase, MgATP, and cyclic AMP lead to a significant increase in the initial Ca2+ accumulation rate. The amounts of Ca2+ that were found to accumulate in the first 200 ms of the reaction are comparable to the quantities of the ion that according to literature data need to be removed from the myofilaments and the myoplasm for induction of relaxation of the myocardial fibers.

Intact vesicles of canine cardiac sarcolemma: evidence from vectorial properties of Na+, K+-ATPase

Most biological membranes are functionally asymmetric. To study biochemical control of cardiac transsarcolemmalion fluxes, it would be of obvious advantage to use isolated vesicles of sarcolemma which retains the low passive permeability characteristics of intact sarcolemma because in such vesicles the membrane should exhibit its normal asymmetric character with respect to enzymic activities. The purpose of this investigation was to attempt identify such vesicles in a cardiac microsomal (membrane vesicular) preparation. We studied activation by Na+ and K+ of Na+, K+-ATPase and its associated K+-phosphatase activities, using as substrates ATP or p-nitrophenylphosphate (pNPP) in the presence of Mg2+. Optimal concentrations of K+ alone (10 mM) stimulated p-nitrophenylphosphatase (pNPPase) activity 1.8-fold, and over 80% of the increase could be inhibited by ouabain. Optimal Na+ plus K+ concentrations (100 mM and 10 mM, respectively) stimulated the rate of ATP hydrolysis 2-fold, but only 11 +/- 1.1% of the increased activity was ouabain-sensitive. Optimal pretreatment with sodium dodecyl sulfate (SDS) (0.3 mg/ml) rendered both activities completely sensitive to inhibition by ouabain and reduced the basal Mg2+-ATPase activity by 70-90%. The K+-stimulated pNPPase activity doubled after preincubation in SDS, but the ATPase activity stimulated by Na+ plus K+ fell by 50% under these conditions. A similar pattern of apparent activation was produced by preincubation with deoxycholate (DOC), except that basal Mg2+-dependent activities were resistant to destruction by this detergent. The incremental responses to activation by ions and substrates, and inhibition by oubain, are consistent with the hypothesis that permeability-intact vesicles of sarcolemma are present in the isolated preparation, and that detergent activation renders the vesicles highly permeable to the ions, substrates, and ouabain.

Effect of anti-inflammatory drugs on the binding of calcium to cellular membranes in various human and guinea-pig tissues

1. Cellular membranes were obtained by centrifugation of homogenates of various human and guinea-pig tissues. These membranes took up added calcium (Ca) when incubated at 25 degrees C for 10 min in histidine buffer at pH 7.4. The uptake of Ca was increased by the presence of adenosine triphosphate (ATP).2. Evidence is presented that the membrane fragments which took up Ca under these conditions were derived from the endoplasmic reticulum.3. The ATP-dependent uptake of Ca by membranes derived from guinea-pig aortic or gastric smooth muscle or from human umbilical arterial muscle was inhibited by indomethacin in a concentration-dependent manner.4. The ATP-dependent uptake of Ca by membranes derived from heart, brain, skeletal muscle or liver of the guinea-pig was not inhibited by indomethacin.5. Endothelial cells were obtained from the lining of the human umbilical vein. Cellular membranes were prepared from the endothelial cells by homogenization and centrifugation. The endothelial membranes took up added Ca in the presence of added ATP and this uptake was inhibited by indomethacin and also by members of a group of pharmacologically-related drugs.