In the fragmented sarcoplasmic reticulum from skeletal muscle of rabbits with experimental uremia, defective calcium ion transport is found. An impairment of all parameters is observed (initial rate of uptake, storing capacity with and without oxalate, and concentrating ability). In vivo administration of 1,25-dihydroxycholecalciferol (1,25-(OH)2-vitamin D3)(2 X 27 ng X kg of body wt-1 X day-1 and 6 X 27 ng X kg-1 X day-1, respectively) improved the kinetic parameters. The low dose improved storing capacity, and the higher dose, in addition to the storing capacity, also corrected concentrating ability and the initial rate of uptake. It is concluded that active calcium transport in the sarcoplasmic reticulum is impaired by uremia and that this defect is responsive to the administration of 1,25-(OH)2-vitamin D3.

The Ca-transport system of sarcoplasmic vesicles of rabbits is altered by experimental uremia. 1. The influx rate constant of the experimental membranes decrease with a resulting decrease of the calcium influx rate.2. The experimental membranes transport a smaller amount of Ca2+ per mol of ATP split than the controls, i.e. their transport ratio is decreased.3. The calcium permeability of the experimental membranes increases with a resulting decreased concentrating ability.4. The phosphatide content but not the cholesterol content of the experimental membranes dccreases with a consequent increase of the cholesterol/phosphatide ratio.5. The fatty acid pattern of total phosphatides of the experimental membranes changes. A relative decrease of palmitic acid and oleic acid occurs and a relative increase of stearic, arachidonic and higher unsaturated fatty acids.6. The altered lipid composition of the membranes does not change the temperature dependence of the kinetics, In recent years it was shown that function and membrane composition of the fragment sarcoplasmic reticulum are affected by different models of disease showed kinetic changes of the sarcoplasmic reticulum during experimental uremia. In the following study these changes are analyzed in detail. Alterations of the lipid-protein interactions of the membrane were found which might be relevant for the understanding of disturbances of active transport processes during uremia.The changed lipid-protein interactions, correlated with kinetic changes, represent a new model of these interactions within intact membranes of fragmented sarcoplasmic reticulum which until now was not available in vifro and thus might allow some more insight into the lipid-protein interactions of the membrane. MATERIALS AND METHODSRabbits (2 -2.5 kg) of either sex were made uremic by 516 nephrectomy (removal of the right kidney, ligation of pole arteries of the left kidney under nembutal anaesthesia in one session. Control animals were sham operated. After the operation the rabbits were kept on pair-feeding conditions and sacrified after 7 days. During this time the serum urea level rose from 41.2 k 5.8 mg% to above 80 m g x .The vesicles were prepared from the psoas muscle and the white muscles of the hind legs according to Nagai et al. [9]. Neither a higher degree of protein nor of mitochondria1 impurities of the pathological preparations could be detected by gel-electrophoresis (Fig. I), electron microscopy (courtesy of D r Agostini) or by means of sodium azide, 5 mM of which showed no depressive action on the ATPase activity in both preparations.The ATPase activity of the vesicles was recorded by a pH stat instrument (Radiometer, Copenhagen) which was standardized by parallel P, measurements according to Rockstein and Herron [lo]. The Catransport activity was measured by the millipore filtration technique. For special conditions of the different kinetic measurements see legends.The vesicle lipids were extracted with a 20-fold amount of chloroform/methanol (2/1) and washed according t...

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(1) The enzymatic removal of lipids from the vesicular membranes of the sarcoplasmic reticulum does not interfere with the fluorescence of the 1-anilino-8-naphthalenesulfonate (ANS) vesicular complex. (2) The fluorescence intensity of the ANS vesicular complex is considerably (50%) reduced by oleic acid (0.5MM) because it displaces ANS from its binding sites. (3) Stearic acid, which also combines with the membranes, interferes neither with ANS binding nor with ANS fluorescence. (4) Of all lipid compounds tested, oleylamine produces the most pronounced fluorescence enhancement of ANS. (5) The complexes formed between oleic acid and cetyltrimethyl ammonium salts or between oleic acid and polylysine produce a much higher fluorescence enhancement than the isolated components. (6) Low concentrations of ether added to ANS-containing vesicular suspensions reduce their fluorescence intensity. It returns to the initial intensity when the ether is removed. (7) A small cyclic change of the fluorescence of the vesicular ANS complex takes place during active calcium uptake.