Reversal of decreased phosphorylation of sarcoplasmic reticulum calcium transport ATPase by 1,25-dihydroxycholecalciferol in experimental uremia

Abstract

When compared to that from shamoperated controls, sarcoplasmic reticulum isolated from skeletal muscle of uremic rabbits had a lower rate of calcium uptake and storing capacity. In vivo administration of 1,25-dihydroxycholecalciferol [1,25(OH)₂D₃] restored the values in uremic animals toward normal. To obtain information about the mechanisms responsible for these differences, phosphorylation of the calcium transport ATPase was studied. The steady-state levels of phosphoprotein in uremic membranes were lower and returned to normal when the secosteroid was administered. Electrophoresis of the membranes phosphorylated with³²P-inosine triphosphate (³²P-ITP) showed that the differences were related to a 100,000 dalton protein. The rate of phosphoprotein formation, determined with³²P-ITP and at 0°C, was considerably lower in uremic than in control animals. Pretreatment with 1,25(OH)₂D₃ prevented this change. The hypothesis is advanced that the vitamin D metabolite affects the steady-state concentration and rate constant of formation of active sites in the Ca-ATPase. These results may partly explain the altered Ca transport function of the sarcoplasmic reticulum in experimental uremia.