Ion pathways in proteins of the sarcoplasmic reticulum. Carriers and channels in biological systems

The sarcoplasmic reticulum is permeable to a number of ions although the permeability can change depending on the physiological state of the membrane. In the resting state, the sarcoplasmic reticulum is relatively impermeable to Ca2+ but this ion is pumped across the membrane when ATP is hydrolyzed by the (Ca2+ + Mg2+)-dependent ATPase.1 Under the appropriate physiological stimulus, Ca2+ flows out of the sarcoplasmic reticulum at rates greatly exceeding those at which it was pumped inward.2 The membrane has permeability to monovalent cations. These ions appear to flow through sites so sparsely dis-tributed that only about two thirds of any population of small vesicles are permeable to K+ or Na+.3 The membrane is also permeable to anions, and, when Ca2+ uptake is assayed in the presence of phosphate or oxalate, Ca2+ oxalate or Ca2+ phosphate precipitates can be found in the interior of vesicles.4 The sites for anion passage may be blocked by anion transport inhibitors such as SITS.