Calcium control of passive permeability to calcium in sarcoplasmic reticulum vesicles

Calcium efflux from sarcoplasmic reticulum vesicles that have been equilibrated with 1-100 mM CaCl2 in the absence of ATP has two apparently first order components. The initial calcium content of each component increases with the total Ca content of the sarcoplasmic reticulum, which reaches 5, 24, and 80 nmol/mg of protein after equilibration with 1, 10, and 100 mM CaCl2, respectively. Initial rates of Ca efflux into a medium containing 10 mM EGTA increase in proportion to Ca in the loading medium up to 20 mM. Above 20 mM, efflux from the slow component clearly saturates, whereas efflux from the fast component continues to increase. The rate constant for the smaller, faster component to efflux (k congruent to 0.5 min-1) is not affected by changing the concentration of Ca either inside or outside the vesicles. The rate constant of the larger, slower component (k congruent to 0.05 min-1) is also unaffected by changes in internal Ca concentration. However, external [Ca2+] diminishes the rate constant of the slow component 6-10-fold. Inhibition by external [Ca2+] is characterized by cooperative interaction between two sites with an apparent Kd of 5.3 X 10(-6) M. The two components may represent two populations of sarcoplasmic reticulum vesicles that differ 10-fold in passive permeability to Ca when external [Ca2+] is less than 10(-6) M, and 60-100-fold when external [Ca2+] is greater than 10(-5) M. The passive permeability in one of these populations seems to be regulated by external, high affinity Ca binding sites.