Luminal calcium regulates calcium release in triads isolated from frog and rabbit skeletal muscle

Abstract

Triads isolated from frog and rabbit skeletal muscle were equilibrated with different external [Ca2+], ranging from 0.025 to 10 mM. Vesicular calcium increased with external [Ca2+] as the sum of a linear plus a saturable component; the latter, which vanished after calsequestrin removal, displayed Bmax values of 182 and 132 nmol of calcium/mg of protein, with Kd values of 1.21 and 1.14 mM in frog and rabbit vesicles, respectively. The effect of luminal [Ca2+] on release kinetics in triads from frog and rabbit skeletal muscle was investigated, triggering release with 2 mM ATP, pCa 5, pH 6.8. In triads from frog, release rate constant (k) values increased sixfold after increasing luminal [Ca2+] from 0.025 to 3 mM. In triads from rabbit, k values increased 20-fold when luminal [Ca2+] increased from 0.05 to 0.7 mM. In both preparations, k values remained relatively constant (10–12 s-1) at higher luminal [Ca2+], with a small decrease at 10 mM. Initial release rates increased with luminal [Ca