LUMINAL CALCIUM REGULATES CALCIUM-RELEASE IN TRIADS ISOLATED FROM FROG AND RABBIT SKELETAL-MUSCLE

Triads isolated from frog and rabbit skeletal muscle were equilibrated with different external [Ca2+], ranging from 0.025 to 10 mM. Vesicula r calcium increased with external [Ca2+] as the sum of a linear plus a saturable component; the latter, which vanished after calsequestrin r emoval, displayed B-max values of 182 and 132 nmol of calcium/mg of pr otein, with K-d values of 1.21 and 1.14 mM in frog and rabbit vesicles , respectively. The effect of luminal [Ca2+ on release kinetics in tri ads from frog and rabbit skeletal muscle was investigated, triggering release with 2 mM ATP, pCa 5, pH 6.8. In triads from frog, release rat e 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 preparatio ns, k values remained relatively constant (10-12 s(-1)) at higher lumi nal [Ca2+], with a small decrease at 10 mM. initial release rates incr eased with luminal [Ca2+] in both preparations; in triads from rabbit the increase was hyperbolic, and in triads from frogs the increase was sigmoidal. These results indicate that, although triads from frog and rabbit respond differently, in both preparations luminal [Ca2+] has a distinctive effect on release, presumably by regulating sarcoplasmic reticulum calcium channels.