BASIC PROPERTIES OF A NOVEL RYANODINE-SENSITIVE, CAFFEINE-INSENSITIVE CALCIUM-INDUCED CALCIUM-RELEASE MECHANISM IN PERMEABILIZED HUMAN VASCULAR SMOOTH-MUSCLE CELLS

The efflux of Ca-45(2+) from preloaded intracellular stores of saponin-permeabilised human uterine artery smooth muscle cultured cells was used to study the mechanisms underlying Ca2+ release from the sarcoplasmic reticulum (SR). The present paper demonstrates directly a functional Ca2+ release mechanism that is dependent on an increase in free Ca2+ (100 nM-30 mu M) and is completely inhibited by 20 mu M Ruthenium red. The amount of Ca2+ released at 30 mu M free Ca2+ was reduced by approximately 50% compared to the release at 10 mu M. This Ca2+-induced Ca2+ release (CICR) mechanism was not sensitive to caffeine. Exposure of cells to low free Ca2+-containing solutions (10 nM) indicated that a component of the CICR mechanism may be functional at basal free Ca2+ levels of 100 nM. application of ryanodine (0.1-100 mu M) induced Ca-45(2+) efflux from the sarcoplasmic reticulum and this release was also inhibited by 20 mu M Ruthenium red.