Ryanodine reduces the amount of calcium in intracellular stores of smooth-muscle cells of the rabbit ear artery

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Abstract
We have investigated the mechanism of action of ryanodine on intact and skinned smooth-muscle cells of the rabbit ear artery. The amplitude of the phasic response induced by low noradrenaline (NA) concentrations (100 nM) was not affected. The phasic contractions induced by both low and high NA concentrations in Ca2+-free solution containing 2 mM EGTA were suppressed by 10 μM ryanodine. The rate of45Ca efflux in Krebs solution was enhanced by 10 μM ryanodine, while the increased45Ca efflux induced by 10 μM NA was inhibited by ryanodine. 10 μM ryanodine did not affect the contractile proteins in saponin-treated smoothmuscle cells. The intracellular Ca2+ stores of these skinned cells could be filled by exposing these cells to a solution containing 0.6 μM Ca2+. After a wash in a Ca2+-free solution, a contraction due to a release of the accumulated Ca2+ could be induced by either 25 mM caffeine or 20 μM inositol 1,4,5-trisphosphate (InsP3) or 10 μM A23187. These contractions did not occur if 10 μM ryanodine was present during Ca2+ loading. The addition of ryanodine during the Ca2+-free wash did not affect the subsequent force development. These observations indicate that ryanodine, in the presence of Ca2+, depletes the intracellular Ca2+ stores, and that this depletion is responsible for the inhibition of the component of the NA-induced contraction which depends on the release of Ca2+ from intracellular stores.