AlF 4 − induces Ca2+ oscillations in guinea-pig ileal smooth muscle

The effects of different compounds that inhibit the isolated plasma-membrane Ca²⁺/Mg²⁺-ATPase on the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and on the corresponding force development have been examined in smooth muscle of the longitudinal layer of the guinea-pig ileum. F^–, in the presence of Al³⁺, induced an increase of the resting force and of the amplitude of the superimposed phasic contractions. The increase of resting force was associated with an increased level of basal [Ca²⁺]_i while the phasic contractions were accompanied by concomitant oscillations in [Ca²⁺]_i. Comparable contractions could be induced by vanadate and the calmodulin antagonist calmidazolium. The oscillations of [Ca²⁺]_i and of force elicited by AlF₄^–were not modified by adrenergic or cholinergic blocking agents but were inhibited by verapamil. These phasic contractions were not affected by depleting the intracellular Ca²⁺ stores with ryanodine. This finding excludes a cytosolic origin of these oscillations. However, hyperpolarization and complete depolarization of the cells inhibited the oscillations. It is concluded that AlF₄^–, vanadate and calmidazolium induce cytoplasmic Ca²⁺ oscillations possibly by acting at the plasma membrane. Indeed all these substances affect by different mechanisms the isolated plasma-membrane Ca²⁺/Mg²⁺-ATPase. The generation of membrane-linked Ca²⁺ oscillations could therefore be related to an inhibition of the plasma-membrane Ca²⁺ pump resulting in an increase of [Ca²⁺]_i. This change in [Ca²⁺]_i could be responsible for the pronounced changes of the electrical and mechanical activity of this tissue.