Cyclic AMP Enhances Inositol Trisphosphate-Induced Mobilization of Intracellular Ca²⁺ in Cultured Aortic Smooth Muscle Cells

Abstract

The effect of cAMP on ATP-induced intracellular Ca⁺ mobilization in cultured rat aortic smooth muscle cells was investigated. Treatment of cells for 3 min at 37°C with dibutyryl cAMP, a membrane-permeable analogue of cAMP, at concentration up to 500 μM resulted in 1.5- to 1.7-fold increase in the peak cytosolic Ca²⁺ concentration when cells were stimulated with 3 to 200 μM ATP either in the presence or absence of extracellular Ca²⁺. Similar results were obtained when 0.5 mM 8-Br-cAMP or 10 AM forskolin was used instead of dibutyryl cAMP. In contrast to the Ca²⁺ response, dibutyryl cAMP did not affect ATP-induced formation of inositol trisphosphate (IP₃). Furthermore, the dibutyryl cAMP treatment did not affect the size of the Ca²⁺ response elicited by 10 μM ionomycin. These results suggest that intracellular cAMP potentiates the ATP-induced Ca²⁺ response by enhancing Ca²⁺ release from the intracellular Ca²⁺ store(s), rather than by increasing the ATP-induced production of IP₃ or by increasing the size of the intracellular Ca²⁺ store. Using saponin-permeabilized cells, we have shown directly that cAMP enhances Ca²⁺ mobilization by potentiating the Ca2+ -releasing effect of IP, from the intracellular Ca²⁺ store.