Ca2+‐stimulated adenylyl cyclases regulate the L‐type Ca2+ current in guinea‐pig atrial myocytes

Key points • Adenylyl cyclases (ACs), the enzymes that synthesize cAMP, are essential for the inotropic response of cardiac muscle to adrenergic stimulation. • Ca2+‐stimulated isoforms of AC have previously been identified in the sino‐atrial node of the heart, where they contribute to pacemaking. • In this study, we demonstrate that Ca2+‐stimulated isoforms of AC regulate the L‐type Ca2+ current in atrial myocytes. • This regulation of the L‐type Ca2+ current provides a feedback mechanism for the Ca2+ released from intracellular stores to control Ca2+ entry to atrial myocytes. • These Ca2+‐stimulated ACs may be significant for the compartmentalization of cAMP signalling in atrial myocytes. Ca2+‐stimulated adenylyl cyclases (ACs) have recently been shown to play important roles in pacemaking in the sino‐atrial node. Here we present evidence that Ca2+‐stimulated ACs are functionally active in guinea‐pig atrial myocytes. Basal activity of an AC in isolated atrial myocytes was demonstrated by the observations that MDL 12,330A (10 μm), an AC inhibitor, reduced L‐type Ca2+ current (ICaL) amplitude, while inhibition of phosphodiesterases with IBMX (100 μm) increased ICaL amplitude. Buffering of cytosolic Ca2+ by exposure of myocytes to BAPTA‐AM (5 μm) reduced ICaL amplitude, as did inhibition of Ca2+ release from the sarcoplasmic reticulum with ryanodine (2 μm) and thapsigargin (1 μm). [Ca2+]i‐activated calmodulin kinase II (CaMKII) inhibition with KN‐93 (1 μm) reduced ICaL, but subsequent application of BAPTA‐AM further reduced ICaL. This effect of BAPTA‐AM, in the presence of CaMKII inhibition, demonstrates that there is an additional Ca2+‐modulated pathway (not dependent on CaMKII) that regulates ICaL in atrial myocytes. The effects of BAPTA could be reversed by forskolin (10 μm), a direct stimulator of all AC isoforms, which would restore cAMP levels. In the presence of BAPTA‐AM, the actions of IBMX were reduced. In addition, inclusion of cAMP in the patch electrode in the whole‐cell configuration prevented the effects of BAPTA. These effects are all consistent with a role for Ca2+‐stimulated AC in the regulation of atrial myocyte ICaL.