Multiple Ca2+-Dependent Mechanisms Regulate L-Type Ca2+Current in Retinal Amacrine Cells

Understanding the regulation of L-type voltage-gated Ca2+current is an important component of elucidating the signaling capabilities of retinal amacrine cells. Here we ask how the cytosolic Ca2+environment and the balance of Ca2+-dependent effectors shape native L-type Ca2+channel function in these cells. To achieve this, whole cell voltage clamp recordings were made from cultured amacrine cells under conditions that address the contribution of mitochondrial Ca2+uptake (MCU), Ca2+/calmodulin (CaM)-dependent channel inactivation (CDI), protein kinase A (PKA), and Ca2+-induced Ca2+release (CICR). Under control conditions, repeated activation of the L-type channels produces a progressive enhancement of the current. Inhibition of MCU causes a reduction in the Ca2+current amplitude that is dependent on Ca2+influx as well as cytosolic Ca2+buffering, consistent with CDI. Including the Ca2+buffer bis-( o-aminophenoxy)- N,N,N',N'-tetraacetic acid (BAPTA) internally can shift the balance between enhancement and inhibition such that inhibition of MCU can enhance the current. Inhibition of PKA can remove the enhancing effect of BAPTA suggesting that cyclic AMP-dependent phosphorylation is involved. Inhibition of CaM suppresses CDI but spares the enhancement, consistent with the substantially higher sensitivity of the Ca2+-sensitive adenylate cyclase 1 (AC1) to Ca2+/CaM. Inhibition of the ryanodine receptor reduces the current amplitude, suggesting that CICR might normally amplify the activation of AC1 and stimulation of PKA activity. These experiments reveal that the amplitude of L-type Ca2+currents in retinal amacrine cells are both positively and negatively regulated by Ca2+-dependent mechanisms.