Multiple Ca2+-dependent mechanisms regulate L-type Ca2+ current in retinal amacrine cells.

Understanding the regulation of L-type voltage-gated Ca(2+) current is an important component of elucidating the signaling capabilities of retinal amacrine cells. Here we ask how the cytosolic Ca(2+) environment and the balance of Ca(2+)-dependent effectors shape native L-type Ca(2+) 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 Ca(2+) uptake (MCU), Ca(2+)/calmodulin (CaM)-dependent channel inactivation (CDI), protein kinase A (PKA), and Ca(2+)-induced Ca(2+) 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 Ca(2+) current amplitude that is dependent on Ca(2+) influx as well as cytosolic Ca(2+) buffering, consistent with CDI. Including the Ca(2+) 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 Ca(2+)-sensitive adenylate cyclase 1 (AC1) to Ca(2+)/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 Ca(2+) currents in retinal amacrine cells are both positively and negatively regulated by Ca(2+)-dependent mechanisms.