β-adrenergic-regulated phosphorylation of the skeletal muscle Ca _v1.1 channel in the fight-or-flight response

Ca_v1 channels initiate excitation-contraction coupling in skeletal and cardiac muscle. During the fight-or-flight response, epinephrine released by the adrenal medulla and norepinephrine released from sympathetic nerves increase muscle contractility by activation of the β-adrenergic receptor/cAMP-dependent protein kinase pathway and up-regulation of Ca _v1 channels in skeletal and cardiac muscle. Although the physiological mechanism of this pathway is well de-fined, the molecular mechanism and the sites of protein phosphorylation required for Ca_v1 channel regulation are unknown. To identify the regulatory sites of phosphorylation under physiologically relevant conditions, Ca_v1.1 channels were purified from skeletal muscle and sites of phosphorylation on the α1 subunit were identified by mass spectrometry. Two phosphorylation sites were identified in the proximal C-terminal domain, serine 1575 (S1575) and threonine 1579 (T1579), which are conserved in cardiac Ca_v1.2 channels (S1700 and T1704, respectively). In vitro phosphorylation revealed that Ca_v1.1-S1575 is a substrate for both cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase II, whereas Ca _v1.1-T1579 is a substrate for casein kinase 2. Treatment of rabbits with isoproterenol to activate β-adrenergic receptors increased phosphorylation of S1575 in skeletalmuscle Ca_v1.1 channels in vivo, and treatment with propranolol to inhibit β-adrenergic receptors reduced phosphorylation. As S1575 and T1579 in Ca_v1.1 channels and their homologs in Ca_v1.2 channels are located at a key regulatory interface between the distal and proximal C-terminal domains, it is likely that phosphorylation of these sites in skeletal and cardiac muscle is directly involved in calcium channel regulation in response to the sympathetic nervous system in the fight-or-flight response.