Ca V 1 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 V 1 channels in skeletal and cardiac muscle. Although the physiological mechanism of this pathway is well defined, the molecular mechanism and the sites of protein phosphorylation required for Ca V 1 channel regulation are unknown. To identify the regulatory sites of phosphorylation under physiologically relevant conditions, Ca V 1.1 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 V 1.2 channels (S1700 and T1704, respectively). In vitro phosphorylation revealed that Ca V 1.1-S1575 is a substrate for both cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase II, whereas Ca V 1.1-T1579 is a substrate for casein kinase 2. Treatment of rabbits with isoproterenol to activate β-adrenergic receptors increased phosphorylation of S1575 in skeletal muscle Ca V 1.1 channels in vivo, and treatment with propranolol to inhibit β-adrenergic receptors reduced phosphorylation. As S1575 and T1579 in Ca V 1.1 channels and their homologs in Ca V 1.2 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-orflight response.adrenalin | calcium channels | cyclic AMP | mass spectometry | protein kinase A T he voltage-gated calcium channels Ca V 1.1 and Ca V 1.2 initiate excitation-contraction coupling in skeletal and cardiac muscle, respectively (1-4), and up-regulation of L-type calcium currents through these channels by epinephrine and norepinephrine via activation of β-adrenergic receptors, G proteins, adenylyl cyclase, and cAMP-dependent protein kinase (PKA) phosphorylation increases contractility in the fight-or-flight response (1, 5-8). In skeletal muscle, Ca V 1.1 channels initiate excitation-contraction coupling by direct protein-protein interactions with the ryanodine-sensitive calcium release channels in the sarcoplasmic reticulum (3). Ca V 1.1 channels also conduct slowly activated, sustained calcium currents (9), which do not participate directly in initiation of excitationcontraction coupling in single muscle twitches (10). However, tetanic stimulation of skeletal muscle is required for development of substantial contractile force (11), and slow calcium currents through Ca V 1.1 channels are necessary for the increase in contractile force with increasing frequency of stimulation of...