Increased Intracellular Magnesium Attenuates β-adrenergic Stimulation of the Cardiac Ca(V)1.2 Channel

Increases in intracellular Mg(2+) (Mg(2+)(i)), as observed in transient cardiac ischemia, decrease L-type Ca(2+) current of mammalian ventricular myocytes (VMs). However, cardiac ischemia is associated with an increase in sympathetic tone, which could stimulate L-type Ca(2+) current. Therefore, the effect of Mg(2+)(i) on L-type Ca(2+) current in the context of increased sympathetic tone was unclear. We tested the impact of increased Mg(2+)(i) on the β-adrenergic stimulation of L-type Ca(2+) current. Exposure of acutely dissociated adult VMs to higher Mg(2+)(i) concentrations decreased isoproterenol stimulation of the L-type Ca(2+) current from 75 ± 13% with 0.8 mM Mg(2+)(i) to 20 ± 8% with 2.4 mM Mg(2+)(i). We activated this signaling cascade at different steps to determine the site or sites of Mg(2+)(i) action. Exposure of VMs to increased Mg(2+)(i) attenuated the stimulation of L-type Ca(2+) current induced by activation of adenylyl cyclase with forskolin, inhibition of cyclic nucleotide phosphodiesterases with isobutylmethylxanthine, and inhibition of phosphoprotein phosphatases I and IIA with calyculin A. These experiments ruled out significant effects of Mg(2+)(i) on these upstream steps in the signaling cascade and suggested that Mg(2+)(i) acts directly on Ca(V)1.2 channels. One possible site of action is the EF-hand in the proximal C-terminal domain, just downstream in the signaling cascade from the site of regulation of Ca(V)1.2 channels by protein phosphorylation on the C terminus. Consistent with this hypothesis, Mg(2+)(i) had no effect on enhancement of Ca(V)1.2 channel activity by the dihydropyridine agonist (S)-BayK8644, which activates Ca(V)1.2 channels by binding to a site formed by the transmembrane domains of the channel. Collectively, our results suggest that, in transient ischemia, increased Mg(2+)(i) reduces stimulation of L-type Ca(2+) current by the β-adrenergic receptor by directly acting on Ca(V)1.2 channels in a cell-autonomous manner, effectively decreasing the metabolic stress imposed on VMs until blood flow can be reestablished.