title: Phosphorylation-dependent modulation of cardiac calcium current by intracellular free magnesium.
abstract: We compared the effects of cytosolic free magnesium (Mg(2+)(i)) on L-type Ca(2+) current (I(Ca,L)) in patch-clamped guinea pig ventricular cardiomyocytes under basal conditions, after inhibition of protein phosphorylation, and after stimulation of cAMP-mediated phosphorylation. Basal I(Ca,L) density displayed a bimodal dependence on the concentration of Mg(2+)(i) ([Mg(2+)](i); 10(-6)-10(-2) M), which changed significantly as cell dialysis progressed due to a pronounced and long-lasting rundown of I(Ca,L) in low-Mg(2+) dialysates. Ten minutes after patch breakthrough, I(Ca,L) density (at +10 mV) in Mg(2+)(i)-depleted cells ([Mg(2+)](i) approximately 1 microM) was elevated, increased to a maximum at approximately 20 microM [Mg(2+)](i), and declined steeply at higher [Mg(2+)](i). Treatment with the broad-spectrum protein kinase inhibitor K252a (10 microM) reduced I(Ca,L) density and abolished these effects of Mg(2+)(i) except for a negative shift of I(Ca,L)-voltage relations with increasing [Mg(2+)](i). Maximal stimulation of cAMP-mediated phosphorylation occluded the Mg(2+)(i)-induced stimulation of I(Ca,L) and prevented inhibitory effects of the ion at [Mg(2+)](i) <1 mM but not at higher concentrations. These results show that the modulation of I(Ca,L) by Mg(2+)(i) requires protein kinase activity and likely originates from interactions of the ion with proteins involved in the regulation of protein phosphorylation/dephosphorylation. Stimulatory effects of Mg(2+)(i) on I(Ca,L) seem to increase the cAMP-mediated phosphorylation of Ca(2+) channels, whereas inhibitory effects of Mg(2+)(i) appear to curtail and/or reverse cAMP-mediated phosphorylation.
authors.affiliation: Department of Physiology and Biophysics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7.
authors.forename: S
authors.initials: S
authors.lastname: Pelzer
authors.forename: C
authors.initials: C
authors.lastname: La
authors.forename: D J
authors.initials: DJ
authors.lastname: Pelzer
mesh_terms: D000818:Animals; D015220:Calcium Channels / Q000502:physiology*; D002227:Carbazoles / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D004553:Electric Conductivity; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006168:Guinea Pigs; D026121:Indole Alkaloids; D007425:Intracellular Membranes / Q000378:metabolism*; D008274:Magnesium / Q000502:physiology*; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000502:physiology
chemical_list: D015220:Calcium Channels; D002227:Carbazoles; D004791:Enzyme Inhibitors; D026121:Indole Alkaloids; D047428:Protein Kinase Inhibitors; C049985:staurosporine aglycone; D000242:Cyclic AMP; D011494:Protein Kinases; D008274:Magnesium