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Am J Physiol Heart Circ Physiol 281: H1532-H1544, 2001;
0363-6135/01 $5.00
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Vol. 281, Issue 4, H1532-H1544, October 2001

Phosphorylation-dependent modulation of cardiac calcium current by intracellular free magnesium

Siegried Pelzer, Chicuong La, and Dieter J. Pelzer

Department of Physiology and Biophysics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7

We compared the effects of cytosolic free magnesium (Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP>) on L-type Ca2+ current (ICa,L) in patch-clamped guinea pig ventricular cardiomyocytes under basal conditions, after inhibition of protein phosphorylation, and after stimulation of cAMP-mediated phosphorylation. Basal ICa,L density displayed a bimodal dependence on the concentration of Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP> ([Mg2+]i; 10-6-10-2 M), which changed significantly as cell dialysis progressed due to a pronounced and long-lasting rundown of ICa,L in low-Mg2+ dialysates. Ten minutes after patch breakthrough, ICa,L density (at +10 mV) in Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP>-depleted cells ([Mg2+]i ~1 µM) was elevated, increased to a maximum at ~20 µM [Mg2+]i, and declined steeply at higher [Mg2+]i. Treatment with the broad-spectrum protein kinase inhibitor K252a (10 µM) reduced ICa,L density and abolished these effects of Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP> except for a negative shift of ICa,L-voltage relations with increasing [Mg2+]i. Maximal stimulation of cAMP-mediated phosphorylation occluded the Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP>-induced stimulation of ICa,L and prevented inhibitory effects of the ion at [Mg2+]i <1 mM but not at higher concentrations. These results show that the modulation of ICa,L by Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP> 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<UP><SUB>i</SUB><SUP>2+</SUP></UP> on ICa,L seem to increase the cAMP-mediated phosphorylation of Ca2+ channels, whereas inhibitory effects of Mg<UP><SUB>i</SUB><SUP>2+</SUP></UP> appear to curtail and/or reverse cAMP-mediated phosphorylation.

whole cell patch-clamp recording; guinea pig ventricular cardiomyocytes; cAMP signaling


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