| dc.description | The effects of cytosolic free magnesium (Mg2+i) on L-type Ca2+, current (ICa,L) were studied in guinea pig ventricular cardiomyocytes. Under basal conditions, the influence of Mg2+i on ICa,L was bimodal. Basal ICa,L density rose with increasing Mg2+i from 1 muM to 17 muM, while higher Mg2+i concentrations led to an inhibition of the ICa,L. The stimulation appeared to be caused by an elevated CAMP level, resulting in cAMP dependent protein kinase (PKA) mediated phosphorylation, whereas inhibition appeared to be caused by a reduction in the level of channel phosphorylation. However, this bimodal effect was eliminated, and the current greatly reduced when phosphorylation was suppressed with K252a. On the other hand, preincubation with forskolin (FSK) and 3-isobutyl-l-methylxanthine (IBMX) produced a large stimulation of the ICa,L and prevented inhibitory effects of the ion at Mg 2+i levels less than 1 mM. In consideration of the results, I postulate that Mg2+i controls the balance of cAMP dependent phosphorylation of the L-type Ca2+ channel. There are four possible sites that Mg2+i can influence the net production of cAMP: at the receptor, at the G-protein, at the adenylyl cyclase and at the phosphodiesterase. The results from the second set of experiments indicate that Mg2+i stimulates the activity of all four sites. However, the apparent affinity seems to be higher at the sites that augment the production of cAMP than the sites responsible for its hydrolysis. In addition to Mg2+i's effect on channel phosphorylation and current amplitude, it has a major influence with ICa,L inactivation. The results lead me to hypothesize that Mg2+i inhibits inactivation by competing with Ca2+ at the Ca2+-binding sites that are required for Ca2+ dependent inactivation. | en_US |
