Magnesium Research


Reduction of perifusate magnesium alters inotropic response of papillary muscle to ion channel modulators Volume 18, issue 3, September 2005


See all figures

Division of Cellular and Molecular Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, India

Magnesium has a significant role in the regulation of ion transport. Marginal deficiency of Mg can therefore affect myocardial excitability and contractility. This study was taken up with the objective of examining the inotropic response of the myocardium to variation in extracellular [Mg] o and identifying the ion channels and pumps mediating the inotropic changes. Electrically stimulated rat papillary muscle was used as the experimental model and mechanical changes were recorded using a physiograph. Channel specific antagonists were used to identify the channels mediating the functional changes. Diastolic Ca 2+ levels were determined in isolated myocytes by the ratiometric method using the fluorescent indicator Fura2-AM. A negative association was observed between the level of [Mg] o and force of contraction, with a peak at 0.48 mM Mg. The force of contraction in Mg deficient medium (0.48 mM) was 158% of control (1.2 mM Mg) (p < 0.001). Inotropic response to the L-type channel antagonist (verapamil–1μm) and NaK ATPase inhibitor (Ouabain–0.3 mM) was augmented in Mg deficiency (p<0.005), indicating activation of the channel and the pump. The response to T-type channel inhibitor (NiCl 2-40μM) was attenuated in Mg deficiency (p < 0.05). The response to the sarcoplasmic reticular Ca pump inhibitor (caffeine-10 mM) and the SR Ca 2+ release channel inhibitor (ryanodine-1μM) were not significantly affected by Mg deficiency. Diastolic level of Ca 2+ increased with a decrease in Mg (p < 0.05). The observations of the study lead to the conclusion that the positive inotropic response in Mg deficiency is mediated by an increase in basal Ca 2+ combined with Ca-induced-Ca release consequent to Ca 2+ influx through L-type Ca channel. Variation in sensitivity to Ca channel blockers and NaK ATPase inhibitor in Mg deficiency can have pharmacological implications.