Insulin modulation of intracellular free magnesium in heart: involvement of protein kinase C

In the present study of rat heart using (31)P-nuclear magnetic resonance, we examined the interaction between beta-adrenergic and insulin receptors in terms of the intracellular free Mg(2+) concentration ([Mg(2+)](i)) regulation. [Mg(2+)](i) was estimated from the separation of the chemical shifts of the alpha- and beta-adenosine triphosphate (ATP) peaks, using the dissociation constant of MgATP 87 microM (established recently). In normal (phosphate-free Krebs-Henseleit) solution, [Mg(2+)](i) was approximately 1.02 mM. Insulin at physiological and pathological concentrations increased [Mg(2+)](i) and contractility in a dose-dependent manner. Insulin (more than 100 micro(u) ml(-1)) suppressed the decrease in [Mg(2+)](i) caused by isoprenaline (100 nM), and these effects of insulin on [Mg(2+)](i) and contractility were blocked by LY333531 (macrocyclic bis (indolyl) maleimide, 100 nM), a protein kinase C (PKC) inhibitor. The isoprenaline-induced decrease in the concentrations of ATP ([ATP]) with insulin application was significantly smaller than that without insulin. Insulin modulates [Mg(2+)](i) and haemodynamics, presumably via activation of PKC, thereby antagonizing the reduction of [Mg(2+)](i) induced by beta-adrenoceptor stimulation.