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 β‐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 α‐ and β‐adenosine triphosphate (ATP) peaks, using the dissociation constant of MgATP 87 μ M (established recently). In normal (phosphate‐free Krebs‐Henseleit) solution, [Mg 2+ ] i was approximately 1.02 m M . Insulin at physiological and pathological concentrations increased [Mg 2+ ] i and contractility in a dose‐dependent manner. Insulin (more than 100 μu ml −1 ) suppressed the decrease in [Mg 2+ ] i caused by isoprenaline (100 n M ), and these effects of insulin on [Mg 2+ ] i and contractility were blocked by LY333531 (macrocyclic bis (indolyl) maleimide, 100 n M ), 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 β‐adrenoceptor stimulation.British Journal of Pharmacology (2000) 130 , 731–738; doi: 10.1038/sj.bjp.0703361