Cardiac Sarcolemmal Vesicles: ATP-Dependent Ca Ion Transport and Inhibition of Protein Kinase Reactions by Amrinone

Abstract

In the cardiac sarcolemma (SL), several Ca²⁺-transporting systems control cytosolic Ca²⁺ concentration which is linked to the contraction/relaxation cycle of the heart.¹ Ca²⁺ ions enter the myocardial cell during each excitation event via dihydropyridine-sensitive, voltage-operated Ca²⁺ channels.^2,3 Both an electrogenic Na⁺/Ca²⁺ exchange process^4,5 and an ATP-driven Ca²⁺ pump participate in the extrusion of Ca²⁺ from the cell. These Ca²⁺ transport processes may be modulated by several membrane-associated protein kinases through protein phosphorylation reactions which are controlled by intracellular messengers like cyclic AMP, Ca²⁺, cyclic GMP and diacylglycerol.^7–9 This modulation is a basic mechanism of mediation of the cellular response to a number of inotropic and chronotropic neurotransmitters, hormones and other agents. For example, it is well documented that the positive inotropic response to β-adrenergic agents includes SR Ca²⁺ pump activation via cyclic AMP and Ca²⁺/calmodulin-dependent phosphorylation of the membrane proteolipid phospholamban.^10,11 SL membrane contains several protein kinases which are controlled either by cyclic AMP, Ca²⁺/calmodulin or Ca²⁺/phospholipid. They catalyse the phosphorylation of different membrane substrates.^7,9 In SL, β-adrenergic stimulation or injection of cyclic AMP-dependent protein kinase into single cardiomyocytes is known to result in enhanced Ca²⁺ flux through slow Ca²⁺ channels.^12,13 Na⁺exchange in cardiac SL vesicles can be regulated by Ca²⁺/calmodulin-dependent phosphorylation/dephosphorylation reaction.¹⁴