Calcium-membrane interactions in the myocardium: Effects of ouabain, epinephrine and 3′, 5′-cyclic adenosine monophosphate

Abstract

Cardiac microsomes, which represent an enriched but not pure preparation of the heart's sarcoplasmic reticulum, can remove calcium from solution by 2 kinetically dissimilar mechanisms. In the presence of adenosine triphosphate (ATP), Ca ++ is taken up by cardiac microsomes by a process designated Ca-binding, which exhibits saturation kinetics. The rate and extent of Ca-binding, and the high affinity of the Ca-binding sites could allow this process to cause the intact cell to relax. When anions that permit Ca ++ to be precipitated within the mlcrosomal vesicles are included along with ATP, much larger amounts of Ca ++ are taken up by cardiac microsomes. This second process, designated Ca-uptake, does not follow saturation kinetics. Instead, the rate of Ca-uptake increases linearly with increasing Ca ++ concentration until Ca-uptake becomes inhibited at higher Ca ++ concentrations. The finding of 2 kinetically distinct Ca ++ transport processes in cardiac microsomes, both of which are highly active in the micromolar range of Ca ++ concentration, suggests that Ca ++ movements in the intact myocardial cell may be controlled by 2 mechanisms. It is suggested that one of these, possibly manifest in vitro as Ca-binding, represents an intracellular release site that initiates systole by delivering Ca ++ to the contractile proteins. The second process, possibly manifest in vitro as Ca-uptake, is suggested to represent the uptake of Ca ++ into an intracellular storage site whose Ca ++ content indirectly determines the amount of Ca ++ that is delivered to the contractile proteins. These 2 intracellular Ca ++ pools can be tentatively related to Ca ++ movements into and out of the myocardial cell, permitting the formulation of a model by which a number of inotropic interventions might modulate myocardial contractility. Cardiac glycosides had no detectible effect on either cardiac microsomal Ca-binding or Ca-uptake. Cyclic adenosine monophosphate (cAMP), which by itself was without effects on cardiac microsomes, more than doubled the rate of Ca-uptake in the presence of a cyclic AMP-dependent protein kinase. The resulting increase in rate of Ca-uptake could explain the actions of epinephrine to enhance contractility at the same time that systole is abbreviated.