CHANGES IN PLATELET FREE Ca2+CONCENTRATION AFTER CHRONIC DIGOXIN TREATMENT

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Abstract
Summary—Cell Na+ and Ca2+concentrations control each other by various mechanisms. In excitable cells from various origins, Ca2+extrusion from the cell and its entry are dependent for a large part on the activity of the Na+, Ca2+‐countertransport system. Cytosolic free Ca2+concentration is also controlled by the Na+–H+ exchange activity. To analyze the changes in cytosolic Ca2+concentration accompanying the reduction of the membrane Na+ gradient, cytosolic free Ca2+concentration ([Ca2+]i) was measured by fluorescent dyes in platelets and erythrocytes from healthy subjects, before and during digoxin treatment (0.25 mg/day for 6 days). [Ca2+]iwas increased in platelets from 169±30 to 321±61 nmol/l (n= 7,P2+]iwas not accompanied by a change in serotonin content (5.43±0.67vs5.49±0.61 10−7mol per 1011cells) and could not be reproduced byin vitroaddition of 10−4mol/l ouabain (198±33vs186±73 nmol/l). The enhanced [Ca2+]iin platelets is thus not a short‐term consequence of a reduced membrane Na+ gradient, but reflects either the overload of intracellular Ca2+stores or an enhancedin vivostimulation by hormones or neurotransmitters.