INHIBITION OF PERIPHERAL-BLOOD MONONUCLEAR CELL-PROLIFERATION BY CARDIAC-GLYCOSIDES

Introduction: Prior studies have shown that ouabain, a cardiac glycosi de that inhibits the sodium, potassium adenosine triphosphatase (Na+,K + ATPase) enzyme, downregulates phytohemagglutinin (PHA)-induced perip heral blood mononuclear cell (PBMNC) proliferation. Objective: This st udy examined and compared the effects of both ouabain and digoxin, a c ardiac glycoside used therapeutically in humans, on PBMNC proliferatio n. Methods: Peripheral blood mononuclear cells were isolated from heal thy human subjects, incubated for 72 hours with and without PHA (2%) i n the presence and absence of ouabain (10(-12) to 10(-4)M) or digoxin (10(-9)M to 10(-6)M), and pulsed with H-3 thymidine. Results: For PHA- stimulated PBMNCs in the ouabain-treated group (n = 10 subjects), the mean (+/-STD) % uptake (% H-3 thymidine uptake in absence of ouabain) was 80.5 +/- 6.0 at 10(-12)M ouabain, 73.1 +/- 8.4 at 10(-10)M, 47.89 +/- 13.1 at 10(-8)M, 6.9 +/- 3.2 at 10(-6)M, and 3.4 +/- 1.6 at 10(-4) M. For PHA-stimulated cells in the digoxin-treated group (n = 9 subjec ts), the mean (+/-STD) % uptake (% H-3 thymidine uptake in absence of digoxin) was 89.8 +/- 9.8 at 10(-9)M digoxin, 92.6 +/- 8.2 at 10(-8)M, 54.3 +/- 19.8 at 10(-7)M, and 1.0 +/- 2.4 at 10(-6)M. Repeated measur es ANOVA demonstrated a significant effect of concentration of both gl ycosides on PBMNC proliferation (P < .01). The inhibitory effect was r eversible, but was largely abbrogated if ouabain was added after 48 ho urs of incubation with PHA. Further, the inhibitory effect extended to PBMNCs stimulated with recall antigen (tetanus) and to fractionated P BMNCs (CD4(+), CD8(+) and CD19(+)) stimulated with mitogens. Additiona lly, dose-response inhibitory effects of glycosides on PBMNC Na+,K+ AT Pase enzyme activity and interleukin-2 (IL-2) secretion by PHA-stimula ted PBMNC were also noted. Neither glycoside had an effect on spontane ous PBMNC proliferation (no PHA) or trypan blue exclusion. Conclusions : These studies demonstrate that both cardiac glycosides inhibited PHA -induced PBMNC proliferation, possibly via Na+,K+ ATPase inhibition, b ut not via cell toxicity. The concentration range over which inhibitio n was observed was similar for both glycosides. The results raise the possibility that therapeutic or toxic doses of digoxin could have an e ffect on cell-mediated immunity in vivo.