Endogenous inhibitors of the Na+, K(+)-pump and platelet Ca2+ handling in hypertension.

Inhibition of the Na(+)-K+ pump by digitalis compounds has been reported to increase intracellular Na+ and Ca2+ concentrations and to stimulate Na(+)-H+ exchange. The activity of endogenous digitalis-like compounds, proposed to promote natriuresis and to raise blood pressure, has been found to be increased in volume expansion and hypertension. The enhanced cytosolic [Ca2+] present in platelets from hypertensive patients may thus originate from inhibition of the Na(+)-K+ pump by endogenous inhibitors, enhanced mobilization of internal Ca2+ stores due to phospholipase C activation and/or structural membrane defects. In unstimulated platelets from essential hypertensives, the increase in [Ca2+]i depends on external Ca2+, thereby underlining the importance of Ca2+ influx. The observation that [Ca2+]i was also enhanced in erythrocytes (p = 0.03) demonstrates that intracellular stores are not required for this rise. Plasma digitalis-like activity was positively correlated with platelet [Ca2+]i (inhibition of renal Na+,K(+)-ATPase, competition with ouabain binding, p less than 0.01). Platelet [Ca2+]i also rose during chronic digoxin administration (p less than 0.02) but not after acute in vitro ouabain treatment. The alkalinisation of platelet cytosol (p = 0.005) also agrees with the stimulation of the Na(+)-H(+)-exchange. In conclusion, these results are compatible with a participation of endogenous Na(+)-K+ pump inhibitors in the control of cytoplasmic [Ca2+] and cell excitability.