Model for the regulation of platelet volume and responsiveness by the trans‐membrane Na⁺/K⁺‐pump

Abstract

The correspondence between K+ uptake in platelets to their responsiveness was studied using 86Rb+ as an analogue of K+. An average 86Rb+ uptake rate of 0.73 (+/- 0.140) x 10(-15) mole Rb+/min-plt (n = 20) was observed. By the use of K(+)-influx inhibitors, we were able to distinguish three distinct 86Rb+ uptake pathways: an ouabain-sensitive (61% +/- 2% inhibitable) pump and two equivalent channels, only one of which is sensitive to furosemide. Other platelet parameters were also examined in conjunction with K(+)-uptake. Platelets incubated with ouabain exhibited an overall rise in their cell volume (MPV) with incubation time (delta MPV = 7.4 x 10(-17) L/min-1 plt-1). Concomitantly, over 24 hours, a steady decrease in platelet number was recorded by blood cell coulter, which correlated inversely with the counts of particles, which by their size resemble white blood cells (r = 0.89). On a cellular level, incubation with ouabain induced greater expression of surface fibrinogen-receptor (GPIIb), increased binding of FITC-labelled fibrinogen, and increased responsiveness to ADP. Our observations suggest the following sequence of events: Ouabain turns off the Na+/K(+)-ATPase pump, which leads to water accumulation in platelets and concomitant increased MPV. Greater expression of fibrinogen receptors on the distended platelet surface corresponds to spontaneous microaggregate formation as well as greater responsiveness to agonists. Our model links volume regulation, the expression of fibrinogen receptors, and the sensitivity of platelets to agonists to the activity of the Na+/K(+)-ATPase pump.

Platelet multielemental composition, lability, and subcellular localization

“…Though the K values for the fractions by DXS are semiquantitative, they reinforce the view that platelet K f is accumulated by the Na+/K+-ATPase pump [27] and sequestered predominantly in the cytoplasmic compartment (Fig. 4C).…”

“…For example, the accumulation of K' within the cytoplasmic pool relates to the operation of the Na+/ K+-ATPase pumps and other pathways of K+ accumulation, which modulate platelet volume (MPV) and reactiv- -. ity to agonists [27]. For Ca(II), other processes have been described to which its accumulation and secretion during platelet activation protein about elemental subcellular distribution helps us to develop insights into cellular organization and functionality.…”

“…Ultracentrifugation was carried out by loading the lysate (-2 ml) onto 28 ml of 30-70% sucrose gradient and spinning (Kontron model centrifuge with SLS-28 head) at 134,000 g, for 2 hr, modifying a previously described procedure [27,28]. After the run, 2 ml fractions were collected from the bottom of the tube by peristaltic pump.…”

The involvement of Na+/K(+)-ATPase in the development of platelet procoagulant response.

“…However, it has been reported that endogenously produced ouabain (probably a stereoisomer of plant ouabain) isolated and purified from bovine blood inhibits rat brain α1 Na + /K + -ATPase with a 1000-fold higher potency than does plant ouabain (Ferrandi et al, 1998). Platelets are likely to possess at least two isoforms of Na + /K + -ATPase, since exogenously added plant ouabain is not able to inhibit its activity totally (Marx et al, 1992). Activated platelets have been reported to express additional binding sites for ouabain, which may result in a rise in the sensitivity of their Na + /K + -ATPase to cardiac glycosides (Bork & Mrsny, 1993).…”

“…The ouabain-induced rise in [Na + ] i is reported to produce water accumulation in platelets and their subsequent swelling. This has been proposed to lead to greater expression of fibrinogen receptors on platelets and to a rise in their sensitivity to stimulators (Marx et al, 1992). The effect of ouabain on the procoagulant activity of platelets has never been studied before.…”