Sodium-hydrogen exchange in human platelets exposed to weak acid: comparison between measurements of platelet swelling and cytoplasmic sodium ion concentration

Increased Na+-H+ exchange activity has been demonstrated in various patholo gical states using the rate constant of platelet swelling (k(s)). This indi rect measure of exchange activity could, however, be influenced by conditio ns that alter platelet distensibility independent of Na+-H+ exchange activi ty. To determine the validity of k(s), the change in light transmission and hence k(s), was measured at 24 degrees and 37 degreesC (temperatures assoc iated with alterations in membrane fluidity), and compared with the rate co nstant (k(Na)) for the increase in cytoplasmic sodium ion concentration ([N a+](i)) and the rate constant of Na+-influx (k(phi Na)) under the same cond itions. Both k(s) and k(Na) were dependent on Na+-H+ exchange; the rate con stants differed in the order k(s) < k(<phi>Na) < k(Na) (P<0.01). Neither k( Na) or k(phi Na) changed with temperature, whereas k(s) was significantly l ower at 24 degrees than at 37 degreesC (7.49 +/- 0.31x10(-3) s(-1) versus 9 .96 +/- 0.42x10(-3) s(-1), P<0.0001). While both k(s) and k(Na) are depende nt on Na+-H+ exchange, k(s) may underestimate the activity of the exchanger and may in part, be influenced by membrane fluidity. This could be importa nt in interpreting data where k(s) has been measured in different groups of subjects.