Influence of human plasma or serum albumin on ADP- or vasopressin-induced calcium increases in human platelets.

By focusing the consequences of loading platelets with the fluorescent calcium indicator, fura-2, in buffer or plasma the influences of plasma constituents on calcium responses in blood platelets has been worked out. Proteins were removed from the pre-incubation medium before agonist stimulation and measurement of intracellular calcium concentration [Ca2+]i. We found that moderate amounts (1-33%, v/v) of plasma added to the buffer during preincubation stimulated the mobilization of cytoplasmic calcium, DELTA[Ca2+]i, and reduced the time from agonist stimulation to peak level of [Ca2+]i in platelets stimulated with ADP or arginine vasopressin A8VP. With the buffer used, calcium response was restored by addition of 33% (v/v) plasma to the same level as found for unwashed platelets in the platelet rich plasma (cf. methods). The presence of human serum albumin during the pre-incubation also influenced the calcium response, but not to the same extent as plasma. From a resting level of 73 +/- 10 nmol l-1, addition of 0.4 mumol l-1 ADP increased the [Ca2+]i by 24 +/- 13 nmol l-1 (n = 20), 65 +/- 30 nmol l-1 (n = 5), and 144 +/- 44 nmol l-1 (n = 22) in platelets pre-incubated with buffer, 5 g l-1 albumin, and 33% (v/v) plasma, respectively. The corresponding values after stimulation with 0.05 mumol l-1) A8VP were 49 +/- 34 nmol l-1, 105 +/- 27 nmol l-1, and 170 +/- 39 nmol l-1 (n = 7). In platelets incubated in buffer only, the DELTAt from stimulation with 0.4 mumol l-1 ADP was 18.9s. When stimulated with ADP (0.4 mumol l-1) or A8VP (0.05 mumol l-1) after pre-incubation with albumin or plasma, DELTAt was clearly reduced. We conclude that other plasma factors, in addition to albumin, contribute to modulate the calcium response to ADP or arginine vasopressin, in human platelets.