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 pre-incubation 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 gl-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 delta t from stimulation with 0.4 mumol l-1 ADP was 18.9s.(ABSTRACT TRUNCATED AT 250 WORDS)