Temperature dependence of the [Ca²⁺]_i signals induced by U-46619, -thrombin, and ADP. The [Ca²⁺]_itransient in a stirred suspension of human platelets (50,000 cells/μL) was monitored with Fura-PE3. The platelets were stimulated at 20°C (dotted traces) or 37°C (solid traces) by 0.1 μmol/L U-46619 (A), 0.02 U/mL human -thrombin (B), or 5 μmol/L ADP (C) in the presence of extracellular calcium (1 mmol/L CaCl₂). Platelet [Ca²⁺]_i was calculated from the fluorescence excitation ratio (340:380 nm) using the kd value for Fura-PE3 at the corresponding temperature. Observations at 25°C and 30°C (not shown) were intermediate between those at 20°C and 37°C. The results shown are from a typical one of six such studies.