Thrombin and ionomycin can raise platelet cytosolic Ca2+ to micromolar levels by discharge of internal Ca2+ stores: studies using fura-2.
In the presence of 1 mM EGTA, the addition of the calcium ionophore ionomycin to human platelets loaded with 30 microM fura-2 could elevate [Ca2+]i from less than 100 nM to a maximum of greater than 3 microM, presumably by discharge of Ca2+ from internal stores. Under the same conditions thrombin could maximally increase [Ca2+]i to a peak of greater than 1 microM which then declined to near resting levels within 3-4 minutes; by contrast in platelets loaded with 1 mM quin2 thrombin could raise [Ca2+]i to only about 200 nM. In the presence of 1 mM Ca2+ the peak response to thrombin in fura-2-loaded platelets was higher (1.4 microM) than that observed in the presence of EGTA (1.1 microM) and the elevation in [Ca2+] was prolonged, presumably by Ca2+ influx. These results with fura-2-loaded platelets indicate that mobilisation of internal Ca2+ can contribute a substantial proportion of the early peak [Ca2+]i evoked by thrombin directly confirming the deductions from previous work with different loadings of quin2. Under natural conditions the major role of Ca2+ influx may be to prolong the [Ca2+]i rise rather than to make it larger.
D000634:Aminoquinolines; D001572:Benzofurans; D001792:Blood Platelets / Q000032:analysis*; D002118:Calcium / Q000032:analysis*; D003600:Cytosol / Q000032:analysis; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D004987:Ethers / Q000494:pharmacology; D005453:Fluorescence; D016257:Fura-2; D006801:Humans; D015759:Ionomycin; D013917:Thrombin / Q000494:pharmacology*
D000634:Aminoquinolines; D001572:Benzofurans; D004987:Ethers; D004533:Egtazic Acid; D015759:Ionomycin; D013917:Thrombin; C035739:Quin2; D002118:Calcium; D016257:Fura-2