The role of Ca²⁺ uptake in the response of human platelets to adrenaline and to 1‐O‐alkyl‐2‐acetyl‐sn‐glycero‐3‐phosphocholine (platelet‐activating factor)

Abstract

1. Thrombin induces Ca2+ uptake into both stirred and unstirred human platelets in the presence or absence of acetylsalicylate. This Ca2 + uptake is closely correlated with adenine nucleotide secretion in accord with previous observations [Massini, P. and Luscher, E. F. (1974) Biochim. Biophys. Actu 372, 109-1211 but a low level of secretion is observed in the absence of significant CaZ + uptake.2. l-O-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-0-alkylAcGEPC) also induces Ca2+ uptake into both stirred and unstirred human platelets in the presence and absence of acetylsalicylate. Aggregation and adenine nucleotide secretion induced by 1-0-alkylAcGEPC can be observed in the absence of added fibrinogen, but addition of fibrinogen causes a very marked shift to the left in the dose/response curves for aggregation and Ca2+ uptake induced by I-0-alkylAcGEPC. In the absence of added fibrinogen a close correlation is observed between Caz+ uptake and adenine nucleotide secretion induced by 1 -0-alkylAcGEPC. In the presence of added fibrinogen significant aggregation can be observed in the absence of detectable Ca2+ uptake.3. Adrenaline induces CaZ+ uptake only into stirred human platelets in the presence of added fibrinogen. Blockade of secretion, e.g. by addition of acetylsalicylate, also prevents Ca2 + uptake. 4. Addition of adrenaline fails to cause breakdown of phosphatidylinositol or phosphatidylcholine in acetylsalicylate-treated platelets under conditions where such a response is observed on addition of thrombin. We conclude that Ca2+ uptake into human platelets induced by thrombin, I-0-alkylAcGEPC and adrenaline is closely associated with the secretory response and in some circumstances, e.g. stimulation by thrombin, is clearly a consequence of this latter response. Previous reports of CaZ+ uptake as an initiating event in the response of human platelets to adrenaline [Owen, N. E., Feinberg, H. and Le Breton, G. C. (1981) Am J. Physiol. 239, H483-4881 have not been confirmed in this study.It is widely accepted that an increase in cytosolic [Ca"] is one mechanism by which excitatory agonists induce platelet aggregation and secretion (cf. [I]). Divalent cation ionophotes which show selectivity for Ca2+ e.g. A23187, ionomycin, induce these responses (21 and direct evidence for an increase in cytosolic [Ca"] on stimulation by thrombin and plateletactivating factor has been obtained in studies where this parameter was monitored directly using Quin I1 [3, 41. However, the source from which Ca2+ is mobilised to permit an increase in the cytosolic concentration of this cation is much less clear. Uptake of 45Ca2+ into human platelets is stimulated by addition of thrombin but such stimulation is closely correlated in time with 5-hydroxytryptamine secretion induced by this agonist [5, 61. A similar correlation of 45Ca2+ uptake with secretion has been suggested for stimulation by collagen, ADP and adrenaline [6]. Furthermore Brass and Shattil [7] were unable to find an enhancement of net 4sCa2+ uptake by ADP or adrena...

Role of catecholamines in platelet function: pathophysiological and clinical significance

“…In fact fluorescent probes such as quin 2 or fura 2 are unable to demonstrate a significant increase in cytoplasmic Ca 2 in response to adrenaline [176,239], whereas other probes such as aequorin and chlortetracycline show an increase in free Ca 2 concentration [205,206,245,246]; these probes seem to confirm the Ca 2 release from a subcompartment sensitive to adrenaline [136]. In fact fluorescent probes such as quin 2 or fura 2 are unable to demonstrate a significant increase in cytoplasmic Ca 2 in response to adrenaline [176,239], whereas other probes such as aequorin and chlortetracycline show an increase in free Ca 2 concentration [205,206,245,246]; these probes seem to confirm the Ca 2 release from a subcompartment sensitive to adrenaline [136].…”

“…The effect of adrenaline-induced activation on intraplatelet free Ca 2 levels are complex: adrenaline per se induces a slight Ca 2 uptake and causes Ca 2 release from stores in relationship to activation of PIP 2 metabolism [212]. Further elevation of intraplatelet Ca 2 levels occurs when adrenaline potentiates the effects of other platelet agonists [213,215,245,246].…”

Platelet activation.

“…According to some investigators, 75 but not others, 76 excitation is accompanied by influx of calcium ions. Binding fibrinogen is thought to occur independently of plateletderived ADP by some, but not all, investigators.…”

Modulation of platelet-activating-factor-induced calcium influx and intracellular calcium release in platelets by phorbol esters

“…Calcium release from intracellular stores may be mediated by products of phosphatidylinositol metabolism, notably inositol trisphosphate (IP3) [5,6]. However, platelet activation by PAF requires calcium influx, and calcium influx accounts for the bulk of the PAF-induced increase in [Ca2+]i for platelets in calcium-containing buffers [1][2][3][4]. Platelets lack voltage-dependent calcium channels such as those described in nerve and muscle [7], so the PAF-activated calcium channel is linked to the PAF receptor by an undefined mechanism.…”

“…Platelet activation by platelet-activating factor (PAF) is mediated in part by increases in free intracellular calcium ([Ca2"],) [1][2][3][4]. There are two functionally distinct pathways by which PAF increases [Ca2+]j: release of calcium from intracellular stores and calcium influx through selective plasma membrane channels.…”