Regulation of platelet cytosolic free calcium by cyclic nucleotides and protein kinase C

Abstract

PAF elicits a rapid, concentration-dependent elevation of platelet cytosolic free calcium ([Car]), measured by quin2. Elevation of [Car] is transient, and the rate of reversal increases with agonist concentration. Adenylate cyclase stimulants (PGI,, PGDJ and 8-bromo CAMP; a guanylate cyclase stimulant (sodium nitroprusside) and 8-bromo cGMP; and a protein kinase C stimulant (phorbol myristate acetate) block the elevation of [Car] induced by PAF, and accelerate its reversal. These results suggest that CAMP, cGMP and 1,2-diacylglycerol (DAG) could act as second messengers to regulate [Car] in platelets. As PAF is known to stimulate platelet phosphoinositide hydrolysis (ergo DAG formation) but fails to elevate platelet CAMP or cGMP. it is proposed that DAG, via activation of protein kinase C, may act as an endogenous modulator of platelet [Car]: an action that contributes to the role of DAG as a bi-directional regulator of platelet reactivity.

Protein kinase C promotes arachidonate mobilization through enhancement of CoA-independent transacylase activity in platelets

“…Halenda et al [13,14] have shown that the active phorbol esters potentiate AA release induced by the Ca2+ ionophore A23187 in platelets, and this potentiation has been interpreted in terms of activating phospholipase A2 by the protein kinase C system. However, phorbol 12-myristate 13-acetate (PMA) alone, which does not mobilize Ca2+ [15,16], is able to release AA [17]. Thus factors other than phospholipase A2 activation, such as inhibition of AA re-incorporation [18] or stimulation of transacylase reactions, might be involved in the protein kinase C-dependent AA release.…”

Cyclic nucleotides attenuate thrombin-evoked alterations in parameters of platelet Na/H antiport. The role of cytosolic Ca.

“…This inhibition is mediated through lowering the Ca, (11)(12)(13)(14)(15)(16)27) and via Ca-independent mechanisms (28). Blunting 'or ablating of agonistevoked Cai response by cyclic nucleotides involve key elements ofCa, homeostasis, including inhibition of(a) agonist-receptor interaction (29); (b) PLC (11,12,30,31); (c) Ca mobilization from intracellular organelles (26,27,30,32); and (d) Ca uptake from the extracellular compartment (27, 30, 32).…”

“…Cyclic nucleotides are cellular messengers that inhibit platelet aggregation and diametrically antagonize Cai in these cells (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). We have thus focused this investigation on the role of cyclic nucleotides in modulating parameters of the Na/H antiport.…”

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“…Using TPA and human platelets we have recently shown that, besides mediating cellular activation, protein kinase C may also regulate agonist-induced Ca2+ flux (MacIntyre et al, 1985). In the present study we examined the effects of TPA on changes in [Ca2+]i in human neutrophils elicited by the chemotactic tripeptide formylmethionylleucylphenylalanine (FMLP), leukotriene B4 (LTB4) and 1-0-hexadecyl-2-acetyl SN glycero-3-phosphorylcholine (platelet-activating factor, PAF).…”

“…Thus, SCH 23390 blocks stereotypy induced both by apomorphine and by the selective D-2 agonist RU 24213 (Molloy & Waddington, 1984, 1985O'Boyle et al, 1984). SK&F 83566 is a potent D-1 antagonist, this action residing in its R-but not S-enantiomer; residual D-2 antagonist activity is weak and shows negligible enantioselectivity (O'Boyle & Waddington, 1984 motor responses which were promptly abolished by tetrodotoxin (TTX), (3x10-7M).…”