Effects of extracellular Ca2+ and the intracellular Ca2+ antagonist 8-(n,n-diethylamino)octyl-3,4,5-trimethoxybenzoate on rabbit platelet conversion of arachidonic acid into thromboxane.

The regulatory role of Ca2+ on the conversion of arachidonic acid (AA) into thromboxane B2 (TXB2) was examined in washed rabbit platelets, whose secretory processes are known to have requirements for extracellular Ca2+. Varying the extracellular free Ca2+ [Caf2+] concentration from less than 10(-8) M to 10(-3) M had no significant effect on the synthesis of immunoreactive TXB2 by rabbit platelets incubated with 1-4 microM AA. On the other hand, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), a putative antagonist of intracellular Ca2+ movement, inhibited AA-stimulated synthesis of TXB2 in a concentration dependent manner--an effect which could be partially overcome by increasing the AA concentration. The TMB-8 inhibition could not be reversed by increasing the [Ca2+f]. Studies examining platelet metabolism of 14C-AA and 14C-prostaglandin H2 demonstrated that TMB-8 inhibited platelet cyclooxygenase, but not thromboxane synthetase. These studies demonstrate the absence of a requirement for [Ca2+f] but suggest the presence of a TMB-8 sensitive intracellular Ca2+ pool in the rabbit platelet synthesis of TXB2 from AA.