2‐aminoethoxydiphenyl borate (2‐APB) and gentamicin inhibit thromboxane A 2 induced calcium responses in cardiac myocytes

Previous work in our laboratory has shown that accelerated idioventricular arrhythmias are induced by the administration of a thromboxane A 2 (TxA 2 ) mimetic, U46619 (U4), into the left atrium of the anesthetized rabbit. Preliminary experiments indicate that application of U4 to cultured myocytes increases intracellular calcium. Therefore, we hypothesize that TxA 2 may produce arrhythmias by directly stimulating cardiac myocytes and altering intracellular calcium via activation of the IP 3 pathway. Ventricular cardiac myocytes from the rabbit heart were isolated and treated with the fluorescent indicator fluo‐4 in 3 series of experiments to examine the effects of U4 and 2 inhibitors of IP 3 mediated calcium release, 2‐APB and gentamicin. In series 1, 5 μM U4 was added to cells that had not been pretreated resulting in an average increase in calcium of 33.7 ± 65.7% (n = 24). In series 2, cells that were pretreated with 1.5 μM 2‐APB exhibited an average calcium increase of 8.1 ± 11.7% upon addition of U4 (p = 0.07; n = 15). In series 3, cells that were pretreated with 1.5 μM gentamicin exhibited an average calcium increase of 6.4 ± 8.5% after addition of U4 (p = 0.03; n = 21). In all 3 series, KCl (50 μM) was administered after U4 as a positive control; cells that did not respond to KCl were discarded from further analysis. These data support the hypothesis that TxA 2 induces a calcium response in cardiac myocytes that involves the IP 3 pathway.