EDTA and EGTA can discriminate tonic contractions induced by thromboxane A2 and phorbol ester in rabbit aortic smooth muscles.

The effects of chelating agents of divalent cations on the contraction induced by STA2, a stable thromboxane A2 analog, were examined in rabbit aortic smooth muscles, comparing with a phorbol ester-induced contraction. Pretreatment of muscles with EDTA (4 mM) resulted in potent inhibition of STA2 (1 nM)-induced contraction. However, STA2 could contract muscles slowly in the presence of EGTA (4 mM). The muscles contracted with STA2 relaxed rapidly after addition of EDTA and/or EGTA. However, STA2 recovered contraction in the presence of EGTA, but not of EDTA. PDBu, phorbol 12,13-dibutyrate, also contracted the muscles slowly and potently. Pretreatment with EDTA, but not EGTA, attenuated PDBu-induced contractions. The muscles contracted with PDBu relaxed after the addition of EDTA, but not of EGTA. The present results imply that the vascular smooth muscle contractions are composed of two distinct components: a short-lived contraction that is related to Ca2+ and a lasting contraction that is related to Mg2+, and EDTA and EGTA are good tools for discriminating the tonic phase of STA2- and/or PDBu-induced contraction.