Pertussis toxin, but not tyrosine kinase inhibitors, abolishes effects of U-50488H on [Ca2+](i) in myocytes

Abstract

The effects of 10-5 M trans-3,4-dichloro-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeacetamidel (U-50488H), a κ-opioid receptor agonist, on cytosolic Ca2+ concentration ([Ca2+](i)) and the [Ca2+](i) transient in quiescent and electrically stimulated rat ventricular myocytes, respectively, were determined after the cells had been pretreated with pertussis toxin (PTX) or a tyrosine kinase inhibitor (genistein or tyrphostin). The [Ca2+](i) was determined with a spectrofluorometric method, with fura 2 as Ca2+ indicator. U-50488H at 10-5 M itself induced a [Ca2+](i) transient in the quiescent cells but inhibited the [Ca2+](i) transient in electrically stimulated cells. The effects of 10-5 M U-50488H on [Ca2+](i), which were blocked by a selective K-opioid receptor antagonist, nor-binaltorphimine (10-6 M), were abolished after pretreatment with PTX (1 μg/ml) for 24 h, but not with genistein (10-4 M) or tyrphostin (5 x 10-5 M) for 30 min. 1-[6-[[(17b)-3-Methoxyestra-1,3,5(10)-trien-17- yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), an inhibitor of phospholipase C, at 10-5 M, but not its inactive structural isomer 1-[6-[[(17b)-3- methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5-pyrrolidinedione (U- 73343), also blocked the Ca2+ responses to U-50488H. The results indicate that activation of phospholipase C on κ-opioid receptor stimulation is via PTX-sensitive G proteins but does not involve protein tyrosine phosphorylation.