RECEPTOR SUBTYPE AND INTRACELLULAR SIGNAL-TRANSDUCTION PATHWAY ASSOCIATED WITH SITUS-INVERSUS INDUCED BY ALPHA-1-ADRENERGIC STIMULATION IN RAT EMBRYOS

In previous studies, we have demonstrated that stimulation of alpha1 but not alpha2 or beta adrenergic receptors in rat embryos grown in culture interferes with normal development of the left/right body axis leading to situs inversus. In the present study, we aimed to determine the alpha1 adrenergic receptor subtype and signal transduction pathway involved in this phenomenon. Rat embryos at Stage 11a by a modified Theiler's staging system were cultured for 50 hr in medium containing various compounds which are known to activate or inhibit different sites of the signal transduction pathways associated with alpha1 adrenergic receptors. They were then examined to determine the sidedness of asymmetric body structures. WB4101, a selective antagonist of alpha1A adrenergic receptor subtype, but not chlorethylclonidine, a selective antagonist of alpha1B adrenergic receptor subtype, inhibited phenylephrine (an alpha1 adrenergic agonist)-induced situs inversus. Neither the protein kinase C (PKC) activators phorbol 12-myristate 13-acetate and SC-9 nor the PKC inhibitor calphostin C caused situs inversus. Furthermore, calphostin C did not block phenylephrine-induced situs inversus. A23187, a Ca2+ ionophore, induced situs inversus; nifedipine, a L-type Ca2+ channel blocker, partially blocked phenylephrine-induced situs inversus. The calmodulin antagonists trifluoperazine, W-7, and W-13 blocked phenylephrine-induced situs inversus, although they did not cause situs inversus by themselves. KN-62, a Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) inhibitor, dose-dependently blocked phenylephrine-induced situs inversus. However, higher concentrations of this compound produced no block in the presence of phenylephrine and in its absence produced a 50% incidence of situs inversus. These results indicate that alpha1 adrenergic stimulation-induced situs inversus is mediated by the alpha1A adrenergic receptor subtype and that activation of CaM kinase II but not PKC may be involved. (C) 1994 Academic Press, Inc.