ADRENERGIC NEUROTRANSMITTERS AND CALCIUM IONOPHORE-INDUCED SITUS-INVERSUS-VISCERUM IN XENOPUS-LAEVIS EMBRYOS

Xenopus laevis embryos at the blastula-early tail bud stage were expos ed to norepinephrine or octopamine dissolved in culture saline until t hey reached the larval stage. The left-right asymmetry of the heart an d gut was then examined. We found that these adrenergic neurotransmitt ers induced situs inversus in the heart and/or gut in up to 35% of tes ted neurula embryos. Norepinephrine-induced situs inversus was blocked by the alpha-1 adrenergic antagonist prazosin. Furthermore, A23187, a calcium ionophore, also increased the incidence of situs inversus up to 54% when late-neurula embryos were exposed to the solution. A23187 treatment initialed before neural groove formation was lass effective. The incidence of situs inversus induced by these reagents decreased t owards the control level (2.2%, 25 untreated embryos out of 1127 embry os in total) in embryos past the stage of neural tube closure. In the present experiments we obtained 22 gut-only situs inversus embryos hav ing an inverted gut and a normal heart. In contrast, such embryos were not observed among the 1127 untreated embryos. An adrenergic signal m ediated by an increase in intracellular free calcium may be involved i n the asymmetrical visceral morphogenesis of Xenopus embryos.