Effect of ouabain on the innervated and noninnervated embryonic chick heart

Attempts to assess the importance of the role played by endogenous catecholamines in the positive inotropic response to ouabain have produced contradictory results. The sympathetic nervous system is not present in the 4-day-old chicken embryo heart but is fully developed after 7 days of embryonic life. This was confirmed by the fact 4-day-old hearts do not respond to tyramine and cocaine while the usual positive inotropic and chronotropic responses were observed when these drugs were administered to 7-day-old hearts. In spite of this difference the positive inotropic response to ouabain was virtually identical at these two stages of development.