Positive inotropic activity of cholera enterotoxin on the embryonic chick heart

The characteristics of the positive inotropic effect of different preparations of cholera toxin on the isolated 4-day-old embryonic chick heart were investigated. Crude (CCT) as well as partially purified and purified (PCT)preparations of cholera toxin were shown to have positive inotropic activity. Contractility was increased within 1 minute and reached a maximum at approximately 40 minutes after exposure to toxin. Activities of all three toxin preparations were abolished by heating, their effects were prevented by addition of antitoxin, but nort propranolol, before exposure of the heart to toxin. The positive inotropic effect of PCT was not reversed by washing, but was reversed by subsequent addition of antitoxin. The effects of CCT were reversed by insulin. Effects of both CCT and PCT were potentiated by theophylline, and both toxin preparations elevated tissue levels of cyclic adenosine 3',5'-monophosphate. A filtrate of CCT (MW is less than 30,000) exhibited similar cardiostimulant activity but displayed certain differences from the other toxin preparations. The filtrate was heat stable, and its effects were blocked by pretreatment with propranolol but not by antitoxin. The filtrate of CCT, unlike the toxin-containing preparations, did not stimulate intestinal fluid secretion in the dog.