Dietary fat and the beta-adrenergic-mediated chronotropic response in the rat

We have investigated the effect of dietary fat on isoproterenol-stimulated and propranolol-inhibited heart rate in the rat. In the first experiment, weanling male Sprague-Dawley rats were fed diets containing 10% butter (10B), 10% corn oil (10C) or 9% butter/1% corn oil (10M) for 4 wk. Heart rate was determined in response to increasing i.v. doses of isoproterenol. The percent stimulation at the highest dose of isoproterenol (100 microM) was found to be significantly higher (P less than 0.05) in the 10C-fed group than either the 10B- and 10M-fed groups. A similar study investigated the effect of propranolol inhibition of heart rate in low-fat (10C and 10B) as well as high-fat (25C) corn oil- and high-fat (25B) butter-fed animals. Heart rate was elevated with a constant i.v. infusion of isoproterenol and the inhibitory effect of propranolol was determined. The ED50 was found not to depend on the level of fat, but was found to be significantly lower in the corn oil-fed animals than in those red butter fat (P less than 0.05). Thus, animals appear to be more sensitive to these beta-adrenergic agonists/antagonists when fed a diet containing corn oil rather than butter fat as the lipid source.