Effects of catecholamines on plasma free fatty acids in fed and fasted cattle

We have studied the effects of 60-min infusions of dopamine (DA), norepinephrine (NE), epinephrine (E), isoproterenol (ISO), phenylephrine, phentolamine, and propranolol alone, of 60-min infusions of NE, E, and ISO in the presence of propranolol, and of 7-min infusions of ISO in the presence of phenylephrine on FFA levels in blood plasma of cows, heifers, and steers fed according to requirements. The effect of 60-min infusions of propranolol on FFA levels was also studied in steers fasted for 4 days. In addition, plasma levels of FFA, free glycerol, glucose, lactic acid, and E were measured during E infusions before and after 4 days of fasting in steers. Plasma FFA increased during NE, E, ISO, and phentolamine infusions, whereas DA and phenylephrine had no effect. Propranolol inhibited the effects of NE, E, and ISO on FFA. Propranolol alone had no significant effect on FFA levels in fed animals, but lowered the elevated FFA levels in fasted animals. The increases in FFA and glycerol during E infusions were more marked in starved than in fed animals, whereas the increases in glucose and lactic acid were smaller. Basal plasma E concentrations were similar in fed and fasted animals, but plasma E increased to higher levels during E infusions in starved than in fed animals. The data suggest that in cattle, as in other species, beta-adrenergic agonists and beta-adrenergic components of catecholamines elevate plasma FFA, whereas alpha-adrenergic agonists and DA have apparently no effect or no direct effect. Low plasma insulin levels, exposition of fat cells to higher circulating levels of E during E infusions, and decreased reesterification of FFA in starved compared to fed animals may contribute to increased lipolytic activity and the enhanced FFA response to E infusions during fasting.