Duodenal infusion of oil in midlactation cows .7. Interaction with niacin on responses to glucose, insulin, and beta-agonist challenges

Metabolic effects of rapeseed oil, niacin, or both were investigated for multiparous Holstein-Friesian cows in midlactation. Cows were continuously infused with water (control), rapeseed oil, niacin, or rapeseed oil plus niacin into the proximal duodenum, using a 4 x 4 Latin square design. Oil infusion resulted in higher plasma concentrations of cholesterol and urea, but a lower plasma concentration of glucose, without changing basal concentrations of insulin, NEFA, and BHBA in plasma or causing a difference in the concentration of metabolites in jugular versus mammary vein plasma. Effects of niacin were small and nonsignificant for almost all of these measurements. Responses of plasma glucose, NEFA, and insulin to challenges by isoproterenol (beta-agonist), glucose, or insulin were studied. Rapeseed oil infusion lowered glucose tolerance. Niacin infusion increased responses by glucose and insulin to glucose challenge. Oil and niacin negatively interacted for responses of plasma concentrations of glucose and insulin to isoproterenol challenge. Positive energy balance in midlactation cows possibly masked potential niacin effects on the studied measurements.