1 Carbon 14-Labeled Palmitic Acid Metabolism in Fasted, Lactating Goats Following Nicotinic Acid Administration

Abstract

Four independent infusions of carbonlabeled palmitate were performed, two in each of two goats to allow examination of fatty acid metabolism following nicotinic acid administration to fasted, lactating goats. Nicotinic acid caused an initial inhibition of lipolysis followed by a rebound period of accelerated adipose fatty acid mobilization. Carbon-labeled palmitate was infused for approximately 4h, beginning at 6.5 or 24.5h following nicotinic acid, to coincide with the depression and rebound phases of lipolysis. During maximum depression, plasma nonesterified fatty acids and ketone bodies were generally less than 5 mg/100ml with less than 25% of ketone body carbon originating from fatty acids, even under fasting conditions. Nonesterified fatty acid turnover rate was less than 1.0 mg/kg per min while turnover time was approximately 1.2min. Transition from depressed to accelerated adipose lipolysis was characterized by increased fatty acid turnover rate, turnover time, and incorporation into ketone body carbon. The maximum rebound period displayed elevated plasma nonesterified fatty acids and ketone bodies, with 75 to 95% of ketone body carbon originating from fatty acids. Nonesterified fatty acid turnover rate generally exceeded 7.0mg/kg per min while turnover time was 2.1 to 3.5min. Fatty acid turnover rate, turnover time, and contribution to ketone body carbon were, in general, positively related to circulating plasma nonesterified fatty acid concentrations. This suggests that nicotinic acid has a marked effect on fatty acid release from adipose tissue but minimal effects on subsequent fatty acid metabolism. Label incorporation was slow for glycerides with negligible incorporation into cholesterol esters and phospholipids. Low specific activities in milk fat indicated minimal direct nonesterified fatty acid contribution.