Effects of abomasal infusion of nicotinic acid on responses to glucose and β-agonist challenges in underfed lactating cows

The objectives were to assess the use of nicotinic acid (NA) to chronically (i.e., 74 h) manipulate plasma nonesterified fatty acid (NEFA) concentrations in partially feed-restricted lactating cows, determine whether the reduction of plasma NEFA altered responses to i.v. glucose tolerance test (ivGTT) and whether NA would attenuate an acute lipolytic stimuli of a beta-agonist challenge (ivBAC). Eight lactating dairy cows [244 +/- 31 d in milk; 696 +/- 63 kg of body weight (BW)] were blocked by breed and body condition score (3.2 +/- 0.4) and randomly assigned to a sequence of 2 treatments in a crossover design. Treatments were 74-h continuous abomasal infusion of NA solution (3 mg/h per kg of BW) as an antilipolytic agent to decrease plasma NEFA concentrations or the same volume of water (200 mL/h), concomitant with partial feed restriction. From 0 to 74 h of each experimental period, cows were feed-restricted to 33% of the ad libitum intake recorded during the prior 5 d. An ivGTT (0.25 g/kg of BW of glucose i.v.) and an ivBAC (4 nmol/kg of BW of isoproterenol hydrochloride, i.v.) were performed at 48 and 72 h, respectively. Intake was 24.1, 8.2, 8.0, and 8.0 kg of dry matter/d before restriction, on d 1, 2 and 3, respectively. Nicotinic acid decreased plasma NEFA and increased insulin and glucose concentrations during feed restriction. Nicotinic acid also led to greater glucose and insulin response areas under the curve during ivGTT [glucose: 6,562 vs. 5,056 (mg/dL) x180 min; insulin: 6,042 vs. 2,502 (mu IU/mL) x180 min] and ivBAC [glucose: 535 vs. 240 (mg/dL) x120 min; insulin: 1,283 vs. 222 (mu IU/mL) x120 min], and enhanced NEFA area under the curve during ivBAC [45,521 vs. 22,862 (mu Eq/L) x120 min]. Milk, fat, and protein yields (29.1, 1.2, and 0.93 kg on d -2, respectively) decreased to 17.9, 0.81, and 0.56 kg for control, and 11.5, 0.54, and 0.39 kg for NA on d 3, respectively. Nicotinic acid may have decreased production by inhibiting the supply of NEFA for energy and milk fat synthesis. Milk urea nitrogen was increased by NA on d 2 (12.8 vs. 19.1 mg/dL) and d 3 (11.6 vs. 17.8 mg/dL), probably due to a greater reliance on mobilized amino acids. Somatic cell count was increased by NA on d 3 (187 vs. 848 x1,000 cells/mL). Patterns of glucose and insulin concentration observed during 74 h of NA infusion reflect a state of insulin resistance, which contrasts with shorter term responses in nonlactating cows. Data suggest that long-term supraphysiological infusion of NA affected intermediary metabolism beyond antilipolysis and did not inhibit acute lipolytic stimuli of ivBAC.