Effects of niacin supplementation on the insulin resistance in Holstein cows during early lactation

Insulin resistance in early lactation includes low glucose concentration, low insulin release and responsiveness and high lipolysis. Niacin is important antilipolytic agent and leads to increase glucose and insulin concentration. The objectives of this study were to determine the influence of niacin on the insulin resistance in cows during early lactation using the difference of value and regression analysis between blood non-esterified fatty acid (NEFA), glucose and insulin concentrations, revised quantitative insulin sensitivity check index and glucose-to-insulin ratio. Niacin supplementation led to a decrease of NEFA concentration and an increase of glucose and insulin concentrations during the first three weeks after calving. Cows in the niacin group which were more resistant to insulin showed higher concentrations of non-esterified fatty acid in comparison with more sensitive cows from the same group, but still lower than the control. The regression analyses suggest the following characteristics of cows supplemented with niacin in comparison with the control group: the insulin response to glucose was more intense; the antilipolytic effect of insulin was lower; insulin efficiency expressed as glucose-to-insulin ratio increase with a decrease in NEFA. The metabolic changes due to niacin supplementation showed a dual influence on the insulin resistance in dairy cows during early lactation: decreased NEFA concentrations led to a decrease in the insulin resistance (due to an increase in insulin efficiency and insulin sensitivity index), but increased concentrations of insulin and glucose possibly caused an increase in the insulin resistance in dairy cows (due to lower insulin sensitivity index and possibly lower antilipolytic effects of insulin). Cattle, nicotinic acid, transition period, glucose metabolism The transitional period, from 3 weeks before to 3 weeks after parturition, is critically important to health, production, and profitability of dairy cows (Butler 2000). Dairy cows undergo tremendous changes during the transition from late gestation to early lactation (Drackley 1999). During early lactation, dairy cows undergo a state of negative energy balance (NEB) and they have to mobilize adipose stores to support energy requirements and milk fat synthesis. Large amounts of fat are mobilized and delivered to the liver via the circulation as non-esterified fatty acids (NEFAs). The concentration of NEFAs in the blood reflects the degree of adipose tissue mobilization (Pullen et al. 1989). Furthermore, NEFAs are used as alternate fuels for much of the rest of the body, and are also converted by the liver to ketones bodies (Drackley 1999), but in excess amounts they may be toxic (Bell 1995; Overton 2001). Elevated plasma NEFA concentrations are correlated with postpartum disorders (Dyk et al. 1995), depressed feed intake and increased triglyceride accumulation in the liver (Grummer 1993). Another important change during this period is the development of insulin resistance in peripheral tissues. Insulin resistance is defined as a condition when higher than normal insulin concentrations are needed to ACTA VET. BRNO 2017, 86: 231-238; https://doi.org/10.2754/avb201786030231 Address for correspondence: Marko R. Cincović Department of Veterinary Medicine Faculty of Agriculture University of Novi Sad, Serbia Phone: +38 121 485 3516 E-mail: mcincovic@gmail.com http://actavet.vfu. cz/ achieve normal metabolic responses (Kahn 1978). Increased plasma NEFA concentrations have been associated with insulin resistance in mammals as fatty acids can cause insulin resistance in skeletal muscles by indirectly interfering with the insulin-stimulated glucose transport activity (Petersen and Schulman 2006). Pires et al. (2007) showed that the induction of hyperlipidaemia causes insulin resistance in Holstein cows by impairing both the sensitivity and maximum responsiveness to insulin. Moreover, they suggest that elevated NEFA concentrations during the periparturient period may potentially interrupt adipose tissue functions and that NEFAs may be a key factor triggering insulin resistance in Holstein cows. As a precursor of the coenzyme nicotinamide adenine dinucleotide (NAD) and with the ability to maintain a large number of oxidation-reduction reactions (Bender 2003), niacin exerts antilipolytic effects and, in large doses, reduces the release of NEFAs from adipose tissue (Pires et al. 2007b; Morey et al. 2011). Antilipolytic effects of niacin are mediated by the activation of G protein-coupled GPR109A receptors, which exist in a functional form in cattle (Kenѐz et al. 2014). Consequently, the inhibition of adenyl cyclase activity occurs, reducing the intracellular levels of cyclic adenosine monophosphate (cAMP) and resulting in the suppression of lipolysis (Wise et al. 2003). We hypothesize that niacin administration to dairy cows in the transition period can influence the insulin responsiveness and resistance in adipose tissue by virtue of niacin-induced changes in NEFA, glucose and insulin concentrations. Materials and Methods Animals, treatments and blood collection A total of 30 clinically healthy, multiparous Holstein-Friesian cows in late gestation were included in the study. The average milk yield in the previous lactation was 7500 ± 950 l. Cows were divided into two groups, 15 cows in the experimental and 15 cows in the control group. The experimental group was treated with niacin 14 days prior to the expected calving date and 14 days after parturition. Niacin (Rovimax®Niacin, F. Hoffmann-La Roche AG, Switzerland) was administered per os with food, 120 g/d per cow. The dose of niacin was determined according to a next characteristic: the bioavailability of unprotected niacin is about 10%; in most experimental models 6 or 12 g/d (10% of 120g/d) of protected nicotinic acid was supplemented to cows. Niacin was in the form of nicotinic acid and in a rumen-unprotected form. The control group was not treated with niacin. All the cows were fed and taken care of in a similar fashion, meeting daily NRC standards (2001). Blood samples were collected by venipuncture from the coccygeal vein before morning feeding and were taken three times, on the week of calving and during the first and second week after parturition. Blood samples for biochemical analysis were collected in sterile 10-ml vacuum tubes with gel separator (BD Vacutainer® SST II Advance, BD Plymouth, UK, they contain no other additives other than the clot activator and gel for serum separation) and appropriately marked. After centrifugation of the blood samples at 4 000 × g for 10 min, plasma was obtained. The samples were kept on dry ice and protected from light until laboratory analysis. Samples were analyzed immediately after sampling. Measurements of metabolic properties Blood samples were taken in the week of calving and in the 1st and 2nd week after calving (weeks 0, 1 and 2). NEFA and glucose concentrations were determined using colorimeters (Randox, UK and Pointe Scientific, USA) and a semi-automatic biochemistry analyser (Rayto RT/1904cv, China). Insulin was measured with the enzymelinked immunosorbent assay (ELISA) kit manufactured by Cusabio (code: CSB-E11993B, China), and the results were read and interpreted by the ELISA reader (Rayto, China). Calculation and statistical analyses The index of insulin resistance was calculated on the basis of the following insulin resistance indicators: the glucose-to-insulin (G:I) ratio and Revised Quantitative Insulin Sensitivity Check Index (RQUICKI). The formula for the glucose-to-insulin ratio is as follows: G:I = glucose (mg/dl) / insulin (μU/ml). The RQUICKI is calculated on the basis of the blood plasma concentrations of glucose in mg/dl, insulin in μU/ml and free fatty acids in mmol/l. The formula is as follows: RQUICKI = 1/[log (glucose mg/dl)+log (insulin μU/ml)+log (NEFA mmol/l)] (Holtenius and Holtenius 2007). The influence of niacin supplementation and the week of and after calving on the glucose, insulin and NEFA concentrations, as well as the RQUICKI values, were analysed by the ANOVA method. All samples (n = 15 cow × 3 week = 45) from the supplemented and control cows were divided, according to the RQUICKI values, into a more resistant group (RQUICKI < 0.5) and a less resistant group 232