PLASMA NONESTERIFIED FATTY-ACID RESPONSE TO A BETA-ADRENERGIC CHALLENGE BEFORE OR AFTER FEEDING IN ENERGY UNDERFED OR OVERFED, DRY OR LACTATING COWS

The effects of adrenergic challenge (in order to evaluate the adipose tissue lipolytic potential) and time of challenge relative to feeding on the response curve of plasma non-esterified fatty acids (NEFA) in f oul underfed or overfed non-lactating non-pregnant cows were studied. Basal NEFA and NEFA response to isoproterenol (ISO; 4 nmol/kg body wei ght) were higher when challenged before than after feeding and higher in underfed than overfed cows. Interaction between feeding level and t ime of challenge was significant (P < 0.001)for the rising part qi NEF A response. Pooled NEFA response curves from several trials (no. = 63 challenges) were analysed in order to obtain a simplified procedure fo r the prediction of the NEFA response. High correlations were found be tween the response area or maximal NEFA response and NEFA response at 15 min (r = 0.95 and 0.98, respectively). This simplified procedure wa s applied on pooled data from 84 challenges in order to evaluate the e ffects of energy balance, physiological status, body condition score ( BCS) and time of challenge relative to feeding. Energy balance had a s ignificant effect on basal plasma NEFA and NEFA response at 15 min aft er ISO challenge (-7.6 and -14.2 mu mol/l when the daily energy balanc e increased by 1 MJ respectively). Time of ISO injection relative to f eeding had a greater effect on stimulated NEFA than on basal NEFA (308 v. 139 mu mol/l). There was a significant effect of BCS (41 mu mol/l per unit of BCS) on the basal plasma NEFA level. The NEFA response at 15 min after ISO challenge was lower (387 mu mol/l) in lactating cows than in dry cows. The NEFA response to ISO at 15 min could provide an efficient method of studying the adipose tissue lipolytic potential of cattle in vivo.