Subclinical ammonia toxicity in steers: effects on blood metabolite and regulatory hormone concentrations

The effects of subclinical NH3 toxicity on circulating and regulatory hormone concentrations were investigated in seven Hereford steers. Ammonium chloride (NH4Cl) was infused via a right jugular vein catheter at a rate of 12 mumol NH4Cl.kg BW-1.min-1 for 240 min. This was preceded (PRE) and followed (POST) by saline infusions of 120 and 180 min, respectively. Blood samples were taken at 20-min intervals via a left jugular vein catheter. Metabolite and hormone concentrations during NH4Cl and POST periods were compared to PRE values using the Student's t-test procedure. Plasma NH3 was elevated rapidly (P less than .001) and peaked at 503 micrograms/dl 220 min into NH4Cl infusion. Plasma urea-N and glucose increased (P less than .001) 39 and 12%, respectively, during NH4Cl infusion and remained elevated 180 min POST. Whole blood L-lactate concentrations peaked (P less than .05) at 18% above PRE between 160 and 240 min into the NH4Cl infusion and gradually returned to PRE values, whereas pyruvate levels were not altered (P greater than .10). Plasma nonesterified fatty acids peaked (P less than .001) at 94% above PRE levels 40 min into NH4Cl infusion, thereafter declining to PRE concentrations. Whole blood acetoacetate and beta-hydroxybutyrate concentrations were not altered (P greater than .10) by NH4Cl administration. Plasma insulin concentration decreased (P less than .05) 26 to 46% during NH4Cl infusion and increased (P less than .05) 89 to 122% during POST. Plasma glucagon levels were not altered by NH4Cl infusion, so molar insulin:glucagon ratio changes resembled those of insulin. Plasma epinephrine, norepinephrine and dopamine did not vary (P greater than .10) with treatment. These results support the hypothesis that the hyperglycemia observed during hyperammonemia may result from an under-utilization of glucose by insulin-sensitive tissues.