Technical note: A method for quantification of saliva secretion and salivary flux of metabolites in dairy cows

Salivary flow and net jugular flux of metabolites were studied during resting and rumination in 3 lactating dairy cows (BW 548 ± 17.2 kg, days in milk 113 ± 4 d). The method was based on the concentration difference between arterial and jugular blood, and jugular blood flow measured by downstream dilution of p-aminohippuric acid (pAH). Cows were surgically prepared with a permanent arterial catheter in A. intercostales dorsales before the trial. On sampling days, cows were prepared with left and right side jugular, and ear vein catheters for blood sampling and infusion of pAH, respectively. Blood was sampled simultaneously from the 2 jugular veins and artery during periods of rest and rumination. Secretion of saliva was set equal to the net water extraction calculated from the increased hemoglobin concentration in jugular blood compared with arterial blood. Arterial and jugular blood flow summed for both sides of the head doubled (P < 0.001) during rumination (437 ± 19, 424 ± 18 L/h, respectively) compared with resting (210 ± 19, 202 ± 18 L/h, respectively), consequently doubling the saliva secretion (P < 0.001, resting = 7.6 ± 0.8 L/h, rumination = 13.8 ± 0.8 L/h). The extraction of inorganic phosphate (Pi) from arterial blood during resting periods was greater compared with rumination (P = 0.004; resting = 21.7% ± 0.9%; rumination = 15.6% ± 0.9%), resulting in a greater Pi concentration in saliva secreted during resting. The concentrations of Pi in saliva were 4.5 ± 0.3 and 3.7 ± 0.3 times the arterial concentration during resting and rumination (P = 0.09), respectively. The urea concentration in saliva was 0.63 ± 0.04 times the arterial level, showing that urea is less efficiently transferred from blood than water, resulting in a greater numerical urea concentration in jugular compared with arterial blood. The water extraction method presented in the present paper offers an alternative way of estimating saliva secretion without the chewing activity constraints associated with other methods, for example, allowing for determination of saliva flow during rumination.