Milk feeding and xylazine treatment induce increased antroduodenal motility in young cattle with opposite effects on duodenal digesta flow rate.

Antroduodenal myoelectrical activity and duodenal digesta flow rate in young cattle were assessed by implanted nichrome wire electrodes and reentrant duodenal cannulation with interposed flow-meter. In addition, a small glass electrode was inserted through a duodenal cannula to continuously measure the pH of duodenal contents. When eating only hay ad libitum, the duodenal myoelectrical activity showed the usual migrating myoelectrical complex (MMC) pattern. Duodenal contents pH rose sharply at the end of a period of repetitive spike activity (RSA), or phase III, from pH less than 2 to greater than 5 and remained high through most of the subsequent period of no spike activity (NSA), or phase I, during which there was no antral spiking and no flow of duodenal contents. Feeding milk (pH 6.5) caused a marked increase in antroduodenal spike activity and intraduodenal flow rate, with a concomitant rapid rise in duodenal contents pH from 1.8 +/- 0.3 (SEM) to 5.0 +/- 0.2 which then slowly declined over the next few hours. Within 5 min after the administration of the alpha 2-adrenergic agonist xylazine (25 and 50 micrograms/kg intramuscular), there was a significant increase (P less than 0.05) in antroduodenal spike frequency and a concomitant marked drop in intraduodenal flow rate with a sustained low pH of intraduodenal contents. The results suggest that: (1) continuous measurement of proximal duodenal pH adds a useful dimension to documenting relationships between antroduodenal myoelectrical activity and duodenal digesta flow rate; and (2) increased spike activity in the antroduodenal region does not necessarily indicate an increased duodenal flow rate is occurring.