Rhythmic contractions of the teat sphincter in bovines: an expulsion mechanism.

Spontaneous and induced rhythmic contractions of the teat sphincter muscle were measured in conscious bovines using a pressure transponding device. Contractions were induced or the frequency of contractions was increased by intravenous injection of phenylephrine. Epinephrine, isoxsuprine, and phentolamine inhibited contractions and prior administration of phentolamine blocked the response to phenylephrine. Contractions in adjacent teats were asyncronous and the frequency of spontaneous contractions (0-20/min) increased with time after milking, presumably due to the effect of the increased muscle tension that resulted from the accumulation of milk within the gland cistern and the teat cistern. Furthermore, the contractions were found to be peristaltic in nature. Squeezing the tip of the teat was found to induce one or more contractions, whereas squeezing the teat elsewhere had no effect. It was hypothesized that peristaltic contractions in response to irritation of the tip of the teat might represent an expulsion mechanism.