The relaxant effects of parathyroid hormone(1-34) and parathyroid hormone-related protein(1-34) on ovine reticulo-ruminal smooth muscle in vivo

The motility of the reticulo-rumen has been measured in trained, conscious sheep using inflated balloons temporarily introduced to selected regions of that forestomach. The frequency and amplitude of the contractions of the r eticulum and both the A and B waves of contraction of the rumen were measur ed under the same conditions before, during and after the administration of an I.V. bolus of either parathyroid hormone (PTH(1-34)) or PTH-related pro tein (PTHrP(1-34)) followed by its I.V. infusion. These two peptides are kn own to share a common receptor in other organs, e.g. the kidney. In this st udy they both showed an inhibitory effect on reticulo-ruminal motility. The effect of PTHrP(1-34) on the rate of ruminal blood flow was also examined and a significant reduction observed, after a transient increase. The secre tion of endogenous PTH(1-34) was stimulated by a 32% reduction in the plasm a calcium ion concentration induced by an I.V. infusion of sodium citrate. Associated with this were significant reductions in reticulo-ruminal motili ty, e.g. the reduction in the mean amplitude of the reticular contractions reflected the reduction in plasma calcium ion concentration. When the PTH(1 -34)/PTHrP(1-34) receptor was blocked with [Asn(10),Leu(11),D-Trp(12)]PTHrP (7-34) before and during the induction of hypocalcaemia, all but one of the parameters of reticulo-ruminal motility were normalized. Indeed, by the da y following the administration of this blocking agent, all these parameters had returned to their normal range. It is concluded that stimulation of th e PTH(1-34)/PTHrP(1-34) receptor in reticulo-ruminal smooth muscle reduces the motility of this tissue and may play a role in the depression of motili ty of the digestive tract which is characteristic of clinical milli fever i n the dairy cow.