52 P. Razdan et al. ter ovulation (Fig. 5). Both groups had a signif- icantly (p = 0.001) higher concentration of P4 in the period, 13 to 24 h after ovulation, com- pared with the period -11 h to 0 h (0 = time of ovulation) and it continued to increase. There were no signiﬁcant differences between the 2 groups before or after the ovulation. P r o s t a g l a n d i n m e t a b o l i t e . The plasma levels of the prostaglandin metabolite (15-ke- todihydro-PGF2α) were signiﬁcantly (p = 0.0001) higher in the E-group compared with the C-group during the fasting period, while there were no signiﬁcant differences between the groups in the prefasting period (Fig. 6). Discussion It has been suggested that the development of embryos during oviductal transit is affected by the rate at which they are transported to the uterus (Hunter 1988). The food deprived sows had a signiﬁcantly greater number of ova in the proximal and middle part of the isthmus com- pared to the control sows, whereas the control sows had most of their ova in the distal part of the isthmus and uterine horns. This indicates that fasting, either directly by inducing a change in the metabolism, or indirectly via the effect of stress and the hormonal changes in- duced by stress, lowers the rate at which the ova are transported through the oviduct. The circu- lar muscle layers of the isthmus are very richly innervated, with high densities of adrenergic nerve terminals (Hunter 1988, Rodriguez-Mar- tinez et al. 1982). The dense adrenergic inner- vation permits the isthmus to act as a physiolog- ical sphincter which may be important for regulating the transport of gametes. It has also been shown in in vitro studies of the oviduct that the isthmus during the period of ova reten- tion in the ampulla-isthmic junction, reacts more strongly to noradrenaline and adrenaline suggesting the existence of an adrenergic sphincter (Rodriguez-Martinez 1984). Acta vet. scand. vol. 42 no. 1, 2001 Mwanza et al. (2000b) found a deviant isthmic motility pattern in fasted sows. In control sows there was a decrease in the luminal isthmic pressure after ovulation; this decrease was de- layed in the fasted sows and they also had higher frequencies of phasic pressure ﬂuctua- tions in the isthmus during the same period. A delay in the opening of the ampullary isthmic junction could have restricted ova displacement through the isthmus (Mwanza et al. 2000b). One explanation could be that stress in general causes an up-regulation of the sympathetic nerve-system, which induces contraction of the smooth muscles, creating a physiological sphincter in the oviduct. The fasted sows had a signiﬁcantly higher blood plasma concentration of the PGF2α-me- tabolite than the control sows from 6 h after ov- ulation and onwards (Fig. 6). Prostaglandin F2α has been shown to increase the in vitro motor activity of the isthmus and its circular muscle layer, especially during day 3 of the oestrous cycle (Rodriguez-Martinez et al. 1985). Pet- tersson et al. (1993) found that treatment with PGF2α increased the frequency of phasic pres- sure ﬂuctuations in a dose-dependent manner, supporting the theory that PGF2α is involved in maintaining intraluminal pressure in the isth- mus during proestrus. Before ovulation, there is a physiological decrease in the concentration of prostaglandin F2α-metabolite, which is asso- ciated with a decrease in isthmic pressure (Mwanza et al. 2000b). Therefore it may be speculated that an increase in the PGF2α.pro- duction, as reﬂected by the increased blood plasma concentration of prostaglandin F2α-me- tabolite, will further enhance the effect of the previously assumed stress-induced adrenergic response. No signiﬁcant difference was observed in P4 levels between the groups (Fig. 5). This is not consistent with earlier ﬁndings where increased P4 levels have been noted in sows deprived of