incubation at 388C, and cell viability was lower than that of cells collected from cows in winter. Also, luteal cells collected in winter and incubated at 408C produced 30 less
progesterone than similar cells incubated at 388C. In order to differentiate between the Ž
. effects of the season on progesterone production by small theca derived and large
Ž .
granulosa derived luteal cells, theca and granulosa cells obtained from first-wave dominant follicles from cows in summer and winter were luteinized in vitro for 9 days at
Ž .
Ž .
388C in the presence of forskolin 10 mM and insulin 2 mgrml; Sonego, 1995 . These cells collected from follicles on day 6 of the estrous cycle in each season were similar,
in terms of diameter, number of granulosa cells and viability of granulosa and theca cells. Progesterone production by granulosa-derived, large luteal cells was only slightly
lower in summer than in winter, though the rate of increase in progesterone production was higher in winter than in summer. In contrast, progesterone production by theca-de-
rived, small luteal cells, dropped markedly in summer to one-fifth of the corresponding winter value.
In summary, chronic exposure to summer HS suppressed progesterone production. This was evident in in vitro studies in which progesterone production by luteal cells
obtained from cows in summer was lower than that by cells obtained in winter. Under certain physiological states, HS lowers plasma progesterone concentration. Inadequate
progesterone secretion may have adverse effects during two physiological periods, before and after insemination. Low plasma progesterone concentration can cause
aberrant follicular development, which leads to abnormal oocyte maturation in the
Ž .
ovulatory follicle and early embryo death Ahmad et al., 1995 . Low plasma proges- terone affects steroidogenesis in the dominant follicle and in the subsequently formed
CL, and it also altered endometrial morphology and function in the subsequent cycle Ž
. Shaham-Albalancy et al., 1996a,b . Low plasma progesterone following AI may also
contribute to increased embryo losses. However, the effectiveness of increasing post-in- semination plasma progesterone in improving the fertility of cattle is debatable: Robin-
Ž .
Ž .
son et al. 1989 found a rise in fertility, whereas Breuel et al. 1990 found no effect.
5. Gonadotrophins
The effect of HS on secretion of gonadotrophins in cattle is poorly documented despite the important roles of LH and FSH in regulating follicular growth, ovulation,
and CL function. The available information regarding LHrFSH secretion is limited to concentrations of these hormones in peripheral blood; there are no data on either GnRH
content of the hypothalamus or LHrFSH content of the pituitary under HS.
Plasma concentrations of tonic LH during the estrous cycle of HS cows have been Ž
. reported, in earlier studies, to be unchanged Gwazdauskas et al., 1981 or increased
Ž .
Roman-Ponce et al., 1981 . Similarly, the preovulatory surge of plasma LH concentra- Ž
. tion decreased in HS heifers
Madan and Johnson, 1973 but not in HS cows
Ž .
Ž .
Gwazdauskas et al., 1981; Rosenberg et al., 1982 . Wise et al. 1988a reported a decrease in LH pulse frequency in the early stages of the estrous cycle, which, in turn
may influence luteinization of the CL. In agreement with this finding, it was shown recently that LH secretion early in the cycle plays an important role in the formation of
Ž .
fully functional CL Peters et al., 1994 . The pattern of tonic LH secretion also
Ž influenced the rate of growth and turnover of dominant follicles in cyclic cows Savio et
. al., 1993 . The effects of HS on tonic LH as well as on GnRH-induced preovulatory
surge concentrations were found to be dependent on the concentration of estradiol in Ž
. plasma Gilad et al., 1993 . Chronic HS during summer decreased the mean and
amplitude of tonic LH as well as of GnRH-induced preovulatory plasma LH concentra- tion surge in cows with low concentrations of plasma estradiol. In contrast, neither tonic
LH concentration nor GnRH-induced LH concentration surge was altered in cows with high concentrations of plasma estradiol. Similar responses were recorded in cows
Ž .
acutely heat-stressed for 16 h in a hot chamber during the winter Gilad et al., 1993 . GnRH-induced plasma FSH surge was lower in lactating cows during both chronic
exposure to HS in summer and an acute 16 h exposure to thermal stress in a hot Ž
. Ž
. chamber Gilad et al., 1993 . These reductions were evident as with LH only in cows
Ž .
with low concentrations of plasma estradiol. In contrast as discussed above , in HS cows that were not treated with GnRH, there was a pronounced increase in plasma
concentrations in the FSH surge that preceded the second-wave dominant follicle, and in the preovulatory FSH surge that was associated with decreased plasma inhibin concen-
Ž .
trations Roth, 1998 . In agreement with the latter study, serum FSH concentrations and content of GnRH in the hypothalmus have been found higher in summer than in winter
Ž .
in primiparous sows Armstrong et al., 1986 . The reason for the discrepancy in
Ž .
responses to HS between cows treated with GnRH lower plasma FSH and intact cows Ž
. higher plasma FSH is unclear.
6. Delayed effects of HS
