by HS. 17a-Hydroxylase activity is rate-limiting in the biosynthesis of androgens, serving as the substrates for estradiol synthesis by the granulosa cells. The reason why
theca cells are susceptible to HS is unclear; it may be related to the fact that theca cells differentiate earlier than granulosa cells.
4. The corpus luteum
The effects of HS on CL function have been examined mainly by measuring plasma concentrations of progesterone. This is an important measure because the hormone is
delivered to the uterus via the circulation, to maintain pregnancy. However, plasma progesterone concentration depends not only on its rate of production by the CL, but
also on the rate of secretion to the circulation; the latter depends on ovarian luteal blood
Ž .
flow, which was found to be lower by 30 in HS rabbits Lublin and Wolfenson, 1996 . Furthermore, possible adrenal release of progesterone, metabolism in the liver, haemodi-
lution or haemoconcentration, the degree of hyperthermia, the type of heat exposure Ž
. acute vs. chronic , the age of the cows, their stage of lactation, and the type of feeding
all contribute to the wide variation among findings on the observed effect of HS on Ž
. plasma progesterone concentration Jonsson et al., 1997; Trout et al., 1998 . Earlier
results were controversial: some show decreased plasma concentration of progesterone Ž
. during HS Wise et al., 1988b; Wolfenson et al., 1988 , others report no change or
Ž .
increased concentration Thatcher and Collier, 1986; Wise et al., 1988a . More recent works also vary in their findings on the effect of HS on plasma
Ž .
progesterone. Exposure of cows Trout et al., 1998; Wilson et al., 1998a or heifers Ž
. Wilson et al., 1998b to HS during the second half of the cycle has been reported to
result in a rise in plasma progesterone concentration that was associated with delayed Ž
. luteolysis. Roth 1998 reported no change in plasma progesterone in lactating cows
exposed to direct solar radiation in summer. Contrary responses have also been Ž
. recorded. Younas et al. 1993 found increased luteal progesterone secretion in cows
Ž .
exposed to fan cooling in summer. Howell et al. 1994 , in a seasonal study, showed a decrease in plasma progesterone concentration in cows in summer, which was not
associated with any change in cross-sectional area of the CL or with the presence of Ž
. luteal cavities. Another seasonal study Jonsson et al., 1997 found a lower concentration
of plasma progesterone in cows in summer than in winter during the life of the second CL after calving, and the difference was not associated with any differences between
seasons, in dry matter intake, body condition score or milk yield. The latter indicates that the decreased progesterone concentration in plasma was directly related to the heat
load and not necessarily to HS-induced nutritional or metabolic changes. It has been suggested that plasma progesterone concentration decreased in cows subjected to
chronic HS, typical of the natural summer environment, and rose in cows subjected to a more acute HS such as exposure to direct solar radiation or heat exposure in a hot
Ž .
chamber Howell et al., 1994 . Evidence for direct suppression of progesterone production by high temperature has
Ž .
been documented in in vitro studies Wolfenson et al., 1993 . Enzymatically dispersed luteal cells collected from cows in summer produced less progesterone during 2 h of
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
