The objective of this experiment was to determine the effects of controlled heat stress on ovarian function of dairy heifers. Estrus was synchronized in Holstein heifers (estrus = d 0), and heifers then were randomly assigned to either heat stress (n = 10; 33 degrees C, 60% relative humidity) or thermoneutral (n = 11; 21 degrees C, 60% relative humidity) treatment. For heat-stressed heifers, ambient temperature was increased from thermoneutrality to heat stress (33 degrees C) between d 9 and 14 (2.4 degrees C/d increase) after the synchronized estrus and remained between 31 and 33.5 degrees C until d 22. From d 11 to 21, the growth and regression of ovarian follicles and corpora lutea were measured by using ultrasonography, and blood was collected daily for serum progesterone and estradiol analyses. The second wave dominant follicle was larger for the heifers in the thermoneutral environment than for heat-stressed heifers, and ovulation of the second wave dominant follicle occurred in 9 of 11 thermoneutral heifers. For 6 of 10 heat-stressed heifers, the second wave dominant follicle regressed and was replaced by an ovulatory third wave dominant follicle. Smaller follicular size in heat stressed heifers was associated with decreased serum estradiol concentrations between d 11 and 21. Serum concentrations of progesterone during the luteal phase were similar, but luteolysis was delayed in heat-stressed heifers compared with onset in heifers in the thermoneutral treatment. Conclusions were that heat stress inhibited the growth and function of the dominant follicle so that most of the heat-stressed heifers had three follicular waves and a delay in corpus luteum regression.

Ovarian follicular growth and steroidogenesis are controlled by the interaction of insulin-like growth factors (IGFs) and gonadotropins. The objective was to determine the temporal and spatial relationships for gonadotropin receptor, steroidogenic enzyme, and IGF system gene expression during the development of preovulatory porcine follicles. Sows (n = 18) were weaned and follicles were monitored by transrectal ultrasonography. Ovaries were collected from sows when the mean diameter of the preovulatory follicular cohort was approximately 2, 4, 6, or 8 mm. mRNA were measured by in situ hybridization for individual follicles within the preovulatory cohort (3 to 5 follicles per sow). Patterns of gene expression detected by in situ hybridization were confirmed by RNase protection analyses of pooled RNA samples. The amount of LH receptor mRNA and steroidogenic enzyme mRNA (17alpha-hydroxylase and aromatase) increased as the mean diameter of the follicular cohort increased from 2 to 6 mm, but then decreased abruptly for 8-mm follicles. Estradiol concentrations in follicular fluid closely followed the expression patterns of steroidogenic enzymes and LH receptor mRNA. FSH receptor mRNA was present in cohorts of 2-mm follicles but declined in 4-mm follicles and was undetectable in 6- and 8-mm follicles. The expression of IGF-I and type I IGF receptor mRNA were similar for follicles of 2, 4, 6, and 8 mm. In contrast, IGF-II mRNA progressively increased in follicles collected from 2-, 4-, and 6-mm cohorts, and then decreased slightly at 8 mm. Type II IGF receptor mRNA was greatest in 8-mm follicles. IGF binding protein-2 (BP-2) mRNA decreased as follicles achieved progressively larger sizes during the preovulatory period (2 to 8 mm), whereas the IGFBP-4 mRNA remained relatively low for follicles in 2- to 6-mm cohorts but then increased markedly in 8-mm follicles. In summary, temporal and spatial patterns of gene expression for gonadotropin receptor, steroidogenic enzyme, and IGF system genes were characterized in preovulatory porcine follicles by using in situ hybridization and RNase protection analyses. The unique patterns of gene expression suggest interdependence among specific genes that may be essential for preovulatory follicular development.

The somatotropin receptor mRNA is controlled by at least two different gene promoters that generate two variants with different exon 1 sequences (1A and 1B). The location of 1A and 1B somatotropin receptor mRNA within cattle tissues and, hence, the tissue specificity of the 1A and 1B promoters are unknown. In addition, the cDNA sequence of the 1B somatotropin receptor has not been determined. Our objective, therefore, was to sequence a cDNA for the 1B somatotropin receptor and to analyze bovine tissues for expression of 1A and 1B somatotropin receptor mRNA. Twenty adult tissues and six fetal tissues were collected at slaughter from each of four cows and two fetuses. Messenger RNA was analyzed using ribonuclease protection assays. The adult liver expressed both 1A and 1B mRNA. All other adult tissues expressed 1B mRNA but not 1A mRNA. The greatest amount of 1B mRNA was detected in liver and adipose (abdominal and subcutaneous) tissues. Other tissues had approximately one-half to one-tenth of the amount of 1B mRNA in the liver or adipose tissue. Fetal tissues (including fetal liver) expressed 1B mRNA and not 1A mRNA. Based on cDNA sequencing, the protein encoded by the 1A and 1B mRNA was nearly identical. We concluded that 1A somatotropin receptor mRNA is specific to adult bovine liver. Other adult and fetal bovine tissues expressed 1B somatotropin receptor mRNA with a predicted protein sequence that was similar to the 1A somatotropin receptor.

Fetal growth is increased when pregnant gilts are treated with recombinant porcine somatotropin. The mechanism for increased fetal growth was examined by measuring the expression of IGF-I and -II and IGF-binding protein-2 (IGFBP-2) mRNA in liver and reproductive tissues of somatotropin-and saline-treated pregnant gilts.Twenty-four pregnant gilts received daily injections of either saline (control; n=12) or 5 mg recombinant porcine somatotropin (n=12) from day 30 to day 43 of gestation.