Adipose tissue, longissimus muscle and anterior pituitary growth and function in clenbuterol-fed heifers.

The present study was conducted to determine the effects of feeding clenbuterol on adipose tissue and longissimus muscle growth in heifers. For 50 d, 14 heifers were fed either a sucrose-based, clenbuterol supplement or a placebo in which the clenbuterol had been omitted. The heifers were slaughtered in two groups, based on initial weight. Adipose tissue from several anatomical sites and longissimus muscle (depending on slaughter group) were obtained fresh at slaughter. Changes in carcass characteristics elicited by clenbuterol were similar to those reported by others for steers and sheep. Subcutaneous (sc) and intramuscular (im), but not perirenal, adipocytes were smaller and there were more cells per g tissue in the adipose tissue depots of the clenbuterol-fed heifers. Clenbuterol decreased lipogenic enzyme activities, fatty acid-binding protein activity, basal lipolysis and acetate incorporation into glyceride-fatty acids (P less than .05) in sc adipose tissue, but had no effect (P greater than .05) on lipogenesis or lipolysis in im adipose tissue. Clenbuterol elicited a 20% increase in type II myofiber diameters (P less than .05) but had no effect on type I myofiber diameters. In vitro growth hormone release by perifused anterior pituitaries was not affected significantly by long-term in vivo exposure to clenbuterol. These data indicate that a depression in lipogenesis is the mechanism by which clenbuterol decreases subcutaneous fat accretion in cattle.

Temporal relationship of the pulsatile fluctuation of luteinizing hormone and progesterone in cattle: a time series cross-correlation analysis.

The temporal relationship between the pulsatile patterns of plasma luteinizing hormone (LH) and progesterone (P4) was studied in mid-luteal (ML) and early-pregnant (EP) dairy cows. Blood samples were collected (via external jugular vein cannulae) at 10-min intervals for 16 h in 5 ML cows (d 10 to 12 of the cycle) and for 10 h in 5 EP cows (d 52 to 56 of gestation). Concentrations of LH and P4 were determined by radioimmunoassays and a time series cross-correlation analysis was utilized to evaluate the temporal relationship between them. A pulsatile pattern was found for both hormones in both groups, and in all animals LH peaks were uniformly followed by P4 peaks. In 80% of the cows in both groups the highest cross-correlation occurred between samples LH(n) and P4 (n + 1) (n = sample number), suggesting that a lag time of about 10 min is necessary for luteal stimulation. Results from both groups demonstrate that P4 is released from the corpus luteum in a pulsatile manner and that its release is at least partially dependent upon the pulsatile pattern of plasma LH. Correlation coefficients between LH and P4 mean level, basal levels, peak frequencies and peak amplitudes obtained in both groups indicate that the conceptus alters the hypothalamic-pituitary-ovarian relationship found in the ML cows, suggesting the existence of another factor(s) acting along with LH to release P4 in EP cows.