Beta-adrenergic receptors in corpora lutea from different stages of the estrous cycle in conscious and slaughtered cattle

The overall objective of this study was to determine the pattern of beta-adrenergic receptors in bovine luteal membranes obtained from slaughtered and from conscious cattle on different days of the estrous cycle. Two approaches were used with respect to the CL collection. In the first approach, CL (n = 20) were obtained from animals slaughtered in a local abattoir. The day of the estrous cycle was estimated based on the morphology of luteal tissue and follicles present. Using 3H-dihydroalprenolol (3H-DHA) as a ligand, we were unable to find specific binding in any of the tissues examined. Therefore, for the second approach we decided to collect CL from mature heifers or cows under local anesthesia on Days 4 (n = 15), 8 (n = 7), 12 (n = 6), and 16 (n = 3) of the estrous cycle by means of colpotomy or by lumbar incision. This procedure was chosen to reduce the influence of stress factors before CL collection. In these samples, specific beta-adrenergic binding sites were found and they varied during the estrous cycle. Maximal binding (Bmax; fmol/mg of protein) was highest on Day 4 (178.3 +/- 15.2); it then decreased significantly (73.2 +/- 14.7, 40.1 +/- 5.5, 10.8 +/- 0.8) on Days 8, 12, and 16, respectively. The dissociation constant (Kd; nM), which followed the same pattern, was 52.0 +/- 14.4, 16.3 +/- 3.9, 14.9 +/- 3.2, and 7.6 +/- 4.0) on the same days, respectively. When the total number of beta-receptors was expressed per whole CL, it was correlated (r = 0.76; p < 0.004) with progesterone plasma concentrations and was highest on Days 8 and 12 compared to Days 4 and 16. Noradrenaline and adrenaline blood concentrations in conscious animals were 0.36 +/- 0.02 and 0.14 +/- 0.01 ng/ml, respectively, whereas in slaughtered cattle they were 32 and 104 times higher, i.e., 11.79 +/- 4.45 and 14.62 +/- 2.06 ng/ml, respectively. We assume therefore that beta-receptors could be involved in the regulation of CL secretion and that long-lasting stress down-regulates specific luteal beta-receptors.