Differential origin and control mechanisms in small and large bovine luteal cells

The ready accessibility of the bovine corpus luteum by enucleation through a slit in the anterior vagina (Casida, 1960) enabled researchers to conduct studies in vitro with accurately dated luteal tissue, without slaughtering the donor animals. These studies have led to the accumulation of a large part of the knowledge upon which our current concepts of control of the corpus luteum are based. In our own laboratory, an in-vitro technique, based on measurements of progesterone produced in a 2-h period by slices of bovine luteal tissue was developed in the early 1960s to study the roles of the pituitary hormones on progesterone synthesis. Luteinizing hormone (LH) proved to be the only significant pituitary luteotrophin (Hansel & Seifart, 1967). Soon thereafter, it was established that LH exerts its luteotrophic effects through the cyclic AMP (cAMP) second messenger system (Savard, 1973). However, there were some reasons to doubt that all of the luteotrophic effects were mediated by cAMP. In bovine luteal slices, levels of LH too low to cause measurable increases in cAMP resulted in increased progesterone synthesis, and progesterone responses to increasing concentrations of LH continued to rise after cAMP production had plateaued. These same techniques also proved useful in early studies on the mechanism(s) of luteolysis in cattle. As early as 1966, Armstrong & Black showed that the steroidogenic ability of LH in bovine luteal slices was lost between the 16th and 18th day of the cycle. Following the demonstrations of the luteolytic effects of prostaglandin (PG) F-2a in sheep (McCracken et al., 1971) and cattle (Hansel et al., 1973), the effects of adding PGF-2a to bovine (Hansel et al., 1973) and ovine (Fletcher & Niswender, 1982) luteal tissue were studied in vitro. Surprisingly, PGF-2a stimulated rather than inhibited progesterone synthesis in vitro by bovine luteal tissues. However, synthesis was inhibited in ovine luteal tissues. This and other fundamental differences between ovine and bovine luteal tissues continue to be matters of considerable interest. Over the years, this simple and useful technique has evolved into sophisticated techniques for cell dispersion and cell separation by fluorescence activated cell sorting. In addition, the development of the instrumentation for measurement of intracellular calcium by computer-driven spectrofluorometers has made it possible to study the role of calcium in the control of steroidogenesis in separated cells.