Structure of a protein catalyzing the formation of 11 cis-retinal in the visual cycle of invertebrate eyes

A pigment made up of a protein able to bind retinal as well as retinol is described. The molecule consists of a dimer with a molecular weight of 50,000 which binds one molecule of retinal. The binding site for retinal is a Schiff base buried in the interior of the protein. Retinol is probably bound to the protein in the same site as for retinal, although not covalently, as suggested by the absorbance spectra. The protein, extracted from honeybee retina, is involved in visual pigment metabolism, and its structure may elucidate the mechanism of the stereospecific photoisomerization of all trans-retinal to 11-cis-retinal.     

Effect of rate of change of luteinizing hormone concentration on in-vitro progesterone secretion within rat corpora lutea during differentiation.

Immature rats were injected with pregnant mares' serum gonadotrophin followed by human chorionic gonadotrophin (hCG). Ovaries were removed 0, 2, 5 or 8 days after hCG and either prepared for morphometric analysis or perifused with 0, 5 or 30 ng luteinizing hormone (LH)/min. In a second study, ovaries were removed on Day 2 or 8 and perifused with 0.1 mg 8-br-cyclic adenosine 5'-phosphate/ml (8-br-cAMP). On Day 0, the granulosa cells of the preovulatory follicles were small (53 +/- 0.5 microns2) with a cytoplasmic to nuclear (Cy:Nu) ratio less than or equal to 1.5. By Day 2, corpora lutea (CL) were present and composed of 95% small luteal cells (diameter less than 125 microns2, Cy:Nu greater than or equal to 3.0) and 5% large luteal cells (diameter greater than 125 microns2, Cy:Nu ratio greater than or equal to 3.0). The percentage of large luteal cells increased to 36 +/- 7% by Day 5, suggesting that they are derived from a select population of small luteal cells. Basal progesterone secretion increased from 38 +/- 5 on Day 0 to 1010 +/- 48 pg/mg/ml on Day 8. The rate of 5 ng LH/min stimulated progesterone secretion on Days 0, 2 and 8; 30 ng LH/min stimulated progesterone secretion on Days 0, 2 and 8, but not on Day 5; 8-br-cAMP stimulated progesterone secretion on both Days 2 and 8. These data demonstrate that once granulosa cells are induced to luteinize they lose their capacity to secrete progesterone in response to 5 ng LH/min and do not regain their responsiveness to LH rate until they completely differentiate. The loss of this LH responsiveness appears to be due to an inability to stimulate sufficient intracellular cAMP concentrations, since cAMP stimulates progesterone secretion on both Days 2 and 8.     

Effect of pulse amplitude of luteinizing hormone, duration and rate of change on progesterone secretion from rat corpora lutea.

Corpora lutea (CL) were obtained from immature rats primed with pregnant mares' serum gonadotrophin followed by human chorionic gonadotrophin (hCG). Two days after hCG, CL were isolated, placed in perifusion culture and exposed to control medium or specific pulses of luteinizing hormone (LH). In Expt 1, a frequency of 1 pulse LH/h (amplitude 500 pg/ml, duration 40 min, 30 ng/min) increased progesterone secretion compared with control values (P less than 0.05). In Expt 2, LH rate was held constant and the amplitude and duration of a single LH pulse varied; 250 and 500 ng LH/ml initially stimulated progesterone secretion equivalently, but increasing the duration of the LH pulse prolonged high progesterone secretion. These observations suggest that at less than or equal to 500 ng LH/ml, once a stimulatory amplitude is obtained, higher amplitudes do not further increase progesterone secretion, while increasing pulse duration further enhances progesterone secretion. In Expt 3, the LH pulse amplitude was 250 ng/ml and the rate set at 0, 5 or 30 ng LH/min; only 30 ng LH/min resulted in sustained stimulation of progesterone (P less than 0.05). Taken together, these data demonstrate that the characteristics which determine whether an LH pulse will be stimulatory include not only amplitude and duration but also the rate at which an amplitude is obtained.     

Effect of alterations in pulsatile luteinizing hormone release on ovarian follicular atresia and steroid secretion on diestrus 1 in the rat estrous cycle

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.