Gonadectomized male Japanese quail maintained on short daylengths (8 h light: 16 h darkness) showed a reliable and repeatable response, in terms of increased gonadotrophin secretion, to a single long photoperiod (20 h light) followed by continuous darkness. Plasma levels of LH were significantly increased within 4 h of the end of the long day. Secretion continued to increase with time and LH and FSH concentrations reached a maximum some 24 h later, thereafter decreasing only slowly over the next 8–10 days. Quail could be used repeatedly at fortnightly intervals and gave a consistent response for at least 15 months. The size of the photoperiodic response and its considerable duration were much greater than found in intact quail. The birds showed a similar (though reduced) response if a short daylength was followed by a single 4-h light pulse interrupting the 16-h dark period. Using this system the time of maximum responsivity to the inductive effects of light lies 10–16 h from 'dawn'. The marked and rapid effects described provide an excellent experimental system in which to study how photoperiodic changes are recognized and how consequent neural activation is translated into enhanced secretion rates of hormones in the peripheral endocrine system.
T. J. Nicholls, B. K. Follett and J. E. Robinson
A. Klein, B. Bruser, J. B. Robinson, P. H. Pinkerton and A. Malkin
We have observed previously that the rate of cortisol catabolism by lymphocytes (CCL) was indicative of the vulnerability of these cells to cortisol. We attempted to ascertain whether cortisol-sensitive lymphocytes (e.g. thymocytes) metabolize cortisol at a different rate from cortisol-resistant cells and whether lymphocytes in which cortisol catabolism is inhibited become cortisol sensitive. The work was facilitated by the observation that an ethanol extract plasma from patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC) had the capacity to inhibit CCL.
The capacity of thymocytes to metabolize cortisol was found to be 11 times lower than that of peripheral lymphocytes. Inhibition of CCL with an ethanol extract of plasma from AIDS/ARC patients made the cells vulnerable to cortisol, causing them to die at a rate seven times greater than that of control samples. It is suggested that these findings may have important implications with regard to the nature of lymphocyte depletion in AIDS/ARC patients or in people at risk of developing the syndrome.
J. Endocr. (1987) 112, 259–264
J. ROBINSON, B. J. MERRY, M. E. LIGHTFOOT and A. K. HALL
The characteristics of the disappearance of progesterone and 20α-dihydroprogesterone from blood were examined in rats made pseudopregnant by administration of gonadotrophins. Measurement of the disappearance of [3H]progesterone from blood indicated that progesterone metabolism can be represented by a two-compartment model in this animal preparation. The disappearance of [3H]progesterone from blood was described by two half-lives of 0·5 and 11·7 min. The metabolic clearance rate for this steroid was 2·9 litre/day, equivalent to a production rate of 3·9 μmol/day. The initial volume of distribution of the injected [3H]progesterone was 5·0 ml, a volume similar in size to the expected plasma volume. The total volume of distribution of [3H]progesterone (i.e. 'inner' and 'outer' pools) was in the range 24·8–35·4 ml.
The disappearance of endogenous progesterone from the blood of pseudopregnant rats after vascular isolation of the ovaries showed two half-lives of 1·1 and 11·4 min respectively. The disappearance of a product of reductive metabolism of progesterone, 20α-dihydroprogesterone, was apparently uniphasic, with a half-life of 41·3 min.
B. Malpaux, J. E. Robinson, N. L. Wayne and F. J. Karsch
Three experiments were conducted to evaluate the role that the increasing day lengths of late winter and spring play in timing the breeding season of the Suffolk ewe. In the first experiment, ewes were denied their normal complement of increasing day length by maintaining them on the photoperiod experienced at the winter solstice. This prevented the breeding season from occurring the subsequent autumn. In the second experiment, ewes were exposed to increases in day length at different time-intervals after the winter solstice: the normal time, later than normal or earlier than normal. Once the summer solstice photoperiod was reached, it was maintained until the end of the study. When increasing photoperiod was provided early, the breeding season was advanced; when it was provided late, reproduction was delayed. In the third experiment, ewes were exposed to a continuously increasing photoperiod matching the maximal rate of rise in natural conditions; this treatment was begun on the spring equinox and continued until mid-autumn. The steadily increasing photoperiod did not alter the time of reproductive onset in the autumn.
These findings support the following conclusions for timing of the breeding season of the Suffolk ewe. (1) The lengthening photoperiod between the winter and summer solstices is required for the occurrence of the breeding season in the autumn. (2) The time of initial exposure to this lengthening photoperiod provides an important cue for determining when the reproductive period occurs. (3) The time of onset of the breeding season does not depend upon the decreasing photoperiod after the summer solstice, nor does it require the photoperiod to stop increasing as the summer solstice approaches. These findings have been incorporated into a conceptual model for temporal regulation of the annual reproductive cycle of the ewe. An important component of this model is a critical role for increasing photoperiod to initiate a process in the late winter–spring which ultimately leads to an obligatory reproductive onset in the autumn.
Journal of Endocrinology (1989) 122, 269–278
M. D. MITCHELL, B. R. HICKS, G. D. THORBURN and J. S. ROBINSON
The rates of production of thromboxane B2 in vitro by intra-uterine tissues obtained from late pregnant monkeys by Caesarean section have been determined. The general quantitative order of rates of production was decidua basalis = decidua parietalis > placenta > chorion > amnion = myometrium. Myometrial production of thromboxane B2 was greater at term than during late pregnancy; no other tissue showed a significant trend with advancing gestation. These data demonstrate that the production of thromboxane B2 by intra-uterine tissues from late pregnant monkeys is both qualitatively and quantitatively different from the production of prostaglandins described previously. It is suggested that prostaglandins rather than thromboxanes are more intimately involved in the onset of labour in the rhesus monkey.
M. D. MITCHELL, B. R. HICKS, G. D. THORBURN and J. S. ROBINSON
The rates of production of 6-oxo-prostaglandin F1α (6-oxo-PGF1α) in vitro by intra-uterine tissues taken from late-pregnant monkeys at Caesarean section have been determined. For tissues obtained between days 140 and 149 of pregnancy (late pregnancy) the general quantitative order of rates of production (per unit weight) was decidua basalis> placenta > decidua parietalis>amnion>chorion = myometrium. When tissues were taken between days 160 and 168 of pregnancy (near term) this order was placenta > decidua parietalis = amnion> myometrium = decidua basalis > chorion. There was a significant reduction near term in the rate of production of 6-oxo-PGF1α by decidua basalis; all other tissues exhibited similar rates of production at the two gestational periods investigated.
M. D. MITCHELL, A. P. F. FLINT, B. R. HICKS, E. J. KINGSTON, G. D. THORBURN and J. S. ROBINSON
Nuffield Department of Obstetrics and Gynaecology, University of Oxford, John Radcliffe Hospital, Headington, Oxford, 0X3 9DU
(Received 21 July 1978)
It now seems likely that prostaglandins play an important role in the mechanisms of parturition in many species (Flint & Hillier, 1976; Thorburn, Challis & Robinson, 1977), including the goat (Thorburn, Nicol, Bassett, Shutt & Cox, 1972; Currie & Thorburn, 1977). This evidence has been further strengthened by the demonstration of the production of prostaglandins in vitro by uterine tissues from goats during late pregnancy (Mitchell, Flint, Robinson & Thorburn, 1978b). The recent discovery of prostacyclin (Moncada, Gryglewski, Bunting & Vane, 1976) has added a new dimension to prostaglandin research since in some biological systems it has a greater potency than other prostaglandins (Moncada et al. 1976; Omini, Moncada & Vane, 1977). Prostacyclin is highly unstable in aqueous media and degrades spontaneously to 6-oxo-prostaglandin F1α (6-oxo-PGF1α; Johnson, Morton, Kinner, Gorman,
J. L. H. O'RIORDAN, J. S. WOODHEAD, G. N. HENDY, J. A. PARSONS, C. J. ROBINSON, H. T. KEUTMANN, B. F. DAWSON and J. T. POTTS Jr
The presence of a single methionine in porcine parathyroid hormone, at position 8, permitted assessment of the role of this residue separate from the second methionine residue found at position 18 of bovine and human parathyroid hormones. Oxidation of the solitary methionine of porcine parathyroid hormone to the sulphoxide destroyed biological activity, but this was restored by subsequent reduction with cysteine. Oxidation of the hormone did not, however, affect its immunological activity; therefore, oxidation of the hormone may bring about dissociation of biological and immunological activity.
Yi Zhang, Yunfeng Liu, Jihong Qu, Alexandre Hardy, Nina Zhang, Jingyu Diao, Paul J Strijbos, Robert Tsushima, Richard B Robinson, Herbert Y Gaisano, Qinghua Wang and Michael B Wheeler
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate pacemaker activity in some cardiac cells and neurons. In the present study, we have identified the presence of HCN channels in pancreatic β-cells. We then examined the functional characterization of these channels in β-cells via modulating HCN channel activity genetically and pharmacologically. Voltage-clamp experiments showed that over-expression of HCN2 in rat β-cells significantly increased HCN current (I h), whereas expression of dominant-negative HCN2 (HCN2-AYA) completely suppressed endogenous I h. Compared to control β-cells, over-expression of I h increased insulin secretion at 2.8 mmol/l glucose. However, suppression of I h did not affect insulin secretion at both 2.8 and 11.1 mmol/l glucose. Current-clamp measurements revealed that HCN2 over-expression significantly reduced β-cell membrane input resistance (R in), and resulted in a less-hyperpolarizing membrane response to the currents injected into the cell. Conversely, dominant negative HCN2-AYA expression led to a substantial increase of R in, which was associated with a more hyperpolarizing membrane response to the currents injected. Remarkably, under low extracellular potassium conditions (2.5 mmol/l K+), suppression of I h resulted in increased membrane hyperpolarization and decreased insulin secretion. We conclude that I h in β-cells possess the potential to modulate β-cell membrane potential and insulin secretion under hypokalemic conditions.
M. D. MITCHELL, ANNE B. M. ANDERSON, JANE D. BRUNT, LINDA CLOVER, D. A. ELLWOOD, J. S. ROBINSON and A. C. TURNBULL
The concentrations of 6-oxo-prostaglandin F1α (6-oxo-PGF1α) have been determined in maternal and foetal plasma from nine chronically catheterized sheep during late pregnancy and parturition. Labour occurred either spontaneously (three sheep) or was induced by continuous intrafoetal infusion of Synacthen (ACTH 1–24; 0·24 mg/24 h; three sheep) or dexamethasone (1 mg/24 h; three sheep). During spontaneous and Synacthen-induced parturition, concentrations of 6-oxo-PGF1α in maternal and foetal plasma remained at basal levels until 24 h before delivery. At varying times during the 24 h before delivery, levels of 6-oxo-PGF1α in maternal and foetal plasma were generally increased. When parturition was induced with dexamethasone, however, no increase was observed in the foetal plasma although the concentration of 6-oxo-PGF1α in maternal plasma was raised close to delivery.