Absence of regulatory effects of 1alpha25-dihydroxyvitamin D3 on 25-hydroxyvitamin D metabolism in rats constantly infused with parathyroid hormone

Unilateral salpingectomy in Sprague Dawley rats and its effect on litter size

The study described a novel, rapidly performed, successful and safe surgical procedure in rats to achieve a reduction in the number of conceptuses. The objectives were to investigate the total foetal count and foetal health in both uterine horns after unilateral salpingectomy compared to the control group. Sixteen female Sprague-Dawley rats (Rattus norvegicus) were allocated to the study of which 10 rats underwent unilateral salpingectomy with six controls before all 16 were mated at 8–10 weeks of age. Gestational length was taken as 22 days, determined from the day of appearance of the copulatory plug. The female rats were terminated on day 19 or 20 of the gestational period. The foetuses in each horn were mapped and counted for comparison between the salpingectomy and control groups. The gravid uteri, individual foetal weights and placental weights were measured and compared between the two groups. This study described a novel, rapidly performed, successful and safe surgical procedure in rats. The mean number of foetuses in the salpingectomy group was significantly smaller than the control group. No significant differences in foetal and placental development were observed between the groups. These observations support future investigation of unilateral salpingectomy in other species as an alternative surgical method for population control.

PTH regulation of c-Jun terminal kinase and p38 MAPK cascades in intestinal cells from young and aged rats

In the present study, we examined the role of Parathyroid hormone (PTH) on the c-Jun N-terminal kinase (JNK) 1/2 and p38 mitogen-activated protein kinase (MAPK) members of the MAPK family as it relates to ageing by measuring hormone-induced changes in their activity in enterocytes isolated from young (3 month old) and aged (24 month old) rats. Our results show that PTH induces a transient activation of JNK 1/2, peaking at 1 min (+threefold). The hormone also stimulates JNK 1/2 tyrosine phosphorylation, in a dose-dependent fashion, this effect being maximal at 10 nM. PTH-induced JNK 1/2 phosphorylation was suppressed by its selective inhibitor SP600125. Moreover, hormone-dependent activation of JNK 1/2 was dependent on calcium, since pretreatment of cells with BAPTA-AM or EGTA blocked PTH effects. With ageing, the response to PTH was significantly reduced. JNK basal protein expression was not different in the enterocytes from young and aged rats, however, basal protein phosphorylation increased with ageing. PTH did not stimulate, within 1-10 min, the basal activity and phosphorylation of p38 MAPK in rat intestinal cells. The hormone increased enterocyte DNA synthesis; the response was dose-dependent and decreased (-40%) with ageing. In agreement with the mitogenic role of the MAPK cascades, this effect was blocked by specific inhibitors of extracellular signal-regulated protein kinase (ERK) 1/2 and JNK 1/2. The results obtained in this work expand our knowledge on the mechanism of action of PTH in duodenal cells.

Age-related changes in the response of intestinal cells to parathyroid hormone

The concept of the role(s) of parathyroid hormone (PTH), has expanded from that on acting on the classical target tissues, bone and kidney, to the intestine where its actions are of regulatory and developmental importance: regulation of intracellular calcium through modulation of second messengers and, activation of mitogenic cascades leading to cell proliferation. Several causes have been postulated to modify the hormone response in intestinal cells with ageing, among them, alterations of PTH receptor (PTHR1) binding sites, reduced expression of G proteins and hormone signal transduction changes. The current review summarizes the actual knowledge regarding the molecular and biochemical basis of age-impaired PTH receptor-mediated signaling in intestinal cells. A fundamental understanding of why PTH functions are impaired with age will enhance our understanding of its importance in intestinal cell physiology.

Effect of aging on the mechanisms of PTH-induced calcium influx in rat intestinal cells

We have investigated the effects of aging on parathyroid hormone (PTH) modulation of intracellular calcium homeostasis and their relationship to signal transduction pathways in isolated rat duodenal cells (enterocytes). PTH (10(-8)-10(-9) M) increased enterocyte (45)Ca(2+) influx and intracellular Ca(2+) concentration ([Ca(2+)](i)) to a greater extent (twofold and 50%, respectively) in aged (24 months) than in young (3 months) animals. The [Ca(2+)](i) response of old cells to the hormone was slower, lacking the early phase of changes in cytosolic Ca(2+). Ca(2+) influx induced by PTH was prevented by the protein kinase A antagonist Rp-cAMPS in both young and aged enterocytes, whereas neomycin and compound U73122, inhibitors of PLC-catalyzed phosphoinositide hydrolysis, abolished hormone-dependent Ca(2+) influx in young but had no effect on aged cells. Higher basal adenylyl cyclase (AC) activity and cAMP content were detected in old enterocytes. PTH increased the absolute levels of cAMP in aged cells and AC activity of microsomes isolated therefrom to a greater extent (>/= twofold) than in young enterocytes/membranes. In young cells, the hormone also induced a rapid and transient release of inositoltrisphosphate (IP(3)) and diacylglycerol (neomycin-sensitive) at 45 sec, and a delayed phase of DAG at 5 min (neomycin-insensitive). The early formation of IP(3) and DAG was blunted in aged animals. These results suggest that both the PLC and adenylyl cyclase cascades are involved in PTH stimulation of Ca(2+) influx in duodenal cells. During aging, however, only the cAMP pathway is operative, mediating a potentiation of the effects of the hormone. Additional studies are required to establish the relative role of PTH-dependent messenger systems in the regulation of intestinal calcium absorption and age-related abnormalities.

Influence of a tumor promoting phorbol ester on the metabolism of 25-hydroxyvitamin D3

When added to primary cultures of chick kidney cells, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) decreased the basal rate of production of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and increased that of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3). The normal stimulatory effect of parathyroid hormone and forskolin on 1,25(OH)2D3 production was abolished or blunted by the presence of TPA and TPA overcame the inhibitory effect of PTH and forskolin on 24,25(OH)2D3 production. The evidence suggests that protein kinase C may be involved in the regulation of 25(OH)D3 metabolism by chick kidney cells.

Vitamin D status regulates 25-hydroxyvitamin D3-1 alpha-hydroxylase and its responsiveness to parathyroid hormone in the chick

We asked this question: Under normal or near-normal metabolic conditions, does the prevailing normal or near-normal vitamin D status dampen the activity of 25-hydroxyvitamin-D3-1 alpha-hydroxylase (1 alpha-hydroxylase) such that it determines not only its "basal" activity but also its responsiveness to stimulation by increased circulating concentrations of parathyroid hormone (PTH)? To answer this question, we measured the activity of 1 alpha-hydroxylase in chicks, with and without administration of PTH, immediately before and during deprivation of vitamin D. Before deprivation of vitamin D, 1 alpha-hydroxylase activity increased only slightly with administration of PTH. With deprivation of vitamin D for 5 and 10 d, while the plasma concentrations of calcium and phosphorus persisted normal and unchanged, 1 alpha-hydroxylase activity not only increased progressively but also became sharply and increasingly responsive to stimulation by administration of PTH. But after 15 d of vitamin D deprivation, and the supervention of hypocalcemia, 1 alpha-hydroxylase activity was not further increased by the administration of PTH. With deprivation of vitamin D, the progressive increase in 1 alpha-hydroxylase correlated inversely with circulating levels of 1,25-dihydroxyvitamin D (1,25-[OH]2D), and the decreasing calcemic response to PTH correlated inversely with the responsiveness of 1 alpha-hydroxylase to PTH (in chicks deprived of vitamin D for 1-10 d). These results demonstrate that: under normal metabolic conditions, the normal vitamin D status regulates the activity of 1 alpha-hydroxylase so as to dampen both its "basal" activity and its responsiveness to stimulation by PTH; and vitamin D deprivation insufficient to cause hypocalcemia enhances both the "basal" activity of 1 alpha-hydroxylase and its responsiveness to stimulation by PTH. The results suggest that the normal dampening of 1 alpha-hydroxylase and both of the demonstrated enhancements of its activity are mediated by normal and reduced levels of circulating 1,25-(OH)2D, respectively. The finding that PTH fails to further stimulate 1 alpha-hydroxylase when vitamin D deprivation is sufficient in duration to cause hypocalcemia confirms the findings of other investigators and again demonstrates that observations made during abnormal metabolic circumstances may bear little on the physiologic regulation of 1 alpha-hydroxylase under normal or near-normal metabolic circumstances.

Changes in serum levels of 1,25-dihydroxyvitamin D3, calcium and phosphorus with age and vitamin D status in chickens

The effects of vitamin D3 (D3) on serum levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), ionic calcium, total Ca and phosphorus in chicks were studied from the time of hatching until sexual maturity. Chicks fed on a diet low in D3 showed a serum level of 1,25(OH)2D3 higher than that in chicks on a normal-D3 diet, for both sexes and at any given age. A dramatic increase in the serum level of 1,25(OH)2D3 occurred in female birds approaching sexual maturity and in laying hens raised on the low-D3 diet the level was five times that of their counterparts raised on a normal-D3 diet. The serum 1,25(OH)2D3 level in adult males in the low-D3 groups was seven times that of those on the normal-D3 diet. The serum level of 25-hydroxyvitamin D3 remained relatively unchanged at weeks 2 and 15 in birds on a low D3 intake as well as in those fed on a normal-D3 diet. Nevertheless, the levels of 25-hydroxyvitamin D3 were different between the two groups. No significant change was observed in the level of ionized serum Ca in relation to dietary regimen, but there was an increase in total Ca concentration in females with the onset of reproduction. The serum P level decreased gradually with age, reaching a minimum value 3 and 8 weeks before laying commenced in the groups on low- and normal-D3 diets respectively. An increase was observed when the hens began laying.(ABSTRACT TRUNCATED AT 250 WORDS)