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D W Dempster

Publications and source records attributed to D W Dempster.

83 records · Page 5Linked to original sources
PubMed1983

Potassium stimulates parathyroid hormone release in the absence of extracellular calcium.

Potassium-stimulated release of many hormones requires the presence of extracellular calcium. At variance with this mechanism, potassium-evoked parathyroid hormone (PTH) release from perifused dispersed bovine parathyroid cells also occurred in calcium-free medium containing 1 mM EGTA. Tetraethylammonium, which presumably suppresses the efflux of potassium from parathyroid cells, also stimulated PTH secretion. Removal of sodium did not suppress potassium-stimulated PTH secretion, but inhibited the release of the hormone evoked by calcium removal and by isoproterenol. Ouabain, on the other hand, suppressed the release of PTH evoked by calcium removal and by potassium, but not by isoproterenol. Unlike high potassium and removal of calcium, isoproterenol caused a parallel increase in cAMP release. In conclusion, PTH secretion is reversibly stimulated by potassium in the absence of extracellular calcium. Our findings suggest that potassium stimulates PTH secretion by a unique mechanism the nature of which remains to be elucidated.

Animals↗
PubMed1982

Preparation of cryosections with a modified Sorvall MT2B ultramicrotome and cryoattachment.

The Sorvall MT2B ultramicrotome and cryoattachment were modified to extent the duration of cutting and to overcome problems in tissue preparation. Microtome and cryochamber were encased and surrounded by an atmosphere of dry nitrogen gas at room temperature. A specially designed cold block sited within the box close to the microtome enabled tissues to be trimmed at low temperatures in the dry environment. Tissues could be readily visualized both on the trimming block and in the chamber with the existing microscope modified to improve its working distance and lighting system. During transfer from storage in liquid nitrogen, through the trimming procedure, to arrival in the cryochamber, the temperature of tissues as determined using thermocouples embedded in PVP on a stub, never exceeded 140 K. Although there was a considerable standing temperature gradient within the cryochamber, the cutting environment, specimen and edge of the knife were thermally stable. Sections could be routinely cut dry and freeze-dried within the chamber, if desired.

Animals↗
PubMed1982

Potassium stimulates parathyroid hormone release from perifused parathyroid cells.

A method for perifusing dispersed bovine parathyroid cells is described. Using this approach, we have studied the effects of raised extracellular potassium and ouabain on calcium-regulated parathyroid hormone (PTH) secretion in vitro. Decreasing calcium from 1.5 to 0.5 mM stimulated PTH release within 5-10 min, and the increased secretory rate was maintained for the duration of the low calcium administration. High potassium (60 mM) promptly stimulated PTH secretion at 1.5 mM calcium. The effect of high potassium was more transient than that of low calcium. Raised potassium significantly enhanced the response to 0.5 mM calcium. Ouabain (10(-3) M) had no significant effect at 1.5 mM calcium, but depressed the secretory response to 0.5 mM calcium. In conclusion, the effect of raised potassium on parathyroid cells is qualitatively similar to its effect on other endocrine and exocrine systems.

Animals↗
PubMed1981

Quantitation and localisation of aluminum in human cancellous bone in renal osteodystrophy.

There is increasing evidence that aluminium toxicity may be responsible for a type of vitamin D-resistant osteomalacia and an unusually severe form of dementia ("dialysis dementia") occurring in some patients with chronic renal failure on regular haemodialysis. High concentrations of Al have been found in blood, bone and brain tissue from these patients. The A1 comes either from the water used during dialysis (added in some public water supplies during purification to precipitate contaminants) or from aluminium salts taken orally to bind phosphates and so restrict their dietary adsorption. Recent X-ray microanalytical studies have demonstrated Al in lysosomes of cerebral cells and at the calcification front in bone of patients dying of dialysis dementia but its concentration at this site in bone has not been measured using this technique. We have examined transiliac bone biopsies from 3 patients with dialysis dementia and 6 non-demented patients on regular haemodialysis, Atomic absorption spectrometry (AAS) reveals high Al content in bone from the 3 demented and 2 of the non-demented patients. All had vitamin D-resistant osteomalacia. Using X-ray microanalysis Al was located in the bone of these five patients only. The Al had a highly focal distribution and was measured at up to 40 times higher concentration than by AAS but only in mineralisation nuclei of the calcification front or less than 2 micrometer into the mineralized bone. The study was done retrospectively on biopsies fixed in 10% buffered formalin, which almost certainly eluted some of the Al. In life, Al levels may have been higher than those we have detected.

Adolescent↗
PubMed1980

Aspects of microprobe analysis of mineralized tissues.

Mineralizing tissues were the first biological specimens to be examined by the electron microprobe. The necessity for performing analyses at high spatial resolution has led to the increasing use of energy dispersive (ED) detectors in CTEMs/STEMs. Because the ED detector accepts x-rays from a large solid angle, it often detects a large extraneous x-ray contribution from the instrument. By carefully redesigning and remaking the specimen stage and its environment from aluminium, the instrumental contribution may be subtracted from spectra recorded on this specimens. Measurements of the apparent concentrations of Ca and P in araldite due to extraneous x-rays from other parts of the specimen, show that accurate quantitative analyses of Ca and P may be performed in unmineralized organic matrix such as osteoid, despite the proximity of fully mineralized tissue. Quantitative microprobe analyses have been performed on 1) intracellular mineral deposits, 2) osteoid, 3) mineralized matrix in trabecular bone from rachitic (R), vitamin D-treated (D) and normal (N) rats. The composition of the intracellular mineral deposits was similar in all 3 groups with mean Ca:P molar ratios = approximately 1.0. The Ca and P levels in osteoid from each group were sufficiently high that, assuming: a) that all of the Ca and P was in ionized form and b) that osteoid has a water content of 59% the calculated [Ca2+] x [HPO4(2-)] products were much greater than that required for spontaneous precipitation of mineral. In the D and N groups, the Ca and P concentrations in mineralized matrix increased with increasing distance into bone, but in the R group they remained constant. The Ca:P molar ratios of the mineralized matrix and of the serum were highest in the R group (bone: 1.85--1.87, serum: 5.76 +/- 1.05), intermediate in the D group (bone: 1.72--1.90, serum: 2.46 +/- 0.36) and lowest in the N group (bone: 1.68--1.80, serum: 1.25 +/- 0.17).

Bone Matrix↗