Reduced Calcium Levels Research Articles

RHEOLOGICAL PROPERTIES OF ALGINATE GELS

AbstractRheological properties of alginate gels formed in various ways were studied using a parallel plate viscoelastometer and an Instron.All the gels studied exhibited linear viscoelastic behaviour. The creep compliance‐time response was fitted by a model consisting of a Maxwell element in series with two or three Kelvin‐Voigt elements. Despite the fact that alginate gels are non‐thermoreversible the crosslinks do not appear to be permanent. Gels formed with an alginate containing a high proportion of mannuronic acid residues are weaker and more elastic than gels prepared with an alginate predominantly composed of guluronic acid residues. When the level of calcium in the gel was high, the reciprocal of the creep compliance was proportional to the square of the polysaccharide concentration. A reduction in calcium level for a given polysaccharide concentration resulted in a more elastic gel. The nature of the temperature dependence varied with the degree of crosslinking and time of measurement after application of stress. The results were explained by assuming that the creep compliance could be regarded as the sum of two components: a time independent factor given by the theory of rubber elasticity, or some modification of this, and a time dependent factor associated with the breakage of crosslinks.

The influence of dietary calcium deficiency and parathyroidectomy on bone collagen structure

Bone collagen synthesized in chicks fed a calcium-deficient diet had the same chain composition as that normally synthesized, but differed in that in both α1- and α2-chains there was an increased conversion of lysine to hydroxylysine, of about 50–70%, equivalent approximately to an extra five residues of hydroxylysine in the α1- and an extra six to seven residues in the α2-chain. This effect was not due to the hyperparathyroidism accompanying the calcium deficiency since increased hydroxylation was also observed when calcium levels were reduced following parathyroidectomy. It is suggested the effect may be directly attributable to reduced calcium levels and that the increased hydroxylation of bone collagen lysine occurring in rickets may also be attributable to the reduced level of calcium existing in vitamin D deficiency. Examination of skin collagen indicated a slight increase in hydroxylation, equivalent only to an extra one to two residues of hydroxylysine, in the α1-chains. A similar slight increase in the α2-chains was not unequivocably demonstrated. The results suggest lysine hydroxylation in bone collagen to be much the more sensitive to changes in calcium level and the possible significance of this, particularly in relation to mineralization, is considered.

Influence of prostaglandins on cation movement in the lens

Rabbit lenses incubated at 36°C for 20 hr in a medium containing 10 −3 m prostaglandin (PG) E 1, E 2 or E 2 α exhibited a marked increase in sodium and decrease in potassium content; 10 −3 m PGF 1 α was without effect. 10 −3 m PGE 1 caused a significant increase in cation permeability as evidenced by increased rubidium-86 efflux. Reduced rubidium-86 uptake was observed in lenses incubated in 10 −3 m PGE 1 and F α. 10 −4 m prostaglandins (E 1, E 2 and F 2 α ) appeared to stimulate the lens cation transport mechanism. Recovery from a cold induced cation shift was enhanced, as was rubidium-86 uptake. Lenses subjected to reduced calcium level in the incubation medium survived better in the presence of 10 −4 m PG. It is suggested that the effects of 10 −4 m PG are mediated via the adenyl cyclase system. The effects of 10 −3 m PG might be due to a large non-specific increase in lens cation permeability.