Repeated measurements of P retention in ponies fed rations with various Ca:P ratios.

This study addresses the question of whether feeding rations rich in P for a period of up to 42 d induces a positive P balance in adult ponies. Biochemical bone markers and parathyroid hormone (PTH; intact as well as whole PTH) were measured to obtain clues as to the effect of P loading on bone metabolism. The experiment had a Latin square design. Each feeding period lasted 42 d, and there were 2 balance trials (ECP1 and ECP2) within each feeding period. Each balance trial lasted 10 d (ECP1: d 11 to 21; ECP2: d 33 to 42). Six ponies aged 2.5 to 7 yr were fed a control diet that provided P and Ca according to the requirement (Control diet: 54 mg Ca·kg BW(-1) · d(-1); 36 mg P · kg BW(-1) · d(-1)), a diet high in Ca and P (HCaHP diet: 146 mg Ca · kg BW(-1) · d(-1); 121 mg P · kg BW(-1) · d(-1)), and a diet with a high P level only and Ca fed to the requirement (HP diet: 54 mg Ca · kg BW(-1) · d(-1); 122 mg P · kg BW(-1) · d(-1)). When fed the Control diet, the ponies showed a zero P and Ca balance over the 42-d period. The HCaHP diet resulted in both P and Ca retention (about 2 g Ca and P/d; P < 0.05). Phosphorus retention (about 2 g P/d) alone was observed when ponies were fed the HP diet, but P retention was only different (P < 0.05) from the Control diet in ECP1. The excretion of P in urine was reduced by greater Ca intake (P < 0.05), and Mg absorption was reduced by high P intake (P < 0.05). Plasma P concentration was raised by high P intake. Plasma Ca levels were not affected by dietary treatment. The greater (P < 0.05) P retentions observed for the HCaHP diet during ECP1 and ECP2 and HP diet during ECP1 could not be explained by processes that could have been indicated by the bone markers or PTH values. It was concluded that dietary-P-induced retention of P in ponies does not seem to be associated with altered bone metabolism in this study.

An unusual case of generalized soft-tissue mineralization in a suckling foal.

An atypical case of severe soft-tissue mineralization in a 3-week-old foal from a herd of Andalusian horses is described. The herd clinical history and the laboratory findings were compatible with a diagnosis of secondary hyperparathyroidism due to a mineral imbalance in the diet (low calcium and high phosphorus intake). Mares showed a marked increase in serum parathyroid hormone (PTH) approximately 10 times normal levels. Serum PTH was marginally elevated in foals. Clinical signs (unthriftiness, painful joints, lameness in one or more limbs, and stiff gait) were more pronounced in foals than in mares. Two foals died and necropsy of one of them revealed extensive soft-tissue mineralization of arterial walls and pulmonary parenchyma. Clinical signs in mares and foals resolved by 4 weeks after diet adjustment.

Diurnal variations in the plasma concentration of parathyroid hormone in dogs.

The plasma concentrations of parathyroid hormone (PTH), ionised calcium (Ca(2+)), total calcium, albumin and inorganic phosphorus, and the pH were measured in blood samples obtained from nine dogs during a period of 26 hours. The plasma pth levels fluctuated slightly during the day, by about 20 pg/ml, but there was a distinct peak (42.8 [8.8] pg/ml) at 07.00. Plasma Ca(2+) showed a diurnal pattern in which two peaks (increases of 0.03 mmol/l) were observed at 05.00 and 17.00, and the plasma concentration of inorganic phosphorus showed a similar pattern. There were no diurnal changes in total calcium or albumin.

Parathyroid gland function in the uremic rabbit.

Rabbits with renal failure have been reported to be hypercalcemic and to have decreased parathyroid hormone (PTH) concentrations. Thus, it would seem that uremic rabbits are resistant to secondary hyperparathyroidism (HPT). The work reported here was designed to investigate parathyroid gland function in uremic rabbits and the effect of diets with different calcium (Ca) and phosphorus (P) content. The relationship between PTH and ionized calcium (Ca2+), parathyroid gland size and parathyroid cell cycle were studied in three groups of rabbits: Group I, rabbits with normal renal function on a standard diet (Ca = 1.2%, P = 0.6%); Group II, partially nephrectomized rabbits on a standard diet; and Group III, partially nephrectomized rabbits on a low Ca (0.6%)-high P (1.2%) diet. Group I rabbits had baseline Ca2+ = 1.71 +/- 0.05 mmol/l and PTH = 26.9 +/- 3.2 pg/ml. During hypo- and hypercalcemic stimulation PTH reached maximal values (PTHmax) of 94.4 +/- 5.5 pg/ml and minimal concentrations (PTHmin) of 3.2 +/- 0.2 pg/ml. Rabbits from Group II were hypercalcemic (baseline Ca2+ = 2.03 +/- 0.06 mmol/l) and had very low PTH levels (1.7 +/- 0.5 pg/ml); however, they reached a PTHmax that was similar to Group I, 92 +/- 8.7 pg/ml. Group III rabbits were hypocalcemic (baseline Ca2+ = 1.22 +/- 0.08 mmol/l) and had very high basal PTH levels (739 +/- 155 pg/ml). Their PTHmax and PTHmin were 801 +/- 169.4 pg/ml and 102.2 +/- 22.2 pg/ml, respectively. Both parathyroid gland size and parathyroid cell proliferation were increased in Group III. In conclusion, our results show that the Ca and P content of the diet markedly influence PTH secretion in the uremic rabbit and that when placed on a low Ca-high P diet uremic rabbits develop secondary HPT.

Mineral metabolism in healthy geriatric dogs.

To study mineral metabolism in geriatric dogs, parathyroid hormone, calcitriol, ionised calcium, phosphorus, blood urea nitrogen and creatinine were evaluated in 35 geriatric dogs (> 10 years) and in 20 young adult dogs (2-5 years). Parathyroid hormone levels were within the normal range in both groups, but values (mean +/- SEM) were greater in the old dogs (34.8 +/- 3.6 vs 21.2 +/- 2.3 pg ml(-1), P=0.005). Calcitriol and ionised calcium were similar in the two groups, and the values for both parameters were within the normal reference range. Plasma phosphorus levels were in the normal range in both groups but tended to be greater in the older dogs (P=0.09). While blood urea nitrogen was similar in the two groups, creatinine levels (mean +/- SEM) were higher in the young dogs (82.2 +/- 3.5 vs 101.7 +/- 4.4 micromol litre(-1)). Even when the dogs were matched for weight, plasma creatinine concentration was still greater in the younger dogs. In conclusion, an increase in parathyroid hormone without changes in calcium, phosphorus and calcitriol has been identified in geriatric dogs.