Stimulation of rat renal medullary Na+/K+-ATPase by arginine vasopressin is mediated by the V2 receptor

ABSTRACT In previous studies, we have demonstrated that 1–10 fmol arginine vasopressin (AVP)/l maximally stimulates the activity of the enzyme Na+/K+-ATPase in the rat renal medullary thick ascending limb (MTAL) of Henle's loop after 4 or 10 min of stimulation when measured using a cytochemical bioassay. We have tested the hypothesis that this stimulation is mediated by the V2 receptor in the MTAL. A cytochemical bioassay was used to investigate the effect of specific V1 and V2/V1 antagonists and a synthetic V2 agonist [1-deamino,8-d-arginine]-vasopressin (dDAVP), on the activity of Na+/K+-ATPase. There was no effect of the V1 antagonist (1 fmol-1 μmol/l) in inhibiting the activity of Na+/K+-ATPase stimulated by 1 fmol AVP/l. In contrast, 100 pmol of the V2/V1 antagonist/l significantly (P < 0·001) inhibited the stimulation of Na+/K+-ATPase activity by 1 fmol AVP/l from 55·5±4·3 (s.e.m.) to 31·9±1·6 mean integrated extinction (MIE) after 4 min of stimulation and from 67·0±3·2 to 36·9±0·7 MIE after 10 min of stimulation. Similarly, the stimulation of Na+/K+-ATPase by 10 fmol dDAVP/l was inhibited by the V2/V1 antagonist from 55·1±1·0 to 26·1±0·5 MIE after 4 min of stimulation. We conclude that the stimulation of Na+/K+-ATPase by AVP is mediated by the V2 receptor in the rat renal MTAL. Journal of Endocrinology (1990) 127, 213–216

Bioactive parathyroid hormone in pregnant rats and fetuses

Parathyroid function at the end of gestation (day 21) was investigated by measuring plasma calcium (PCa), immunoreactive parathyroid hormone (iPTH), bioactive parathyroid hormone (bioPTH; cytochemical bioassay), and bone histology in intact and thyroparathyroidectomized (TPTX; day 12, ether anesthesia) rats and their fetuses. In pregnant intact rats, PCa was significantly lower, and iPTH, bioPTH, and osteoclast number were higher than in nonpregnant rats. In fetuses, PCa was higher than maternal PCa and correlated with fetal bioPTH. TPTX suppressed maternal bioPTH and decreased iPTH and osteoclast number, whereas fetal iPTH and bioPTH were decreased with no change in osteoclast number. Fetal PCa was near normal and was correlated with maternal PCa but not with fetal bioPTH. The fetomaternal calcium gradient was maintained and even increased. This study shows that there is maternal physiological hyperparathyroidism and functional fetal parathyroid glands at the end of gestation in the rat. Parathyroid hormone does not seem to be responsible for maintaining the high fetomaternal calcium gradient in TPTX animals.

Inhibition of arginine vasopressin-stimulated Na+K+ATPase activity by difluoromethyl ornithine in the rat renal medulla

ABSTRACT Arginine vasopressin (AVP) stimulates Na+K+ ATPase and ornithine decarboxylase (ODC) activity in the rat medullary thick ascending limb. The effect of difluoromethyl ornithine (DFMO), a specific inhibitor of ODC activity, on AVP-stimulated Na+K+ATPase activity was evaluated using a cytochemical bioassay. Peaks in Na+K+ATPase activity in cultured rat renal segments which occurred after tissue had been exposed to 1 fmol AVP/l were completely inhibited by the addition of 20 mmol DFMO/l to the culture medium containing AVP. The addition of 20 mmol DFMO/l to the culture medium containing AVP in the concentration range 0·001–10 fmol/l inhibited completely the stimulation of Na+K+ATPase activity by AVP. The response of Na+K+ATPase to increasing doses of ATP (10–40 g polypeptide/l) was not influenced by the addition of 20 mmol DFMO/l to the culture medium containing AVP, suggesting that the prevention of AVP-stimulated Na+K+ATPase activity by DFMO was not due to a direct effect on the enzyme. Journal of Endocrinology (1989) 121, 435–439

Effects of estrogen on mineral metabolism in postmenopausal women as evaluated by multiple assays measuring parathyrin bioactivity.

Data on the effect of estrogen on immunoreactive parathyrin (iPTH) in postmenopausal women are conflicting. We administered estrogen or placebo to 21 postmenopausal women for 12 weeks and measured PTH bioactivity (bioPTH), using the renal cytochemical bioassay. Before treatment, there was a negative correlation between nephrogenous cAMP and the tubular maximum for urinary phosphate excretion and a positive correlation between values measured by a mid-region-specific PTH RIA and those measured in an immunoradiometric assay for intact PTH. Values measured by the midregion-specific RIA were also positively correlated with nephrogenous cAMP. BioPTH values were not correlated with other indices of PTH activity but were increased compared with values for younger subjects. After estrogen treatment there was no change in bioPTH activity despite an early decrease in serum osteocalcin and a later increase in nephrogenous cAMP. PTH concentrations measured by mid-region-specific or intact RIAs were unchanged, but sample size may have been insufficient to exclude the possibility of significant changes in these values. The effects of estrogen on mineral metabolism in postmenopausal women are time-dependent. Early effects are independent of PTH, and later effects are variably associated with increased PTH activity.

Sequential presentation of a case of hyperthyroidism with autonomously functioning nodules and Graves' disease in the presence of IgG thyroid stimulators

The rare occurrence of hyperthyroidism with an autonomously functioning nodule which following 131I therapy presented as toxic diffuse goitre (Graves' disease) is described in a 60 year old male. This progression was characterised by the presence of varying concentrations of IgG thyroid stimulators, thyroid stimulating immunoglobulins and thyroid growth stimulating immunoglobulins, as measured by cytochemical bioassay. It is postulated that the presence of the nodule and its associated hypersecretion of thyroid hormones may have protected the gland from the effects of IgG stimulators by bringing about inhibitory short-loop feedback on normal thyroid cells. It is further suggested that following therapeutic ablation of the nodule, normal thyroid cells became sensitive to the thyroid stimulators with the evolution of typical features of toxic diffuse goitre.