Effects of thyroid deficiency on the vasopressin receptors in the kidney of developing and adult rats. A comparative study of hormonal binding and adenylate cyclase activation

The effects of propylthiouracil (PTU) treatment on renal vasopressin sensitive adenylate cyclase in young and adult rats were studied by measuring the binding of tritiated vasopressin and adenylate cyclase activation by vasopressin in kidney medulla plasma membranes. Thyroxine therapy completely corrected the effects of PTU treatment on the vasopressin-adenylate cyclase system. Thus, the abnormalities observed after a such treatment are directly related to thyroid deficiency and not to toxic effects of PTU. The inability of the kidney to normally concentrate urine in developing and adult animals with induced hypothyroidism was mainly related to the reduction of the number of binding sites without significant changes in the basal and guanylyl-imidodiphosphate (Gpp(NH)p)-stimulated adenylate cyclase activities, the apparent dissociation constant (Kbind) of labeled vasopressin from its specific receptor and the apparent activation constant (Kact) of vasopressin for adenylate cyclase. These results also show that thyroid deficiency has more effect on the ontogenesis of receptors than on their turnover, and demonstrate that a normal antidiuretic response occurs at very low receptor occupancy. Since, on the one hand, the hypothyroidism-induced abnormalities in renal medulla responsiveness to vasopressin were reversible and, on the other, only a permanent therapy consisting of two daily physiological doses of thyroxine from birth to the age of sacrifice fully restored them, the responsiveness of developing kidney to thyroid hormones appears to be fundamentally different from that of the CNS.     

Hormonal regulation of postnatal development of renal tubular transport processes

Renal tubular transport of p-aminohippurate (PAH) is immature at birth. Repeated saturation of transport sites by treatment with various organic anions is without any influence on the postnatal development of kidney transport capacity. Hormonal regulation of postnatal maturation of PAH transport must therefore be taken into consideration. It was tried to stimulate immature PAH transport by treating rats of different ages with thyroid hormones, corticosteroids or testosterone, respectively. In rats with immature kidney function, renal PAH excretion can be stimulated by daily treatment with thyroid hormones. Experiments on renal cortical slices have shown that PAH excretion is preferentially stimulated by an increase of transport capacity. Whereas thyroid hormones stimulate the renal excretion of PAH both in young and in adult rats, dexamethasone treatment is more effective in rats with immature kidney function. Dexamethasone treatment is without any influence on PAH accumulation in renal cortical slices. Kidney weight and the protein content of kidney tissue was increased after dexamethasone treatment. Repeated testosterone administration did not stimulate the PAH transport in rats of different ages. The data have demonstrated the influence of thyroid hormones or of dexamethasone on renal tubular transport processes in rats with immature kidney function. Treatment with such hormones could be useful in the management of renal insufficiency in full-term and pre-term neonates with immature kidney function.     

Phosphatidylinositol metabolism in rat hepatocytes stimulated by glycogenolytic hormones. Effects of angiotensin, vasopressin, adrenaline, ionophore A23187 and calcium-ion deprivation

1. The effects on phosphatidylinositol metabolism of three Ca(2+)-mobilizing glycogenolytic hormones, namely angiotensin, vasopressin and adrenaline, have been investigated by using rat hepatocytes. 2. All three hormones stimulate both phosphatidylinositol breakdown and the labelling of this lipid with (32)P. 3. The response to angiotensin occurs quickly, requires a high concentration of the hormone and is prevented by [1-sarcosine, 8-isoleucine]angiotensin, a specific angiotensin antagonist that does not prevent the responses to vasopressin and to adrenaline. This response therefore seems to be mediated by angiotensin-specific receptors. 4. [1-Deaminocysteine,2-phenylalanine,7-(3,4-didehydroproline),8-arginine] vasopressin, a vasopressin analogue with enhanced antidiuretic potency, is relatively ineffective at stimulating phosphatidylinositol metabolism. This suggests that the hepatic vasopressin receptors that stimulate phosphatidylinositol breakdown are different in their ligand selectivity from the antidiuretic vasopressin receptors that activate renal adenylate cyclase. 5. Incubation of hepatocytes with ionophore A23187, a bivalent-cation ionophore, neither mimicked nor appreciably changed the effects of vasopressin on phosphatidylinositol metabolism, suggesting that phosphatidylinositol breakdown is not controlled by changes in the cytosol Ca(2+) concentration. This conclusion was supported by the observation that hormonal stimulation of phosphatidylinositol breakdown and resynthesis persists in cells incubated for a substantial period in EGTA, although this treatment somewhat decreased the phosphatidylinositol response of the hepatocyte. The phosphatidylinositol response of the hepatocyte therefore appears not to be controlled by changes in cytosol [Ca(2+)], despite the fact that this ion is thought to be the second messenger by which the same hormones control glycogenolysis. 6. These results may be an indication that phosphatidylinositol breakdown is an integral reaction in the stimulus-response coupling sequence(s) that link(s) activation of alpha-adrenergic, vasopressin and angiotensin receptors to mobilization of Ca(2+) in the rat hepatocyte.     

Restriction of stress-induced activation of lipid peroxidation by small doses of thyroid hormones]

The experiments on 57 female rats demonstrated that small doses of thyroid hormones (thyroidin) significantly (55-118%) restrict stress induced increase in the concentration of initial and terminal products of lipid peroxidation (LP) in the myocardium and in the blood plasma. After hormone injection stress decreases the activity of key antioxidant enzyme, superoxide dismutase of erythrocytes (SOD), to a lesser degree and increases the rate of malonyldialdehyde (MDA) production induced by Fe2+ in homogenates of the myocardium to the same degree as well in comparison with rats that had not been injected thyroidin. In normal rates thyroidin does not influence the concentration of products of LP, increases the activity of SOD and decreases increment of MDA induced by Fe2+ in homogenates of the myocardium. Thus, small doses of thyroid hormones restrict significantly stress induced activity of LP membranes, increasing the power of antioxidant systems both in the myocardium and in the organism.     

Thyroid hormone levels in the peripheral blood of the ground squirrel Citellus parryi during winter hibernation

In the Arctic ground squirrel Citellus parryi, thyroxine and triiodothyronine concentrations in the peripheral blood have been determined by RIA in various seasons and during a short hibernation period. It was shown that during hibernation (December-February), hormonal concentration is higher than during homoiothermal period (August-October). The highest concentration of the thyroid hormones was found in hibernating animals in January. The data obtained indicate that thyroid hormones are involved in control of hibernation of the species investigated.     

Stimulation by thyroid hormone of phosphate transport in primary cultured renal cells

The regulation by thyroid hormone of phosphate transport in primary cultured chick renal cells was examined. The more physiologically active L-analogs of triiodothyronine and thyroxine, but not the D-analogs of the hormones, stimulated the Na+-dependent phosphate uptake system. Na+-independent phosphate uptake and Na+-dependent uptakes of alpha-methylglucoside and L-proline were unaffected. The increase in Na+-dependent phosphate uptake was concentration dependent, exhibited an induction period, and was blocked by inhibitors of RNA and protein synthesis. The stimulation of phosphate uptake by triiodothyronine was due to an increased Vmax rather than to an altered affinity for phosphate. These findings demonstrate that thyroid hormone acts directly on renal cells to modulate phosphate transport and suggest that the renal cell system may serve as a model to examine the mechanism by which thyroid hormone controls gene expression and regulates plasma membrane transport function.     

Thyroid Hormone Receptors in the Developing Rat Brain

It is widely believed that the adult mammalian brain is insensitiveto thyroid hormones unlike the neonatal brain which is criticallydependent on these hormones for the development of normal structureand function. Recent studies have demonstrated the presenceof limited capacity, high affinity, triiodothyronine (T3) bindingnuclear sites in tissues that are considered responsive to thyroidhormones. Furthermore, there is evidence from studies on peripheraltissues that these T3 binding sites act as true receptors ininitiating thyroid hormone action. This report examines whetherthe higher sensitivity of neonatal brain to thyroid hormonesand the purported decline in sensitivity in adulthood are relatedto changes in the concentration and affinity characteristicsof thyroid hormone receptors in rat cerebral nuclei. Analysisof Scatchard plots of in vitro T3 binding data indicate thatcerebral nuclei from adult rats contain T3 specific nuclearbinding sites at a concentration comparable to that presentduring the period when the brain is critically dependent onthe presence of thyroid hormones and exceed that in the liver,a tissue generally considered thyroid sensitive. Neonatal thyroidectomysignificantly increased the number of binding sites. The resultsshow that the apparent unresponsiveness of the cerebral cortexof adult rats to thyroid hormones is not due to the absenceor a low density of T3 nuclear binding sites. The significanceof these results is discussed.     

Effect of thyroxine in vitro on the transmembrane potential of rat liver mitochondria

It has been shown in in vitro experiments that a certain latent period after the addition of thyroxine (T4) and triiodothyronine (T3) was necessary for the manifestation of their effects on transmembrane potential (TMP) of the rat liver mitochondria. The duration of the lag-period decreased upon an increase in the concentrations of these hormones, and T4 at a dose of 2.10(-4) M produced a fall in TMP immediately after its addition. The rate of TMP fall was in proportion with the concentrations of thyroid hormones introduced into the cell, with T3 30-40% more effective than T4. It was established that the action of I2 resembled that of thyroid hormones, namely, a fall in TMP, mitochondrial swelling, activation of transhydrogenase Kl was ineffective. It is suggested that the appearance of the lag-period upon the action of thyroid hormones might be explained by the period of time necessary for the formation of the active iodine forms, as well as by the formation of fatty acids (donators of H+) by mitochondrial phospholipases. All these factors lead to TMP fall resulting in decreased formation of sufficient ATP quantities in mitochondria.     

Effect of thyroid hormones on the gene expression of calcium transport systems in rat muscles

It has been previously shown that modification of thyroid hormone levels have a profound impact on cardiac function, predominantly through a direct regulation of the sarcoplasmic reticulum protein levels. Nevertheless, little is known about the regulation of calcium transport systems in skeletal muscle due to the altered concentration of thyroid hormones. Thus, the goal of our study was to find out whether altered thyroid status could change the gene expression of the Na(+)/Ca(2+) exchanger (NCX), the inositol 1,4,5-trisphosphate (IP(3)) receptors and ryanodine receptors (RyRs) in slow and fast skeletal muscles of rats. A hyperthyroid state was maintained in rats by triiodothyronine (T(3)) administration, while methimazole was employed for inducing hypothyroidism. After a period of 2-10 months of T(3) treatment we observed a significant increase in mRNA levels of the NCX, RyRs and IP(3) receptors. This increase was more pronounced in the slow soleus than in the fast extensor digitorum longus (EDL) muscle. It is tempting to speculate that thyroid hormones also alter calcium concentration and thus influence the process of excitation-contraction coupling in the skeletal muscle.     

Prolactin as a regulator of fluid and electrolyte metabolism in mammals

The evidence that prolactin is a fluid and electrolyte regulator in mammals is reviewed, with particular emphasis on the renal actions of prolactin. Prolactin receptors are found in mammalian kidneys. Prolactin modulates renal formation of cyclic AMP and polyamines and it leads to demonstrable histological changes in the proximal tubules. The renal actions of prolactin primarily involve modulation of the effects of other hormones and are therefore critically dependent on the background physiological situation. Prolactin seems able to cause a prolonged reduction in water, sodium, and potassium excretion, a pattern that is imitated by no other hormone with the possible exception of growth hormone. Prolactin preparations can cause an acute antidiuresis, which may in part be related to contamination of prolactin preparations with vasopressin. However, most of the described effects cannot be explained by vasopressin contamination. This is particularly so with the effects of prolactin on water movements across fetal skin, the amniotic membrane, and in the eye where prolactin and vasopressin have diametrically opposite effects. It is concluded that prolactin is a regulator of fluid and electrolyte metabolism in mammals but that it is a modulator rather than a primary controlling factor.     

Microtubules and brain development: The effects of thyroid hormones

The data accumulated during the past twenty years suggest that thyroid hormones have a direct effect on the differentiation of both the neurons and the glial cell during the critical period of brain development. A fast survey of the available data (which is presented in the introduction of this article) on the mechanism of action of thyroid hormones and on their different effects during brain development suggests that the most dramatic effect of hypothyroidism is a hypoplastic neuropile. Both in vivo, during the critical period of nerve cell differentiation and in vitro, when added to primary cultures of embryonic nerve cells thyroid hormones stimulate neurite outgrowth. Since neurite outgrowth requires massive microtubule assembly the assumption was made that thyroid hormones stimulate nerve cell differentiation by changing the concentration and/or activity of the different proteins (tubulin and “microtubule associated proteins”, MAPs) which co-polymerize to form microtubules.

Preliminary information was obtained by following the kinetics of microtubule assembly in crude brain supernatants. The data showed that: (1) the rate of in vitro microtubule assembly increases with age during brain development; (2) hypothyroidism, when produced in the rat at late pregnancy, slows this evolution; (3) early replacement therapy with thyroid hormones restores normal rates of assembly; (4) the addition of purified MAPs to normal young or 15-day-old hypothyroid brain preparations restores normal rates of polymerization. These and other data suggested that thyroid hormones regulate microtubule assembly by changing the concentration and/or activity of one or more of the MAPs.

Further analysis revealed that striking qualitative changes in MAPs composition occur during brain development. For instance, the TAU fraction, a group of 4–5 proteins with a molecular weight of 60–68 K which is present in adult brain, is absent at early stages of postnatal development: two other entities are present, TAU slow and TAU fast, with different molecular weights, lower activity and different peptide mapping. This latter observation suggests that different TAU genes are expressed during brain development; a conclusion which has been confirmed by cell-free translation of the mRNas coding for these proteins. Analysis of the TAU fraction prepared from hypothyroid rat brains also revealed that a group of TAU proteins. “TAU₃”, is almost missing, whereas thyroid hormone administration markedly increases its concentration. Two-dimensional gel electrophoresis showed that the TAU fraction is composed with more than 15 entities, with at least five of them being under thyroid hormone control.

The precise physiological significance of the heterogeneity of MAPs and of the changes in MAPs composition seen during development and in hypothyroid rat brain remains to be determined. The assumption is made that these changes might be of utmost importance to regulate the number and length of the microtubules, and therefore the number and length of the neurites which are formed during the differentiation process of the different neurons. Thyroid hormones would be in these respects one of the epigenic factors required to synchronize sequentially the expression of the genes coding for these proteins in the different nerve cells.     

Synergistic stimulation of the Ca2+ influx in rat hepatocytes by glucagon and the Ca2+-linked hormones vasopressin and angiotensin II

Glucagon was added to isolated rat hepatocytes, either alone or together with vasopressin or angiotensin II, and the effects on the initial 45Ca2+ uptake rate were investigated. Addition of glucagon alone which increased cyclic AMP content of the cells slightly increased the initial 45Ca2+ uptake rate. When glucagon was added together with vasopressin or angiotensin II--both of which when added separately increase the initial 45Ca2+ uptake rate but did not affect the cellular content of cyclic AMP--the measured initial 45Ca2+ uptake rate was larger than the sum of that seen with each hormone alone. This indicates that glucagon and Ca2+-linked hormones synergistically enhanced the Ca2+ influx in rat hepatocytes. These effects of glucagon can be mimicked by dibutyryl cyclic AMP or forskolin, suggesting that cyclic AMP augments both the resting Ca2+ and the vasopressin- or angiotensin II-stimulated influx. Measurement of the initial 45Ca2+ uptake rate as a function of the extracellular Ca2+ concentration indicated that the increase in the Ca2+ influx resulting from single or combined glucagon and vasopressin administration occurred through a homogeneous population of Ca2+ gates. These hormones were found to raise both the apparent Km for external Ca2+ and the apparent Vmax of the Ca2+ influx. The maximal increase in these two parameters was observed when the two hormones were added together. This suggests that glucagon and vasopressin synergistically stimulate the same Ca2+ gating mechanism. The dose-response curves for the action of glucagon or vasopressin applied in the presence of increasing concentrations of vasopressin or glucagon, respectively, showed that each hormone increases the maximal response to the other without affecting its ED50. It is proposed that glucagon and the Ca2+-linked hormones control the cellular concentration of two intermediates which are both necessary to allow Ca2+ entry into the cells.     

Activation of adenylate cyclase in renal medulla by bovine growth hormone. An artifact attributable to vasopressin.

The effect of bovine growth hormone on adenylate cyclase activity was studied in bovine and rat renal medulla. Highly purified growth hormone (lot B1003A) increased adenylate cyclase activity in plasma membranes from bovine renal medulla from 132+/-6 pmol cyclic AMP formed/mg protein per 10 min to 364+/-10 pmol cyclic AMP formed/mg protein per 10 min. Similar results were seen with homogenates of rat renal medulla. The minimum effective concentration of bovine growth hormone required to activate adenylate cyclase was 0.5 mug/ml and maximum activation was detected at 500 mug/ml. The amount of vasopressin determined by radioimmunoassay to contaminate the growth hormone caused an increase in adenylate cyclase activity comparable to that of the corresponding concentration of growth hormone that was tested. Dialysis of growth hormone and vasopressin resulted in parallel reductions in the effect of each hormone on adenylate cyclase activity. Similarly, both growth hormone and vasopressin produced increases in short circuit current in isolated toad bladders but these effects were not detectable after dialysis of the hormones. In contrast, the effect of growth hormone on the uptake of 35SO2-4 by cartilage from hypophysectomized rats was not decreased after dialysis. These results indicate that available preparations of growth hormone are contaminated by small but physiologically significant amounts of vasopressin and that the activation of adenylate cyclase activity in renal medulla in response to growth hormone can be explained by this contamination rather than by an effect of growth hormone per se.     

Thyroid hormones influencing renal electrolyte excretion in saline loaded rats of different ages

Thyroid hormones are involved in the postnatal development of kidney functions. The influence of pretreatment with thyroid hormones on renal electrolyte excretion was measured in rats of different ages. Pretreatment with triiodothyronine or tetraiodothyronine (10 micrograms/100 g b.wt. i.p. for 3 days, once a day) cannot alter the low sodium excretion in young rats receiving a saline load. In 33-, 55- and 105-day-old rats the pretreatment with thyroid hormones is followed by a preferential increase of renal sodium excretion. Both thyroid hormones are effective.     

Hormone-specific responses and biosynthesis of sulfolipids in cell lines derived from mammalian kidney.

The established cell lines isolated from mammalian kidney were characterized by its receptor activities against hormones and the ability to synthesize sulfolipids localized in the renal tubule. The level of 3':5'-cyclic AMP in JTC-12.P3 (monkey kidney) cells increased in 2 min as much as 2.5-5-fold on activation with 1.0 unit/ml of bovine parathyroid hormone or 1.9 units/ml of synthetic parathyroid hormone (1-34) resulting in intracellular cyclic AMP concentration of more than 40 pmol/mg protein. Prostaglandin E1 (14 micronM) and isopropylnorepinephrine (10 micronM) were also found to increase the concentration of cyclic AMP by more than 30- and 9-fold, respectively. Addition in medium of calcitonin, arginine vasopressin, adrenocorticotropic hormone and glucagon caused no significant changes of cyclic AMP level in the cell. In contrast, MDCK, a cell line isolated from canine kidney, reacted to arginine vasopressin, isopropylnorepinephrine and prostaglandin E1 and only slightly to parathyroid hormone. MDBK cell line derived from bovine kidney or fibroblast cell lines from rat lung and guinea pig kidney did not react to any of the hormones specific to kidney, i.e. arginine vasopressin, calcitonin or parathyroid hormone in the presence of theophylline. However, in the presence of 2 mM isobutylmethylxanthine, small but significant elevation of cellular cyclic AMP levels in response to calcitonin, arginine vasopressin, isopropylnorepinephrine and prostaglandin E1 was observed. The cell lines JTC-12, MDCK and MDBK, when incubated with H235SO4, incorporated the isotope into sulfolipids assigned as sulfatides and ceramide dihexoside sulfate or in MDCK also into cholesterol sulfate. The results suggested that JTC-12, MDCK and MDBK cell lines are epithelial origin and also JTC-12 and MDCK originated most probably from renal tubular cells of cortex and medulla, respectively.     

Distinct effects of glucagon and vasopressin on proline metabolism in isolated hepatocytes. The role of oxoglutarate dehydrogenase.

The hormonal regulation of gluconeogenesis and ureogenesis in isolated rat hepatocytes with 5 mM-proline as precursor was studied, with the following results. (1) The formation of glucose and urea in a 30 min interval were stimulated more by vasopressin than by glucagon, and the effects of the two hormones in combination were additive. (2) The rates of gluconeogenesis during the 30 min were constant under control, glucagon-stimulated and glucagon-plus-vasopressin-stimulated conditions. The stimulated rate in the presence of vasopressin diminished with time; glucagon in combination with vasopressin prevented this diminution, resulting in an additive effect. (3) Coincident with these changes in gluconeogenesis, vasopressin caused a decrease in cell oxoglutarate concentration, which, in contrast with the decrease caused by glucagon, was greater, but not sustained unless glucagon was also present. Changes in cell glutamate concentration similar to those observed for oxoglutarate occurred. (4) The data suggest that activation of oxoglutarate dehydrogenase (EC 1.2.4.2) by glucagon and vasopressin by different mechanisms may explain the relative effects of the hormones alone and in combination on gluconeogenesis from proline.     

Ontogeny of renal renin in congenital hypothyroid rats.

In the present study we have found age-related differences between the renal renin activity (RRA) and the immunoreactive renal renin (IRR) profiles during the neonatal development of the rat. RRA was markedly greater in newborn rats than in adult ones, while IRR was low at birth and progressively increased until adulthood. These observations suggest the existence of a control mechanism operating either at the level of the translation of the template or at a post-translational level which varies throughout the development. Since thyroid hormones have been demonstrated to affect several renin-angiotensin components, the neonatal RRA and IRR profiles in congenital hypothyroid rats were evaluated to determine whether renal renin activity or its synthesis could be endogenously regulated by thyroid hormones in the early stages of life. Although significant differences were observed in the RRA profiles of congenital hypothyroid and control rats, no changes were found in the relative amount of immunoreactive protein. These findings indicate that thyroid hormone deficiency does not directly affect renal renin expression during the critical period of the normal morpho-functional development of the newborn.     

Metabolism of bromide and its interference with the metabolism of iodine

The present knowledge about the metabolism of bromide with respect to its goitrogenic effects, including some conclusions drawn from our recent research on this subject, is reviewed. Firstly, the biological behavior of bromide ion is compared with that of chloride and iodide. Secondly, the details about distribution and kinetics of bromide ions in the body and in 15 different organs and tissues of the rat are given. Significant correlation between the values of the steady-state concentration of bromide in the respective tissue and of the corresponding biological half-life was found in most tissues examined. A remarkably high concentration of radiobromide was found in the skin, which represents, due to its large mass, the most abundant depot of bromide in the body of the rat. Thirdly, the effects of excessive bromide on the rat thyroid are summarized, along with the interference of exogenous bromide with the whole-body metabolism of iodine. It is suggested that high levels of bromide in the organism of experimental animals can influence their iodine metabolism in two parallel ways: by a decrease in iodide accumulation in the thyroid and skin (and in the mammary glands in lactating dams), and by a rise in iodide excretion by kidneys. By accelerating the renal excretion of iodide, excessive bromide can also influence the pool of exchangeable iodide in the thyroid. Finally, our recent results concerning the influence of high bromide intake in the lactating rat dam on iodine and bromide transfer to the suckling, and the impact of seriously decreased iodine content and increased bromide concentration in mother's milk on the young are discussed. We must state, however, that the virtue of the toxic effects of excessive bromide on the thyroid gland and its interference with the biosynthesis of thyroid hormones, as well as the exact mechanism of bromide interference with postnatal developmental processes remains to be elucidated.     

Activation of adenylate cyclase in renal medulla by bovine growth hormone: An artifact attributable to vasopressin

The effect of bovine growth hormone on adenylate cyclase activity was studied in bovine and rat renal medulla. Highly purified growth hormone (lot B1003A) increased adenylate cyclase activity in plasma membranes from bovine renal medulla from 132 ± 6 pmol cyclic AMP formed/mg protein per 10 min to 364 ± 10 pmol cyclic AMP formed/mg protein per 10 min. Similar results were seen with homogenates of rat renal medulla. The minimum effective concentration of bovine growth hormone required to activate adenylate cyclase was 0.5 μg/ml and maximum activation was detected at 500 μg/ml. The amount of vasopressin determined by radioimmunoassay to contaminate the growth hormone caused an increase in adenylate cyclase activity comparable to that of the corresponding concentration of growth hormone that was tested. Dialysis of growth hormone and vasopressin resulted in parallel reductions in the effect of each hormone on adenylate cyclase activity. Similarly, both growth hormone and vasopressin produced increases in short circuit current in isolated toad bladders but these effects were not detectable after dialysis of the hormones. In contrast, the effect of growth hormone on the uptake of ³⁵SO₄²⁻ by cartilage from hypophysectomized rats was not decreased after dialysis. These results indicate that available preparations of growth hormone are contaminated by small but physiologically significant amounts of vasopressin and that the activation of adenylate cyclase activity in renal medulla in response to growth hormone can be explained by this contamination rather than by an effect of growth hormone per se.