Effects of parathyroid hormone on H+ and NH 4 + excretion in toad urinary bladder

Summary The urinary bladder ofBufo marinus excretes H⁺ and NH ₄ ⁺ , and the H⁺ excretion is increased after the animal is placed in metabolic acidosis. The present study was done to determine if parathyroid hormone could stimulate the bladder to increase the excretion of H⁺ and/or NH ₄ ⁺ . Parathyroid hormone added to the serosal solution in a final concentration of 10 g/ml was found to increase H⁺ excretion by 50% above the control hemibladders, while there was no effect on NH ₄ ⁺ excretion. Parathyroid hormone had no effect on H⁺ excretion when added to the mucosal solution. We also performed experiments utilizing theophylline and dibutyryl cyclic AMP which mimicked those of the parathyroid hormone experiments. A dose-response analysis was performed and the results indicate that 1 g/ml of parathyroid hormone was the minimal effective dose. These results suggest that parathyroid hormone can stimulate H⁺ excretion in the toad urinary bladder and this effect seems to be mediated by cyclic AMP. In addition, it was found that parathyroid hormone has no effect on NH ₄ ⁺ excretion.     

Action of steroids on H+ and NH 4 + excretion in the toad urinary bladder

Summary This study was done to determine if steroid compounds will stimulate the urinary bladder of the toad to increase its capacity to acidify the urine and excrete NH ₄ ⁺ , Aldosterone, 17-estradiol, dexamethasone, pregnenolone, and cholesterol were tested on the bladder. All compounds tested were found to stimulate the rate of acidification by the bladder, above that of a paired control hemibladder. In contrast, only the steroids aldosterone and 17-estradiol were found to stimulate NH ₄ ⁺ excretion in the bladder. Cycloheximide was found to block the action of aldosterone on the NH ₄ ⁺ excretion, but did not have a significant effect on the stimulation of acidification by aldosterone. We conclude that steroids stimulate H⁺ and NH ₄ ⁺ excretion in the toad urinary bladder. In addition, the NH ₄ ⁺ excretory system seems to be more specific to this effect than is the H⁺ excretory system.This work was presented in part at the 62nd annual meeting, Federation of American Societies for Experimental Biology, Atlantic City, N.J., April 1978.     

The effect of catecholamines on H+ and NH+4 excretion in toad urinary bladder

The catecholamines epinephrine and norepinephrine, when placed on the toad urinary bladder in vitro, at a final concentration of 50 microM, caused a significant increase in H+ and NH+4 excretion by the bladder. Isoprenaline in a final concentration of 50 microM also increased H+ and NH+4 excretion in the bladder. Propranolol at a concentration of 50 microM blocked the stimulation of H+ excretion by isoprenaline but propranolol at 100 microM was required to block the stimulation of NH+4 by isoprenaline. The dose-response analysis indicates that the concentration of epinephrine used (50 microM) is at or near the maximal effective dose. These findings indicate that catecholamines stimulate H+ and NH+4 excretion in the toad urinary bladder and evidence suggests this may be mediated via the beta receptor mechanism.     

Insulin-stimulated sodium transport in toad urinary bladder

Mammalian and teleost insulins increase active sodium transport by the toad urinary bladder at subnanomolar concentrations. This stimulation is evident within 15 min and persists for hours. Porcine proinsulin and a cross-linked derivative of bovine insulin are less effective than porcine insulin in stimulating the short-circuit current (SCC), indicating the specificity appropriate for activation of sodium transport through an insulin receptor. The initial stimulation by insulin of the SCC is not blocked by pretreatment with actinomycin D, puromycin, cycloheximide, or tunicamycin. However, in the presence of any one of these inhibitors the sustained increase in SCC is blocked and the rise is short-lived, lasting only 45 to 90 min. In amphotericin-treated bladders, the addition of insulin did not further stimulate SCC.     

Stimulation of ammonium ion excretion in the toad urinary bladder by an extract of human urine

It is well known that ammonium ion excretion is increased during metabolic acidosis in mammals. The purpose of this study was to determine whether we could isolate from human urine during metabolic acidosis a factor that would stimulate NH₄⁺ and/or H⁺ excretion in toad urinary bladder. Extracts of urine from six human subjects collected during NH₄Cl-induced acidosis were prepared. These extracts were tested for their effect on NH₄⁺ excretion in hemibladders mounted between plastic chambers. The extracts significantly increased NH₄⁺ excretion in the toad urinary bladder. We found no effect on H⁺ excretion by these extracts. This ammoniuretic activity was not present in the urine when the same individuals were in metabolic alkalosis. We conclude that during metabolic acidosis a humoral factor is present which stimulates the excretion of NH₄⁺. The factor could act as a permease in the bladder cell or as a stimulator of an NH₄⁺ transport system.     

Effects of parathyroid hormone on H+ and NH+4 excretion in toad urinary bladder.

The urinary bladder of Bufo marinus excretes H+ and NH+4, and the H+ excretion is increased after the animal is placed in metabolic acidosis. The present study was done to determine if parathyroid hormone could stimulate the bladder to increase the excretion of H+ and/or NH+4. Parathyroid hormone added to the serosal solution in a final concentration of 10 mug/ml was found to increase H+ excretion by 50 per cent above the control hemibladders, while there was no effect on NH+4 excretion. Parathyroid hormone had no effect on H+ excretion when added to the mucosal solution. We also performed experiments utilizing theophylline and dibutyryl cyclic AMP which mimicked those of the parathyroid hormone experiments. A dose-response analysis was performed and the results indicate that 1 mug/ml of parathyroid hormone was the minimal effective dose. These results suggest that parathyroid hormone can stimulate H+ excretion in the toad urinary bladder and this effect seems to be mediated by cyclic AMP. In addition, it was found that parathyroid hormone has no effect on NH+4 excretion.     

Stimulation of ammonium ion excretion in the toad urinary bladder by an extract of human urine.

It is well known that ammonium ion excretion is increased during metabolic acidosis in mammals. The purpose of this study was to determine whether we could isolate from human urine during metabolic acidosis a factor that would stimulate NH+4 and/or H+ excretion in toad urinary bladder. Extracts of urine from six human subjects collected during NH4Cl-induced acidosis were prepared. These extracts were tested for their effect on NH+4 excretion in hemibladders mounted between plastic chambers. The extracts significantly increased NH+4 excretion in the toad urinary bladder. We found no effect on H+ excretion by these extracts. This ammoniuretic activity was not present in the urine when the same individuals were in metabolic alkalosis. We conclude that during metabolic acidosis a humoral factor is present which stimulates the excretion of NH+4. The factor could act as a permease in the bladder cell or as a stimulator of an NH+4 transport system.     

Stretch induces granule exocytosis in toad urinary bladder

Evidence from thin sections and freeze-fracture is presented showing that stretch induces a burst of exocytosis in granular cells of the toad urinary bladder epithelium. Since the role of granule exocytosis in hormonally-induced permeability changes in this tissue has not yet been clarified, we propose that the stretch factor is an important parameter to consider and standardise in future physiological and morpho-functional studies using this model transporting system.     

Amide transport channels across toad urinary bladder

Summary Urea and other small amides cross the toad urinary bladder by a vasopressinsensitive pathway which is independent of somotic water flow. Amide transport has characteristics of facilitated transport: saturation, mutual inhibition between amides, and selective depression by agents such as phloretin. The present studies were designed to distinguish among several types of transport including (1) movement thought a fixed selective membrane channel and (2) movement via a mobile carrier. The former wold be characterized by co-transport (acceleration of labele amide flow in the direction of net flow in the opposite direction). Mucosal to serosal (MS) and serosal to mucosal (SM) permeabilities of labeled amides were determined in paired bladers. Unlabeled methylurea, a particularly potent inhibitor of amide movement, was added to either the M or S bath, while osmotic water flow was eliminated by addition of ethylene glycol to the opposite bat. Co-transport of labeled methylurea and, to a lesser degree, acetamide and urea with unlabeled methylurea was observed. Co-transport of the nonamides ethylene glycol and ethanol could not be demonstrated. Methylurea did not alter water permeability or transmembrane electrical resistance. The demonstration of co-transport is consistent with the presence of ADH-sensitive amide-selective channcels rather than a mobile carrier.     

Mode of action of amiloride in toad urinary bladder

Summary The effect of amiloride on the sensitivity to Na of the mucosal border of toad urinary bladder was investigated by recording Na concentration-dependent transepithelial potential difference (V _t ) and the intracellular potential. When mucosal Na concentration was normal, amiloride added to the mucosal solution at 10^–4 m markedly reduced the mucosal membrane potential (V _m ) and altered the potential profile from a two-step type to a well type. Similar changes were observed when Na was totally eliminated from the mucosal medium. The serosal membrane potential was insensitive to amiloride and elimination of mucosal Na. In the absence of amiloride, theV _t could be described by the Goldman-Hodgkin-Katz equation in the range of mucosal Na concentration from 0 to 16mm, and amiloride extended this concentration range. By using the Goldman-Hodgkin-Katz equation, Na permeability was calculated from the data ofV _t 's obtained in the allowed ranges of Na concentration and compared before and after the addition of amiloride. The results show that Na permeability decreases to 1/600 of control when the maximum dose of amiloride (10^–4 m) is applied. The relationship between Na permeability and amiloride concentration is well explained on the basis of assumptions that amiloride binds to the Na site of the mucosal border in one-to-one fashion and in a competitive manner with Na and that Na permeability reduces in proportion to increase in number of the sites bound with amiloride.     

Reserve of vasopressin-sensitive adenylate cyclase in toad urinary bladder

Studies have been performed on the effect of vasopressin on cyclic AMP content of toad bladders. A prompt increase in cyclic AMP content occurred after exposure to vasopressin, which reached maximal values within 8 min and remained elevated up to 30 min. By a comparison of the dose-response characteristics of vasopressin on cyclic AMP content, with those Na⁺ transport and osmotic water flow, it was shown that supramaximal concentrations of vasopressin with respect to physiological function generate more cyclic AMP than is required for maximal stimulation of Na⁺ transport and water flow. Thus, it would seem that a reverse of hormone-sensitive adenylate cyclase is present in this tissue.     

Metabolic cost of sodium transport in toad urinary bladder

Summary The metabolic cost of active sodium transport was determined in toad bladder at different gradients of transepithelial potential, , by continuous and simultaneous measurements of CO₂ production and of transepithelial electric current. Amiloride was used to block active sodium transport in order to assess the nontransport-linked, basal, production of CO₂ and the passive permeability of the tissue. From these determinations active sodium transport,J _Na, and suprabasal CO₂ production, , were calculated. Since large transients inJ _Na and frequently accompanied any abrupt change in , steady state conditions were carefully defined.Some 20 to 40 min were required after a change in before steady state of transport activity and of CO₂ production were achieved. The metabolic cost of sodium transport proved to be the same whether the bladder expended energy moving sodium against a transepithelial electrical potential grandient of +50 mV or whether sodium was being pulled through the active transport pathway by an electrical gradient of –50 mV. In both cases the value of the ratio averaged some 20 sodium ions transported per molecule of CO₂ produced.When the Na pump was blocked by 10^–2 m ouabain, the perturbations of the transepithelial electrical potential did not elicit changes ofJ _Na nor, consequently, of .The independence of the ratio from over the range ±50 mV indicates a high degree of coupling between active sodium transport and metabolism.