Cyclic AMP and sodium transport. Quantitative and temporal relationships in toad urinary bladder

Differences in c-AMP levels in epithelial cells from pelvic and pectoral regions of the toad skin

Arginine vasopressin (AVP) stimulated active Na+ transport (JNa+) and osmotic water flow (JH2O) across the pelvic skin but only JNa+ across the pectoral skin of the toad, Bufo woodhouseii. Isolated epithelial cells from the pelvic skin had a maximal c-AMP level of 11.16 pmoles/mg protein after 5 min of AVP treatment while that of pectoral skin was 3.64 pmoles/mg protein. The c-AMP level of both skin areas fell to unstimulated values after 20 min of AVP treatment; however, JH2O (pelvic skin) and JNa+ (pelvic and pectoral skin) remained elevated during 3 hr of treatment. Dibutyryl c-AMP and theophylline stimulated JH2O across the pelvic but not the pectoral skin. Maintaining toads in water for 12-24 hr resulted in a substantial lowering of JH2O across the pectoral skin which was not reversible by treatment with c-AMP and theophylline.

Correlation between cAMP in isolated frog skin epithelium and stimulation of sodium transport and osmotic water flow by antidiuretic hormone and phosphodiesterase inhibitors

The cAMP level in isolated frog skin epithelia was stimulated by a range of concentrations of the antidiuretic hormone arginine vasotocin (AVT) and compared to active sodium transport measured as short-circuit current (SCC). The response of SCC and osmotic water flow (OWF) to AVT was investigated in a separate series. SCC was approximately three times more sensitive to AVT than OWF. Measurable increments of cAMP above the basal level were found only with AVT concentrations eliciting half-maximal response or more of SCC. Two models are offered to explain the findings: (A) Total SCC depends on epithelial cAMP with a sigmoidal relationship. (B) Epithelial cAMP exists in two separate pools of which only one is accessible to AVT, and SCC stimulation depends on cAMP in the AVT-controlled pool in a simple saturable fashion. Correlation between cAMP and SCC after stimulation with theophylline resembled that after AVT. Papaverine (10 microM) induced only small changes in SCC inspite of a substantial increase in cAMP level. Higher concentrations of papaverine inhibited both basal and AVT-stimulated SCC, while the cAMP level was further increased. This effect of papaverine may be due to a simultaneous block of another rate-determining process. Papaverine had no effect on basal or AVT-stimulated OWF despite a marked stimulation of the cAMP level. Thus, the role of cAMP as mediator of the hydroosmotic action of AVT must be questioned.

Transport properties of toad kidney epithelia in culture

The characteristics of a continuous line of toad kidney epithelial cells (A6) are described. These cells form a monolayer epithelium of high transepithelial electrical resistance (about 5,000 omega . cm2). The cells generate a transepithelial potential difference (apical surface negative) of about 9 mV. The short-circuit current is equivalent to net sodium flux. Net sodium flux is stimulated by aldosterone and by analogues of cAMP. The stimulation is readily reversible. Neither urea permeability nor osmotic water flow is altered by analogues of cAMP. Amiloride eliminates 90% of the short-circuit current. Thus A6 cells form an epithelium with several differentiated properties including hormonal regulation of transport.

Mechanisms for the effects of acetylcholine on sodium transport in frog skin

In frog skin (Rana temporaria) acetylcholine applied to the serosal surface produces either a sustained inhibiton or sustained stimulation of short-circuit current (SCC). The former effect is accompanied by a reduction and the latter by an increase in total tissue conductance. Both effects of acetylcholine can be accounted for, within experimental error, by changes in net sodium flux across the tissue. By use of selective agonists and antagonists it is concluded that acetylcholine interacts with muscarinic receptors in the serosal membrane. The effects of cholinoceptor agents are also seen with isolated epithelium. The stimulatory effect of acetylcholine is potentiated by theophylline and blocked by inhibitors of prostaglandin synthetase and by mepacrine. It is suggested that acetylcholine stimulates transport by liberating prostaglandins which may then activate adenylcyclase. The inhibitory effect of acetylcholine is correlated with a reduction in cyclic AMP content of the epithelium. Calcium appears to be an important determinant of the type of response seen eith acetylcholine, but the mechanism is not known.

Toad urinary bladder epithelial cells in culture: maintenance of epithelial structure, sodium transport, and response to hormones

Epithelial cells from the toad urinary bladder have been grown in continuous culture. Many of the cells resemble the granular cell type of the urinary bladder. They form an epithelium with typical tight junctions and gap junctions. The transport properties of two cell lines have been examined. When cells of the line designated TB-M or of line TB-6c are grown on collagen-coated nucleopore filters, epithelia are formed that have transepithelial potential differences of 40 and 20 mV, resistances of 5000 and 10,000 omega-cm2, and short-circuit currents (ISC) of 8.5 and 2.5 muA/cm2, respectively. Net mucosa to serosa sodium transport accounts for all of ISC in line TB-M and for 70% of ISC in line TB-6c. Vasopressin, which stimulates adenylate cylase and ISC in the intact bladder, has no effect on the cells in culture. Cyclic AMP stimulates ISC and lowers resistance in both lines. Aldosterone stimulates ISC in both lines. This is accompanied by a fall in resistance in line TB-M and no change in resistance in line TB-6c. Amiloride inhibits ISC in TB-M cells under basal conditions and after stimulation by aldosterone. In line TB-6c amiloride has no effect under basal conditions but lowers ISC of aldosterone-treated cells to the basal level. Thus, the cells have retained the ability to form oriented, high-resistance epithelial membranes that manifest hormone-sensitive transepithelial sodium transport.

Cyclic AMP levels in isolated frog skin epithelium: effects of phosphodiesterase inhibitors, oxytocin and catecholamines

Direct measurements of cyclic AMP were performed in the isolated epithelium of frog skin. Phosphodiesterase inhibitors (methylxanthines, papaverine) and activators of adenylyl cyclase (oxytocin, catecholamines) significantly increased the cyclic AMP content. Propranolol completely blocked the generation of cAMP induced by beta-adrenergic agonists but had little or no effect on that induced by oxytocin. Phentolamine enhanced the cAMP production by adrenalin and noradrenalin. At supramaximal concentrations, oxytocin and isoproterenol produced similar increments in cAMP, while exposure to both agents roughly doubled the increase in cAMP. The results suggest the presence of independent receptors for oxytocin and catecholamines in frog skin, with additive effects on cAMP generation.

Kinetics of cyclic AMP in toad bladder

Tissue distributions of cyclic [3H]AMP and [14C]inulin in toad bladder were determined and their kinetics analyzed. We found that both the epithelial and the other cells of the toad bladder handle cyclic AMP similarly. Moreover, we found that the distribution of cyclic AMP did not differ from that of inulin, an extracellular marker. Kinetic analysis suggests that the rate coefficient of cyclic AMP metabolism is much larger than the exchange rate coefficient, which explains why distribution of both cyclic AMP and inulin are similar.

Effects of the antidiuretic hormone, arginine vasotocin, theophylline, filipin and A23187 on cyclic AMP in isolated frog skin epithelium (Rana temporaria)

A method for measuring cAMP in frog skin epithelium was developed. The epithelia were isolated after collagenase-treatment. cAMP was extracted by boiling water and the extract was purified on dry Al2O3. The change with time of the cAMP level after addition of arginine vasotocin (AVT) was studied. The hormone caused a rapid increase in cAMP level with a maximum after 3-5 min, whereafter the cAMP level declined. Incubation with AVT made the epithelia refractory to a second dose of AVT, which indicates that the decline in cAMP level was caused by a feedback mechanism and not by inactivation of the hormone. cAMP appeared evenly distributed in all cell-layers of the epithelia both before and after stimulation with AVT. Theophylline caused a rapid increase in the cAMP level, which remained elevated for at least 45 min. Addition of the ionophore A23187 or of filipin had no effect on the cAMP level. However, in the presence of theophylline, A23187 enhanced the cAMP level, whereas filipin had no effect. Therefore the involvement of cAMP in the action of A23187 has to be considered.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

Study of enzymes regulating vasopressin-stimulated cyclic AMP metabolism in separated mitochondria-rich and granular epithelial cells of toad urinary bladder

The epithelial cells of the toad urinary bladder are morphologically heterogenous. In order to relate the effect of vasopressin on cyclic AMP metabolism to cell type, the epithelial cells were separated by the density gradient technique of Scott, Sapirstein and Yoder (Science 184:797, 1974). The separation was verified by electron-microscopy and by observing that the band of cells enriched in mitochondria-rich cells was enriched in carbonic anhydrase activity compared to the band of granular cells. A large portion of cells collected from the gradient was considered to be nonviable, precluding further study of their function as intact cells. Vasopressin-stimulated adenylate cyclase activity in homogenates of granular cells was simular to that in homogenates of mitochondria-rich cells. Cyclic nucleotide phosphodiesterase activity was also similar in the two types of cell. Thus, the enzymes known to be involved in cyclic AMP metabolism in response to vasopressin appear to be located in both major cell types.

The membrane action of antidiuretic hormone (ADH) on toad urinary bladder

Radioactive tracer and electrical techniques were used to study the transport of nonelectrolytes and sodium, respectively, across toad urinary bladders in the presence and absence of ADH. The permeability of lipophilic molecules was roughly proportional to bulk phase oil/water partition coefficients both in the presence and absence of hormone; i.e., ADH elicited a general nonselective increase in the permeation of all nine solutes tested. The branched nonelectrolyte, isobutyramide, was less permeable than its straight-chain isomer, n-butyramide, in control tissues. ADH reduced the discrimination between these structural isomers. Hydrophilic solutes permeated more rapidly than expected. In the presence of hormone, there was no change in the permeation of large hydrophilic solutes considered to move via an extracellular pathway, but there was a marked increase in the permeability of water and other small hydrophilic solutes. Collectively, these results suggest that ADH acts to increase the motional freedom or fluidity of lipids in the cell membrane which is considered to be the preferred pathway for the permeation of lipophilic and small hydrophilic molecules. At concentrations of cAMP and ADH which elicit equivalent increments in the shortcircuit current, the effects of these agents on nonelectrolyte transport and membrane electrical conductance are divergent. Such observations suggest that some membrane effects of ADH may not be directly dependent upon cAMP. ADH in the mucosal solution increased the permeability of the toad bladder when the surface charge on the outer surface of the apical membrane was screened with the polyvalent cation, La-3+. These experiments emphasize that interaction of ADH with membranes of toad urinary bladder may account for at least some effects of this hormone.

Immunofluorescent localization of cyclic AMP in toad urinary bladder: possible intercellular transfer

By use of an immunofluorescent cytochemical staining technique, adenosine 3',5'-monophosphate (cyclic AMP) has been localized in toad bladder epithelial cells. Within 2 minutes after addition of vasopressin, staining intensity increases in both mitochondria-rich and granular cells. This finding, taken together with the precise anatomical relation between these two epithelial cell types and the observation that after separation of the two cell types vasopressin stimulates cyclic AMP accumulation in only mitochondria-rich cells, suggests that cyclic AMP may be transferred from mitochrondria-rich to granular cells as part of the response of the toad urinary bladder to vasopressin.

Calcium release in relation to permeability changes in toad bladder epithelium following antidiuretic hormone

1. Methods for measuring the release of (45)Ca from isolated urinary bladders of toads (Bufo marinus) pre-loaded with this isotope have been devised. One method allowed separate collection from the mucosal and serosal surfaces of the bladders.2. Reducing the ambient calcium concentration reduced the rate of (45)Ca efflux suggesting that efflux of radiolabel represents calcium exchange.3. Antidiuretic hormone, theophylline and prostaglandin E(1) all increased calcium efflux, while lanthanum and amphotericin were without effect. Cyclic AMP caused only an inhibition of calcium release.4. The increase in (45)Ca efflux due to antidiuretic hormone came exclusively from the mucosal side. Experiments with EGTA suggest that the calcium entering the mucosal solution arises mainly from superficial sites in the mucosal membrane.5. The release of (45)Ca by hormone was not influenced by removal of sodium from the bathing solution. Low pH and amiloride reduced or abolished calcium release to hormone.6. The time course of calcium release from the mucosal surface due to hormone was rapid (commencing between 0.5 and 1.5 min after hormone application). Thus calcium release precedes the increase in sodium transport and hydro-osmotic flow following hormone, and appears to be at least as rapid as cyclic AMP generation in the tissue.7. The relationship between calcium release or exchange and the permeability changes in the bladder to water and to sodium, following hormone, are discussed.

Partition of tissue functions in epithelia: localization of enzymes in "mitochondria-rich" cells of toad urinary bladder

The mucosal epithelium of the toad urinary bladder reabsorbs sodium, acidifies the urine, and is responsive to neurohypophyseal hormnones. Mucosal epithelial cells, consisting of two major morphologic cell types, "mitochondria-rich" and "granular," were removed from the bladder and separated by density gradient centrifugation. The mitochondria-rich cells contained three times as much carbonic anhydrase activity as the granular cells. Oxytocin caused a 235 percent increase in the adenosine 3',5'-monophosphate content of mitochondria-rich cells but had no effect on the granular cells. The evidence indicates that the mitochondria-rich cell, which accounts for only 15 percent of the mucosal cells, plays a major role in the mediation of sodium ion and hydrogen ion transport in the toad bladder and is a specific site of action of neurohypophyseal hormones.