Differential effects of vasopressin on the water, calcium and lysosomal enzyme contents of mitochondria-rich and lysosome-rich (granular) epithelial cells isolated from bullfrog urinary bladder

Maximal flux responses after multiple challenges with vasopressin

Antidiuretic hormone (ADH) increases transepithelial flux of water and particular solutes across the amphibian urinary bladder and mammalian collecting duct by increasing the permeability of the apical surface. We find that if each challenge with ADH is ended by replacing the medium bathing both the mucosal and serosal surfaces of the toad bladder, then rechallenge with the same supramaximal dose of ADH 36-100 min later produces flux equivalent to or greater than the original response, but rechallenge after 15 min produces only 68% of the original response. If the medium bathing the mucosal surface is neither replaced nor returned to its original volume, complete recovery of the osmotic flux response to ADH does not occur. Maximal restimulation by ADH occurs with transepithelial osmotic gradients between 119 and 180 mosmol/kg during both challenges (the serosal bath is always isotonic amphibian Ringers). In addition, ADH-containing serosal baths that have maximally activated transport across bladders for 30-60 min can be reused and again produce maximal activation of ADH responses in fresh bladders or in the original bladders after washing. These results are in contradistinction to reports of desensitization of transepithelial flux upon rechallenge with ADH after an initial stimulation under many conditions. Our findings suggest that desensitization in vitro may result from experimental design rather than intrinsic biological characteristics of the system.

Role of calcium and prostaglandins in the antidiuretic hormone response. Effect of ionophore A23187

The calcium ionophore A23187 (IP) inhibited the antidiuretic hormone (ADH)-stimulated hydro-osmotic response in toad urinary bladder but had no effect on the osmotic transfer of water in the absence of hormone. Extracellular calcium was necessary for this effect at lower but not at higher IP concentrations. The hydro-osmotic response to exogenous cyclic AMP was unaltered by IP, but the same response produced by inhibition of phosphodiesterase was reduced significantly. Cyclic AMP concentrations in isolated toad bladder epithelial cells were reduced by 50% with IP or exogenous prostaglandin E2 (PGE2). Indomethacin, a prostaglandin synthesis inhibitor, prevented the inhibitory actions of the IP on the ADH-mediated response. Collectively, these observations suggest a key role for cellular calcium in modulating the actions of antidiuretic hormone and are consistent with the hypothesis that the ionophore, through increasing intracellular calcium, stimulates the synthesis of prostaglandins which have a negative feedback on adenylcyclase. This effect would terminate the action of the hormone.

Extracellular Ca2+ and the effect of antidiuretic hormone on the water permeability of the toad urinary bladder: an example of flow-induced alteration of flow

The extracellular Ca2+ requirement for antidiuretic hormone (ADH) stimulation of water permeability in the toad urinary bladder has been critically examine. The polarity of the tissue was maintained with 1 mM Ca2+ in the mucosal bathing medium and a serosal bath nominally free of Ca2+. Under these conditions, ADH-induced osmotic water flow was inhibited by more than 60% while enhancement of the diffusional permeability to water was unaffected. Structural studies revealed that low serosal Ca2+ led to parallel alterations in epithelial architecture that amounted to a significant distortion of the osmotic water pathway. Prevention of these alterations, or restoration of normal cell-cell contact showed that the reduction of serosal Ca2+ did not restrict hormonal action per se, but that it resulted in a weakening of cell-cell junctions such that intercellular space distension during water flow occurred to a point where the geometric conditions for maintenance of osmotic flow were compromised. We conclude that extracellular Ca2+ is not a requirement for the molecular aspects of ADH action but that, in its absence, a direct measurement of ADH-induced osmotic flow proves to be an inaccurate index of the hormone-generated changes in epithelial transport characteristics. Under certain conditions the ADH-effect on the tissue's hydraulic permeability is probably best assessed by measurement of the diffusional permeability to water; although accuracy in this determination is difficult, it is not as strongly dependent on tissue geometry.

Enhanced sensitivity to stimulation of sodium transport and cyclic AMP by antidiuretic hormone after Ca2+ depletion of isolated frog skin epithelium

The role of Ca2+ in the stimulation by antidiuretic hormone (ADH) of active sodium transport across the isolated epithelium of frog skin was investigated. This has been done by bathing the blood side with Ca2+-free solution containing 0.1 mM EGTA. This Ca2+ depletion halved the resistance but had no significant effect on the short-circuit current (SCC). The sensitivity of both cAMP- and SCC-stimulation to ADH was increased 40-fold by Ca2+ depletion. Sensitivity to stimulation by theophylline was only changed a little, while stimulation by exogenous cAMP was completely unaltered. The increase in sensitivity to ADH was dependent on the duration of preincubation in Ca2+-free solution, which indicates that a slowly exchanging Ca2+ pool is involved in the determination of sensitivity to ADH. We suggest this pool is of cellular origin and the increased sensitivity is due to the decrease of a Ca2+ inhibition of the ADH-stimulated adenylate cyclase. But a direct effect of Ca2+ on binding of ADH to the receptor cannot be excluded. Our results are not compatible with the hypothesis that entry of extracellular Ca2+ is an obligatory step in the natriferic action of ADH, although it may be so in the hydroosmotic action of ADH. We also found the maximal response to ADH to be higher after Ca2+ depletion. This is in agreement with the hypothesis of intracellular Ca2+ as a modulator of the sodium permeability of the outward-facing membrane.

Heterogeneity of toad bladder granular cell luminal membranes. Distribution of filipin-sterol complexes in freeze-fracture

Glutaraldehyde-fixed toad urinary bladders were incubated with the sterol specific antibiotic, filipin, and the distribution of the resulting filipin-sterol complexes was studied in freeze-fracturing replicas. While the luminal membrane of some cells had a heavy, homogeneous labelling, most had clusters of filipin-sterol complexes scattered on an otherwise poorly labelled membrane. These clusters were seen to overlie cytoplasmic granules, which are found just beneath the luminal plasma membrane of this cell type, and which were always heavily labelled. These results suggest that (a) the granule membranes have a high cholesterol content and (b) sites of interaction between granule and luminal membranes might be enriched in cholesterol. In addition, filipin-sterol complexes were absent from vasopressin-induced particle aggregates on the granular cell luminal membrane.

Quantitative analysis of exocytosis and endocytosis in the hydroosmotic response of toad bladder

This study concerns the timing and magnitude of exocytosis and endocytosis in the granular cells of toad bladder during the hydroosmotic response to antidiuretic hormone. Granule exocytosis at the luminal cell surface is extensive within 5 min of the administration of a physiological dose of hormone. Hydroosmosis becomes detectable during this time period. The amount of membrane added to the luminal surface by exocytosis during 60 min of exposure to hormone can be of the same order of magnitude as the extent of the luminal plasma membrane. Endocytosis, demonstrated by peroxidase uptake from the luminal surface, becomes extensive during the period 15-45 min after hormone administration. Thus, maximal endocytic activity occurs later than the period of most extensive exocytosis and seems to correlate with the onset of the decline in water movement. The amount of membrane retrieved from the luminal surface by endocytosis during 60 min of stimulation is at least three quarters of that added by exocytosis. The bulk membrane movement in ADH stimulated preparations does not require the presence of an osmotic gradient. Colchicine inhibits the hydroosmotic response, the exocytosis of granules, and endocytosis at the luminal surface. These results strengthen our view that the bulk circulation of membrane at the cell surface, via exocytosis and endocytosis, is closely related to the permeability changes occurring at the surface.

Comparison of the effects of increased intracellular calcium and antidiuretic hormone on active sodium transport in frog skin. A study with the calcium ionophore A23187

The addition of the Ca2+ ionophore A23187 (1 microM) to the inside solution of the frog skin resulted in an approx. 40% transient increase in the active influx of Na+ and ionic conductance, which decayed to an approx. 13% steady-state stimulation after 1--2 h. A23187 had no effect from the outside solution. A23187's stimulatory action is most likely the result of the ionophore's ability to increase intracellular Ca2+. This contention is supported by the following experimental results: (1) reintroduction of Ca2+ into a Ca2+-free inner solution stimulated Na+ transport only in the presence of A23187: (2) Mg2+ would not mimic these effects, and (3) EGTA in the inner solution would inhibit the A23187 response. The stimulation of active transport and ionic conductances elicited by A23187 were found to be very similar to those caused by antidiuretic hormone. Several lines of evidence suggest that A23187 may by-pass steps in the normal antidiuretic hormone stimulatory process: (1) A23187 and antidiuretic hormone are apparently non-additive; (2) A23187 acts three times faster than antidiuretic hormone; (3) A23187 stimulates antidiuretic hormone-insensitive frog skins, and (4) results from other laboratories indicate that A23187 does not increase cyclic AMP concentrations. It is speculated that an increase in free intracellular Ca2+ may be a step in the normal antidiuretic hormone stimulatory process. This increase in intracellular Ca2+ may in turn stimulate active sodium transport by increasing the Na+ permeability of the outer 'rate-limiting' membrane.

Vasopressin-like effects of psychotropic drugs in amphibian epithelia

Amphibian epithelia have been used as models for studying the effects of psychotropic drugs on membrane transport. Several of these agents added to the internal or to the external media, at concentrations greater than 10(-3) M, had inhibitory, "ouabain-like" effects on Na transport. In contrast, stimulatory, "vasopressin-like" effects were seen at lower concentrations. The stimulation was additive to that of oxytocin if the drug was present in the external solution but nonadditive if in the internal solution. On water transport, harmala alkaloids had a vasopressinomimetic action in toad skin, while inhibition was seen with Li and amitriptyline. To account for these multiple effects, it is hypothesized that psychotropic drugs act on the following cell targets: the Na pump, the cyclic nucleotide system, microtubules, and membrane calcium sites at the outer barrier of the epithelium. Direct, biochemical evidence is needed to substantiate this hypothesis.