The mechanism of cyclic AMP stimulation of secretion in the dogfish rectal gland

The effects of dietary sodium loading on the activity and expression of Na, K-ATPase in the rectal gland of the European dogfish (Scyliorhinus canicula)

cDNA fragments of both the alpha- and beta-subunits of the Na, K-ATPase and a cDNA fragment of the secretory form of Na-K-Cl cotransporter from the European dogfish (Scyliorhinus canicula) were amplified and cloned using degenerate primers in RT-PCR. These clones were used along with a sCFTR cDNA from the related dogfish shark, Squalus acanthias to characterise the expression of mRNAs for these ion transporters in the dogfish rectal gland subsequent to an acute feeding episode. Following a single feeding event where starved dogfish were fed squid portions (20 g squid/kg fish), there was a delayed and transient 40-fold increase in the activity of Na, K-ATPase in crude rectal gland homogenates. Increases in enzyme activity were apparent 3 h after the feeding event and peaked at 9 h before returning to control values within 24 h. These increases in activity were accompanied by small and transient decreases in plasma sodium and chloride concentrations lasting up to 3 days. Significant increases in the expression of mRNAs for alpha- and beta-subunits of the Na, K-ATPase, the Na-K-Cl cotransporter and CFTR chloride channel were detected but not until 1-2 days after the feeding event. It is concluded that the transient increase in Na, K-ATPase activity is not attributable to increases in the abundance of alpha- and beta-subunit mRNAs but must be associated with some, as yet unknown, post-transcriptional activation mechanism.

Intracellular [Ca2+] and inositol phosphates in avian nasal gland cells

Isolated cells from the nasal salt gland of ducklings (Anas platyrhynchos) were evaluated as a model system for the study of the muscarinic activation of exocrine ion secretion. Cells loaded with the fluorescent probe indo-1 were used to study changes in intracellular Ca2+ concentration [( Ca2+]i) after stimulation. Changes in inositol phosphate generation and oxygen consumption were also determined. Loading with the acetomethoxy ester form of indo-1 (indo-1/AM) was rapid, and intracellular cleavage of the ester was essentially complete. Leakage of the dye was negligible over the time course of measurements (up to 20 min). Resting [Ca2+]i was approximately 100 nM. Stimulation with carbachol resulted in progressive increases in the generation of inositol phosphates and rapid four- to fivefold increases in [Ca2+]i. At normal extracellular Ca2+ concentrations, [Ca2+]i remained elevated (approximately 3 times resting levels) for as long as stimulation continued. Experiments showed that the increases in [Ca2+]i were comprised of a combination of release of Ca2+ from intracellular stores and an enhanced entry of Ca2+ from the extracellular medium. It is specifically this latter process that produces the sustained elevations in [Ca2+]i that are the essential signal for secretory activity.

Cyclic AMP stimulation of Cl- secretion by the opercular epithelium: the basolateral chloride uptake mechanism

The isolated, short-circuited opercular epithelium of Fundulus heteroclitus, secretes Cl- by a mechanism dependent on the presence of serosal Na+ and inhibited by bumetanide and furosemide. Under serosal Na+-free conditions the active Cl- secretion is abolished. However, subsequent elevations of intracellular cyclic AMP (cAMP) levels with isoproterenol or forskolin stimulated Cl- secretion markedly. This stimulation was unaffected by SITS, DIDS, methazolamide, and HCO-3-free solutions, but was blocked by furosemide and bumetanide. Determinations of relative intracellular 36Cl- levels showed a Na+ dependence of intracellular 36Cl- in epithelia not stimulated by isoproterenol and a Na+ independence of intracellular 36Cl- in isoproterenol stimulated epithelia. In both conditions, the intracellular 36Cl- was bumetanide sensitive. The results indicate that cAMP stimulation of Cl- secretion can occur by a Na+-independent, loop diuretic-inhibitable mechanism, which may be operative even in the presence of Na+. Whether this is a separate Cl- uptake mechanism or a cAMP-induced alteration in the normal Na+-dependent mechanism could not be determined. In either instance, an alternative to the Na+ gradient as a source of energy for Cl- uptake into the cell across the basolateral membrane is required.

Cyclic AMP-dependent stimulation of Na,K-ATPase in shark rectal gland

Scatchard analysis of 3H ouabain bound to isolated rectal gland cells as a function of increasing ouabain concentrations produced a concave curvilinear plot that was resolved into two specific sites with either a high (I) or low (II) affinity for ouabain. Cyclic cAMP/theophylline (+/- furosemide, 10(-4) M) increased the amount of 3H ouabain bound to the high-affinity site I. Vanadate, a phosphate congener which promotes formation of the ouabain-binding state of the enzyme, mimicked the effects of cAMP/theophylline at low concentrations of ouabain, suggesting that cAMP/theophylline increases binding to site I by enhancing the rate of turnover of resident enzyme. Enhanced 86Rb uptake seen following cAMP/theophylline administration was primarily associated with increased flux through the high-affinity ouabain site, and this stimulation was not obliterated by the co-administration of furosemide. A model was presented which suggested the presence of two noninteracting pools of enzyme or isozymes which exhibit either a high or low affinity for ouabain. Cyclic AMP both stimulated turnover via site I, and modified the kinetics of binding of 3H ouabain to site II. The (ave) Kd of 3H ouabain for site II was increased from 3.6 microM (controls) to 0.5 microM (cAMP/theophylline) and the Hill coefficient was modified from 0.45 (controls) to 1.12 (cAMP/theophylline), suggesting a transition from a negative- to a noncooperative binding state. While furosemide reversed the effects of cAMP/theophylline on site II kinetics, it did not obliterate cAMP/theophylline effects on site I. This suggests that cAMP may alter the intrinsic turnover rate of this particular pool of Na,K-ATPase in shark rectal gland.

Cyclic AMP stimulation of Cl- secretion by the opercular epithelium: the apical membrane chloride conductance

Chloride secretion (Isc) by the opercular epithelium of the teleost, Fundulus heteroclitus, is stimulated by elevations in intracellular cyclic AMP (cAMP) elicited by beta-adrenergic agonists, such as isoproterenol, and is accompanied by a small but significant increase in the transepithelial conductance (Gt). Cupric ions (Cu2+) have been shown to block the apical membrane Cl- channels in this epithelium, leading to a reduction in both the Isc and Gt (Degnan, '85). In the present studies, the effects of Cu2+ on cAMP-elevated and cAMP-depleted epithelia were observed to define the actions of cAMP in this stimulatory process. At a concentration of 5 X 10(-4) M in the mucosal solution, Cu2+ inhibited the Isc 79.8% and reduced the Gt 39.2%. Isoproterenol produced an attenuated stimulation of the Isc in these tissues compared to untreated controls, but had no effect on the Gt. In tissues bathed bilaterally with Cl- -free Ringer, the Isc was virtually abolished and the Gt was reduced 37.0%; neither Cu2+ nor isoproterenol had any effects on the Isc or Gt under this condition. Simultaneous 2 2Na and 3 6 Cl unidirectional flux determinations indicated that the only effects of both isoproterenol and Cu2+ were on the active Cl- secretory flux. An inhibitor of adenylate cyclase, 2',5' dideoxyadenosine (DDA), reduced the Isc and Gt 39.8% and 20.8% respectively. This inhibitor had no additional effects in Cu2+ -treated tissues and the action of Cu2+ on the Gt was reduced in DDA-treated tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell solute composition and potassium effects in slices of the rectal gland of the dogfish shark (Squalus acanthias)

Cell solute composition and apparent intracellular ionic concentrations (subscript i) were determined in rectal gland slices of the dogfish shark (Squalus acanthias). On aerobic incubation in elasmobranch Ringer's at 15 degrees C, the steady state values were: Ki+, 151 mM, close to its apparent electrochemical equilibrium; Cai2+, 1.7 mM; Mgi2+, 7.5 mM; Cl1-, 97 mM (maintained against its electrochemical potential gradient); the sum (Nai+ + Ki+) was about 66 meq/kg cell water lower than in the medium. Of free amino acids, only the effectively impermeable anionic taurine (32 mM) and glutamate (3.7 mM) significantly contribute to the Donnan system. Variations of the Ringer's K+ concentrations, or the presence of 0.5 mM ouabain, demonstrated the permeability of the cell membrane to na+, K+, and Cl-. Cell volume changes: 0.5 mM ouabain produced a gradual swelling and ionic changes consistent with an inhibition of the sodium pump. In high-K+ media the cells swelled massively and the membrane depolarized. A linear relationship between cell volume changes and the Nernst potential for K+ was found. These properties are consistent with a pump-and-leak system for cell volume maintenance. The K+ -induced cell swelling was found to be reversible on transfer of the tissue to regular Ringer's. The extrusion of cell water was associated with an influx of Na+ into, and an efflux of K+ from, the cells (both fluxes down-hill), as well as extrusion of Cl- against its concentration gradient.2+

Mechanism of NaCl secretion in rectal gland tubules of spiny dogfish (Squalus acanthias). III. Effects of stimulation of secretion by cyclic AMP

Segments of rectal gland tubules (RGT) the spiny dogfish (Squalus acanthias) were perfused in vitro to study the cellular mechanism by which NaCl secretion is stimulated. Transepithelial PD (PDte), transepithelial resistance (Rte), the PD across the basolateral membrane (PDbl), the fractional resistance of the lumen membrane (FR1), and the cellular activities for Cl-, Na+, and K+ (alpha cell x) were measured. In series 1 the effects of stimulation (S) (dbcAMP 10(-4, adenosine 10(-4), and forskolin 10(-6) mol x 1(-1) on these parameters were recorded and compared to nonstimulated state (NS). PDte increased from -1.9 +/- 0.2 mV to -11.0 +/- 0.9 mV (n = 51). PDbl depolarized from -86 +/- 1 to -74 +/- 1.4 mV (n =52), Rte fell from 29 +/- 2.8 to 21 +/- 2 omega cm2 (n = 23), and FR1 fell from 0.96 +/- 0.005 to 0.79 +/- 0.04 (n = 9). alpha cell K+ was constant (123 +/- 13 versus 128 +/- 17 mmol x 1(-1) (n = 6), but alpha cell cl- fell significantly from 48 +/- 4 to 41 +/- 3 mmol x 1(-1) (n = 7). alpha cell Na+ increased from 11 +/- 2.1 to 29.5 +/- 6.6 mmol x 1(-1) (n = 4). In series 2 the conductivity properties were examined by rapid K+, and Cl- concentration steps on the basolateral and luminal cell side respectively in NS and S states. In NS-segments reduction of bath K+ led to a hyperpolarization of PDbl with a mean slope of 28 +/- 1.3 mV/decade (n = 9) (as compared to 19 mV/decade for S-state).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of NaCl secretion in the rectal gland of spiny dogfish (Squalus acanthias). I. Experiments in isolated in vitro perfused rectal gland tubules

Rectal gland tubules (RGT) of spiny dogfish were dissected and perfused in vitro. Transepithelial PD (PDte), resistance (Rte), the PD across the basolateral membrane (PDbl) and intracellular chloride and potassium activities (a cell Cl-, a cell K+) were measured. In a first series, 67 RGT segments were perfused with symmetric shark "Ringers" solution. The bath perfusate contained in addition db-cAMP 10(-4), forskolin 10(-6), and adenosine 10(-4) mol X 1(-1). PDte was -11 +/- 1 (n = 67) mV lumen negative, Rte 27 +/- 2 (n = 47) omega cm2, PDbl -75 +/- 0.4 (n = 260) mV. a cell K+ and a cell Cl- were 109 +/- 22 (n = 4) and 38 +/- 4 (n = 36) mmol X 1(-1) respectively. These data indicate that Cl-secretion across the RGT must be an uphill transport process, whereas secretion of Na+ could be driven by the lumen negative PDte. Intracellular K+ is 14 mV above equilibrium with respect to the basolateral membrane PD and Cl- is 23 mV above equilibrium across the apical membrane. In series 2, the conductivity properties of the apical and basolateral membrane as well as that of the paracellular pathway were examined in concentration step experiments. Decrease of the basolateral K+ concentration led to a rapid hyperpolarization of PDbl with a mean slope of 19 mV per decade of K+ concentration change. Addition of 0.5 mmol X 1(-1) Ba2+ to the bath solution lead to a marked depolarization and abolished the response to K+ concentration steps. In the lumen a Cl- concentration downward step led to a depolarization of the lumen membrane; resulting in a mean slope of 18 mV per decade of Cl- concentration change. When dilution potentials were generated across the epithelium, the polarity indicated that the paracellular pathway is cation selective. In series 3 the equivalent short circuit current (Isc = PDte/Rte) was determined as a function of symmetrical changes in Na+ concentration, with Cl- held at 276 mmol X 1(-1), and as a function of symmetrical changes in Cl- concentration, with Na+ held at 278 mmol X 1(-1). Isc was a saturable function of Na+ concentration (Hill coefficient 0.9 +/- 0.1, K1/2 4.4 mmol X 1(-1), n = 7) and also a saturable function of Cl- concentration (Hill coefficient 2.0 +/- 0.1, K1/2 75 mmol X 1(-1), n = 11).(ABSTRACT TRUNCATED AT 400 WORDS)

An endogenous peptide stimulates secretory activity in the elasmobranch rectal gland

Extraction and partial purification of peptide material from the intestine of the elasmobranch Scyliorhinus canicula yielded a fraction that shows potent stimulatory activity in the rectal gland. The extracted material appears to contain an endogenous peptide (or peptides) that represents the natural hormone responsible for the control of rectal gland secretion in vivo.

Ouabain binding in rectal gland of Squalus acanthias

In an attempt to examine the mechanisms of activation of (Na, K)-ATPase when epithelial transport is stimulated, the binding of ouabain to rectal gland tissue was measured before and after stimulation with dibutyryl cAMP and theophylline. Stimulation significantly altered the characteristics of ouabain binding to slices of Squalus acanthias rectal gland and to isolated rectal gland cells, accelerating the rate of binding and increasing the amount of ouabain bound at equilibrium when low concentrations of ouabain (10(-9) to 10(-7) M) were present in the medium. Scatchard plots of ouabain binding were nonlinear, suggesting at least two classes of binding sites, one of higher and one of lower affinity. Stimulation with cAMP and theophylline appeared to increase the affinity of the high-affinity site. Ouabain binding was increased by cAMP and theophylline even in the presence of furosemide (10(-4) M) or bumetanide (10(-5) M), and when Li+ was substituted for Na+, or NO3- for Cl- -maneuvers known to inhibit rectal gland secretion. The changes in ouabain binding induced by cAMP and theophylline do not appear, therefore, to be secondary to secretory activity but may reflect a change in the configuration, environment or location of existing enzyme so as to enhance its activity. Stimulation of ouabain binding cannot be demonstrated in whole homogenates of rectal gland, indicating that intact cells are necessary for the cyclic AMP-induced increase in ouabain binding to become manifest.

Amphotericin B and the elasmobranch rectal gland: implications for the relationship between oxygen consumption and ion transport

In slices of the dogfish rectal gland, amphotericin B produced increases in oxygen consumption and ouabain binding similar to those produced by cyclic AMP, except that the increases induced by the latter were inhibited by furosemide whereas those resulting from the addition of amphotericin B were unaffected by the diuretic. Unlike cyclic AMP, however, amphotericin B failed to stimulate secretion by the isolated perfused gland. These results support the suggestion that the increases in ouabain binding and oxygen consumption produced by cyclic AMP result simply from an increased sodium entry into the cells, and as such can be mimicked by amphotericin B, whereas secretion rate itself depends on some additional, cyclic AMP-induced process. The implications of this for determinations of the relationship between rates of oxygen consumption and ion transport are discussed.