Amiloride increases the sensitivity of particulate guanylate cyclase to atrial natriuretic factor

ATP binding is required for physiological activation of retinal guanylate cyclase

ATP bound to retinal guanylate cyclase (retGC)/membranes prior to the assay (pre-binding effect) and during the assay (direct effect) further enhances retGC activity stimulated by GC-activating proteins (GCAPs). Here we investigate differences between these two effects. We found that the pre-binding effect, but not the direct effect, was absent in membranes pre-washed with Mg(2+)-free hypotonic buffers, that the pre-binding effect, but not the direct effect, was strictly limited to GCAP-stimulated retGC activity, and that these two effects were independent and additive rather than being synergistic. Pre-incubation with amiloride enhanced GCAP2-activated retGC activity in a manner similar to that by ATP pre-binding; however, amiloride did not directly stimulate the retGC activity. These results indicate that these two effects are mechanistically different. Levels of retGC activation by these effects and conditions required for these effects indicate that only the mechanism involving ATP pre-binding is physiologically relevant to retGC activation.

Regulation of natriuretic peptide (urodilatin) release in a human kidney cell line

BACKGROUND

To identify the molecular mechanisms underlying the release of a renal natriuretic peptide (NP) we selected a human kidney cell line (HEK 293) that displays several characteristics of distal tubular cells.

METHODS

Cells were exposed to different extracellular and intracellular stimuli, and the effect on NP release was measured with a specific urodilatin radioimmunoassay, as well as with an atrial NP (ANP) radioimmunoassay.

RESULTS

In the absence of stimuli, HEK 293 cells showed a basal release of urodilatin immunoreactivity and ANP immunoreactivity. Raising the osmolality of the secretion medium with sodium chloride and various other osmolytes rapidly increased cellular NP secretion. Elevation of intracellular cAMP levels by forskolin plus 3-isobutyl-1-methylxanthine and administration of phorbol-12-myristate-13-acetate together with the calcium-ionophore A23187 also resulted in respective increases in the amount of secreted peptide. HEK 293 cells exhibit the endogenous expression of both particulate and soluble guanylyl cyclases. In the presence of 8-Br-cGMP, cell cultures showed the enhanced secretion of an ANP immunoreactive peptide only, indicating that guanylyl cyclase activation provoked the secretion of ANP immunoreactivity but not of urodilatin immunoreactivity.

CONCLUSIONS

The human embryonic kidney cell line HEK 293 represents a renal cellular model system in which we have identified a rapid and regulated release of NPs in response to the osmotic effect of increased extracellular sodium chloride and various intracellular stimuli.

Distribution of membrane bound guanylyl cyclases in human intestine

The quantification and distinction of particulate guanylyl cyclases in the human intestine were considered by an enzymatic approach, which comprised the signal transduction from receptor binding to cGMP formation, and, in addition, by showing the expression of an intracellular portion of these transmembrane proteins. Basal guanylyl cyclase (GC) activities were 50 to 80 pmol cGMP formation/min/mg protein and were stimulated up to twofold by heat stable enterotoxin, but were not significantly influenced by atrial natriuretic factor. Enzymatic analysis of colonoscopic specimens pointed to the prevalence of guanylyl cyclase C in the terminal ileum and in the large bowel including colon ascendens, colon descendens, sigmoid, and rectum. The availability of sequence information on human guanylyl cyclases permitted the development of a polymerase chain reaction approach for distinguishing the expression of GC-A and GC-C in human tissue samples. The expression levels of particulate guanylyl cyclases found by polymerase chain reaction in surgical biopsy specimens confirmed the enzymatic data, in that substantial expression of GC-C was found not only in the small intestine but also in the large bowel. According to the restriction mapping of amplificates, GC-C prevailed over GC-A throughout the human intestine, particularly in the mucosal layers.

Adenosine 5'-triphosphate, phorbol ester, and pertussis toxin effects on atrial natriuretic peptide stimulation of guanylate cyclase in a human renal cell line

We examined adenosine 5'-triphosphate (ATP), pertussis toxin (PT) and phorbol myristate acetate (PMA), a protein kinase C (PKC) activator, modulation of atrial natriuretic peptide (ANP)-stimulated cell-membrane guanylate cyclase (ANP-s-GC) activity and ANP stimulation of whole-cell cGMP accumulation (ANP-s-cGMP) in an ANP-receptor-transduction cell model, the human renal cell line (SK-NEP-1). Acute and long-term effects of PMA on PKC isotype activity are different: Acute (20-min) PMA activation of PKC inhibits ANP-s-cGMP and ANP-s-GC; whereas, long-term (36-h) PMA treatment inhibits slightly less by only partially down-regulating PKC activity, the type-III PKC isotype being 36-h resistant. Long-term 10(-7)M PMA treatment of cells neither affected membrane basal GC activity nor ANP-s-GC activity but partially inhibited ATP enhancement of ANP-s-GC. This partial inhibition was completely reversed by the PKC inhibitor H7 and a PKC inhibitory antibody but only partially reversed by the antibody to the catalytic domain of PKC type III. The EC50 for ATP and its non-phosphorylating analog ATP gamma S in the presence of acute PMA inhibition of ANP-s-cGMP was similar (approximately 10(-9)). This enhancement of PMA inhibition was two orders of magnitude more sensitive (EC50 10(-7)M) than inhibition of ANP-s-cGMP that we previously reported for acute PMA treatment of whole SK-NEP-1 cells. The three- to four-fold ATP enhancement of cell membrane ANP-s-GC was not blocked by 12-hour preincubation of cells with 150 ng/mL PT but was completely blocked if 2-x-10(-7)M PMA was then added for 20 minutes, indicating that acute activation of PKC by PMA does not require a functional "G-type" protein. Acute PMA inhibition of ANP-s-cGMP was reversed by permeabilizing SK-NEP-1 cells to a specific PKC inhibitory peptide, further confirming that PMA inhibition was mediated through PKC activation. These data demonstrated that ANP-s-GC and ANP-s-cGMP were modified through non-phosphorylating interactions with ATP, by multiple PMA activatable PKC isoforms, and that neither were affected by PT-sensitive guanine-nucleotide-binding (G)-protein(s).

Different ATP effects on natriuretic peptide receptor subtypes in LLC-PK1 and NIH-3T3 cells

We have observed different ATP interactions in two guanylate cyclase (GC)-coupled natriuretic peptide (NP) receptor subtypes, designated NPR-A and NPR-B. The NPR-A is selectively expressed by LLC-PK1 epithelial cells and the NPR-B by NIH-3T3 fibroblast cells. In LLC-PK1 membranes, ATP-Mg2+ potentiated ANP-stimulated GC activity (ANP-s-GC). In contrast, in NIH-3T3 membranes, ATP-Mg2+ inhibited ANP-s-GC but enhanced CNP-stimulated GC activity (CNP-s GC). ATP in the presence of Mn2+ inhibited LLC-PK1 and NIH-3T3 membrane ANP-s-GC and CNP-s-GC. These are the first data suggesting that the ATP-Mg2+ produces different effects between membrane NPR-A and -B subtypes. We have also demonstrated that GC of NPR-B is sensitive to methylene blue.

Phosphorylation of atrial natriuretic factor R1 receptor by serine/threonine protein kinases: evidences for receptor regulation

The 130 kDa atrial natriuretic factor receptor (ANF-R1) purified from bovine adrenal zona glomerulosa is phosphorylated in vitro by serine/threonine protein kinases such as cAMP-, cGMP-dependent and protein kinase C. This phosphorylation is independent of the presence of ANF (99-126) and there is no detectable intrinsic kinase activity associated with the ANF-R1 receptor or with its activated form. In bovine adrenal zona glomerulosa cells, TPA (phorbol ester) induces a marked inhibition of the ANF-stimulated cGMP accumulation as well as of the membrane ANF-sensitive guanylate cyclase catalytic activity without any change in the binding capacity or affinity for 125I-ANF. However, we have demonstrated a significant 32P incorporation in the ANF-R1 receptor of the TPA-treated cells. The effect of TPA on the zona glomerulosa ANF-R1 receptors was abolished by calphostin C, a specific protein kinase C inhibitor. Altered ANF actions due to blunted response of guanylate cyclase to ANF could be a consequence of the ANF receptor phosphorylation by excessive activity of protein kinase C and might be involved in the pathogenesis of hypertension.

Calcium and calmodulin regulate atrial natriuretic factor stimulation of cyclic GMP in a human renal cell line

We examined calcium and calmodulin regulation of atrial natriuretic factor stimulation of particulate-membrane guanylate cyclase (ANF-s-GC) in SK-NEP-1 cells. W7 and trifluoropiperazine, but not W5, inhibited whole cellular ANF-stimulated cyclic GMP accumulation (ANF-s-cGMP). EGTA and LaCl3 decreased ANF-s-GC and calmodulin reversed this inhibition. A23187-induced inhibition of ANF-s-cGMP was only partly reversible by IBMX. H7 or staurosporine counteracted the inhibitory effect of A23187. Calcium inhibited basal and ANF-s-GC. These data suggest that at low concentrations of calcium, ANF-s-GC was calcium-calmodulin dependent but high concentrations of calcium inhibited ANF-s-GC through phosphodiesterase, through inhibition of GC, and probably through protein kinase C.

Kinetic characterization of atrial natriuretic factor-sensitive particulate guanylate cyclase

The present investigation describes kinetic characteristics of membrane-bound and Triton X-100-solubilized atrial natriuretic factor (ANF)-sensitive guanylate cyclase from bovine adrenal cortex. The kinetic analysis of both enzyme forms suggests that in the presence of manganese, ANF induces or stabilizes at least two apparent GTP*Mn2(+)- and in addition two Mn2(+)-binding sites. Addition of the natriuretic drug amiloride favors this state. ATP increases the vmax in the presence of ANF for GTP*Mg2+, but not for GTP*Mn2+ as a substrate. With GTP*Mg2+, amiloride has no effect on basal or ANF-stimulated activity, but slightly reduces the effect of ATP. Under all conditions tested, the enzyme follows regular Michaelis-Menten kinetics in the presence of Mg2+ and exhibits positive cooperativity with Mn2+. Positive cooperativity is also retained after Triton extraction. The results indicate that Triton extraction has no major influence on the kinetic properties of particulate guanylate cyclase when the extraction procedure is done carefully. The data also support the suggestion that multiple interactions of subunits might occur upon activation of the enzyme by ANF in the presence of Mn2+.

Characterisation of atrial natriuretic peptide receptors in bovine ventricular sarcolemma

Analysis of [125I]-ANP binding data in an isolated bovine ventricular sarcolemmal membrane fraction revealed a single high affinity binding site (Kd approximately 5 x 10(-11) M). The ring deleted ANP analogue des [QSGLG]-ANP (4-23)-NH2 bound with a 1000-fold lower affinity indicating the absence of C-type receptors in this preparation. ANP stimulated guanylate cyclase activity by up to 2-fold with half-maximal activation at approximately 10(-9) M. Crosslinking [125I]-ANP to its receptor with disuccinimidyl suberate (DSS) revealed two radiolabelled bands of 120 kDa and 65 kDa on non-denaturing SDS-PAGE. Radioactive signals from both bands were lost by reducing the sample with beta-mercaptoethanol prior to electrophoresis, in which case a radioactive fragment of less than 5 kDa migrated with the dye front. These results suggest that the binding of ANP to both high and low molecular weight "receptor" proteins may be associated with the hydrolysis of the peptide.

Purification and properties of active atrial-natriuretic-peptide receptor (type C) from bovine lung

Atrial-natriuretic-peptide (ANP) receptor, previously identified as a 140 kDa protein with a disulphide-linked homodimeric structure, was purified from bovine lung by (NH4)2SO4 fractionation and affinity chromatography on ANP-Affi-Gel 10. The purified receptor had a binding capacity of 4.2 nmol of ANP/mg of protein and an affinity constant of 6.5 pM. The isoelectric point of the receptor was 5.8, consistent with the acidic nature of the protein (amino acid analysis revealed a predominance of glutamic acid and aspartic acid residues). Treatment with endoglycosidase H and glycopeptidase F revealed that the receptor has three complex types of oligosaccharide chains per 70 kDa subunit. Deglycosylation of the receptor did not affect its binding activity. Reduction with dithiothreitol and reoxidation by dialysis revealed a strong tendency of the receptor subunits to dimerize via disulphide cross-linking; however, carboxymethylation of the reduced receptor indicated that the intersubunit disulphide bond is not necessary for the ligand-binding activity.

Urodilatin and beta-ANF: binding properties and activation of particulate guanylate cyclase

Urodilatin (ANF-(95-126] and beta-ANF, the antiparallel dimer of ANF-(99-126), are naturally occurring members of the ANF family. We studied their receptor binding properties in human platelets and Triton-solubilized membranes from bovine adrenal cortex and their ability to activate particulate guanylate cyclase in bovine adrenal cortex. In human platelets containing R2-receptors not coupled to particulate guanylate cyclase urodilatin binds with similar affinity as ANF-(99-126) (KD: 55 pM), whereas beta-ANF has an affinity lower than the truncated ANF-(103-123) (KD: 295 pM and 154 pM). Scatchard analysis indicates one binding site for urodilatin as well as for beta-ANF. In adrenal cortex containing predominantly R1-receptors coupled to particulate guanylate cyclase, urodilatin binds with a higher affinity (KD: 30 pM) than ANF-(99-126) (KD: 52 pM) and stimulates to a similar extent to ANF-(99-126) (about two fold at 1 muM), whereas beta-ANF has a smaller affinity (KD: 120 pM) and stimulates particulate guanylate cyclase to a lower extent than ANF-(99-126). The data from platelets and adrenal cortex show that beta-ANF has low binding affinities but stimulates particulate guanylate cyclase, whereas urodilatin appears to be a physiological R1-agonist.

Amiloride enhances atrial natriuretic factor stimulation of cGMP accumulation in rat glomeruli

The effects of amiloride on ANF binding and ANF stimulation of cGMP were evaluated in rat glomeruli. Amiloride increased ANF binding to whole glomeruli and to glomerular membrane preparations. In contrast, amiloride enhanced ANF-stimulated cGMP accumulation only at 37 degrees C in whole glomeruli, but not at 4 degrees C in whole glomeruli or at 37 degrees C in membrane preparations. These data suggest that under physiological conditions amiloride augments ANF-stimulated intracellular cGMP accumulation. The discrepancies between amiloride augmentation of ANF binding and failure to increase ANF-stimulated cGMP accumulation may result from ANF receptor heterogeneity. This is the first report of an amiloride augmentation of ANF-stimulated cGMP accumulation in renal tissue.

Effect of ATP and amiloride on ANF binding and stimulation of cyclic GMP accumulation in rat glomerular membranes

We investigated ANF binding and stimulation of cGMP accumulation in isolated rat glomerular membranes in the presence and absence of amiloride and ATP. Amiloride enhanced high affinity binding of ANF without affecting its stimulation of cGMP. In contrast ATP decreased binding and decreased basal cGMP accumulation without affecting the ability of ANF to stimulate cGMP. These data indicate that ANF binding and stimulation of cGMP accumulation can be regulated independently supporting further the concept of receptor heterogeneity in renal glomerular membranes.