Prostaglandin E binding by plasmatic membranes of thyroid gland cells]

The binding of prostaglandins E (PGE) with human and bovine thyrocyte plasmatic membranes was investigated by the new method with material from the kit for radioimmunoassay of PGE. There was a high affinity and a low affinity site for the specific PGE binding on human and bovine thyrocytes plasmic membranes. The effect of thyrotropin and cyclic nucleotides on the PGE binding by the membranes was revealed.     

Sequence of events underlying the allosteric transition of rod cyclic nucleotide-gated channels

Activation of cyclic nucleotide-gated (CNG) ion channels involves a conformational change in the channel protein referred to as the allosteric transition. The amino terminal region and the carboxyl terminal cyclic nucleotide-binding domain of CNG channels have been shown to be involved in the allosteric transition, but the sequence of molecular events occurring during the allosteric transition is unknown. We recorded single-channel currents from bovine rod CNG channels in which mutations had been introduced in the binding domain at position 604 and/or the rat olfactory CNG channel amino terminal region had been substituted for the bovine rod amino terminal region. Using a hidden Markov modeling approach, we analyzed the kinetics of these channels activated by saturating concentrations of cGMP, cIMP, and cAMP. We used thermodynamic mutant cycles to reveal an interaction during the allosteric transition between the purine ring of the cyclic nucleotides and the amino acid at position 604 in the binding site. We found that mutations at position 604 in the binding domain alter both the opening and closing rate constants for the allosteric transition, indicating that the interactions between the cyclic nucleotide and this amino acid are partially formed at the time of the transition state. In contrast, the amino terminal region affects primarily the closing rate constant for the allosteric transition, suggesting that the state-dependent stabilizing interactions between amino and carboxyl terminal regions are not formed at the time of the transition state for the allosteric transition. We propose that the sequence of events that occurs during the allosteric transition involves the formation of stabilizing interactions between the purine ring of the cyclic nucleotide and the amino acid at position 604 in the binding domain followed by the formation of stabilizing interdomain interactions.     

Stimulation of cyclic AMP production by vasoactive agents in cultured bovine aortic and pulmonary artery endothelial cells

The ability of selected vasoactive agents to influence cyclic AMP levels of confluent, early-passaged bovine calf aortic and pulmonary artery endothelial cells was investigated. Among the agents tested, only the catecholamines (isoproterenol, epinephrine, norepinephrine) and prostaglandins (PGE1, PGE2, PGF2 alpha) resulted consistently in increased cyclic AMP production in both cell populations. The degree of cyclic AMP stimulation obtained with other vasoactive compounds (angiotensins I and II, bradykinin, and serotonin) tended to be either very small or difficult to reproduce. Isoproterenol stimulation was blocked completely by propanolol, a beta-blocking agent, but not by phentolamine, and alpha-blocking agent. These results reveal that bovine calf aortic and pulmonary artery endothelial cells are responsive to catecholamines and prostaglandins, and therefore presumably possess both sensitive adenylate cyclases and plasma membrane receptors for these compounds.     

Solution structure of the Mesorhizobium loti K1 channel cyclic nucleotide‐binding domain in complex with cAMP

Cyclic nucleotide‐sensitive ion channels, known as HCN and CNG channels, are crucial in neuronal excitability and signal transduction of sensory cells. HCN and CNG channels are activated by binding of cyclic nucleotides to their intracellular cyclic nucleotide‐binding domain (CNBD). However, the mechanism by which the binding of cyclic nucleotides opens these channels is not well understood. Here, we report the solution structure of the isolated CNBD of a cyclic nucleotide‐sensitive K⁺ channel from Mesorhizobium loti. The protein consists of a wide anti‐parallel β‐roll topped by a helical bundle comprising five α‐helices and a short 3₁₀‐helix. In contrast to the dimeric arrangement (‘dimer‐of‐dimers’) in the crystal structure, the solution structure clearly shows a monomeric fold. The monomeric structure of the CNBD supports the hypothesis that the CNBDs transmit the binding signal to the channel pore independently of each other.     

Separate Binding and Functional Sites for ω co-Conotoxin and Nitrendipine Suggest Two Types of Calcium Channels in Bovine Chromaffin Cells

Purified adrenomedullary plasma membranes contain two high-affinity binding sites for 125I-omega-conotoxin, with KD values of 7.4 and 364 pM and Bmax values of 237 and 1,222 fmol/mg of protein, respectively. Dissociation kinetics showed a biphasic component and a high stability of the toxin-receptor complex, with a t1/2 of 81.6 h for the slow dissociation component. Unlabeled omega-conotoxin inhibited the binding of the radioiodinated toxin, adjusting to a two-site model with Ki1 of 6.8 and Ki2 of 653 pM. Specific binding was not affected by Ca2+ channel blockers or activators, cholinoceptor antagonists, adrenoceptor blockers, Na+ channel activators, dopaminoceptor blockers, or Na+/H+ antiport blockers, but divalent cations (Ca2+, Sr2+, and Ba2+) inhibited the toxin binding in a concentration-dependent manner. The binding of the dihydropyridine [3H]nitrendipine defined a single specific binding site with a KD of 490 pM and a Bmax of 129 fmol/mg of protein. At 0.25 microM, omega-conotoxin was not able to block depolarization-evoked Ca2+ uptake into cultured bovine adrenal chromaffin cells depolarized with 59 mM K+ for 30 s, whereas under the same conditions, 1 microM nitrendipine inhibited uptake by approximately 60%. When cells were hyperpolarized with 1.2 mM K+ for 5 min and then Ca2+ uptake was subsequently measured during additions of 59 mM K+. Omega-conotoxin partially inhibited Ca2+ uptake in a concentration-dependent manner. These results suggest that two different types of Ca2+ channels might be present in chromaffin cells. However, the molecular identity of omega-conotoxin binding sites remains to be determined.     

Function and Dysfunction of CNG Channels: Insights from Channelopathies and Mouse Models

Channels directly gated by cyclic nucleotides (CNG channels) are important cellular switches that mediate influx of Na⁺ and Ca²⁺ in response to increases in the intracellular concentration of cAMP and cGMP. In photoreceptors and olfactory receptor neurons, these channels serve as final targets for cGMP and cAMP signaling pathways that are initiated by the absorption of photons and the binding of odorants, respectively. CNG channels have been also found in other types of neurons and in non-excitable cells. However, in most of these cells, the physiological role of CNG channels has yet to be determined. CNG channels have a complex heteromeric structure. The properties of individual subunits that assemble in specific stoichiometries to the native channels have been extensively investigated in heterologous expression systems. Recently, mutations in human CNG channel genes leading to inherited diseases (so-called channelopathies) have been functionally characterized. Moreover, mouse knockout models were generated to define the role of CNG channel proteins in vivo. In this review, we will summarize recent insights into the physiological and pathophysiological role of CNG channel proteins that have emerged from genetic studies in mice and humans.     

The influence of exogenous eicosanoids on the radiation response of cultured bovine aortic endothelial cells

The radioprotection by several eicosanoids was investigated in cultures of bovine aortic endothelial cells. One hour before irradiation (0-500 cGy, 137Cs gamma rays) 10 micrograms/ml of PGD2, PGE1, PGI2, misoprostol (PGE1-analog), 16,16-dimethyl PGE2, PGA2, or 1 microgram/ml LTC4 was added. Radiation decreased incorporation of [3H]thymidine at 4 h, cell number/culture at 24 h, and cell survival as measured by colony formation. Under these conditions the eicosanoids were not radioprotective. Two eicosanoids, PGD2 and PGA2, appeared to be toxic. Because receptors might mediate eicosanoid-induced radioprotection, radioligand binding of PGE2 and LTC4 and levels of adenosine 3',5'-cyclic monophosphate (cAMP) were measured. Evidence for a receptor was equivocal; there was nonspecific binding and metabolism of LTC4. The level of cAMP was elevated by 16-16-dimethyl-PGE2 in the presence of isobutyl methylxanthine; however, this combination of the prostaglandin and the methylxanthine was not radioprotective. These investigations suggest that an elevated cAMP level alone does not lead to eicosanoid-induced radioprotection of bovine aortic endothelial cell monolayers in vitro.     

A single intracellular cysteine residue is responsible for the activation of the olfactory cyclic nucleotide-gated channel by NO

The activation of cyclic nucleotide-gated (CNG) channels is the final step in olfactory and visual transduction. Previously we have shown that, in addition to their activation by cyclic nucleotides, nitric oxide (NO)-generating compounds can directly open olfactory CNG channels through a redox reaction that results in the S-nitrosylation of a free SH group on a cysteine residue. To identify the target site(s) of NO, we have now mutated the four candidate intracellular cysteine residues Cys-460, Cys-484, Cys-520, and Cys-552 of the rat olfactory rCNG2 (alpha) channel into serine residues. All mutant channels continue to be activated by cyclic nucleotides, but only one of them, the C460S mutant channel, exhibited a total loss of NO sensitivity. This result was further supported by a similar lack of NO sensitivity that we found for a natural mutant of this precise cysteine residue, the Drosophila melanogaster CNG channel. Cys-460 is located in the C-linker region of the channel known to be important in channel gating. Kinetic analyses suggested that at least two of these Cys-460 residues on different channel subunits were involved in the activation by NO. Our results show that one single cysteine residue is responsible for NO sensitivity but that several channel subunits need to be activated for channel opening by NO.     

Mechanism of allosteric modulation of rod cyclic nucleotide-gated channels

The cyclic nucleotide-gated (CNG) channel of retinal rod photoreceptor cells is an allosteric protein whose activation is coupled to a conformational change in the ligand-binding site. The bovine rod CNG channel can be activated by a number of different agonists, including cGMP, cIMP, and cAMP. These agonists span three orders of magnitude in their equilibrium constants for the allosteric transition. We recorded single-channel currents at saturating cyclic nucleotide concentrations from the bovine rod CNG channel expressed in Xenopus oocytes as homomultimers of alpha subunits. The median open probability was 0.93 for cGMP, 0.47 for cIMP, and 0.01 for cAMP. The channels opened to a single conductance level of 26-30 pS at +80 mV. Using signal processing methods based on hidden Markov models, we determined that two closed and one open states are required to explain the gating at saturating ligand concentrations. We determined the maximum likelihood rate constants for two gating schemes containing two closed (denoted C) and one open (denoted O) states. For the C left and right arrow C left and right arrow O scheme, all rate constants were dependent on cyclic nucleotide. For the C left and right arrow O left and right arrow C scheme, the rate constants for only one of the transitions were cyclic nucleotide dependent. The opening rate constant was fastest for cGMP, intermediate for cIMP, and slowest for cAMP, while the closing rate constant was fastest for cAMP, intermediate for cIMP, and slowest for cGMP. We propose that interactions between the purine ring of the cyclic nucleotide and the binding domain are partially formed at the time of the transition state for the allosteric transition and serve to reduce the transition state energy and stabilize the activated conformation of the channel. When 1 microM Ni2+ was applied in addition to cyclic nucleotide, the open time increased markedly, and the closed time decreased slightly. The interactions between H420 and Ni2+ occur primarily after the transition state for the allosteric transition.     

Antagonistic regulation of native Ca2+- and ATP-sensitive cation channels in brain capillaries by nucleotides and decavanadate

Regulation by cytosolic nucleotides of Ca2+- and ATP-sensitive nonselective cation channels (CA-NSCs) in rat brain capillary endothelial cells was studied in excised inside-out patches. Open probability (Po) was suppressed by cytosolic nucleotides with apparent KI values of 17, 9, and 2 microM for ATP, ADP, and AMP, as a consequence of high-affinity inhibition of channel opening rate and low-affinity stimulation of closing rate. Cytosolic [Ca2+] and voltage affected inhibition of Po, but not of opening rate, by ATP, suggesting that the conformation of the nucleotide binding site is influenced only by the state of the channel gate, not by that of the Ca2+ and voltage sensors. ATP inhibition was unaltered by channel rundown. Nucleotide structure affected inhibitory potency that was little sensitive to base substitutions, but was greatly diminished by 3'-5' cyclization, removal of all phosphates, or complete omission of the base. In contrast, decavanadate potently (K1/2 = 90 nM) and robustly stimulated Po, and functionally competed with inhibitory nucleotides. From kinetic analyses we conclude that (a) ATP, ADP, and AMP bind to a common site; (b) inhibition by nucleotides occurs through simple reversible binding, as a consequence of tighter binding to the closed-channel relative to the open-channel conformation; (c) the conformation of the nucleotide binding site is not directly modulated by Ca2+ and voltage; (d) the differences in inhibitory potency of ATP, ADP, and AMP reflect their different affinities for the closed channel; and (e) though decavanadate is the only example found to date of a compound that stimulates Po with high affinity even in the presence of millimolar nucleotides, apparently by competing for the nucleotide binding site, a comparable mechanism might allow CA-NSC channels to open in living cells despite physiological levels of nucleotides. Decavanadate now provides a valuable tool for studying native CA-NSC channels and for screening cloned channels.     

Modulation of synaptic function by cGMP and cGMP-gated cation channels

Cyclic nucleotide-gated cation channels have been studied intensively in the primary sensory neurons of the visual and olfactory systems. Using both anatomical and physiological methods we have shown that they have a much more widespread distribution in the nervous system. In many retinal ganglion cells cGMP, but not cAMP, activates a non-selective conductance that has many of the properties of CNG channels. As many neurons also contain cGMP-dependent protein kinases (PKGs), we have used a variety of cGMP analogues to distinguish the actions of cGMP. Sp-8-Br-PET-cGMPS is a potent non-hydrolyzable cGMP analogue that is an agonist of PKG. We found that Sp-8-Br-PET-cGMPS acts as a competitive inhibitor of at least the rod CNG channel. Rp-8-Br-cGMPS has shown the opposite effects, namely as an agonist of the rod CNG channel and an inhibitor of PKG. In dissociated cell cultures and slices of rodent visual cortex cGMP had multiple rapid and reversible effects on transmission at glutamatergic synapses. Extracellular application of 8-Br-cGMP or Sp-8-Br-PET-cGMPS reduced stimulus evoked EPSPs in cortical slices. In cortical cultures both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. The effects on both EPSPs and EPSCs were presynaptic. The effects on evoked EPSPs may be due, in part, to reduced calcium influx through voltage-gated calcium channels. The effects on spontaneous EPSCs may be due, in part, to modulation of calcium fluxes through internal stores. Similar modulations of synaptic transmission have been found at gabaergic synapses. On postsynaptic cells, PKG activation produced a dramatic enhancement of the responses to applied NMDA. No effects were detected on applied AMPA/kainate or GABA. Together the results suggest that cGMP may use multiple mechanisms to modulate synaptic efficacy and that its actions may include regulating synaptic plasticity and the relative strength of excitatory and inhibitory drive through neural pathways.     

Block of inwardly rectifying K+ currents by extracellular Mg2+ and Ba2+ in bovine pulmonary artery endothelial cells

Using whole-cell patch clamp technique, we investigated the blocking effects of extracellular Ba2+ and Mg2+ on the inwardly rectifying K+ (KIR) currents of bovine pulmonary artery endothelial cells (BPAEC). The BPAEC KIR channel has recently been identified as Kir2.1 of the Kir2.0 subfamily. Block of KIR currents by Mg2+ (3-30 mM) was instantaneous, and increased with hyperpolarization slightly (Kd at -160 and 0 mV was 9.5 and 23.2 mM, respectively). The apparent fractional electrical distance (delta) of the Mg2+ binding site is calculated to be 0.07 from the outer mouth of the channel pore. Ba2+ (0.3-10 microM) time-dependently blocked the KIR currents with a much higher potency and stronger voltage-dependence (Kd at -160 and 0 mV was 1.0 and 41.6 microM, respectively). The Ba2+ binding site had a delta value of 0.34. Our data suggest that Mg2+ binds to a very superficial site of the KIR channel, while Ba2+ binds to a much deeper site, sensing much more of the membrane electric field. Thus, the BPAEC Kir2.1 appears to be pharmacologically different from the Kir2.1 reported before in bovine aortic endothelial cells (BAEC), which has 2 sites for Mg2+ block (a deep site in addition to a shallow one), and a superficial and low-sensitivity site for Ba2+ block.     

Natriuretic peptides and nitric oxide stimulate cGMP synthesis in different cellular compartments

Cyclic nucleotide-gated (CNG) channels are a family of ion channels activated by the binding of cyclic nucleotides. Endogenous channels have been used to measure cyclic nucleotide signals in photoreceptor outer segments and olfactory cilia for decades. Here we have investigated the subcellular localization of cGMP signals by monitoring CNG channel activity in response to agonists that activate either particulate or soluble guanylyl cyclase. CNG channels were heterologously expressed in either human embryonic kidney (HEK)-293 cells that stably overexpress a particulate guanylyl cyclase (HEK-NPRA cells), or cultured vascular smooth muscle cells (VSMCs). Atrial natriuretic peptide (ANP) was used to activate the particulate guanylyl cyclase and the nitric oxide donor S-nitroso-n-acetylpenicillamine (SNAP) was used to activate the soluble guanylyl cyclase. CNG channel activity was monitored by measuring Ca2+ or Mn2+ influx through the channels using the fluorescent dye, fura-2. We found that in HEK-NPRA cells, ANP-induced increases in cGMP levels activated CNG channels in a dose-dependent manner (0.05-10 nM), whereas SNAP (0.01-100 microM) induced increases in cGMP levels triggered little or no activation of CNG channels (P < 0.01). After pretreatment with 100 microM 3-isobutyl-1-methylxanthine (IBMX), a nonspecific phosphodiesterase inhibitor, ANP-induced Mn2+ influx through CNG channels was significantly enhanced, while SNAP-induced Mn2+ influx remained small. In contrast, we found that in the presence of IBMX, both 1 nM ANP and 100 microM SNAP triggered similar increases in total cGMP levels. We next sought to determine if cGMP signals are compartmentalized in VSMCs, which endogenously express particulate and soluble guanylyl cyclase. We found that 10 nM ANP induced activation of CNG channels more readily than 100 muM SNAP; whereas 100 microM SNAP triggered higher levels of total cellular cGMP accumulation. These results suggest that cGMP signals are spatially segregated within cells, and that the functional compartmentalization of cGMP signals may underlie the unique actions of ANP and nitric oxide.     

Cyclic nucleotide-gated ion channels in sensory transduction

Cyclic nucleotide-gated (CNG) channels, directly activated by the binding of cyclic nucleotides, were first discovered in retinal rods, cones and olfactory sensory neurons. In the visual and olfactory systems, CNG channels mediate sensory transduction by conducting cationic currents carried primarily by sodium and calcium ions. In olfactory transduction, calcium in combination with calmodulin exerts a negative feedback on CNG channels that is the main molecular mechanism responsible for fast adaptation in olfactory sensory neurons. Six mammalian CNG channel genes are known and some human visual disorders are caused by mutations in retinal rod or cone CNG genes.     

Substituted diphenylbutylpiperidines bind to a unique high affinity site on the L-type calcium channel. Evidence for a fourth site in the cardiac calcium entry blocker receptor complex

Fluspirilene binds with high affinity to a single class of sites in purified porcine cardiac sarcolemmal membrane vesicles at a Kd of 0.6 nM and a Bmax that is in approximately 1:1 stoichiometry with other Ca2+ entry blocker receptors. Fluspirilene binding is modulated by various classes of L-type Ca2+ channel effectors. Metal ion channel inhibitors (e.g. Cd2+) stimulate binding primarily by increasing ligand affinity, whereas channel substrates (e.g. Ca2+) inhibit binding. Dihydropyridine, aralkylamine, and benzothiazepine Ca2+ entry blockers partially inhibit binding with Ki values equivalent to their respective Kd values, indicating close coupling between binding sites for the former agents and the diphenylbutylpiperidine site. All of these agents function as mixed inhibitors and affect both Kd and Bmax of fluspirilene binding. Only other substituted diphenylbutylpiperidines (e.g. pimozide) inhibit binding competitively. Diphenylbutylpiperidines, on the other hand, block nitrendipine, D-600, and diltiazem binding through a noncompetitive mechanism with Ki values much reduced from their measured Kd values, suggesting that coupling between the diphenylbutylpiperidine site and receptors for diverse Ca2+ entry blockers is more indirect. In addition, high affinity sites have been detected for fluspirilene in bovine aortic sarcolemmal vesicles, rat brain synaptic membranes, and GH3 rat anterior pituitary cell plasma membranes. Fluspirilene also effectively blocks Ca2+ flux through L-type Ca2+ channels in GH3 cells. Together, these results suggest that fluspirilene binds with high affinity to a unique fourth site in the Ca2+ entry blocker receptor complex and that substituted diphenylbutylpiperidines represent a new structural class of potent L-type Ca2+ channel inhibitors.     

Characterization of the interaction between factor Xa and bovine aortic endothelial cells

Cultured bovine aortic endothelial cells incubated with Factor Xa activate prothrombin. Factor V, synthesized by the endothelial cells, or plasma Factor V and calcium are required for the reaction. In the present study, it has been demonstrated that 125I-Factor Xa binds specifically to endothelial cells. In addition, the activation of prothrombin by Factor Xa and aortic endothelial cells has been further characterized. The binding of 125I-Factor Xa to endothelial cells was saturable and reversible. The equilibrium dissociation constant (Kd) for 125I-Factor Xa binding was 3.6 X 10(-9) M, with 39000 molecules bound per cell. 125I-Factor Xa, inactivated by diisopropylfluorophosphate did not bind specifically to endothelial cells, indicating that the active site of Factor Xa was required for binding. Factor Xa, but not activated protein C, competed with 125I-Factor Xa for binding. Autoradiograms of sodium dodecyl sulfate-polyacrylamide gels of cell lysates indicated that the radiolabeled material that bound to the cells had electrophoretic mobility identical to Factors Xa alpha and Xa beta. Although Factor X partially inhibited the binding of 125I-Factor Xa, Factor Xa did not inhibit the binding of 125I-Factor X, indicating that the zymogen and enzyme bound to different receptors. The relationship of the 125I-Factor Xa binding which was measured in these studies to aortic endothelial cell prothrombin activation is unclear since an anti-Factor V IgG blocked prothrombin activation but not Factor Xa binding. Additionally, 125I-Factor Xa binds to nonvascular cells; these cells do not activate prothrombin in the presence of Factor Xa. Moreover, the calcium requirements for each reaction and the saturation curves of 125I-Factor Xa binding and prothrombin activation differ. Although these data do not exclude a relationship between Factor Xa binding and prothrombin activation, the binding of 125I-Factor Xa to aortic endothelium measured in these studies may be related to a separate cellular function. To further characterize prothrombin activation by Factor Xa and endothelial cells, the rates of thrombin generation by intact bovine aorta or endothelial cells derived from this tissue were compared and were found to be equivalent. These data indicate that vascular endothelium may serve as a physiologic surface for hemostasis.     

Conformational changes in S6 coupled to the opening of cyclic nucleotide-gated channels.

In cyclic nucleotide-gated channels (CNG), direct binding of cyclic nucleotides in the carboxy-terminal region is allosterically coupled to opening of the pore. A CNG1 channel pore was probed using site-directed cysteine substitution to elucidate conformational changes associated with channel opening. The effects of cysteine modification on permeation suggest a structural homology between CNG and KcsA pores. We found that intersubunit disulfide bonds form spontaneously between S399C residues in the helix bundle when channels are in the closed but not in the open state. While MTSET modification of pore-lining residues was state dependent, Ag(+) modification of V391C, in the inner vestibule, occurred at the same diffusion-limited rate in both open and closed states. Our results suggest that the helix bundle undergoes a conformational change associated with gating but is not the activation gate for CNG channels.     

Atrial natriuretic peptide binding, cross-linking, and stimulation of cyclic GMP accumulation and particulate guanylate cyclase activity in cultured cells

The stimulation of cyclic GMP accumulation and particulate guanylate cyclase activity by atrial natriuretic peptide (ANP) was compared to the affinity and number of ANP receptors in eight cultured cell types. At 100 nM, ANP increased cyclic GMP by 13-fold in bovine adrenal cortical, 35-fold in human lung fibroblast, 58-fold in canine kidney epithelial, 60-fold in bovine aortic smooth muscle, 120-fold in rat mammary epithelial, 260-fold in rat Leydig, 300-fold in bovine kidney epithelial, and 475-fold in bovine aortic endothelial cells. ANP (1 microM) increased particulate guanylate cyclase activity by 1.5-, 2.5-, 3.1-, 3.2-, 5.0-, 7.0-, 7.8-, and 8.0-fold in bovine adrenal cortical, bovine aortic smooth muscle, human lung fibroblast, canine kidney epithelial, rat mammary epithelial, rat Leydig, bovine kidney epithelial, and bovine aortic endothelial cells, respectively. Specific 125I-ANP binding to intact rat Leydig (3,000 sites/cell; Kd = 0.11 nM), bovine aortic endothelial (14,000 sites/cell; Kd = 0.09 nM), bovine adrenal cortical (50,000 sites/cell; Kd = 0.12 nM), human lung fibroblast (80,000 sites/cell; Kd = 0.32 nM), and bovine aortic smooth muscle (310,000 sites/cell; Kd = 0.82 nM) cells was saturable and high affinity. No specific and saturable ANP binding was detected in bovine and canine kidney epithelial and rat mammary epithelial cells. Two ANP-binding sites of 66,000 and 130,000 daltons were specifically labeled by 125I-ANP after cross-linking with disuccinimidyl suberate. The 130,000-dalton ANP-binding sites bound to a GTP-agarose affinity column, and the specific activity of guanylate cyclase was increased by 90-fold in this fraction. Our results demonstrate that the increase in cyclic GMP accumulation and particulate guanylate cyclase activity by ANP does not correlate with the affinity and number of ANP-binding sites. These results suggest that multiple populations of ANP receptors exist in these cells and that only one receptor subtype (130,000 daltons) is associated with particulate guanylate cyclase activity.