The cyclic GMP-induced inward current in neuron R14 of Aplysia californica: similarity to a FMRFamide-induced inward current

Injecting cGMP into Aplysia neuron R14 induced an inward current similar to one elicited by application of FMRFamide to the outside of that cell. In contrast, injection of cAMP into R14 caused a long-lasting outward current and conductance increase. Phosphodiesterase inhibitors increased the cGMP and FMRFamide-induced inward currents in R14. The cGMP-induced inward current is voltage dependent and is largely carried by Na+. It is also strongly and inversely dependent on both external [Ca2+] and [Cl-], although these ions are not significant current carriers. Changing external [K+] had no effect. Voltage and ion dependencies of the cGMP-induced inward current are similar to those of an inward current induced by FMRFamide. Thus cGMP may be a second messenger to FMRFamide in producing a slow inward current in R14. cGMP does not appear to be a second messenger to FMRFamide in most Aplysia neurons.     

Effect of nitric oxide on hyperpolarization-activated current in substantia gelatinosa neurons of rats

Central sensitization is the hyperexcitability of spinal processing after peripheral nerve injury or inflammation. This phenomenon may be associated with nitric oxide (NO) signal pathway in synapse. Here, we have investigated the effect of NO on hyperpolarization-activated inward current (I(h)) in substantia gelatinosa (SG) neurons, using the whole-cell patch clamp technique. I(h) was increased by the application of sodium nitro prusside (SNP, a NO donor) or 8Br-cGMP. The stimulatory effects of NO were abolished by guanylyl cyclase inhibitor, ODQ, suggesting that the effect of NO was mediated by cGMP. However, this effect of NO was not prevented by the pretreatment with KT5823, PKG inhibitor. Taken together, the activation of I(h) in SG neurons could be mediated by NO-cGMP dependent pathway. These results reveal an involvement of NO in excitability of SG neuron via the activation of I(h) may be associated with central sensitization.     

Effects of manipulating the nitric oxide/cyclic GMP pathway on bovine oocyte meiotic resumption in vitro

The objective of this study was to examine the effects of manipulating the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) pathway on bovine oocyte nuclear maturation in vitro. Cumulus-enclosed oocytes (CEO) were recovered from abattoir-derived ovaries and cultured in M199+FCS for 7 or 21h in the presence of various molecules affecting the NO/cGMP pathway, and then fixed and stained for evaluation of the stage of nuclear maturation. Cyclic GMP levels were also measured in cumulus-oocyte complexes after 3 and 6 h of culture. The iNOS inhibitor, aminoguanidine (AG, 10 and 50 mM) and the NO donor sodium nitroprusside (SNP, 100 and 500 microM) significantly inhibited GVBD after 7h of culture. However, a lower concentration of SNP (0.01 microM) stimulated GVBD. The inhibitory effects of AG and SNP were reversible, indicating that they were not toxic effects. Although SNP (500 microM) increased cGMP levels in cumulus-oocyte complexes after 3 h of culture, the inhibitor of soluble guanylate cyclase ODQ and the protein kinase G (PKG) inhibitor KT5823 did not reverse the inhibitory effect of SNP on meiosis, suggesting that SNP does not inhibit meiosis through the cGMP/PKG pathway. Similarly, an analogue of cGMP (8-Bromo-cGMP 0.5, 1, 3, and 6 mM), as well as activation of guanylate cyclase with Protoporphyrin IX or atrial natriuretic peptide, or inhibition of the enzyme with ODQ, did not have any significant effect on GVBD after 7 h of culture, supporting the idea that the effects of AG and SNP were not due to altered cGMP levels. Atrial natriuretic peptide, Protoporphyrin IX and SNP 500 microM increased cGMP levels after 3 h but not 6 h of culture. In conclusion, soluble and particulate guanylate cyclases could be activated in bovine cumulus-oocyte complexes, but accumulation of cGMP was probably not responsible for the effects of NO on meiosis.     

Activation of the sensory current in salamander olfactory receptor neurons depends on a G protein-mediated cAMP second messenger system.

Olfactory receptor neurons respond to odor stimulation with an inward cationic current. Under whole-cell patch clamp, individual, isolated olfactory receptors were exposed to pharmacological agents known to interact with distinct enzymes in a putative second messenger cascade, and their response to odors was measured. IBMX prolonged the odor-evoked current and also reduced its amplitude. cAMP and cGMP induced a current electrically identical to the odor current, but the current showed desensitization only with cAMP. GTP-gamma-s prolonged and GDP-beta-s interfered with the odor-evoked current. The long latency seen in the odor response appears to be mainly due to the loading of the G protein and secondarily to the requirement for cAMP accumulation. The main source of the response decay appears to be cyclic nucleotide hydrolysis.     

Inhibition of nitric oxide synthase enhances contractile response of ventricular myocytes from streptozotocin-diabetic rats

The contractile hyporesponsiveness of the streptozotocin diabetic rat heart in vitro to β-adrenergic agonists is eliminated when the heart is perfused with N^G-nitro-l-arginine methyl ester (l-NAME), a non-selective inhibitor of nitric oxide synthase (NOS). The following study evaluated the hypothesis that an increased production of NO/cGMP within the diabetic myocyte inhibits the β-adrenergic-stimulated increase in calcium current and contractile response. Male Sprague-Dawley rats were given an intravenous injection of streptozotocin (60 mg/kg). After 8 weeks, L-type calcium currents were recorded in ventricular myocytes using the whole cell voltage-clamp method. Shortening of isolated myocytes was determined using a video edge detection system. cAMP and cGMP were measured using radioimmunoassay. Nitric oxide production was determined using the Griess assay kit. Basal cGMP levels and nitric oxide production were elevated in diabetic myocytes. Shortening of the diabetic myocytes in response to isoproterenol (1 μM) was markedly diminished. However, there was no detectable difference in the isoproterenol-stimulated L-type calcium current or cAMP levels between control and diabetic myocytes. Acute superfusion of the diabetic myocyte with l-NAME (1 mM) decreased basal cGMP and markedly enhanced the shortening response to isoproterenol but did not alter isoproterenol-stimulated calcium current. These data suggest that increased production of NO/cGMP within the diabetic myocyte suppressed β-adrenergic stimulated shortening of the myocyte. However, NO/cGMP apparently does not suppress shortening of the myocyte by inhibition of the β-stimulated calcium current.     

A large contribution of a cyclic AMP-independent pathway to turtle olfactory transduction

Although multiple pathways are involved in the olfactory transduction mechanism, cAMP-dependent pathway has been considered to contribute mainly to the transduction. We examined the degree of contribution of cAMP-independent pathway to the turtle olfactory response by recording inward currents from isolated cells, nerve impulses from cilia and olfactory bulbar responses. The results obtained by the three recordings were essentially consistent with each other, but detail studies were carried out by recording the bulbar response to obtain quantitative data. Application of an odorant cocktail to the isolated olfactory neuron after injection of 1 mM cAMP from the patch pipette elicited a large inward current. Mean amplitude of inward currents evoked by the cocktail with 1 mM cAMP in the patch pipette was similar to that without cAMP in the pipette. Application of the cocktail after the response to 50 microM forskolin was adapted also induced a large inward current. Application of the odorant cocktail to the olfactory epithelium, after the response to 50 microM forskolin was adapted, brought about an appreciable increase in the impulse frequency. The bulbar response to forskolin alone reached a saturation level around 10 microM. After the response to 50 microM forskolin was adapted, 11 species of odorants were applied to the olfactory epithelium. The magnitudes of responses to the odorants after forskolin were 45-80% of those of the control responses. There was no essential difference in the degree of the suppression by forskolin between cAMP- and IP3- producing odorants classified in the rat, suggesting that certain part of the forskolin-suppressive component was brought about by nonspecific action of forskolin. Application of a membrane permeant cAMP analogue, cpt-cAMP elicited a large response, and 0.1 mM citralva after 3 mM cpt- cAMP elicited 51% of the control response which was close to the response to citralva after 50 microM forskolin. A membrane permeant cGMP analogue, db-cGMP elicited a small response and the response to 0.1 mM citralva was unaffected by db-cGMP. It was concluded that cAMP- independent (probably IP3-independent) pathway greatly contributes to the turtle olfactory transduction.     

Inhibitory effect of nitric oxide on voltage-dependent calcium currents in rat dorsal root ganglion cells

The effect of nitric oxide (NO) on calcium current (I(Ca)) and intracellular calcium concentration ([Ca(2+)](i)) in primarily cultured dorsal root ganglion (DRG) neurons was investigated from neonatal rats. I(Ca) and [Ca(2+)](i) were simultaneously recorded using perforated-patch technique in combination with fluorescence measurement from single DRG neurons. NO donors, sodium nitroprusside (SNP) and S-nitro-N-acetylpenicillamine (SNAP), inhibited I(Ca) in small-diameter neurons without significant change in voltage-dependence of activation and activation time constants. SNP and SNAP also reduced the transient [Ca(2+)](i) peak accompanied by I(Ca). Inhibition by NO was reproducible, but gradually desensitized. In some DRG neurons, SNP and SNAP increased basal [Ca(2+)](i) in concentration of 10 microM with little effect on NO-induced inhibition of I(Ca). 8-Br-cGMP, a permeable cGMP analog, mimicked the effects of SNP and SNAP. These results suggest that, in DRG neurons, NO has inhibitory effect on I(Ca), which is independent of NO-induced increase of basal [Ca(2+)](i), through cGMP-dependent pathway.     

NO donor sodium nitroprusside inhibits excitation-contraction coupling in guinea pig taenia coli

In guinea pig taenia coli, the nitric oxide (NO) donor sodium nitroprusside (SNP, 1 microM) reduced the carbachol-stimulated increases in muscle force in parallel with a decrease in intracellular Ca(2+) concentration ([Ca(2+)](i)). A decrease in the myosin light chain phosphorylation was also observed that was closely correlated with the decrease in [Ca(2+)](i). With the patch-clamp technique, 10 microM SNP decreased the peak Ba(2+) current, and this effect was blocked by an inhibitor of soluble guanylate cyclase. Carbachol (10 microM) induced an inward current, and this effect was markedly inhibited by SNP. SNP markedly increased the depolarization-activated outward K(+) currents, and this current was completely blocked by 0.3 micorM iberiotoxin. SNP (1 microM) significantly increased cGMP content without changing cAMP content. Decreased Ca(2+) sensitivity by SNP of contractile elements was not prominent in the permeabilized taenia, which was consistent with the [Ca(2+)](i)-force relationship in the intact tissue. These results suggest that SNP inhibits myosin light chain phosphorylation and smooth muscle contraction stimulated by carbachol, mainly by decreasing [Ca(2+)](i), which resulted from the combination of the inhibition of voltage-dependent Ca(2+) channels, the inhibition of nonselective cation currents, and the activation of Ca(2+)-activated K(+) currents.     

Two types of P2x-purinoceptors in neurons of the guinea pig ileum submucous plexus

Effects of exogenous adenosine 5′-triphosphate (ATP) on dissociated guinea pig ileum submucous neurons were studied using a conventional whole-cell patch-clamp technique. With the holding potential of −50 mV, application of 50–1,000 μM ATP evoked an inward current (ATP-induced current) in most (90%) of the tested neurons (n-35). ATP-induced currents were observed regardless of whether or not guanosine 5′-triphosphate (GTP, 0.2 mM) and ATP (2 mM) were present in the intracellular solution, or GTP was replaced with equimolar concentration of guanosine 5′-O-3-thiotriphosphate (n-5). In 26 of 29 neurons studied, which responded to ATP, applications of 50–1,000 μM ATP induced slowly declining currents. ATP receptors did not appear to be completely desensitized during a long pulse (up to 4 min) of 200 μM ATP. Suramin (200 μM) accelerated an increase to peak of the current induced by 200 μM ATP without affecting the maximum response amplitude (n−4_. In about 10% of the neuronsn−3), 50 μM ATP evoked rapidly declining (about 1 sec) currents. Application of 100 μM α,β-Me-ATP to these neurons evoked similar responses. The above results suggest that submucous neurons express two specific subtypes of ionotropic P_2x-purinoceptors, which might be involved in distinct excitatory processes in these neurons.     

Nitric oxide modulation of voltage-gated calcium current by S-nitrosylation and cGMP pathway in cultured rat hippocampal neurons

Nitric oxide (NO) plays an important role in many physiological and pathophysiological processes in the brain. In this study, we examined the mechanistic effects of an NO donor, diethylenetriamine/nitric oxide adduct (DETA/NO) on the voltage-gated calcium currents in cultured rat hippocampal neurons. DETA/NO stimulated the calcium currents and slightly increased the channel sensitivity to depolarizing voltages. The effect of DETA/NO on the calcium current was blocked by either depleting the NO in DETA/NO or by pretreating the neurons with NEM, a thiol-specific alkylating agent, suggesting an involvement of S-nitrosylation in the current response to NO. In addition, activation of the cGMP pathway by 8-Br-cGMP inhibited the calcium current in the neurons. Also, inhibition of guanylyl cyclase by 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ) increased the current response to DETA/NO. Taken together, our results demonstrate that both S-nitrosylation and cGMP pathway are involved in the NO modulation of the hippocampal calcium current.     

Immunohistochemical assessment of cyclic guanosine monophosphate (cGMP) and soluble guanylate cyclase (sGC) within the rostral ventrolateral medulla

Functional evidence suggests that nitric oxide (NO) signalling in the rostral ventrolateral medulla (RVLM) is cGMP-dependent and that this pathway is impaired in hypertension. We examined cGMP expression as a marker of active NO signalling in the C1 region of the RVLM, comparing adult (>18 weeks) Wistar–Kyoto (WKY, n = 4) and spontaneously hypertensive rats (SHR, n = 4). Double label immunohistochemistry for cGMP-immunoreactivity (IR) and C1 neurons [as identified by phenylethanolamine N-methyltransferase (PNMT-IR) or tyrosine hydroxylase TH-IR)], or neuronal NO synthase (nNOS) neurones, failed to reveal cGMP-IR neurons in the RVLM of either strain, despite consistent detection of cGMP-IR in the nucleus ambiguus (NA). This was unchanged in the presence of isobutylmethylxanthine (IBMX; 0.5 mM, WKY, n = 4, SHR n  = 2) and in young animals (WKY, 10-weeks, n = 3). Incubation of RVLM-slices (WKY, 10-weeks, n = 9) in DETA-NO (100 μm; 10 min) or NMDA (10 μM; 2 min) did not uncover cGMP-IR. In all studies, cGMP was prominent within the vasculature. Soluble guanylate cyclase (sGC)-IR was found throughout neurones of the RVLM, but did not co-localise with PNMT, TH or nNOS-IR neurons (WKY, 10-weeks, n = 6). Results indicate that within the RVLM, cGMP is not detectable using immunohistochemistry in the basal state and cannot be elicited by phosphodiesterase inhibition, NMDA receptor stimulation or NO donor application. Kellysan Powers-Martin and Anna M. Barron contributed equally.     

Responses to putative second messengers and odorants in water nose olfactory neurons of Xenopus laevis

Using the whole-cell mode of the patch-clamp technique, we attempted to record inward currents in response to cAMP, inositol 1,4, 5-trisphosphate (IP(3)) and odorants from sensory neurons in the olfactory epithelium of the Xenopus laevis lateral diverticulum (water nose). Dialysis of 100 microM of IP(3) induced inward currents, while dialysis of 1 mM of cAMP into olfactory neurons did not induce any response under the voltage-clamp conditions. Changes in membrane conductance were examined by applying ramp pulses. The slope of the current-voltage (I-V) curve during the IP(3)-induced response was steeper than that after the response, indicating that IP(3) increased the membrane conductance. The water nose olfactory neurons have been shown to respond to both amino acids and volatile odorants. The slopes of I-V curves during responses to amino acids and a volatile odorant, lilial, were similar to those before the responses, suggesting that the total membrane conductance was not changed during responses to amino acids and the volatile odorant.     

Melittin activates TRPV1 receptors in primary nociceptive sensory neurons via the phospholipase A2 cascade pathways

Previous studies demonstrated that melittin, the main peptide in bee venom, could cause persistent spontaneous pain, primary heat and mechanical hyperalgesia, and enhance the excitability of spinal nociceptive neurons. However, the underlying mechanism of melittin-induced cutaneous hypersensitivity is unknown. Effects of melittin applied topically to acutely dissociated rat dorsal root ganglion neurons were studied using whole-cell patch clamp and calcium imaging techniques. Melittin induced intracellular calcium increases in 60% of small (<25 μm) and medium (<40 μm) diameter sensory neurons. In current clamp, topical application of melittin evoked long-lasting firing in 55% of small and medium-sized neurons tested. In voltage clamp, melittin evoked inward currents in sensory neurons in a concentration-dependent manner. Repeated application of melittin caused increased amplitude of the inward currents. Most melittin-sensitive neurons were capsaicin-sensitive, and 65% were isolectin B4 positive. Capsazepine, the TRPV1 receptor inhibitor, completely abolished the melittin-induced inward currents and intracellular calcium transients. Inhibitions of signaling pathways showed that phospholipase A₂, but not phospholipase C, was involved in producing the melittin-induced inward currents. Inhibitors of cyclooxygenases (COX) and lipoxygenases (LOX), two key components of the arachidonic acid metabolism pathway, each partially suppressed the inward current evoked by melittin. Inhibitors of protein kinase A (PKA), but not of PKC, also abolished the melittin-induced inward currents. These results indicate that melittin can directly excite small and medium-sized sensory neurons at least in part by activating TRPV1 receptors via PLA2-COXs/LOXs cascade pathways.     

Changes in the ionic mechanisms of electrical excitability in the somatic membrane of rat dorsal root ganglion neurons during ontogenesis. Relationship between calcium channels and intracellular metabolism

It was found during experiments on rat sensory neurons that the relationship between high-threshold calcium channels and the system of intracellular cyclic nucleotide metabolism declined in the course of postnatal ontogenesis. Intracellullar administration of the cAMP-ATP-Mg²⁺ complex led to restoration after dialysis-induced decline in peak amplitude of high-threshold calcium currents in 70% of cells belonging to the first age group of 5–9-day-old animals, as against 26% of those examined in the 2nd (45-day-old) and only 10% of all those investigated in the third (90-day-old) group. Kinetics and voltage-dependence of high-threshold calcium current in the neuronal soma were identical in rats of all three age groups. The effect of recovery in calcium conductivity produced by intracellular application of the cAMP-ATP-Mg²⁺ complex was different in neurons with different inward current combinations. This recovery did not occur in cells with "fast" sodium and high-threshold calcium currents only. Conventional effects of intracellular cAMP application were seen in neurons mainfesting a "slow" TTX-resistant sodium inward current together with the two main inward currents.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vo.. 18, No. 6, pp. 827–832, November–December, 1986.     

Nitric oxide stimulates embryonic somatotroph differentiation and growth hormone mRNA and protein expression through a cyclic guanosine monophosphate-independent mechanism

In the pituitary gland, NO is locally synthesized by gonadotroph and folliculo-stellate cells. Many reports have shown that NO can modulate the growth hormone (GH) secretion. However, its role on mice embryo GH regulation remains unclear. In addition, it is unknown whether the regulation is associated with the proliferation of pituitary cells. In this study, we have investigated the regulatory effects of NO on somatotroph differentiation, proliferation and GH mRNA and protein expression using primary cell cultures of mice fetal pituitaries (embryonic days 16.5, ED 16.5). Our results show that incubation of pituitary cells in the presence of sodium nitroprusside (SNP; 1 mM), a NO donor, for 4.5 h resulted in a significant increase in GH mRNA and protein expression (P < 0.05) and the stimulation of SNP can be inhibited by hemoglobin, a NO scavenger. But the addition of cyclic guanosine monophosphate (cGMP; 3.0 mM), the second messenger of multiple NO actions cannot influence GH mRNA and protein expression. The cyclic nucleotide cellular efflux pumps existed in the pituitary cells can transport the majority of de novo-produced cGMP and effectively block cGMP accumulation. For maintaining intracellular concentration of cGMP, probenecid (0.5 mM), a blocker of cGMP efflux pump, combined with cGMP (3.0 mM) was used to treat the pituitary cells. This also cannot influence GH mRNA and protein expression. In addition, the ratio of GH-positive cells is increased significantly after the stimulation of SNP (P < 0.05). However, SNP cannot modulate the pituitary cell proliferation. From these results we conclude that NO can increase GH mRNA and protein expression in fetal pituitary cells and cGMP is not involved in this hormonal regulation. Stimulation of NO on the somatotroph differentiation does not occur due to pituitary cell proliferation.     

Involvement of Cyclic AMP in ABA- and Ca2--Mediated Signal Transduction of Stomatal Regulation in Vicia faba

The focus of this study is to investigate possible involvementof cyclic AMP in regulation of Vicia stomatal movements. Thepresence of 0.1 mM 8-Br-cAMP, a membrane-permeable analogueof cAMP, alone in the incubation medium did not affect stomatalopening in the light in leaf epidermal peel experiments. However,addition of 0.1 mM 8-Br-cAMP completely reversed exogenous ABA-and C^a2+-induced inhibition of stomatal opening. Consistentwith these results, patch-clamping experiments showed that intracellularaddition of 0.5 mM or 1 mM cAMP significantly reversed the inhibitionof whole-cell inward K⁺ currents by internally supplied 13 µMCa²⁺ or 10 µM ABA in stomatal guard cell protoplasts,respectively. Furthermore, intracellular addition of either10 µM prostaglandin E₁ (PGE1, an adenylate cyclase activator)or 1 mM 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesteraseinhibitor) mimicked the effect of exogenous cAMP on the removalof ABA- or Ca²⁺ inhibition of inward K⁺-current. These resultssuggest that a cAMP signaling pathway is involved in signaltransduction in stomatal regulation by interacting with ABAand Ca²⁺ signaling cascades. A hypothetical mechanism by whichcAMP may regulate K⁺ in stomatal guard cells is also discussed. (Received May 6, 1999; Accepted August 27, 1999)     

Transduction pathways mediated by second messengers including cAMP in the sugar receptor cell of the blow fly: study by the whole cell clamp method

The whole cell clamp method was directly applied to the sensory receptor neurons isolated from the adult labellar hair of the blow fly Phormia regina to locate the signal transduction pathways mediated by second messengers. First, the cAMP-mediated transduction pathway was examined to specify its location in the sugar receptor cell. Sugar receptor cell was identified by recording inward current flow under the voltage clamp applying sucrose solution to the surface of the taste neurons. When cyclic nucleotides, such as cGMP and cAMP, were introduced into the sugar receptor cell, inward current was observed (cGMP, 70pA; cAMP, 300pA at 350microM). Inhibitors and activators for the second messengers (GDPbetaS and forskolin) and non-cyclic nucleotides were also examined. Second, non-nucleotide second messengers (IP3 and Ca2+) were examined. The sugar receptor cell was activated when it was injected with IP3 or Ca2+. All the obtained results suggest that the cAMP-mediated signal transduction pathway plays a major role in the sugar receptor cell. The possibility of other transduction pathways mediated by IP3 or Ca2+ was not excluded.     

cAMP和cGMP对棉铃虫神经细胞高电压激活钙通道的调节作用

用全细胞膜片钳法研究了cAMP和cGMP对棉铃虫Helicoverpa armigera 3龄幼虫胸腹神经节细胞高电压激活钙通道的调节作用。细胞外液中加入腺苷酸环化酶（AC）激活剂福斯克林（forskolin） 0.1 mmol/L,对于Ba²⁺介导的钙通道电流激活电压、峰电压、峰电流变化以及通道激活和电流达到峰值的时间无影响。电极内液中加入1 mmol/L的cGMP则明显抑制峰电流,且抑制作用呈时间依赖性和浓度依赖性,而对激活电压、峰电压无影响。结果提示,棉铃虫神经细胞高电压激活钙通道的活动可能不受细胞内cAMP水平提高的影响,但被cGMP抑制。     

Photoreceptor channel activation by nucleotide derivatives

Cyclic nucleotide activated sodium currents were recorded from photoreceptor outer segment membrane patches. The concentration of cGMP and structurally similar nucleotide derivatives was varied at the cytoplasmic membrane face; currents were generated at each concentration by the application of a voltage ramp. Nucleotide-activated currents were analyzed as a function of both concentration and membrane potential. For cGMP, the average K0.5 at 0 mV was 24 microM, and the activation was cooperative with an average Hill coefficient of 2.3. Of the nucleotide derivatives examined, only 8-[[(fluorescein-5-yl-carbamoyl)methyl]thio]-cGMP (8-Fl-cGMP) activated the channel at lower concentrations than cGMP with a K0.5 of 0.85 microM. The next most active derivative was 2-amino-6-mercaptopurine riboside 3',5'-monophosphate (6-SH-cGMP) which had a K0.5 of 81 microM. cIMP and cAMP had very high K0.5 values of approximately 1.2 mM and greater than 1.5 mM, respectively. All nucleotides displayed cooperativity in their response and were rapidly reversible. Maximal current for each derivative was compared to the current produced at 200 microM cGMP; only 8-Fl-cGMP produced an identical current. The partial agonists 6-SH-cGMP, cIMP, and cAMP activated currents which were approximately 90%, 80%, and 25% of the cGMP response, respectively. 5'-GMP, 2-aminopurine riboside 3',5'-monophosphate, and 2'-deoxy-cGMP produced no detectable current. The K0.5 values for cGMP activation, examined from -90 to +90 mV, displayed a weak voltage dependence of approximately 400 mV/e-fold; the index of cooperativity was independent of the applied field.(ABSTRACT TRUNCATED AT 250 WORDS)