SEARCH · PubMed Health

Results for “Chloride Channel Agonists”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6Linked to original sources

PubMed1988

Apical membrane chloride channels in a colonic cell line activated by secretory agonists.

We characterized the anion channel responsible for the increase in apical membrane Cl secretion using a model salt-secreting epithelium, the T84 colonic cell line. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated secretagogues, prostaglandin E2, forskolin, and 8-bromo-cAMP, evoked activity of an outwardly rectifying Cl channel in previously quiet cell-attached membrane patches. The channel remained active in excised, inside-out membranes, where its single-channel conductance was 40-45 pS at 0 mV with 160 mM NaCl in pipette and bath. Selectivities were PCl/PNa = 50 and for halides I(1.8)/Br(1.4)/Cl(1.0)/F(0.4). This halide sequence illustrates that the ability of various anions to undergo transepithelial secretion is determined by the selectivity of the basolateral membrane Cl entry step rather than by the apical Cl channel. Open-channel probability increased with depolarization, an effect that would adjust the rate of Cl exit across secretory cell apical membranes with agonist-induced changes in apical membrane potential. Comparison with the properties of Cl channels detected in other cell types suggests that this cAMP-stimulated Cl channel is uniquely present in the apical membranes of salt-secreting epithelial cells.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1999

A single hydrophobic residue confers barbiturate sensitivity to gamma-aminobutyric acid type C receptor.

Barbiturate sensitivity was imparted to the human rho1 homooligomeric gamma-aminobutyric acid (GABA) receptor channel by mutation of a tryptophan residue at position 328 (Trp328), which is located within the third transmembrane domain. Substitutions of Trp328 with a spectrum of amino acids revealed that nearly all hydrophobic residues produced receptor channels that were both directly activated and modulated by pentobarbital with similar sensitivities. Previous studies with ligand-gated ion channels (including GABA) have demonstrated that even conservative amino acid substitution within the agonist-dependent activation domain (N-terminal extracellular domain) can markedly impair agonist sensitivity. Thus, the lack of significant variation in pentobarbital sensitivity among the Trp328 mutants attests to an intrinsic difference between pentobarbital- and the GABA-dependent activation domain. Compared with the heterooligomeric alphabetagamma receptor channel, the mode of modulation for homooligomeric Trp328 mutants by pentobarbital was more dependent on the GABA concentration, yielding potentiation only at low concentrations of GABA (fractions of their respective EC50 values), yet causing inhibition at higher concentrations. Agonist-related studies have also demonstrated that residue 328 plays an important role in agonist-dependent activation, suggesting a functional interconnection between the GABA and pentobarbital activation domains.

Amino Acid Sequence↗

PubMed1994

Activation of the cystic fibrosis transmembrane regulator by cyclic AMP is not correlated with inhibition of endocytosis.

Based on the observation (Bradbury et al. (1992) Am. J. Physiol. 262, C752-C759) that conditions known to activate the cystic fibrosis transmembrane regulator protein (CFTR) increase the rate of exocytosis and decrease the rate of endocytosis, it was proposed that activation of the CFTR may involved cAMP-dependent fusion of CFTR containing endosomes with the apical membrane. We have tested this hypothesis in two cell lines derived from epithelia that express defective chloride transport in cystic fibrosis (CF): the human colonic cell line, T84, and the tracheal cell line 9HTEo-. The dose-dependence of forskolin- and CPT-cAMP-induced inhibition of endocytosis were compared with the dose-dependence of chloride channel activation. Endocytosis was determined from the uptake of FITC-dextran, and assayed in purified endosomes. Chloride channel activity was measured from the rate of I-efflux. If the fusion hypothesis is correct: (1) concentrations of agonist that inhibit endocytosis should activate chloride channel activity, and (2) the relationship between endocytosis and channel activation should be similar for forskolin and CPT-cAMP. Results in both cell lines were inconsistent with these postulates, suggesting that either chloride channel activation and the inhibition of endocytosis are separate effects of cAMP, or that the increase in apical CFTR resulting from agonist-dependent inhibition of endosomal fusion is minimal.

Cell Line↗

PubMed1999

Activation of Ca(2+)- and cAMP-sensitive K(+) channels in murine colonic epithelia by 1-ethyl-2-benzimidazolone.

1-Ethyl-2-benzimidazolone (EBIO) caused a sustained increase in electrogenic Cl(-) secretion in isolated mouse colon mucosae, an effect reduced by blocking basolateral K(+) channels. The Ca(2+)-sensitive K(+) channel blocker charybdotoxin (ChTX) and the cAMP-sensitive K(+) channel blocker 293B were more effective when the other had been added first, suggesting that both types of K(+) channel were activated. EBIO did not cause Cl(-) secretion in cystic fibrosis (CF) colonic epithelia. In apically permeabilized colonic mucosae, EBIO increased the K(+) current when a concentration gradient was imposed, an effect that was completely sensitive to ChTX. No current sensitive to trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2, 2-dimethylchromane (293B) was found in this condition. However, the presence of basolateral cAMP-sensitive K(+) channels was demonstrated by the development of a 293B-sensitive K(+) current after cAMP application in permeabilized mucosae. In isolated colonic crypts EBIO increased cAMP content but had no effect on intracellular Ca(2+). It is concluded that EBIO stimulates Cl(-) secretion by activating Ca(2+)-sensitive and cAMP-sensitive K(+) channels, thereby hyperpolarizing the apical membrane, which increases the electrical gradient for Cl(-) efflux through the CF transmembrane conductance regulator (CFTR). CFTR is also activated by the accumulation of cAMP as well as by direct activation.

Animals↗

PubMed1997

Target sites of anthelmintics.

This paper reviews sites of action of anthelmintic drugs including: (1) levamisole and pyrantel, which act as agonists at nicotinic acetylcholine receptors of nematodes; (2) the avermectins, which potentiate or gate the opening of glutamategated chloride channels found only in invertebrates; (3) piperazine, which acts as an agonist at GABA gated chloride channels on nematode muscle; (4) praziquantel, which increases the permeability of trematode tegument to calcium and results in contraction of the parasite muscle; (5) the benzimidazoles, like thiabendazole, which bind selectively to parasite beta-tubulin and prevents microtubule formation; (6) the proton ionophores, like closantel, which uncouple oxidative phosphorylation; (7) diamphenethide and clorsulon, which selectively inhibit glucose metabolism of Fasciola and; (8) diethylcarbamazine, which appears to interfere with arachidonic acid metabolism of filarial parasites and host. The review concludes with brief comments on the development of anthelmintics in the future.

Amino Acid Sequence↗

PubMed2006

Functional inhibition of intestinal and uterine muscles by non-permeant triphenylethylene derivatives.

We have previously shown that the triphenylethylene antiestrogen tamoxifen reversibly inhibited spontaneous contractile activity in isolated duodenal muscle. Now, we have synthesized different quaternary ammonium salts of tamoxifen by changing the substituents on the nitrogen of the alkylaminoethoxy side-chain, to obtain plasma membrane impermeable compounds. Synthesized molecules were N-desmethyl-tamoxifen-hydrochloride, ethylbromide-tamoxifen and butylbromide-tamoxifen, which differed in the size of their ionic side-chain. All compounds rapidly and reversibly inhibited spontaneous and CaCl(2)-induced contractions in mouse duodenum and uterus. Dose-response analyses revealed a structure-activity relationship where the larger the side-chain the higher the inhibitory potency. Fourier analyses on triphenylethylene-relaxed duodenal tissues showed that harmonic components of contractile activity were readily recovered upon exposure to the L-type calcium channel agonist 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-pyridine-3-carboxilic acid methyl ester (BAY-K644). Likewise, BAY-K644 completely reversed triphenylethylene-induced effects on uterine tonic tension. Our experiments suggest that impermeant tamoxifen derivatives relax visceral smooth muscle through a membrane-mediated non-genomic mechanism that involves inhibition of L-type calcium channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1999

The pharmacological profile of GABA receptors on cultured insect neurones.

Neuronal cultures of the cockroach, Periplaneta americana, were used to study the pharmacological profile of GABA receptors using the whole-cell-voltage clamp technique. The results indicated that insect GABA receptors are linked to a chloride channel that can be activated by both GABA(A) and GABA(C) receptor agonists. The receptors are blocked by GABA(A) chloride channel blockers and some insecticides but not by competitive GABA(A) receptor antagonists. The GABA(C) receptor competitive antagonists were either full or partial agonists of the cockroach GABA receptors. The receptors were modulated by the enantiomers of lindane. In conclusion, insect GABA receptors appear to have a distinct pharmacological profile that does not conform to either vertebrate GABA(A) or GABA(C) receptors.

Journal Article↗

PubMed1984

In vitro characterization of agonist, antagonist, inverse agonist and agonist/antagonist benzodiazepines.

The efficacy of coupling between the benzodiazepine receptor and chloride channel as well as the coupling to the GABA receptor is differentially affected by different benzodiazepine ligands. In general, the order of efficacy with regard to allosteric effects of benzodiazepine ligands on the chloride channel ([35S]TBPS) and GABA receptor ([3H]muscimol), is: agonist greater than agonist/antagonist greater than partial agonist greater than antagonist; with inverse agonists acting in a manner opposite to the classical benzodiazepine agonists. The chloride ionophore is allosterically modulated both by benzodiazepine and GABA receptors.

Animals↗

PubMed1993

Modulatory action of arachidonic acid on GABAA/chloride channel receptor function in adult and aged brain cortex membranes.

Effect of arachidonic acid on binding parameters of two binding sites on the GABAA receptor and on GABA activated Cl- uptake was investigated in synaptic plasma membrane and in synaptoneurosomes from brain cortex of adult (4-months old) and aged (27-months old) rats. The ligands used were [3H]muscimol, a GABA agonist and [35S]-t-butylbicyclophosphorothionate ([35S]TBPS), a convulsant that binds to the site near the chloride channel. Arachidonic acid increases significantly GABAA agonist binding and concomitantly decreases [35S]TBPS binding in a concentration dependent manner. The analysis of binding parameters in adult brain showed a significant decrease by AA of KD value for low but not for high affinity of [3H]muscimol binding. Concomitantly, AA enhances Bmax values for high affinity binding and has no effect on Bmax value for low affinity binding in synaptic plasma membrane (SPM) from adult brain. In synaptic plasma membrane from aged brain AA increases low and high affinity binding of agonist to GABAA receptor, modified significantly KD value by about 30 and 66%, respectively. On the other hand, AA significantly decreases of [35S]TBPS binding to chloride channel recognition site. Scatchard's analysis indicates that this inhibition results from a decrease of total number of binding sites. Moreover, the affinity of [35S]TBPS binding was increased (KD = 37.0 nM for AA-treated membrane, as compared to 69.3 nM KD in control membrane). GABA-dependent chloride uptake into synaptoneurosomes is also inhibited by AA in a concentration dependent manner in adult brain. In aged brain synaptoneurosomes AA has similar inhibitory effect on GABA-activated chloride uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1997

Regulatory volume decrease of cardiac myocytes induced by beta-adrenergic activation of the Cl- channel in guinea pig.

A new method was developed to automatically measure the thickness of a single ventricular myocyte of guinea-pig heart. A fine marker was attached on the cell's upper surface and changes in its vertical position were measured by focusing it under the microscope. When the osmolarity of the bath solution was varied, the cell thickness reached a new steady level without any obvious regulatory volume change within the period of observation up to 15 min. The cell thickness was 7.8 +/- 0.2 microns (n = 94) in the control Tyrode solution and was varied to 130.4 +/- 3.1% (n = 10), 119.1 +/- 1.1% (n = 50), 87.2 +/- 1.9% (n = 9), and 75.6 +/- 3.2% (n = 5) of control at 50, 70, 130, and 200% osmolarity, respectively. The application of a Cl- channel blocker, 500 microM anthracene-9-carboxylic acid (9AC) did not modify these osmotic volume changes. We discovered that the application of isoprenaline induced a regulatory volume decrease (RVD) in cells inflated by hypotonic solutions. This isoprenaline-induced RVD was inhibited by antagonizing beta-adrenergic stimulation with acetylcholine. The isoprenaline-induced RVD was mimicked by the external application of 8-bromoadenosine 3':5'-cyclic monophosphate. The RVD was inhibited by blocking the cAMP-dependent Cl- channel (ICl, rAMP) with 9AC but was insensitive to 4,4'-diisothiocyanostilbene-2,2'-dissulphonate (DIDS). Taken together these data suggest an involvement of ICl, cAMP activation in the RVD. Whole cell voltage clamp experiments revealed activation of ICl, cAMP by isoprenaline under the comparable conditions. The cardiac cell volume may be regulated by the autonomic nervous activity.

Adrenergic beta-Agonists↗

PubMed1996

Characterization of the GABA response on identified dialysed Lymnaea neurons.

1. The effect of gamma-aminobutyric acid has been studied on identified, internally perfused dialysed neurons of Lymnaea stagnalis L. (Pulmonata, Basommatophora). It was shown that: On the majority of neurons GABA (10(-8)-10(-3) M) depolarized the membrane with a decrease in input resistance and activated a Cl- dependent inward current with -20 +/- 4 mV E(GABA). In some cells, the outward current with -67 +/- 8 mV E(GABA) was also recorded. 2. The GABA-induced inward current was fast (1.5 +/- 0.3 sec, n = 4) or slow (3.2 +/- 0.2 sec, n = 3) peaking in a voltage-independent manner. The inactivation phase could be fitted by one or two exponentials characterized with fast (tau = 0.7 sec) and slow (tau = 3.6 sec) time constants. The outward current component was slow and activated at more positive Vh(-30-20 mV). 3. The agonist effects (GABA and muscimol) indicated the involvement of GABA(A) receptors and Cl-permeability changes in activating inward current. Picrotoxin (10(-5)-10(-4) M) and Cd2+ completely inhibited the GABA-activated inward current also affecting E(GABA). Furosemide was without effect on the peak value of GABA-induced inward current, but slightly modified the slope of inactivation. 4. High concentrations of Ca-ions and their substitution with Ba-ions in extracellular saline failed to alter the GABA-induced inward current. However, omission of Cl-ions from extracellular media shifted E(GABA) to the right by 18 +/- 8 mV (n = 4). 5. Omission of Cl-ions from intracellular saline led to inhibition of the fast component of GABA-induced inward current. Full recovery followed readdition of Cl-ions. 6. The results are in agreement with the data obtained on cloned Lymnaea GABA receptors.

Animals↗

PubMed1995

Effect of the Ca2+ channel activator CGP 28392 on reactivation of oxygen evolution of Ca(2+)-depleted photosystem II.

The effect of the Calcium channel activator, CGP 28392, on the reactivation of oxygen evolution in Ca(2+)-depleted Photosystem II (PS II) particles has been investigated. Ca(2+)-binding is associated with a functional water splitting complex of PS II. In the presence of the activator, a low affinity site of Ca(2+)-binding is converted into a high affinity binding site. Following removal of the extrinsic proteins (17 and 23 kDa), any effect of the activator is no longer observed. Ca2+ channel inhibitors can inhibit the Ca(2+)-dependent reactivation of oxygen evolution. The activator partially protects against this type of inhibition. It is suggested that the extrinsic proteins form a Ca2+ channel-like structure at the donor side of PS II.

Calcium↗

PubMed1989

Desensitization and capsaicin-induced release of substance P-like immunoreactivity from guinea-pig ureter in vitro.

Substance P-like immunoreactivity was assayed in superfusates of guinea-pig ureters following stimulation of afferent fibres with capsaicin, potassium chloride and the calcium channel agonist Bay K 8644. Capsaicin-evoked release of substance P-like immunoreactivity was calcium-dependent but unaffected by cobalt. Under appropriate conditions release was dose-related (ED50 = 610 nM) and reproducible. Selective desensitization to capsaicin could be demonstrated following prolonged exposure to different doses of capsaicin. No desensitization to capsaicin was observed following afferent fibre stimulation with a combination of Bay K 8644 and K+, which released a similar amount of substance P-like immunoreactivity as a desensitizing capsaicin stimulus. These data suggest that depletion of releasable substance P-like reactivity is unlikely to account for selective desensitization of ureteric primary afferent fibres to capsaicin.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed2003

Sodium channel inhibition by anandamide and synthetic cannabimimetics in brain.

Anandamide is a prominent member of the endocannabinoids, a group of diffusible lipid molecules which influences neuronal excitability. In this context, endocannabinoids are known to modulate certain presynaptic Ca(2+) and K(+) channels, either through cannabinoid (CB1) receptor stimulation and second messenger pathway activation or by direct action. We investigated the susceptibility of voltage-sensitive sodium channels to anandamide and other cannibimimetics using both biochemical and electrophysiological approaches. Here we report that anandamide, AM 404 and WIN 55,212-2 inhibit veratridine-dependent depolarization of synaptoneurosomes (IC(50)s, respectively 21.8, 9.3 and 21.1 microM) and veratridine-dependent release of L-glutamic acid and GABA from purified synaptosomes [IC(50)s: 5.1 microM (L-glu) and 16.5 microM (GABA) for anandamide; 1.6 microM (L-glu) and 3.3 microM (GABA) for AM 404, and 12.2 (L-glu) and 14.4 microM (GABA) for WIN 55,212-2]. The binding of [3H]batrachotoxinin A 20-alpha-benzoate to voltage-sensitive sodium channels was also inhibited by low to mid micromolar concentrations of anandamide, AM 404 and WIN 55,212-2. In addition, anandamide (10 microM), AM 404 (10 microM) and WIN 55,212-2 (1 microM) were found to markedly block TTX-sensitive sustained repetitive firing in cortical neurones without altering primary spikes, consistent with a state-dependent mechanism. None of the inhibitory effects we demonstrate on voltage-sensitive sodium channels are attenuated by the potent CB1 antagonist AM 251 (1-2 microM). Anandamide's action is reversible and its effects are enhanced by fatty acid amidohydrolase inhibition. We propose that voltage-sensitive sodium channels may participate in a novel signaling pathway involving anandamide. This mechanism has potential to depress synaptic transmission in brain by damping neuronal capacity to support action potentials and reducing evoked release of both excitatory and inhibitory transmitters.

Analysis of Variance↗

PubMed1999

Chloride transport in the ferret trachea.

A net secretion of chloride stimulated by carbamylcholine was observed in whole trachea. Luminal anthracene-9-carboxylic acid inhibited the net secretion of chloride and the transepithelial potential difference across the isolated trachea. Submucosal ouabain inhibited the net secretion of chloride and submucosal ouabain, or submucosal bumetanide inhibited the transepithelial potential difference across the isolated trachea. Short-circuited flat sheets of trachea manifested a net secretion of chloride induced by carbamylcholine. Serosal ouabain inhibited the short-circuit current and net chloride flux across isolated flat sheets of trachea.

Animals↗

PubMed1995

Studies of porcine and human sperm suggesting a role for a sperm glycine receptor/Cl- channel in the zona pellucida-initiated acrosome reaction.

It is known that Cl- is required for the zona pellucida (zona)- and progesterone-initiated mammalian sperm acrosome reaction (AR). Recent evidence indicates the involvement of a unique progesterone receptor resembling a GABAA receptor/Cl- channel in the latter event. In the study reported here, we investigated whether the in vitro porcine sperm AR initiated by heat-solubilized porcine zonae involves a different receptor/Cl- channel than does the progesterone-initiated AR. Capacitated porcine sperm failed to undergo the zona- and progesterone-initiated AR when suspended in a medium deficient in Cl-. We then employed 1) strychnine, an antagonist of neuronal glycine receptor/Cl- channels at nanomolar concentrations and of GABAA receptor/Cl- channels at micromolar concentrations and 2) (+)-bicuculline, an antagonist of neuronal GABAA receptor/Cl- channels that inhibits glycine receptor/Cl- channels only at higher concentrations. We report here that the zona-initiated AR was inhibited by 50 nM strychnine, but the progesterone-initiated AR required 1 microM for inhibition. Moreover, 1 microM (+)-bicuculline inhibited the progesterone-initiated AR, but the zona-initiated AR required 50 microM for inhibition. In addition, glycine initiated the porcine sperm AR, and that event was inhibited by strychnine and (+)-bicuculline with the same order of potency observed for the zona-initiated AR. Finally, glycine initiated the AR in human sperm, and that initiation was inhibited by 50 nM strychnine. This work provides the first evidence suggesting that a glycine receptor/Cl- channel is involved in the zona-initiated mammalian sperm AR.

Acrosome↗

PubMed1997

Tetrahydrobiopterin regulates cyclic GMP-dependent electrogenic Cl- secretion in mouse ileum in vitro.

1. Basal electrogenic Cl- secretion, measured as the short-circuit current (Isc), was variable in ileum removed from tetrahydrobiopterin (BH4)-deficient hph-1 mice and wild-type controls in vitro, although values were not significantly different. 2. The basal nitrite release and mucosal cyclic guanosine 3',5'-monophosphate (cyclic GMP) production were similar in control and BH4-deficient ileum. 3. Mucosally added Escherichia coli heat-stable toxin (STa, 55 ng ml-1) increased the nitrite release, cyclic GMP levels and the Isc in control ileum, but its secretory actions were reduced in BH4-deficient ileum. 4. L-Arginine (1 mM) increased the nitrite release, cyclic GMP production and the Isc in control ileum, but the actions were reduced in BH4-deficient ileum. 5. Serosal carbachol (1 mM) stimulated maximum short-circuit currents of similar magnitude in both control and BH4-deficient ileum, whilst nitrite release and cyclic GMP production were minimal. 6. E. coli STa and L-arginine increased electrogenic Cl- secretion across intact mouse ileum in vitro by releasing nitric oxide and elevating mucosal cyclic GMP. The inhibition of these processes in the hph-1 mouse ileum suggests that BH4 may be a target for the modulation of electrogenic transport, and highlight the complexity of the interactions between nitric oxide and cyclic GMP in the gut.

Animals↗

PubMed1997

Role of G proteins in alpha1-adrenergic inhibition of the beta-adrenergically activated chloride current in cardiac myocytes.

alpha1-Adrenergic receptor stimulation can inhibit the Cl- current activated by beta-adrenergic receptor agonists in guinea-pig ventricular myocytes. We investigated the role of G proteins in mediating this type of alpha-adrenergic response. The combined alpha- and beta-adrenergic agonist norepinephrine (NE) activated the Cl- current with an EC50 value of 53 nM. Preincubation of myocytes with PTX decreased the EC50 value for NE activation of the Cl- current to 5.9 nM, and addition of the alpha1-adrenergic receptor antagonist prazosin did not cause any further change in sensitivity to NE. These results suggest that the alpha1-adrenergic inhibition of beta-adrenergic responses is mediated through a PTX-sensitive G protein. However, PTX pretreatment also increased the sensitivity of the Cl- current to the selective beta-adrenergic agonist isoproterenol (Iso), which indicates that the PTX treatment increases the sensitivity to beta-adrenergic stimulation alone and that this could account for the PTX-induced change in sensitivity to NE. Consistent with this idea, the selective alpha1-adrenergic receptor agonist methoxamine was still able to inhibit the Cl- current activated by Iso in PTX-treated myocytes. However, the sensitivity to methoxamine was significantly decreased. In control cells, the Cl- current activated by 30 nM Iso was inhibited by methoxamine with an EC50 value of 8.3 microM, but in PTX-treated cells, the EC50 value was 284 microM. The EC50 for methoxamine inhibition was similarly increased when the Cl- current was activated by 300 nM Iso. These data suggest that the effects of PTX on alpha1-adrenergic responses can actually be explained by changes in the sensitivity to beta-adrenergic stimulation. To verify the role for a G protein in mediating the inhibitory alpha1-adrenergic response, we examined the effect of methoxamine on the Cl- current activated in cells dialyzed with the nonhydrolyzable GTP analogue guanosine-5'-O-(3-thio)triphosphate. Pre-exposure to methoxamine resulted in an attenuated response upon subsequent exposure to Iso alone. We conclude that alpha1-adrenergic inhibition of beta-adrenergic responses is mediated by a G protein-dependent mechanism that appears to be PTX-insensitive.

Adrenergic alpha-Antagonists↗