One the importance of agonist concentration-gradients within isolated tissues. Increased maximal responses of rat vasa deferentia to (--)-noradrenaline after blockade of neuronal uptake

A new nitrosyl ruthenium complex nitric oxide donor presents higher efficacy than sodium nitroprusside on relaxation of airway smooth muscle

Nitric oxide (NO) has been demonstrated to be the primary agent in relaxing airways in humans and animals. We investigated the mechanisms involved in the relaxation induced by NO-donors, ruthenium complex [Ru(terpy)(bdq)NO(+)](3+) (TERPY) and sodium nitroprusside (SNP) in isolated trachea of rats contracted with carbachol in an isolated organs chamber. For instance, we verified the contribution of K(+) channels, the importance of sGC/cGMP pathway, the influence of the extra and intracellular Ca(2+) sources and the contribution of the epithelium on the relaxing response. Additionally, we have used confocal microscopy in order to analyze the action of the NO-donors on cytosolic Ca(2+) concentration. The results demonstrated that both compounds led to the relaxation of trachea in a dependent-concentration way. However, the maximum effect (E(max)) of TERPY is higher than the SNP. The relaxation induced by SNP (but not TERPY) was significantly reduced by pretreatment with ODQ (sGC inhibitor). Only TERPY-induced relaxation was reduced by tetraethylammonium (K(+) channels blocker) and by pre-contraction with 75mM KCl (membrane depolarization). The response to both NO-donors was not altered by the presence of thapsigargin (sarcoplasmic reticulum Ca(2+)-ATPase inhibitor). The epithelium removal has reduced the relaxation only to SNP, and it has no effect on TERPY. The both NO-donors reduced the contraction evoked by Ca(2+) influx, while TERPY have shown a higher inhibitory effect on contraction. Moreover, the TERPY was more effective than SNP in reducing the cytosolic Ca(2+) concentration measured by confocal microscopy. In conclusion, these results show that TERPY induces airway smooth muscle relaxation by cGMP-independent mechanisms, it involves the fluxes of Ca(2+) and K(+) across the membrane, it is more effective in reducing cytosolic Ca(2+) concentration and inducing relaxation in the rat trachea than the standard drug, SNP.

Gap junctions in vascular tissues. Evaluating the role of intercellular communication in the modulation of vasomotor tone

Integration and coordination of responses among vascular wall cells are critical to the local modulation of vasomotor tone and to the maintenance of circulatory homeostasis. This article reviews the vast literature concerning the principles that govern the initiation and propagation of vasoactive stimuli among vascular smooth muscle cells, which are nominally the final effectors of vasomotor tone. In light of the abundance of new information concerning the distribution and function of gap junctions between vascular wall cells throughout the vascular tree, particular attention is paid to this integral aspect of vascular physiology. Evidence is provided for the important contribution of intercellular communication to vascular function at all levels of the circulation, from the largest elastic artery to the terminal arterioles. The thesis of this review is that the presence of gap junctions, in concert with the autonomic nervous system, pacemaker cells, myogenic mechanisms, and/or electrotonic current spread (both hyperpolarizing and depolarizing waves through gap junctions), confers a plasticity, adaptability, and flexibility to vasculature that may well account for the observed diversity in regulation and function of vascular tissues throughout the vascular tree. It is hoped that the summary information provided here will serve as a launching pad for a new discourse on the mechanistic basis of the integrative regulation and function of vasculature, which painstakingly accounts for the undoubtedly complex and manifold role of gap junctions in vascular physiology/dysfunction.

Effects of kappa-opioid receptor agonists on stimulated phosphoinositide hydrolysis in rat kidney

To determine the effects of kappa-opioid receptor agonists on phosphoinositide metabolism in rat renal cortex, tissue slices labelled with [3H]inositol were stimulated with norepinephrine or carbachol alone or in combination with the kappa-opioid receptor agonists, ethylketocyclazocine, trans-3,4-dichloro-N-methyl-N-[2-(pyrrolindinyl)-cyclohexyl)- benzeneacetamide (U50,488) and nalorphine. Both norepinephrine and carbachol stimulated phosphoinositide hydrolysis (measured in a LiCl buffer) concentration- and time-dependently. The EC50 and maximal stimulation of phosphoinositide hydrolysis for norepinephrine and carbachol were approximately 3 microM and 0.15 dpm released/dpm incorporated, respectively. Concentrations up to 1 mM of ethylketocyclazocine, U50,488 or nalorphine alone did not affect phosphoinositide hydrolysis. However, ethylketocyclazocine and U50,488 decreased 10 microM norepinephrine-stimulated phosphonositide hydrolysis concentration-dependently, each with an approximate IC50 of 30 microM. In contrast, nalorphine had no effect on norepinephrine-stimulated phosphoinositide hydrolysis. In addition, concentrations of up to 1 mM ethylketocyclazocine or U50,488 did not alter carbachol-stimulated phosphoinositide hydrolysis. The inhibitory effect of U50,488 and ethylketocyclazocine on norepinephrine-stimulated phosphoinositide hydrolysis was blocked by the selective kappa-opioid receptor antagonist, nor-binaltorphimine. These results indicate that kappa 1-opioid receptor stimulation may affect phosphoinositide metabolism in rat renal cortex by modulating the subcellular effects of renal alpha 1-adrenoceptor activation.

Modulation of alpha 1-adrenergic contractility in isolated vascular tissues by heptanol: a functional demonstration of the potential importance of intercellular communication to vascular response generation

After years of intensive investigation, the mechanism(s) underlying syncytial vascular smooth muscle responses both in vitro and in vivo is still poorly understood. Neither perivascular innervation nor regenerative electrical events appear sufficient to coordinate responses among vascular smooth muscle cells in many blood vessels. The implication of these observations is that another mechanism is required for organizing syncytial vascular responses. Although gap junctions are ubiquitously distributed among vascular wall cells throughout the vascular tree, their contribution to the modulation of vasomotor tone is still considered controversial. Resolution of the long standing debate awaits a clear demonstration that gap junctions modulate contraction or relaxation responses to vascular smooth muscle. Despite the absence of specific gap junctional uncoupling agents, it has still been possible to identify reasonable experimental conditions under which the contribution of gap junctions to contractile responses in isolated vascular tissues could be evaluated. Studies in isolated preparations known to contain gap junctions have indicated that alpha 1-adrenergic receptor-mediated contractile responses of diverse isolated vascular tissues, are significantly modulated by selective disruption of intercellular communication with the well studied lipophilic uncoupling agent heptanol. Interpretation of these pharmacological studies is explicitly dependent on the selectivity of the uncoupling actions of heptanol. Considerable experimental evidence suggests that, at the concentrations used, in the preparations thus far examined, heptanol does indeed have selective uncoupling actions. In fact, recent experiments provide empirical support for an operational definition of the selectivity of heptanol, and a functional role for gap junctions in modulating contractile responses in isolated vascular tissues. The operational definition states only that there exists a narrow, albeit identifiable, concentration range over which it is reasonable to assume that the effects of heptanol are primarily related to its uncoupling actions on gap junctions. The functional role for gap junctions is defined by their requisite contribution to tension development during contraction of isolated tissues. Experimentally, this can be visualized as a significant diminution in the contractile responses of isolated vascular tissues in the presence of selective uncoupling heptanol concentrations. Thus, a cogent interpretation of available data is that they provide compelling indirect evidence for a principle role of gap junctions in modulating the alpha 1-adrenergic contractility of isolated vascular tissues.

Mechanism of soman-induced contractions in canine tracheal smooth muscle

The actions of the irreversible organophosphorus cholinesterase (ChE) inhibitor soman were investigated on canine tracheal smooth muscle in vitro. Concentrations of soman greater than or equal to 1 nM increased the amplitude and decay of contractions elicited by electric field stimulation. The effect on decay showed a marked dependence on stimulation frequency, undergoing a 2.4-fold increase between 3 and 60 Hz. Soman also potentiated tensions due to bath applied acetylcholine (ACh). Little or no potentiation was observed for contractions elicited by carbamylcholine, an agonist that is not hydrolyzed by ChE. Concentration of soman greater than or equal to 3 nM led to the appearance of sustained contractures. These contractures developed with a delayed onset and were well correlated with ChE activity. Alkylation of muscarinic receptors by propylbenzilylcholine mustard antagonized the actions of soman on both spontaneous and electrically-evoked muscle contractions. The results are consistent with a mechanism in which the toxic actions of soman are mediated by accumulation of neurally-released ACh secondary to inhibition of ChE activity. An important factor in this accumulation is suggested to be the buffering effect of the muscarinic receptors on the efflux of ACh from the neuroeffector junction.

Antagonism of the effects of 5-hydroxytryptamine on the rabbit isolated vagus nerve by BRL 43694 and metoclopramide

Depolarization and reduction in the C fibre compound action potential (C spike) in response to 5-HT were recorded simultaneously from rabbit isolated vagus nerve. 5-HT (0.1-100 mumol/l) was applied cumulatively and EC50 and IC50 values measured from individual concentration-response curves. Blockade of 5-HT responses by the 3-indazole carboxamide, BRL 43694, was investigated and compared with the blocking action of metoclopramide. BRL 43694 was a selective antagonist of 5-HT responses. A concentration of 10 nmol/l BRL 43694, which nearly abolished the depolarization and reduction of the C spike evoked by 5-HT (100 mumol/l), had no effect on similar responses evoked by DMPP (100 mumol/l) or GABA (100 mumol/l). Blockade of 5-HT responses by BRL 43694 (0.3 nmol/l) was slow in onset, a plateau blockade occurring after equilibrium of tissue with antagonist for 2 to 3 h. Metoclopramide induced a blockade of rapid onset. The maximal blockade was apparent within 30 min of application. Full recovery in the responsiveness of the tissue to 5-HT was observed within 30 min of washing out metoclopramide. BRL 43694 at concentrations of 0.3, 1, 3 and 10 nmol/l caused a progressive rightward shift of the concentration-response curves to 5-HT. At the highest concentration of antagonist, there was some depression of the maximal 5-HT response. The apparent pA2 estimated from the Schild equation was 10.03 +/- 0.09 (mean +/- SEM, n = 20) against 5-HT depolarization and 10.31 +/- 0.1 against C spike reduction. Schild plots had slopes not significantly different from 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Supersensitivity to noradrenaline in preparations of rat cauda epididymis after vasectomy

Preparations of cauda epididymides were taken from rats unilaterally vasectomised by medial transection of the vas deferens. Responses of preparations from the operated side to field stimulation (10 pulses, 70 V, 1 ms, 0.1-20 Hz) declined in parallel with decreases in catecholamine fluorescence. There were leftward shifts in the mean cumulative concentration-response curves to noradrenaline on vasectomised cauda epididymides of 5-, 10-, and 12-fold at days 2, 7 and 28, respectively compared to unoperated cauda epididymides following vasectomy. Nisoxetine (0.1 mumol/l) enhanced the potency of noradrenaline upon unoperated but not upon vasectomised segments. There were leftward shifts of 7- and 8-fold in the mean cumulative concentration-response curves to acetylcholine in vasectomised cauda epididymides at days 2 and 7, respectively, following surgery. Responses to methoxamine and carbachol were unaffected by vasectomy. Thus neurotransmission to the smooth muscle of the rat cauda epididymis resembles that to the vas deferens, in that vasectomy results in the development of prejunctional supersensitivity to noradrenaline and acetylcholine.

Effect of monoamine uptake inhibitors on norepinephrine-stimulated phosphatidylinositol hydrolysis in rat cortex

The effects of the monoamine uptake inhibitors cocaine, nisoxetine, and desipramine (DMI) on norepinephrine (NE) stimulated phosphatidylinositol (PI) hydrolysis were investigated. Rat cortical tissue slices were labeled with [3H]inositol. Slices were then stimulated, in vitro, with NE in LiCl containing buffer in the presence and absence of monoamine uptake inhibitors. Cocaine and nisoxetine, but not DMI, potentiated NE-stimulated PI hydrolysis with a significant decrease in the EC50. Nisoxetine appeared to be more potent than cocaine with respect to the potentiation of NE-stimulated PI hydrolysis. The potentiating effect of cocaine was biphasic and dependent upon the concentrations of cocaine and NE. The NE concentration-effect curve was shifted to the right 100-fold in the presence of 0.1 microM prazosin. Cocaine at 10 microM did not potentiate NE-stimulated PI hydrolysis in the presence of 0.1 microM prazosin. Cocaine at 10 microM did not affect significantly the binding of [3H]prazosin or the NE-[3H]prazosin competition binding to cortical membranes. The results suggest that NE-uptake inhibition by cocaine and nisoxetine is the mechanism for the enhancement of NE-stimulated PI hydrolysis.

Inhibition of presynaptic alpha-2-adrenoceptor and opioid receptor agonist responses in the rat vas deferens by chronic imipramine treatment

The effects of chronic imipramine administration on agonist responses in rat isolated smooth muscle preparations were investigated. The administration of 20 mg/kg imipramine two times a day for 4 and 11 days resulted in an equivalent subsensitivity (approximately 8-fold) of clonidine-induced inhibition of electrically evoked contractions in the rat vas deferens (presynaptic alpha 2-adrenoceptor response). Imipramine (4 days) resulted in a marked inhibition of the ability of [D-Ala2, D-Leu5] enkephalin to decrease electrically evoked contractions of the vas deferens (presynaptic opioid receptor response) but did not significantly affect the carbachol-induced increase in electrically evoked contractions (muscarinic receptor response). In the absence of cocaine the contractile effects of norepinephrine and tyramine in the vas deferens were, respectively, potentiated and inhibited, following imipramine (4 days), suggesting a decrease in the activity of the neuronal uptake mechanism. When determined in the presence of cocaine, the potency of the postsynaptic effects of norepinephrine in the vas deferens (alpha 1-adrenoceptor response) was not significantly altered by imipramine (4 days). With regard to other postsynaptic receptors, imipramine (4 days) decreased slightly the potency of phenylephrine in the aorta (alpha 1-adrenoceptor response) and increased slightly the potency of carbachol in the trachea (muscarinic receptor response) and the potency of serotonin in the rat aorta (5HT2-receptor response). Thus, chronic imipramine administration decreased the potency of presynaptic alpha 2- and opioid agonist responses in the vas deferens but caused very little or no changes in the potencies of agonists at postsynaptic sites.

Ranitidine potentiates ileum contractions caused by GABA and electrical stimulation

GABA-evoked contractions of the guinea pig ileum were significantly potentiated by the histamine H2-receptor antagonist ranitidine in concentrations above 10 microM. To help define the mechanism of this interaction, the present study compared the effects of ranitidine on contractile responses of the guinea pig ileum to GABA, acetylcholine (A Ch) and electrical stimulation of intrinsic cholinergic neurons. Ranitidine, at concentrations that potentiated responses to GABA, also potentiated contractions induced by transmural electrical stimulation. The ability of ranitidine to amplify these latter responses was antagonized by atropine. Contractile responses to exogenous A Ch, however, were unaffected by ranitidine at any concentration. These results suggest that prejunctional, rather than postjunctional mechanisms, are of primary importance in the interaction between ranitidine and GABA.

Simultaneous measurement of contractile effects in the circular and longitudinal smooth muscle of the rat vas deferens by drugs perfused externally or via the lumen

The effects of noradrenaline and barium chloride were studied in the rat isolated vas deferens by perfusion of drugs either externally or through the lumen of the organ. Two effects were recorded simultaneously in the same preparation: (a) isometric contractions, due to the tension elicited by drugs on the external (longitudinal) smooth muscle layer and (b) pressure of internal perfusion, due to contractions of the internal (circular) smooth muscle layer. It was found with the longitudinal muscle that: (a) the potency, expressed as pD2 values, and the maximum response to noradrenaline were lower if the drug was perfused internally rather than externally; (b) the differences in maximum effects were pronounced on the prostatic half but were not observed on the epididymal half; (c) the maximum response obtained by internal perfusion could be increased by simultaneously adding the same dose of drug externally; (d) when barium chloride was used instead of noradrenaline no significant differences were observed on pD2 values, but differences on maximal responses were similar to that observed for noradrenaline; (e) it was possible to block completely the effect of internal or external noradrenaline on the longitudinal muscle, by perfusing external phenoxybenzamine. In these conditions the responses of the circular muscle to the agonist were only partly blocked. With the circular muscle, the differences related to internal and external perfusion were less marked than in the longitudinal muscle. However, unlike the latter, the circular layer was slightly more sensitive to drugs applied internally, in relation to pD2 values. It is suggested that the difference in pD2 values may be due to the removal of noradrenaline by the neuronal uptake process, whereas the difference in maximal effect is due to the inaccessibility of part of the receptor population when drugs are added through the lumen.

The quantification of relative efficacy of agonists

While much pharmacological effort has been expended in the measurement, quantification, and comparison of agonist affinity for the classification of drugs and drug receptors, inordinately less emphasis has been placed on the quantification of the other property of agonists, namely intrinsic efficacy. This is unfortunate as the existing studies of the relative intrinsic efficacy of agonists show this to be a most useful scale for the classification of agonists and the prediction of tissue responses. This paper will review some of the theories that describe efficacy on a molecular level, the methods of measuring relative efficacy, and the factors in these procedures which can lead to artifacts and misleading information for the classification of drug receptors. Lastly, the value of the quantification of efficacy will be discussed in terms of the design of agonists for therapeutic advantage.

The relative contribution of affinity and efficacy to agonist activity: organ selectivity of noradrenaline and oxymetazoline with reference to the classification of drug receptors

Oxymetazoline demonstrated a pronounced organ selectivity, when compared to noradrenaline, by being a potent full agonist in rat anococcygeus muscle and a partial agonist in rat vas deferens. Responses of rat anococcygeus muscles to oxymetazoline were relatively more sensitive to antagonism by phenoxybenzamine (Pbz) an alkylating alpha-adrenoceptor antagonist. Therefore, although oxymetazoline was more potent than noradrenaline in this tissue, after Pbz (0.3 microM for 10 min), the responses to oxymetazoline were completely inhibited while those to noradrenaline were only partially inhibited. Schild analysis with phentolamine, corynanthine, prazosin and yohimbine indicated no alpha-adrenoceptor heterogeneity within the rat anococcygeus muscle or between this tissue and rat vas deferens. Measurement of agonist Kd values and Schild analysis of oxymetazoline antagonism of responses to noradrenaline (after alkylation) confirmed the homogeneity of alpha-adrenoceptors with respect to these two agonists. The above profiles of activity would be predicted if oxymetazoline had a higher affinity but lower efficacy than noradrenaline. Experimentally this was confirmed when it was found that oxymetazoline had 5 times the affinity but 0.2 to 0.3 times the efficacy of noradrenaline. These results serve as a caveat to the use of selective receptor desensitization and/or selective receptor alkylation (or protection from alkylation) as means of differentiating drug receptors. Theoretical modelling and these experimental results indicate that high affinity/low efficacy agonists are much more sensitive to receptor coupling. The implications for therapeutic selectivity could be important in that high affinity/low efficacy agonists theoretically have a much greater potential for organ selectivity.

A pharmacological method to estimate the pKi of competitive inhibitors of agonist uptake processes in isolated tissues

An equation is derived from a mathematical model (proposed by Furchgott) which, under certain circumstances, estimates the pKI (-log dissociation constant) of a competitive inhibitor of agonist uptake by utilizing the sensitization of isolated tissues, to the substrate-agonist, by uptake inhibition. The method is theoretically more sound and appears to be improved by the use of potency-ratios of the substrate-agonist and an agonist which is not a substrate for the uptake process since this allows for the detection and correction of receptor and toxic effects of uptake inhibitors. The pKI values of cocaine, desmethylimipramine and imipramine for the neuronal uptake of norepinephrine were estimated by this method in guinea-pig tracheae and left atria. Also, the pKI values for 17 beta-oestradiol, corticosterone, clonidine and metanephrine for the extraneuronal uptake of isoproterenol were estimated in guinea-pig tracheae (and cat left atria for 17 beta-oestradiol). All estimates were consistent with literature pKI values obtained biochemically with radiolabelled substrates.