Effects Of Several Agonists Research Articles

Dual effects of ATP on isolated arteries of the bovine eye

Although the presence of purinoreceptors has been shown in many human and animal arteries, there is few data yet about their role in the arteries of the eye. The purpose of the present study was to evaluate the effects of several agonists of purinoreceptors on isolated arteries of the bovine eye. Responses of isolated preparations of bovine ophthalmic (OA) and posterior ciliary arteries (PCA) to agonists of purinoreceptors (ATP, α,β-methylene-ATP–α,β-meATP, 2-methylthioATP–2meSATP, uridine-5′-triphosphate–UTP) as well as agonists of adreno-, cholino-, adenosine and histamine receptors were recorded by a standard organ bath method. ATP induced contractions of the intact vessels but caused relaxation of α,β-meATP-pretreated arteries. Contractile responses of PCA to high concentrations of ATP and α,β-meATP were significantly stronger than responses of OA, as well as relaxative responses to ATP and adenosine were significantly stronger in PCA than in OA. We suggest that there are several subtypes of functionally active purinoreceptors in both OA and PCA, although the potency of agonists of purinoreceptors to produce mechanical responses is higher in PCA than in OA. Purinoreceptors can be potential targets for new drugs, treating vascular pathology of the eye.

Shift in sphingolipid metabolism leads to an accumulation of ceramide in senescence

Ceramide mediates the effects of several agonists leading to differentiation, apoptosis or senescence. We previously showed that ceramide becomes elevated in senescent fibroblasts. In the present study, senescent cultures of Wi-38 fibroblasts and human umbilical-vein endothelial cells were compared to low-passage cultures in order to identify which of the several pathways is predominantly responsible for the increased ceramide. We found that senescent cells take up the ceramide precursor [ 3H]palmitic acid and convert it to ceramide at essentially equivalent rates to their low-passage counterparts, suggesting that, as a whole, the inherent steps are unaltered. Analysis of subsequent steps, however, revealed changes in ceramide metabolism. The rate of ceramide conversion to sphingomyelin was reduced while glucosylceramide synthesis differed between the cell lines, while the rate of the reverse reactions tended to be increased in senescent cells. We also found a decrease in acidic but not alkaline ceramidase. The data show an overall change in favor increased ceramide levels. Of all of the pathways, neutral sphingomyelinase appears to be the most likely source of the senescence-associated ceramide. The relevance to mitosis and apoptosis are discussed.

Mass Spectrometric Analysis of Agonist Effects on Posttranslational Modifications of the β-2 Adrenoceptor in Mammalian Cells

Posttranslational modifications (PTMs) of the beta-2 adrenoceptor (B2AR) play a fundamental role in receptor regulation by agonists. We have examined the effects of several agonists on net levels of B2AR palmitoylation and phosphorylation using epitope tagging in stably transfected human embryonal kidney (HEK) 293 cells, immunoaffinity purification, and mass spectrometry combined with the method of stable isotope labeling by amino acids in cell culture (SILAC). Palmitoylation of Cys341 was confirmed and did not change detectably after 30 min exposure of cells to saturating concentrations of dopamine, epinephrine, or isoproterenol. However, all of these agonists produced a marked increase in net phosphorylation. Phosphorylation of the third cytoplasmic loop was increased to a similar degree by all three agonists, whereas differences between agonists were observed in net phosphorylation of the carboxyl-terminal cytoplasmic domain (isoproterenol approximately epinephrine >> dopamine). Interestingly, agonist-induced phosphorylation of the carboxyl-terminal cytoplasmic domain was observed exclusively in a proximal portion (between residues 339-369). None of the agonists produced detectable phosphorylation in a distal portion of the cytoplasmic tail, which contains all sites of agonist-induced phosphorylation identified previously by in vitro reconstitution. These results provide insight to agonist-dependent regulation of the B2AR in intact cells, suggest the existence of significant differences in regulatory phosphorylation events occurring between in vitro and in vivo conditions, and outline a general analytical approach to investigate regulated PTM of receptors in mammalian cells.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of α 2 adrenoceptors and GABA B but not κ-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the μ-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25–4 mg/kg), baclofen (2–32 mg/kg) or U50,488H (2–16 mg/kg) (agonists of α 2 adrenoceptors, GABA B and κ-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001–10 pg/mouse, i.c.v.) and cantharidin (0.001–10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001–1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of κ-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Taurine release modified by GABAergic agents in hippocampal slices from adult and developing mice

In order to characterize the possible regulation of taurine release by GABAergic terminals, the effects of several agonists and antagonists of GABA receptors on the basal and K+-stimulated release of [3H]taurine were investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice using a superfusion system. Taurine release was concentration-dependently potentiated by GABA, which effect was reduced by phaclofen, saclofen and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) at both ages, suggesting regulation by both GABA(B) and GABA(C) receptors. The involvement of GABA(A) receptors could not be excluded since the antagonist bicuculline was able to affect both basal and K+-evoked taurine release. Furthermore, several GABA(B) receptor effectors were able to inhibit K+-stimulated taurine release in the adults, while the GABA(C) receptor agonists trans-4-aminocrotonic acid (TACA) and cis-4-aminocrotonic acid (CACA) potentiated this release. The potentiation of taurine release by agents acting on the three types of GABA receptors in both adult and developing hippocampus further indicates the involvement of transporters operating in an outward direction. This inference is corroborated by the moderate but significant inhibition of taurine uptake by the same compounds.

Adenosine Regulates Tissue Factor Expression on Endothelial Cells

The aim of this study was to evaluate the inhibitory activity of adenosine on tumor necrosis factor-α (TNF), thrombin-, or phorbol 12-myristate 13-acetate (PMA)-induced tissue factor (TF) expression on human umbilical vein endothelial cells (HUVECs). This inhibitory effect of adenosine was found to be counteracted by the non-selective adenosine receptor (AR) antagonist, 8-(p-sulfophenyl) theophylline. To clarify the role of ARs (A1, A2a, A2b, and A3) in this regulation, we evaluated the effect of several agonists and antagonists specific for AR-subclass on TF expression. The selective A2aAR agonist, 2-p-(2-carboxyethyl) phenethylamino-5′-N-ethylcarboxamido adenosine hydrochloride (CGS 21680), the A3AR agonist, N6-2-(4-aminophenyl) ethyladenosine (APNEA), and the A1AR antagonist, 1,3-dipropyl-8-(2-amino-4-chlorophenyl) xanthine (PACPX) each inhibited TF activity expression induced by TNF, thrombin, or PMA on HUVECs. In contrast, the selective A1AR agonist, chloro-N6-cyclopentyladenosine (CCPA) and the A2AR antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX) enhanced each stimulant-induced TF activity expression. All agonist or antagonist alone did not alter the basal TF expression on HUVECs. Our results suggest that stimulation of A2aAR and A3AR down-regulates and that of A1AR up-regulates the endothelial cell TF expression induced by TNF, PMA, or thrombin. Thus, it appears that adenosine itself may exert anticoagulant activity on vascular endothelial cells via its A2a and A3 receptors, particularly during ischemic or atherosclerotic processes which are known to be associated with local increased levels of adenosine.

(S)-4-carboxy-3-hydroxyphenylglycine activates phosphatidyl inositol linked metabotropic glutamate receptors in different brain regions of the neonatal rat

In the present investigation, effects of several agonists and antagonists of metabotropic glutamate receptors (mGluRs) which are coupled to phosphatidyl inositol (PI) hydrolysis were evaluated in slices of neonatal rat hippocampus, striatum, cortex and cerebellum. The rank order of potency of agonists in the PI hydrolysis assay was identical in all brain regions: quisqualic acid (Quis) > (RS)-3,5-dihydroxyphenylglycine (3,5-DHPG) > 1S,3R-aminocyclopentane dicarboxylic acid (1S,3R-ACPD) > > l-glutamate (Glu). All agonists were equiefficacious in the four brain regions tested. The responses to 3,5-DHPG, a highly selective Class I mGluR agonist, were attenuated by (S)-4-carboxyphenylglycine ((S)-4CPG), (+)-α-methyl-4-carboxyphenylglycine ((+)-MCPG) and 1-aminoindan-1,5-dicarboxylic acid (UPF-523) with a rank order of potency of (+)-MCPG ≥ (S)-4CPG ≥ UPF-523 in the different brain regions. These results suggest little selectivity among these putative mGluR antagonists in the different brain regions studied. Interestingly, (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4C3HPG), a compound reported to act as an antagonist at Class I mGluRs, produced concentration-dependent increases in PI hydrolysis in all four brain regions suggesting that (S)-4C3HPG acts as an agonist. In striatum, hippocampus and cortex, (S)-4C3HPG was equiefficacious to Quis, 3,5-DHPG, 1S,3R-ACPD and Glu. However, in the cerebellum, (S)-4C3HPG displayed weak agonist activity (37% of that of a maximally effective concentration of Quis). The effects of (S)-4C3HPG in the PI hydrolysis assay appeared to be mediated by the activation of an mGluR subtype since it was significantly blocked by (S)-4CPG, an mGluR antagonist. In addition, the agonistic effects of (S)-4C3HPG appear to be unrelated to inhibition of [ 3H]-Glu uptake into rat hippocampal or cerebellar synaptosomes. These results demonstrate a unique pharmacological profile of (S)-4C3HPG which can be interpreted as (S)-4C3HPG being a highly selective mGluR5 agonist or alternatively, that the effects of (S)-4C3HPG may be mediated through a novel Class I mGluR subtype(s), yet to be identified.

Presynaptic inhibition of calcium-dependent and -independent release elicited with ionomycin, gadolinium, and alpha-latrotoxin in the hippocampus.

1. Presynaptic inhibition of synaptic transmission in the hippocampus was investigated by comparing the effects of several agonists on miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs). 2. The Ca2+ ionophore ionomycin increased the frequency of mEPSCs and mIPSCs but did not affect their amplitude. Ionomycin-induced release required extracellular Ca2+ and was prevented by pretreatment with botulinum neurotoxin serotype F, like evoked synaptic transmission. Unlike evoked transmission, however, this increase did not involve activation of voltage-dependent Ca2+ channels because it was insensitive to Cd2+. 3. Both the lanthanide gadolinium and alpha-latrotoxin produced increases in the frequency of mEPSCs and mIPSCs, but their actions were independent of extracellular Ca2+. 4. Adenosine, the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen, and a mu-opioid receptor agonist strongly reduced the frequency of synaptic currents triggered by all three secretagogues. 5. We conclude that activation of these presynaptic receptors can reduce high frequencies of vesicular glutamate and GABA release by directly impairing transmitter exocytosis. Presynaptic inhibition of gadolinium- and alpha-latrotoxin-induced release indicates that this impairment occurs without changes in intraterminal Ca2+ homeostasis and when vesicle fusion is rendered Ca2+ independent, respectively. 6. The inhibition of ionomycin-induced release provides additional evidence for a direct, neurotransmitter receptor-mediated modulation of the proteins underlying vesicular docking or fusion as an important component of presynaptic inhibition of evoked synaptic transmission.

Effects of β-adrenergic agonists and other putative transmitters on progesterone production by dispersed ovine luteal cells

The effects of several agonists on progesterone production by dispersed ovine luteal cells derived from corpora lutea at different stages of maturation were examined. Adrenaline significantly ( P<0.05) stimulated basal progesterone production by luteal cells from Day 3 and Day 10 of the oestrous cycle, the latter being more sensitive. Propranolol (a general β antagonist) inhibited the effects of adrenaline but phentolamine (an α antagonist) did not. While isoproterenol (a general β agonist) had similar stimulatory effects to adrenaline, on Day 3 neither dobutamine (a selective β 1 agonist) nor salbutamol (a selective β 2 agonist), though both stimulatory, were as effective. However, in Day 10 cells, salbutamol significantly ( P<0.05) stimulated progesterone production. Luteinizing hormone and cAMP were also stimulatory in both groups of cells, while histamine, melatonin and acetylcholine were without effect. These results suggest that a mixed population of β-adrenergic receptors are present in ovine luteal cells during the early luteal phase, but that β 2 receptors predominate by Day 10.

Potentiation of responses to sympathetic nerve stimulation and vasoconstrictor agents by SK&F 103829 in the feline mesenteric circulation.

1. The amplification of vasoconstrictor effects of several agonists and sympathetic nerve stimulation, caused by 5-HT2 receptor activation, was studied in the autoperfused mesenteric circulation of anaesthetized cats. To produce long lasting and selective 5-HT2 receptor stimulation we used SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulphonyl]-1H3-benzazepin-7-ol methensulphonate). We assessed that SK&F 103829 was a strong contractile partial agonist in isolated preparations of rat tail artery and calf pulmonary artery. 2. The intrinsic activity of SK&F 103829 with respect to 5-hydroxytryptamine (5-HT) was 0.8 in rat tail artery and 0.6 in calf pulmonary artery. SK&F 103829-induced contractile responses were surmountably antagonized by ketanserin with a potency expected from its affinity for 5-HT2 receptors. SK&F 103829 surmountably antagonized the effects of 5-HT in rat tail artery with a pKp of 5.8. 3. Concentrations of SK&F 103829 causing greater than threshold constrictions enhanced vasoconstrictor responses of sympathetic nerve stimulation, noradrenaline, angiotensin II, methoxamine and alpha, beta-methylene ATP in the mesenteric arterial bed. Increases in mesenteric arterial pressure by noradrenaline, observed in the presence of prazosin, were also potentiated by SK&F 103829. 4. Ketanserin prevented both the constrictor effect of SK&F 103829 and the SK&F 103829-evoked potentiation of the responses to noradrenaline and angiotensin II in the mesenteric arterial bed. Ketanserin, however, failed to abolish (once established) the SK&F 103829-evoked potentiation of the constrictor effects caused by both noradrenaline and angiotensin II. 5. Short lasting constrictor effects of 5-HT were reversed to dilator effects by SK&F 103829 in both the mesenteric arterial and venous bed, thereby revealing the existence of vasodilator 5-HT receptors.6. The main finding is consistent with a sensitization of the mesenteric arterial bed to vasoconstrictor responses mediated through alpha 1- and alpha2-adrenoceptors, angiotensin II receptors and purinoceptors by SK&F 103829-evoked activation of 5-HT2 receptors. This property, together with the direct constrictor effect of the mesenteric arterial bed suggest that SK&F 103829 can reduce portal venous flow thereby being a useful therapeutic principle for the treatment of portal hypertension.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum.

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Mediation by the same muscarinic receptor subtype of phasic and tonic contractile activities in the rat isolated portal vein.

1. The effects of several agonists on the phasic and tonic contractile responses to muscarinic receptor stimulation have been investigated in the rat portal vein in vitro. 2. Neither chemical denervation with 6-hydroxydopamine nor the presence of the alpha 1-adrenoceptor antagonist, prazosin, influenced the spontaneous or the stimulated myogenic activity of the portal vein. 3. Indomethacin and NG-nitro-L-arginine were used to investigate the influence of vasoactive factors in this preparation. They slightly increased the frequency and the amplitude of the spontaneous myogenic activity of the portal vein, respectively. NG-nitro-L-arginine but not indomethacin enhanced the maximal phasic response to carbachol. Both indomethacin and NG-nitro-L-arginine failed to influence the tonic response to carbachol. 4. Muscarinic agonists increased phasic activity according to the rank order of potency: acetylcholine > muscarine > methacholine > carbachol > aceclidine > bethanechol. These effects were superimposed on a sustained contracture at higher concentrations. Oxotremorine was more potent than arecoline in increasing the mechanical phasic activity, without inducing a sustained contracture. Pilocarpine and McN A343 were weak agonists, producing submaximal effects only on phasic activity. 5. The muscarinic antagonists AF-DX116, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), P-fluorohexahydrosiladiphenidol (pFHHSiD) and pirenzepine antagonized the phasic and tonic mechanical responses to carbachol. Although the tonic contracture was slightly more sensitive to all antagonists studied, the rank order of potency: 4-DAMP > pFHHSiD > pirenzepine > AF-DX 116 was the same for both types of responses, which is indicative of a M3-receptor subtype. 6. The tonic contractile response of the rat portal vein to carbachol was more susceptible to partial receptor inactivation with propylbenzilylcholine mustard than the phasic contractile response. The dissociation constants (KA) obtained from an analysis according to Furchgott & Bursztyn (1967) were found to be 4.32 +/- 0.31 1AM for the phasic and 3.56 +/- 0.21 1AM for the tonic type of carbachol-induced response, respectively. Since the EC50-values for both carbachol-induced effects were different (phasic0.232 +/- 0.02 1AM; tonic 2.75 +/- 0.1 1AM) the phasic type of response appears to involve a large receptor reserve.

Glomerular endothelial cells respond to calcium-mobilizing agonists with release of EDRF

To determine whether glomerular endothelial cells (GEN) may play a role in the local control of glomerular function by releasing endothelium-derived relaxing factor (EDRF), the effect of several agonists on GEN cytosolic calcium concentration ([Ca2+]i) and GEN EDRF release was determined. Bradykinin, ATP, thrombin, and platelet-activating factor (PAF) all increased [Ca2+]i in GEN in a concentration-dependent manner, whereas serotonin, acetylcholine, phenylephrine, and endothelin-1 were without effect. Coincubation of glomerular mesangial cells (GMC) with GEN augmented mesangial cell guanosine 3',5'-cyclic monophosphate (cGMP) content five- to sixfold, Bradykinin elicited a further concentration-dependent increase in GMC cGMP content in the presence but not absence of GEN. The GEN-dependent bradykinin-stimulated GMC cGMP accumulation was abolished by hemoglobin and methylene blue, blunted by gossypol, and augmented by superoxide dismutase. Other agonists capable of augmenting GEN [Ca2+]i also stimulated GMC cGMP accumulation in the presence but not in the absence of GEN. Thus cultured GEN release a factor that stimulates cGMP accumulation in adjacent mesangial cells which has the pharmacological characteristics of EDRF.

Mechanisms of release of atrial natriuretic factor I. Effect of several agonists and steroids on its release by atrial minces

The effect that several substances may have on ANF release by atrial slices and on its tissular content was investigated. α - and β -adrenergic and cholinergic agonists, vasopressin, met-enkephaline, dexamethasone and DOC, in concentration ranging from 10 −4 to 10 −8 M, were added into the incubation media and incubated 1 and 4 hours. No changes were observed in ANF concentration either in the media or in its tissular concentration as measured by a specific radioimmunoassay. When intact rats were previously treated with DEXA, DOC or DEXA + DOC and their atria incubated “in vitro”, an increase in the release of ANF was observed in the Dexa-treated group only, but all treated groups had higher tissular ANF concentration. It is concluded that neither α - or β -adrenergic, nor cholinergic agonists or vasopressin and met-enkephaline stimulate ANF release “in vitro”. On the other hand steroids may regulate ANF release and synthesis in the intact rat. It seems likely that the ANF released into the media corresponds to a short peptide.

Electrical membrane events in response to alpha-adrenoceptor stimulation.

Strips of rabbit main pulmonary artery (RMPA) were used to study the effects of several agonists on tension development and membrane potential of the vascular smooth muscle cells. The following alpha-adrenoceptor agonists were employed: methoxamine and St 587 (alpha 1-selective), B-HT920 (alpha 2-selective) and clonidine, which stimulates preferentially alpha 2-adrenoceptors. By the use of the selective antagonists prazosin and yohimbine it was not possible to differentiate convincingly between alpha 1- and alpha 2-adrenoceptors in the RMPA. Methoxamine and B-HT920 produced depolarization of similar magnitude of the membrane of the vascular smooth muscle cells. In spite of these results, which point to a uniform alpha-adrenoceptor in the RMPA, contractions to alpha 1- and alpha 2-agonists differed in some important aspects. Contractions in response to alpha 2-agonists were highly susceptible to the inhibitory effects of calcium withdrawal and calcium antagonists whereas contractions to alpha 1-agonists were much less so. Reduction of the membrane potential of the vascular cells by K+ at 12 mmol/l had no effect on the concentration-contraction curve of methoxamine but shifted that of B-HT920 to the left. Conversely hyperpolarization of the membrane of the vascular smooth muscle cells by strychnine totally suppressed contraction to B-HT920 and caused only a rightward shift of the concentration-contraction curve of methoxamine and St 587. Interaction of alpha 1- and alpha 2-agonists with an apparently uniform alpha-adrenoceptor induces in the RMPA contraction which seems to be triggered by different membrane processes.

Stimulant and blocking effects of optical isomers of pindolol on the sinoatrial node and trachea of guinea pig. Role of beta-adrenoceptor subtypes in the dissociation between blockade and stimulation.

The blocking and stimulant potencies of (-)-pindolol and (+)-pindolol were estimated on right atria and tracheae of guinea pig. Blocking affinities were estimated for beta-adrenoceptor subtypes by using several agonists. Binding affinities of (-)-pindolol and (+)-pindolol were also estimated for beta-adrenoceptors labelled with 3H-(-)-bupranolol in membranes of ventricular myocardium and lung of guinea pig. Both (-)-pindolol and (+)-pindolol caused tracheal relaxation with intrinsic activities of 0.3. The concentration-effect curve for (-)-pindolol exhibits a high-sensitivity and a low-sensitivity relaxant component; the curve for (+)-pindolol was nearly monophasic. The EC50's were (-log mol/l) 9.2 and 6.1 for (-)-pindolol and 7.6 for (+)-pindolol. Using subtype-selective blockers it was found that the relaxant effects of (+)-pindolol and those of the high-sensitivity component of (-)-pindolol are mediated through beta 2-adrenoceptors. The low-sensitivity component of relaxation of (-)-pindolol was antagonized by beta-blockers less than expected from their affinities for beta-adrenoceptors. Both (-)-pindolol and (+)-pindolol caused an increase of atrial beating rate with an intrinsic activity of 0.2. The concentration-effect curve of (-)-pindolol was biphasic; the curve of (+)-pindolol was monophasic. The EC50's were (-log mol/l) 9.1 and 7.0 for (-)-pindolol and 7.5 for (+)-pindolol. From the use of subtype-selective antagonists we conclude that the positive chronotropic effects of (+)-pindolol are mediated predominantly by beta 2-adrenoceptors. On the other hand, the high-sensitivity component of the positive chronotropic effects of (-)-pindolol appears to be mediated predominantly through beta 1-adrenoceptors, although beta 2-adrenoceptors may also participate. The low-sensitivity component of the positive chronotropic effects of (-)-pindolol is resistant to blockade by subtype-selective antagonists at concentrations causing at least 98% beta-adrenoceptor occupancy. Only high but non-depressant concentrations of non-selective (-)-bupranolol antagonized the low-sensitivity component of (-)-pindolol. (-)-Pindolol antagonized the effects of several agonists to similar extent in both trachea and right atrium. (+)-Pindolol was less potent as antagonist of the relaxant effects of (-)-noradrenaline on trachea than against those of (-)-adrenaline, (-)-isoprenaline and (+/-)-salbutamol.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies with Isolated Human Coronary Arteries: Some General Observations, Potential Mediators of Spasm, Role of Calcium Antagonists

We studied the basic mechanisms involved in human coronary artery smooth-muscle contraction in an attempt to gain insight into the pathophysiology and pathopharmacology of coronary spasm. Coronary arteries were obtained from recipient hearts of patients receiving cardiac transplantation at Stanford University. Coronary ring segments were studied in an organ bath using standard Tyrode's solution. Observation included characterizations of spontaneous contractile activity, factors affecting basal tone, type of agonist-induced contractile response, receptor distribution, activity of cardioactive drugs, and species differences. Agents mediating contraction included histamine, acetylcholine, norepinephrine, potassium, and prostaglandins. Because of recent clinical interest in calcium antagonists, diltiazem was studied extensively. Diltiazem demonstrated noncompetitive antagonism to the effects of several agonists and competitive antagonism to calcium. The exact cause and mechanism of coronary spasm is still unknown, but we have identified new areas for future investigation. We studied the basic mechanisms involved in human coronary artery smooth-muscle contraction in an attempt to gain insight into the pathophysiology and pathopharmacology of coronary spasm. Coronary arteries were obtained from recipient hearts of patients receiving cardiac transplantation at Stanford University. Coronary ring segments were studied in an organ bath using standard Tyrode's solution. Observation included characterizations of spontaneous contractile activity, factors affecting basal tone, type of agonist-induced contractile response, receptor distribution, activity of cardioactive drugs, and species differences. Agents mediating contraction included histamine, acetylcholine, norepinephrine, potassium, and prostaglandins. Because of recent clinical interest in calcium antagonists, diltiazem was studied extensively. Diltiazem demonstrated noncompetitive antagonism to the effects of several agonists and competitive antagonism to calcium. The exact cause and mechanism of coronary spasm is still unknown, but we have identified new areas for future investigation.

Studies with Isolated Human Coronary Arteries: Some General Observations, Potential Mediators of Spasm, Role of Calcium Antagonists

We studied the basic mechanisms involved in human coronary artery smooth-muscle contraction in an attempt to gain insight into the pathophysiology and pathopharmacology of coronary spasm. coronary arteries were obtained from recipient hearts of patients receiving cardiac transplantation at Stanford University. Coronary ring segments were studied in an organ bath using standard Tyrode's solution. Observation included characterizations of spontaneous contractile activity, factors affecting basal tone, type of agonist-induced contractile respnse, receptor distribution, activity of cardioactive drugs, and species differences. Agents mediating contraction included histamine, acetylcholine, norepinephrine, potassium, and prostaglandins. Because of recent clinical interest in calcium antagonists, diltiazem was studied extensively. Diltiazem demonstrated noncompetitive antagonism to the effects of several agonists and competitive antagonism to calcium. The exact cause and mechanism of coronary spasm is still unknown, but we have identified new areas for future investigation.