Application Of Isoproterenol Research Articles

Blockade of isoproterenol-induced synaptic potentiation by tetra-9-aminoacridine in the rat amygdala

The effects of tetrahydro-9-aminoacridine (THA) on β-adrenoceptor activation-induced synaptic potentiation were studied in brain slices of the rat amygdala using intracellular recording techniques. To exclude the involvement of N-methyl- d-aspartate (NMDA) receptors, all the experiments were performed in the presence of NMDA receptor antagonist, d-APV (50 μM). Bath application of isoproterenol (Iso; 15 μM) results in a long-lasting enhancement of the amplitude of excitatory postsynaptic potentials (EPSPs) to 200 ± 6% of baseline. Forskolin, which directly activates adenyl cyclase, produces a similar effect suggesting that Iso may act through a cyclic AMP-dependent mechanism. Pretreatment of the slices with THA (300 μM) completely abolishes the Iso- and forskolin-induced synaptic potentiation. We hypothesize that the locus of THA/β-adrenoceptor interaction is presynaptic; the underlying mechanism is likely due to THA's depression of transmitter release via a presynaptic blockade of voltage-dependent Ca 2+ channels.

Electrophysiological and electrochemical responses of NMDA in the cerebellum: Interactions with noradrenergic pathway

In the present experiments, we measured N-methyl-D-asparate (NMDA)-induced norepinephrine (NE) release and extracellular action potentials in the cerebellar cortex of urethane-anesthetized rats. The overflow of NE was measured using a Nafion coated-carbon fiber electrode and in vivo chronoamperometry. We found that both NMDA and quisqualate evoked NE release. Our previous work demonstrated that the electrophysiological activity of cerebellar Purkinje neurons could be either excited or inhibited by local NMDA application. It was reported that bicuculline antagonized NMDA-induced inhibition in Purkinje neurons, suggesting that a GABAergic mechanism was activated during NMDA application. We and others previously found that GABA-mediated electrophysiological depressions were enhanced by NE acting via β-adrenergic receptors while these responses were decreased by activation of α-adrenergic receptors. Since NMDA evokes overflow of endogenous NE, the electrophysiological depression induced by NMDA may contain an NE-mediated modulatory component. In this study, we first examined the interaction of NMDA with a β-adrenergic agonist. We found that local application of isoproterenol facilitated NMDA- or GABA-mediated electrophysiological depressions of the Purkinje neurons. Application of phenoxybenzamine, which antagonized the α-adrenergic response of synaptically released NE, also facilitated NMDA-elicited depression. In contrast, the depression induced by GABA, which did not induce NE overflow, was not potentiated by phenoxybenzamine. The facilitation of NMDA-induced depression by phenoxybenzamine was antagonized by the β-adrenergic antagonist timolol. Taken together, these data suggested that the noradrenergic pathway is involved in NMDA-induced electrophysiological responses in the cerebellum. NMDA may induce neuronal depression through modulation of GABAergic inhibition via NMDA-evoked NE release onto cerebellar Purkinje neurons.

Electrophysiological and electrochemical responses of NMDA in the cerebellum: interactions with noradrenergic pathway

In the present experiments, we measured N -methyl-D-asparate (NMDA)-induced norepinephrine (NE) release and extracellular action potentials in the cerebellar cortex of urethane-anesthetized rats. The overflow of NE was measured using a Nafion coated-carbon fiber electrode and in vivo chronoamperometry. We found that both NMDA and quisqualate evoked NE release. Our previous work demonstrated that the electrophysiological activity of cerebellar Purkinje neurons could be either excited or inhibited by local NMDA application. It was reported that bicuculline antagonized NMDA-induced inhibition in Purkinje neurons, suggesting that a GABAergic mechanism was activated during NMDA application. We and others previously found that GABA-mediated electrophysiological depressions were enhanced by NE acting via β-adrenergic receptors while these responses were decreased by activation of α-adrenergic receptors. Since NMDA evokes overflow of endogenous NE, the electrophysiological depression induced by NMDA may contain an NE-mediated modulatory component. In this study, we first examined the interaction of NMDA with a β-adrenergic agonist. We found that local application of isoproterenol facilitated NMDA- or GABA-mediated electrophysiological depressions of the Purkinje neurons. Application of phenoxybenzamine, which antagonized the α-adrenergic response of synaptically released NE, also facilitated NMDA-elicited depression. In contrast, the depression induced by GABA, which did not induce NE overflow, was not potentiated by phenoxybenzamine. The facilitation of NMDA-induced depression by phenoxybenzamine was antagonized by the β-adrenergic antagonist timolol. Taken together, these data suggested that the noradrenergic pathway is involved in NMDA-induced electrophysiological responses in the cerebellum. NMDA may induce neuronal depression through modulation of GABAergic inhibition via NMDA-evoked NE release onto cerebellar Purkinje neurons.

Endothelin-1 inhibitsl-type Ca currents enhanced by isoproterenol in guinea-pig ventricular myocytes

To investigate the action of endothelin-1 (ET-1) on L-type Ca currents (ICa,L) in guinea-pig ventricular cells, whole-cell currents were recorded at approximately 36-37 degrees C in enzymatically isolated myocytes. ET-1 (> or =10 nM) suppressed the basal ICa,L to 79+/-8% of control at 20 nM. Bath application of isoproterenol (ISO; 10 nM) enhanced ICa,L to 192+/-28% with about a -10-mV shift of its relationship with membrane potential. ET-1 concentration dependently inhibited this ISO-enhanced ICa,L with a half-maximally inhibitory concentration (IC50) of 168 pM. The inhibitory actions of ET-1 were antagonised by BQ-123 (300 nM), cyclo(D-Asp-L-Pro-D-Val-L-Leu-D-Trp), a specific ETA receptor antagonist. Histamine-enhanced ICa,L was also suppressed by ET-1, but ICa, L potentiated by internal adenosine 3',5'-cyclic monophosphate (cAMP) was unaffected. Preincubation of myocytes with pertussis toxin (PTX, at 5 microgram/ml for >60 min at 36 degrees C) completely occluded the ET-1 action. Thus, stimulation of ETA receptors by subnanomolar ET-1 inhibits ICa,L via PTX-sensitive G-proteins.

Restricted Propagation of Cytoplasmic Ca2+ Oscillation into the Nucleus in Guinea Pig Cardiac Myocytes as Revealed by Rapid Scanning Confocal Microscopy and Indo-1

Two-dimensional images of cytoplasmic and nuclear free Ca2+ movements in cardiac myocytes were obtained at 67-msec intervals using a Ca(2+)-sensitive fluorescence probe, indo-1, and a rapid scanning confocal laser microscope, Nikon RCM8000. Isolated guinea pig ventricular cells were loaded with indo-1 and stimulated at 0.5 Hz through patch pipettes. On stimulation, nuclear Ca2+ concentration ([Ca2+]) was observed to rise and fall following cytoplasmic [Ca2+] with an obvious delay. Application of isoproterenol significantly increased the peak [Ca2+] on stimulation in both the cytoplasm and nucleus with no substantial change in the basal [Ca2+]; the increase in peak [Ca2+] produced by application of isoproterenol was larger in the cytoplasm than in the nucleus. Under a low [Na+] condition, the basal [Ca2+] was increased from the control values in both the cytoplasm and nucleus; no difference in basal [Ca2+] was observed between the two regions. The increase in peak [Ca2+] by low [Na+] in the cytoplasm was significantly larger than that in the nucleus. When the cells were voltage clamped at 0 mV for 3 sec, no difference in the steady state [Ca2+] was observed between the cytoplasm and nucleus. Nuclear [Ca2+] was also observed to increase following a Ca2+ wave, a local increase in [Ca2+] propagating within the cytoplasm, with a delay. Thus, we demonstrated in isolated myocardial cells that cytoplasmic Ca2+ movements, although hampered by the nuclear envelope, are propagated into the nucleus, a mechanism through which factors affecting cytoplasmic Ca2+ may influence intranuclear events.

Noradrenergic enhancement of GABA-induced input resistance changes in layer V regular spiking pyramidal neurons of rat somatosensory cortex

Previous in vivo studies have shown that microiontophoretic application of norepinephrine (NE) and isoproterenol (ISO) can enhance gamma-aminobutyric acid (GABA)-induced depressant responses of rat somatosensory cortical neurons. In the present investigation we have examined the transmembrane electrophysiological events which are associated with interactions between NE and GABA in layer V pyramidal neurons of rat barrel field cortex. Intracellular recordings were made from electrophysiologically identified cells in a superfused cortical tissue slice preparation before, during and after bath or microdrop application of GABA, NE and ISO, alone or in combination. GABA application produced a small depolarization from resting membrane potential associated with a reduction (22%) in input resistance. NE and ISO (10–100 μM) also produced in some cases small membrane depolarizations (1–4 mV) but little concomitant changes in input resistance. Simultaneous application of NE with GABA potentiated amino acid-induced changes in input resistance in 4 cases and antagonized ( n = 4) 4r had no effect ( n = 4) on GABA-associated membrane events in 8 other cases. When the alpha-blocker, phentolamine (20 μM), was added to the medium, NE-induced enhancement of the GABA response was observed in 3 of 5 cases (60%), suggesting both, a beta-adrenergic mediation and a possible alpha-receptor masking of this noradrenergic-potentiating action. Consistent with this interpretation was the finding that the beta-agonist, ISO (10–100 μM), produced net increases in GABA-induced input resistance changes in 64% of cases tested (9 of 14). The potentiating effect of NE and ISO was mimicked by the adenyl cyclase activator, forskolin ( n = 2), and a membrane permeant analog of cyclic-AMP, 8-bromo-cyclic AMP ( n = 3); and could also be demonstrated when the GABA A agonist muscimol (0.5–1 μM) was substituted for GABA. The reversal potential for GABA and GABA + NE remained the same. These findings suggest that previous demonstrations of NE-potentiating effects on GABA inhibition may be mediated by beta-receptor/cyclic-AMP-linked actions on mechanisms which regulate GABA A receptor-induced membrane conductance changes.

Stable Expression and Coupling of Cardiac L-Type Ca 2+ Channels With β 1 -Adrenoceptors

A number of neurotransmitters modulate cardiac dihydropyridine-sensitive L-type Ca2+ channels through several homologous G protein-coupled receptors. Previous studies that have examined receptor-Ca2+ channel interactions have suffered because of the coexpression of various receptor subtypes in native cells. To study the functional coupling of a particular receptor subtype to these channels, rabbit cardiac Ca2+ channel alpha 1 and skeletal beta and alpha 2/delta subunits were stably expressed in baby hamster kidney cells. In this stable cell line, Ca2+ channels remained at high levels (> 1000 fmol/mg protein, or 2700 channels per cell) over extended times. The expressed recombinant Ca2+ channels displayed the voltage dependence of activation and inactivation, unitary conductance, and pharmacology characteristic of native cardiac L-type Ca2+ channels. Subsequent coexpression of the beta 1-adrenoceptors (150 to 300 fmol/mg protein) with the Ca2+ channels resulted in cell responsiveness to the extracellular application of isoproterenol. These results indicate that heterogeneous expression in mammalian cells provides a useful system for studying both biophysical analysis of Ca2+ channel properties and receptor-coupled regulatory processes.

Activation of Ca(2+)-activated K+ channels by beta agonist in rabbit coronary smooth muscle cells.

Isoproterenol (ISO), a beta agonist, causes hyperpolarization of coronary smooth muscle cells via an increase in K+ conductance. This hyperpolarization may cause the coronary vasorelaxation by decreasing the cytoplasmic Ca2+ concentration. It is well known that the activation of beta adrenoreceptors stimulates the adenylate cyclase activity, and the resulting K+ channel phosphorylation by cAMP-dependent protein kinase may be responsible for ISO-induced increase in K+ channel activity. However, it is not clear whether the increase in K+ channel activity by ISO is exclusively due to the activation of adenylate cyclase or not. In this research, the effect of ISO on the isometric tension and the mechanism of ISO-induced K+ channel activation were investigated in various patch clamp conditions. The summarized results are as follows. ISO- and pinacidil induced vasorelaxation was significantly inhibited by the application of TEA or by increasing the external K+ concentration. In the whole cell clamp mode, application of ISO increased K+ outward current, and this effect was completely eliminated by propranolol. In the cell-attached patch, application of ISO or forskolin increased Ca(2+)-activated K+ channel activity. Application of ISO to the bath in the outside-out patches or GTP in the inside-out patches stimulated Ca(2+)-activated K+ channels. From the above results, both A-kinase dependent channel phosphorylation and direct GTP-binding protein mediated effect might be responsible for the the activation of Ca(2+)-activated K+ channel by ISO in rabbit coronary smooth muscle cells. And this K+ channel activation also contributes to the ISO-induced vasorelaxation.

Norepinephrine modulates excitatory amino acid-induced responses in developing human and adult rat cerebral cortex

These experiments were designed to assess the ability of norepinephrine and its β-receptor agonist, isoproterenol, to modulate responses induced by activation of excitatory amino acid receptors in brain slices obtained from developing human cortex or adult rat cortex. Human cortical slices were obtained from children undergoing surgery for intractable epilepsy (9 months to 10 yr of age). For comparison, slices were also obtained from rats (2–3 months of age). Iontophoretic application of glutamate, N-methyl-d-aspartate or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) produced excitatory responses consisting of membrane depolarizations accompanied by action potentials. Iontophoretic or bath application of norepinephrine or isoproterenol enhanced responses evoked by glutamate or N-methyl-d-aspartate. Depolarizations occurred with shorter latencies and their amplitudes increased. Action potential frequency was also increased and responses were of longer duration. In contrast, norepinephrine or isoproterenol had no effect on responses induced by AMPA. The enhancement of responses induced by N-methyl-d-aspartate or glutamate was antagonized by the β-adrenergic receptor antagonist propranolol. Similar findings were obtained from neurons in humans or rats.These results suggest that norepinephrine, possibly via β-receptors, potentiates responses mediated by glutamate and N-methyl-d-aspartate receptors without affecting those mediated by AMPA receptors. These effects were observed at all ages studied, indicating that the ability of norepinephrine to modulate excitatory neuronal transmission is well developed in human cortex by 9 months of age.

Pathway specificity of noradrenergic plasticity in the dentate gyrus.

Previous experiments have described highly specific effects of noradrenergic agonists on synaptic transmission in the dentate gyrus (DG). For example, perfusion of hippocampal slices with the beta-noradrenergic agonist isoproterenol induces a long-lasting potentiation (LLP) of extracellularly recorded responses following stimulation of the medial perforant path (PP), and long-lasting depression (LLD) of responses evoked by stimulation of the lateral PP (Dahl D, Sarvey JM, 1989, Proc Natl Acad Sci USA 86:4776-4780). To examine the possible interactions of LLP, LLD, and long-term potentiation induced by tetanic stimulation (LTP), the authors recorded extracellular field potentials evoked in the DG by stimulation of the lateral or medial perforant path following LTP and LLP or LLD, invoked in different orders. After establishment of LLP or LLD by bath application of isoproterenol, subsequent tetanization of the respective afferents resulted in additional potentiation of the medial PP-evoked response and return of the lateral PP-evoked response to baseline levels. In other slices, application of isoproterenol after establishment of LTP resulted in further potentiation of medial PP-evoked responses but no change in the potentiated response evoked by lateral PP stimulation. Thus the pathway specificity was maintained irrespective of the history of previous potentiation or depression. Experiments using the specific beta 1 antagonist metoprolol further confirmed pathway specificity. Perfusion with 20 microM of metoprolol appeared to reduce LTP evoked by stimulation of the medial but not lateral PP. In a subsequent experiment, metoprolol in the absence of tetanization produced LLD of the medial PP-evoked response and LLP of the lateral PP-evoked response, opposite to the effects of ISO.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of ATP-Sensitive K + Channels by Cyclic AMP-Dependent Protein Kinase in Cultured Smooth Muscle Cells of Porcine Coronary Artery

The effects of cyclic AMP-dependent protein kinase on ATP-sensitive K + channels in cultured smooth muscle cells of the porcine coronary artery were investigated using the patch-clamp technique. Extracellular application of isoproterenol (1mM), a beta agonist, or forskolin (2×10 −5M), an activator of adenylate cyclase, activated these channels in cell-attached patch configurations, which were not blocked by phorbol 12-myristate 13-acetate (10 −6M), an activator of protein kinase C. Cyclic AMP-dependent protein kinase activated these channels in inside-out patch configurations. These results suggest that cyclic AMP-dependent phosphorylatlon modulates ATPsensitive K + channels, in addition to its well known effects on Ca 2+-activated K + channels. The activation of ATP-sensltive K + channels by cyclic AMP-dependent phosphorylation contributes to hyperpolarizatlon of the membrane and to the relaxation of vascular smooth muscle cells.

Intravenous Isoproterenol as a Marker for Epidural Test-Dosing in Children

The purpose of this study was to determine if isoproterenol would be an effective marker of intravascular injection in anesthetized children. Forty-four ASA 1 children, aged 2 mo to 10 yr, were randomly assigned to two groups. Children in group 1 (n = 21) received 0.05 microgram/kg isoproterenol, and children in group 2 (n = 23) received 0.075 microgram/kg isoproterenol. A blinded observer continuously recorded heart rate and arterial blood pressure. Measurements were recorded before the surgical incision at steady-state halothane concentration of 1.2 minimum alveolar concentration adjusted for age. Isoproterenol produced a graded increase in heart rate with mean maximum increases of 16.5 +/- 8.7 beats/min in group 1 and 21.5 +/- 9.2 beats/min in group 2. No episodes of hypotension and arrhythmia were noted. Isoproterenol, 0.075 microgram/kg, is more sensitive but still is an imperfect marker of an intravascular injection. It produces a heart rate increase in 96% of children anesthetized with halothane and nitrous oxide in 50% oxygen. The application of isoproterenol as an epidural test dose appears promising, but cannot be recommended until its full reliability and neurotoxicity are evaluated.

Cell-type-specific regulation of nerve growth factor (NGF) synthesis in non-neuronal cells: comparison of Schwann cells with other cell types

We have previously shown that after peripheral nerve lesion the synthesis of NGF is induced in cells of the nerve sheath (Heumann et al., 1987a). Further analysis led to the identification of growth factors and intracellular mechanisms responsible for this induction in sciatic fibroblasts (Lindholm et al., 1988; Hengerer et al., 1990). The present work aimed at the elucidation of the regulation of NGF synthesis in Schwann cells. A variety of cytokines and peptide growth factors, including interleukin-1 (IL-1) and platelet-derived growth factor (PDGF), which are known to increase NGF-mRNA in fibroblasts and astrocytes, failed to do so in Schwann cell cultures. Forskolin (FK), an activator of adenylate cyclase, increased the level of NGF-mRNA eightfold within 3 hr of incubation. The effect of FK on NGF-mRNA was mimicked by analogs of cAMP but not by dideoxyforskolin, an FK derivative not activating adenylate cyclase. Application of norepinephrine and isoproterenol also augmented the NGF-mRNA content. Pretreatment of Schwann cells with N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-8), an inhibitor of cyclic-nucleotide-dependent protein kinases, decreased both basal and elevated levels of NGF-mRNA. Ionomycin, a Ca2+ ionophore, and phorbol 12-myristate 13-acetate (TPA), an activator of protein kinase C, potentiated the effect of FK in an H-8-sensitive manner. We show that the action of FK is independent of changes in mRNA stability and of protein synthesis. Thus, in cultured Schwann cells upregulation of NGF-mRNA expression seems to be mainly achieved by a cAMP-triggered transcriptional activation of the NGF gene. Another striking difference between various glial cell types was revealed by application of transforming growth factor beta-1 (TGF-beta 1), which is the strongest inducer of NGF-mRNA in cultured astrocytes (Lindholm et al., 1990). Schwann cells responded to TGF-beta 1 by decreasing basal as well as FK-induced NGF-mRNA levels. Together with previously published work, our results show that cell-type-specific mechanisms not only account for the different control of NGF expression in neurons as compared to glial cells, but also reveal a surprising specificity of regulatory mechanisms in different non-neuronal cell types, even those derived from the same tissue such as fibroblasts and Schwann cells of peripheral nerves.

Modulation of Ca currents in isolated frog atrial cells studied with photosensitive probes. Regulation by cAMP and Ca 2+: A common pathway?

We have studied the regulation of cardiac Ca current by intracellular cyclic AMP (cAMP) and Ca 2+, using photosensitive, caged compounds and the whole-cell, patch-clamp technique in isolated frog atrial cells. Although both low voltage activated (LVA) and high voltage activated (HVA) Ca channels were found to be present in these cells, only the HVA Ca currents were sensitive to modulation by isoproterenol or dihydropyridines (DHPs). The application of extracellular isoproterenol, as well as the photorelease of intracellular cAMP or Ca 2+ at micromolar and submicromolar concentrations, respectively, had no effect on LVA Ca currents. In contrast, these agents: (i) increased the amplitude of currents through HVA channels, carried by either Ca 2+ or Ba 2+ with a similar time-course, (ii) slowed the decay of the current when Ba 2+ was the permeating ion, and (iii) modulated the agonist effect of the DHP Bay-K 8644. The strong similarities between the effects of cAMP and Ca 2+ suggest that both of these intracellular messengers might eventually lead to the phosphorylation of HVA Ca channels. It is possible that Ca-dependent phosphorylation of the channels may account for the potentiation of Ca current induced by repetitive stimulation.

Induction of kinetic cell death and its underlying mechanisms.

Using a modified Langendorff system, we established an experimental model of kinetic cell death in the rat heart. After calcium (5.5 mM) was loaded for 20 min and 10(-7) mol isoproterenol was then delivered to the perfusion medium, the hearts developed tonic contracture. Histological examination of the myocardium revealed widespread kinetic cell death. Stimulation of alpha-adrenoceptors with phenylephrine did not induce kinetic cell death, even after calcium loading. In our kinetic cell death model in the rat, the outflow of creatine phosphokinase into the perfusion medium was increased after isoproterenol application, with a peak occurring at 45 min. The peak transient increase in the intracellular calcium ion concentration (the Ca-transient) was measured using Fura2/AM, and was found to be augmented by increasing the calcium concentration of the perfusion medium. These results suggest that in the presence of an increased intracellular calcium concentration, even slight stimulation of beta-adrenoceptors can easily induce myocardial injury.

Beta-adrenergic agonists stimulate the oxidative pentose phosphate pathway in the rat heart.

The oxidative pentose phosphate pathway is poorly developed in the rat heart compared with other organs, since the activity of glucose-6-phosphate dehydrogenase (G-6-PDH), the first and rate-limiting enzyme of the oxidative pentose phosphate pathway, is low. As a consequence, the available pool of 5-phosphoribosyl-1-pyrophosphate and the rate of adenine nucleotide biosynthesis are limited. Isoproterenol, 24 hours after subcutaneous administration at 0.1, 1, and 25 mg/kg, stimulated the activity of G-6-PDH in whole hearts dose-dependently from 4.3 +/- 0.16 (control) to 6.6 +/- 0.35, 10.3 +/- 0.82, and 11.5 +/- 0.56 units/g protein, respectively. The activity of 6-phosphogluconate dehydrogenase, another of the enzymes in the oxidative pentose phosphate pathway, remained unchanged. G-6-PDH activity started to increase 12 hours after isoproterenol application, when the glycogenolytic and functional response was over, and reached a peak value between 24 and 48 hours. This stimulating effect was also demonstrated in cardiac myocytes that were isolated 28 hours after isoproterenol application. beta-receptor blockade with atenolol reduced the isoproterenol-induced increase in cardiac G-6-PDH activity by 90%. Cycloheximide, which inhibits translation, and actinomycin D, which interferes with transcription, attenuated it by 83% and 78%, respectively. These results indicate that cardiac beta-adrenergic receptors and enzyme protein synthesis are involved in this effect. Other beta-sympathomimetic agents such as dopamine, dobutamine, fenoterol, salbutamol, and terbutaline also stimulated myocardial G-6-PDH activity in a time- and dose-related manner. The calcium antagonist D 600 (gallopamil) reduced the isoproterenol-elicited stimulation by 65%, and verapamil blunted the fenoterol-induced increase by 50%. This suggests that Ca2+ ions also contribute to the stimulation of the cardiac oxidative pentose phosphate pathway.

Properties of a potassium-selective ion channel in human melanoma cells

Currents through ion channels were measured from cells of a human melanin-producing melanoma cell line (IRG 1) with the patch clamp technique. In these cells the most frequently observed channel is a potassium channel. The channel activates slowly at depolarizing voltage steps but does not inactivate. Single channel potassium currents can be measured in cell-attached patches at the resting potential of melanoma cells. The channel has a conductance of approximately 10 pS. As measured from the reversal potentials of single channel currents, the permeability ratio for sodium and potassium, PNa/PK, is between 0.03 and 0.04. Open probability is increased at positive potentials. Mean open times are prolonged at voltage steps to more positive potentials. Closed time histograms are fitted by two exponentials. The slow shut time is decreased at positive potentials. In whole cell measurements, cell conductance measured between -20 and + 70 mV was reduced by 10 mM tetraethylammonium chloride from 6.4 +/- 1.2 nS (n = 4) to 0.8 +/- 0.3 nS (n = 3). Application of isoproterenol decreases the probability of the channel being open without any change in the single channel conductance. A possible role of the 10 pS potassium channel in the growth of melanoma cells is discussed.

Potentiation of gamma-aminobutyric acid-mediated inhibition by isoproterenol in the cerebellar cortex: Receptor specificity

A large body of electrophysiological data has supported the hypothesis that an important role of norepinephrine in the central nervous system is to modulate the actions of other transmitter systems, particularly those utilizing the amino acid neurotransmitters. Noradrenergic potentiation of inhibitory responses, induced by locally-applied or synaptically-released gamma-aminobutyric acid (GABA) on cerebellar Purkinje neurons, has been observed by a number of investigators, who have suggested that activation of beta-adrenergic receptors plays a critical role in mediating this modulatory effect of norepinephrine (NE). Two postsynaptic receptors for GABA, termed A and B, have been identified and both subtypes have been found in the cerebellum of the rat. The purposes of this investigation were first to identify the subtype(s) of GABA receptor responsible for mediating the inhibitory effects of locally-applied GABA in the cerebellar cortex and second to identify which subtype of GABA receptor is modulated by a beta-adrenergic input. Inhibitory responses of cerebellar Purkinje neurons, in urethane-anesthetized rats, to iontophoretic or pressure-applied isoguvacine, a selective GABA Aagonist, to baclofen, a GABA B agonist or to GABA itself, were examined before, during and after local application of isoproterenol or norepinephrine. Isoguvacine, but not baclofen, induced consistent and dose-dependent inhibition of the firing ofPurkinje cells. At ejection currents that had no effect on spontaneous firing rate, iontophoretically-applied isoproterenol potentiated isoguvacine-induced inhibition. These data suggest that GABA A, rather than GABA B receptors, mediate GABA-induced inhibitions of cerebellar Purkinje neurons. Moreover, it appears that the modulation of GABA function by beta adrenergic agonists involves an interaction between a beta-adrenergic input and the GABA A receptor complex.

Comparison of the effects of intra-arterial and locally applied vasoactive agents on pulpal blood flow in dog canine teeth determined by laser doppler velocimetry

The vasoactive agents norepinephrine, 5-hydroxytryptamine, isoproterenol and bradykinin, at concentrations which changed local arterial pressure without changing systemic arterial pressure significantly, were injected intra-arterially (i.a.) into the maxillary artery or applied directly in a deep dentinal cavity on the buccal surface of canine teeth. The probe of a laser Doppler velocimeter was placed in the cavity to monitor pulpal blood flow. Bolus i.a. injections of the vasoconstrictors norepinephrine and 5-hydroxytryptamine produced a statistically significant ( p < 0.05) reduction in pulpal blood flow, 21.1 ± 3.7 and 30.7 ± 15.2%, and the local arterial pressure from the lateral nasal artery increased with norepinephrine but decreased with 5-hydroxytryptamine. The i.a. injections of the vasodilators isoproterenol and bradykinin were found to decrease both local arterial pressure and pulpal blood flow, 17.7 ± 6.0 and 22.7 ± 4.2%, respectively ( p < 0.05). However, local application of isoproterenol and bradykinin caused a biphasic response: an increase in pulpal blood flow, 8.6 ± 1.6 and 9.4 ± 1.1% ( p < 0.05), followed by a decrease, 19.1 ± 11.9 and 5.3 ± 2.1% ( p > 0.005). Local application of norepinephrine and 5-hydroxytryptamine caused a decrease in pulpal blood flow, 23.7 ± 5.2% ( p < 0.05) and 9.3 ± 5.2% ( p > 0.05), respectively. These findings were in accordance with those from other reliable methods, such as injections of radioisotope-labelled 15 μm microspheres and the 133Xe washout technique, making laser Doppler flowmetry a reliable alternative. In addition, the biphasic response to the locally applied vasodilators and the reduction of pulpal blood flow after i.a. injection of vasodilators suggest that regulation of pulpal blood flow is determined by the combined effects of the low compliance environment and the stealing of perfusion to the pulp by the neighbouring tissues.

Influence of isoproterenol on myocardial energetics. Experimental and clinical investigations.

The influence of isoproterenol on myocardial performance and energetics was investigated in normal guinea pig myocardium and in patients with normal left ventricular function. The in vitro experiments were performed by simultaneous isometric force and heat measurements using sensitive antimony-bismuth thermopiles. Following the application of isoproterenol (10(-8) M) isometric peak twitch tension and tension-time integral increased significantly by 185% and 142%, respectively. Tension-independent heat which reflects high energy phosphate hydrolysis of excitation-contraction coupling increased by 183%. Tension-dependent heat reflecting the high energy phosphate hydrolysis of the crossbridges increased by 417%. The ratio of tension-dependent heat to tension-time integral increased by 131%. The recovery/initial heat ratio, reflecting the efficiency of the recovery metabolism, and the resting metabolism did not significantly change. In the patients the effect of isoproterenol on myocardial energetics was evaluated in terms of myocardial efficiency. Following isoproterenol administration, left ventricular systolic stress-time integral decreased by 49% due to reductions in end-diastolic pressure, end-diastolic volume and duration of systole. Pressure-volume work remained unchanged. Myocardial oxygen consumption per minute increased in proportion to heart rate. The ratio of myocardial oxygen consumption per beat to left ventricular systolic stress-time integral increased significantly by 95%. External myocardial efficiency was unaltered. Thus, isoproterenol increases the energy turnover of excitation-contraction coupling and increases the energy consumption of the crossbridges disproportionately to developed tension-time integral in the guinea pig heart.