Norepinephrine Dose-response Curves Research Articles

Inotropic responsiveness of the heart in catecholamine cardiomyopathy

Inotropic responsiveness to norepinephrine (NE) or Ca 2+ was studied in 15 rabbits with catecholamine cardiomyopathy (CM) and the results were compared with 11 controls. The former group was prepared by infusing NE (2 μg/min/kg) for 90 minutes, 2 to 3 days prior to study, and injury was confirmed and quantified histologically. Comparison of NE dose-response curves relating percent increase of left ventricular (LV) dP dt max to NE dose in six controls and 10 CM hearts revealed significant depression of CM contractility. Responses to graded infusions of Ca 2+ were also determined in five controls and in five CM animals. With the increases in serum Ca 2+ identical in the two groups, the percent increases in dP dt max were less in CM hearts. Further, NE dose-response curves obtained during continuous CaCl 2 infusion were sharply attenuated in CM hearts and initial LV end-diastolic pressure (LVEDP) in the CM hearts was considerably higher than in the controls. It is concluded that inotropic sensitivity to NE is reduced in CM hearts. This cannot be ascribed to altered β-receptors alone, because responsiveness to Ca 2+ was also reduced. Myocardial damage was indicated by histology and elevated LVEDP. Moreover, the normalized percent data suggest that the remaining viable fibers in the CM hearts were functioning subnormally and thus reduced performance was not due solely to the decreased number of contracting units.

Artificial buffers do not inhibit contractile responses in the smooth muscle of rat portal vein and guinea pig taenia coli.

Effects of substitution for NaHCO3 (and 5% CO2) in the physiological solution with equimolar N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) or morpholinopropane sulfonic acid (MOPS) (and 100% O2) at various pH levels on the contractility of smooth muscle were examined. At pH 7.4, spontaneous contraction in rat portal vein was not inhibited by the artificial buffer solutions, as compared to findings in the case of bicarbonate buffer solution. Norepinephrine-dose response curve in the case of rat portal vein and the histamine-dose response curve in cases of guinea pig taenia coli remained unchanged in the artificial buffer solutions. At pH 7.2 and 7.0, the spontaneous contraction in portal vein was reversibly inhibited either in artificial or in bicarbonate buffer solutions. The norepinephrine- and histamine-dose response curve shifted downwards and/or to the right in these low pH solutions. Thus, HEPES and MOPS had no inhibitory effect on the smooth muscle contractility. Since low pH strongly inhibited the contractility, attention should be directed to the temperature-dependent decreases in pKa of the artificial buffers (i.e., if pH of the solution is adjusted at room temperature and then warmed, pH decreases).

ARTIFICIAL BUFFERS DO NOT INHIBIT CONTRACTILE RESPONSES IN THE SMOOTH MUSCLE OF RAT PORTAL VEIN AND GUINEA PIG TAENIA COLI

AbstractEffects of substitution for NaHCO3 (and 5% CO2) in the physiological solution with equimolar N-2-hydroxyethylpiperazine-N’-2-ethanesulfonic acid (HEPES) or morpholinopropane sulfonic acid (MOPS) (and 100% O2) at various pH levels on the contractility of smooth muscle were examined. At pH 7.4, spontaneous contraction in rat portal vein was not inhibited by the artificial buffer solutions, as compared to findings in the case of bicarbonate buffer solution. Norepinephrine-dose response curve in the case of rat portal vein and the histamine-dose response curve in cases of guinea pig taenia coli remained unchanged in the artificial buffer solutions. At pH 7.2 and 7.0, the spontaneous contraction in portal vein was reversibly inhibited either in artificial or in bicarbonate buffer solutions. The norepinephrine- and histamine-dose response curves shifted downwards and/or to the right in these low pH solutions. Thus, HEPES and MOPS had no inhibitory effect on the smooth muscle contractility. Since low pH strongly inhibited the contractility, attention should be directed to the temperature-dependent decreases in pKa of the artificial buffers (i.e., if pH of the solution is adjusted at room temperature and then warmed, pH decreases).

Effects of reserpinization, surgical denervation and in vitro chemical denervation with 6-hydroxydopamine on the contractile response of isolated rabbit ear artery to propranolol.

The contractile effect of propranolol in isolated rabbit ear artery was assessed in reserpinized and in surgically and chemically denervated blood vessels. Reserpinization and surgical denervation either had no effect on or enhanced the ear artery contractile response to 10(-6) to 10(-4) M propranolol. In contrast, the contractile response to propranolol was nearly abolished after denervation of the ear artery in vitro with 6-hydroxydopamine (6-OHDA). Dose-response curves to norepinephrine were shifted to the left by factors of 4.9 in 6-OHDA denervated ear arteries and 15.6 in untreated arteries in the presence of 10(-7) M desipramine. The diluent for 6-OHDA shifted both the norepinephrine and propranolol dose-response curves to the right. It is proposed that propranolol caused a contractile response in ear artery by an action on the postsynaptic tissues of this vessel. 6-OHDA denervation caused nonspecific desensitization in rabbit ear artery leading to the loss of response of this vessel to propranolol.

Reversal of corticosterone-induced supersensitivity of vascular smooth muscle to noradrenaline by arachidonic acid and prostacyclin

In the isolated perfused hindlimb preparation of rats treated with corticosterone (2 × 20 mg/kg daily for 2 days), the dose-response curve to noradrenaline was shifted to the left, indicating supersentivity of the vascular bed to noradrenaline. Perfusion with arachidonic acid (10 −5 M) and prostacyclin (10 −9 M) for 5 min reversed the supersensitivity induced by corticosterone. The metabolite of prostacyclin, 6-keto PGF 1α(10 −9 M), was ineffective in this respect. In rats which had received desoxycorticosterone acetate (2 × 5 mg/kg daily for 7 days), there was supersenstivity of the hindlimb preparation to noradrenaline similar to that in corticosterone-treated rats. In that case, however, administration of arachidonic acid not reverse the leftward shift of the dose-response curve. Administration of indomethacin (2 × 2.5 mg/kg for 7 days) prior to the perfusion experiment also resulted in a shift of the noradrenaline dose-response curve to the left, which was less pronounced than the shift induced by cortiscosterone. Combined administration of corticosterone and indomethacin caused the same increase in noradrenaline sensitivity as did corticosterone alone. Since glucocorticoids inhibit the release of arachidonic acid from phospholipids, it is concluded that corticosterone may enhance the sensitivity to noradrenaline by affecting the biosynthesis of prostaglandins.

Significance of sodium, sympathetic innervation, and central adrenergic structures on renal vascular responsiveness in DOCA-treated rats.

The effects of sodium, sympathetic innervation, and central adrenergic structures on the development of changes in renal vascular reactivity were studied in unilaterally nephrectomized rats treated with a single implant of deoxycorticosterone acetate (DOCA; 100 mg/kg). Vascular reactivity to norepinephrine (NE) and vasopressin (ADH) was assessed in isolated kidneys perfused with a synthetic medium. Influence of sodium was determined by placing DOCA-treated rats on high, normal, and low sodium intakes. Neural influence was studied by means of local denervation of the renal artery and by intravenous (iv) and intraventricular (ivt) administration of 6-hydroxydopamine (6-OHDA). Marked changes in renal vascular reactivity in DOCA-treated rats were already apparent prior to the rise in blood pressure. Dose-response curves for NE and ADH showed parallel leftward shifts and decreased threshold doses. Normal sodium intake, local denervation, and peripheral sympathectomy had no effect on the development of these vascular changes in DOCA-treated rats. However, sodium deficiency and ivt administration of 6-OHDA totally prevented development of enhanced vascular reactivity. These results imply that increased vascular reactivity is a major factor in the development of DOCA-hypertension.

Modifications by lidocaine and its N-dealkylated metabolites of the response of the isolated rabbit aorta to transmural electrical stimulation.

Contractile responses of rabbit aortic strips to transmural stimulation and to exogenously applied norepinephrine (NE), potassium chloride (KCl), and histamine were studied in the presence of lidocaine or its metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX). Lidocaine, 10(-4) and 5 x 10(-4) M, attenuated the contractile response to transmural stimulation, whereas MEGX, 2 x 10(-5) and 10(-4) M, potentiated, but 10(-4) M, potentiated the response to transmural stimulation. The suppression induced by lidocaine and MEGX was not reversed by excess calcium, 2.2 and 4.4 mM, but was partially reversed by cocaine, 3 x 10(-6) M. Lidoncaine, 5 x 10(-4) M, and MEGX, 2 x 10(-3) M, shifted the dose-response curve of NE to the right, whereas GX, 5 x 10(-4) M, shifted the curve to the left. The maximum tension developed by K+ was attenuated by lidocaine, 5 x 10(-4) M, MEGX, 5 x 10(-4) and 2 x 10(-3) M, and GX, 2 x 10(-3) M. It may be concluded that lidocaine attenuates the response to stimulation of sympathetic nerves innervating the arterial wall by interfering with the release of NE. In contrast, the metabolites, MEGX and GX, potentiate the response, possibly by increasing the release of NE. MEGX in high concentrations appears to have the same mechanism of inhibitory action as does lidocaine.

Evaluation of long-term chemical sympathectomy in adult rabbits

Administration of 6-hydroxydopamine (6-OHDA) will produce long-term sympathectomy in newborn animals. This investigation was designed to determine whether or not a long-term sympathectomy can be achieved by repeated administration of 6-OHDA in adult rabbits. Chronic treatment with 6-OHDA lowered blood pressure on average of 9 mmHg; the carotid sinus reflex was depressed, in contrast to the pressure response to intravenously administered epinephrine, which was doubled. In a constant-flow hindlimb preparation, the response to norepinephrine (NE) in 6-OHDA-treated rats was 50% larger and lasted 3 times longer. NE uptake in vitro, which is proportional to the number of adrenergic nerve endings, was found to be diminished by 80-85% in aortas from 6-OHDA-treated animals, and the dose-response curve for NE was slightly shifted to the left. The number of ganglionic cells in the superior cervical ganglia in treated animals was decreased by 80%. In conclusion, results from 6-OHDA-treated adult animals were entirely consistent with the effects of long-term sympathectomy.

Direct and indirect actions of dopamine on tracheal smooth muscle.

The effects of dopamine (DA) were studied on guinea-pig isolated tracheal chains. At a low concentration (10−6M) DA occasionally produced a small contraction; this was followed by a dose-dependent relaxation 3×10−6–3×10−3 M). On a molar basis, DA was about 40 times less potent than noradrenaline (NA) in relaxing tracheal chains, and about 2700 times less potent than isoprenaline (ISO). The maximum degree of relaxation obtained with each drug was the same. Pretreatment of the guinea-pig with reserpine (5 mg/kg) resulted in a 3-fold shift of the DA curve to the right without concomitantly affecting the ISO doseresponse curve. Reserpine completely abolished the relaxant effects of tyramine, but a small contractile response remained. Desipramine (DMI), at a concentration of 10−5 M, caused a 4-fold shift of the DA curve to th right. The same concentration of DMI resulted in a shift to the left of the NA dose-response curve by about 8-fold. Benztropine (10−5 M) and haloperidol (10−5 M and 3×10−5 M) did not affect the DA dose-response curve. The DA-induced relaxation was inhibited by propranolol (10−8–10−6 M) in a dose-dependent manner. The higher concentrations of propranolol (10−7 and 10−6 M) unmasked the contractile effect of DA. In the presence of propranolol, phentolamine (10−5 M) abolished the contractile effect of DA. It is concluded that DA has both direct and indirect actions on guinea-pig isolated tracheal smooth muscle. The relaxant effects of DA are predominantly due to a direct action on smooth muscle β-adrenoceptors, with a component due to release of NA from adrenergic nerves. The contractile effects, which under normal conditions are masked by the relaxant effect of DA, are mediated by functional α-adrenoceptors. There is no evidence for either specific dopaminergic nerves, uptake mechanisms or receptors in guinea-pig trachealis muscle.

Sulpiride: A weak antagonist of norepinephrine and 5-hydroxytryptamine

Antagonism of norepinephrine (NE) and 5-hydroxytryptamine (5-HT) by sulpiride was studied in vitro on rabbit aortic strips. Concentrations of sulpiride ranging from 3 × 10 −5 to 3 × 10 −3 M caused a progressive shift of the dose response curves of both NE and 5-HT to the right without inhibiting the responses to KCl. pA 2 values of sulpiride calculated from Schild plots against NE and 5-HT were 4.6 and 4.4, respectively. Thus, sulpiride, which has been previously shown to be the most potent antagonist of the dopamine vascular receptors, is a very weak α-adrenergic or 5-HT antagonist.

α- AND β-ADRENOCEPTORS IN THE DETRUSOR MUSCLE AND BLADDER BASE OF THE PIG AND β-ADRENOCEPTORS IN THE DETRUSOR MUSCLE OF MAN

1 The presence and type of adrenoceptors in the smooth muscle of the pig and human urinary bladder was assessed on the basis of the relative potency of alpha- and beta-adrenoceptor agonists and antagonists.2 In isolated, carbachol-contracted bladder strips from the pig detrusor muscle the relaxing potency of isoprenaline was four times that of salbutamol and ritodrine and thirty times that of noradrenaline.3 Propranolol caused a parallel shift to the right of the noradrenaline dose-response curve which was not changed by phentolamine.4 Propranolol and butoxamine showed, in contrast to practolol, a dose-dependent antagonism of the response to isoprenaline. A pA(2) value of 9.2 +/- 0.2 and 6.8 +/- 0.2 (mean +/- s.e. mean) for the first two antagonists was calculated.5 In the bladder base of the pig, propranolol caused a parallel shift to the right and phentolamine a shift to the left of the dose-response curve to noradrenaline.6 In the human detrusor muscle the potency and maximum effect of isoprenaline and salbutamol were less than those in the pig detrusor muscle. The potency of isoprenaline was sixty times that of salbutamol.7 Whereas a parallel shift to the right of the dose-response curve to isoprenaline was obtained with propranolol, no antagonism was obtained with butoxamine or practolol.8 The results are interpreted as indicating the presence of beta(2)-adrenoceptors in the detrusor muscle of the pig and beta-adrenoceptors with neither beta(1)- nor beta(2)-characteristics in the detrusor muscle of man. An indication of the presence of alpha-adrenoceptors in the bladder base but not in the detrusor muscle of the pig was obtained.

Direct evidence that the greater contractility of resistance vessels in spontaneously hypertensive rats is associated with a narrowed lumen, a thickened media, and an increased number of smooth muscle cell layers.

The mechanical and morphological properties of segments of certain precisely defined resistance vessels (approximately 150 micrometer lumen diameter) in the mesenteric bed of spontaneously hypertensive (SHR) and normotensive (WKY) rats have been compared in vitro under carefully controlled conditions and also after fixation. At a given transmural pressure, the relaxed SHR vessels (compared with the WKY vessels) would have had a 16% smaller lumen diameter (P less than 0.01) and a 49% thicker media (P less than 0.005), so that the media volume per unit segment length was 31% greater (P less than 0.05). The smooth muscle cells were arranged circumferentially in about four layers in the SHR vessels and in about three layers in the WKY vessels. The SHR active wall tension in response to potassium was 53% greater (P less than 0.02) and to norepinephrine was 50% greater (P less than 0.01) than for WKY. However, the ED50 values for the norepinephrine dose-response curves were similar (approximately 5 micrometer). Activation with potassium plus norepinephrine gave greater responses in both vessel types, than with either agent alone, but the SHR responses were on average only 19% greater than the WKY (P less than 0.10). However, under these conditions, the SHR vessels would have been able to contract against 45% greater transmural pressures (P less than 0.001) because of their smaller lumen. On maximal activation, the mean force developed by each cell (approximately 3.85 micro N) was the same in both vessel types, even though on average (P = 0.10) the SHR cells had a 21% greater cross-sectional area. The results support the Folkow hypothesis that in genetic hypertension the increased peripheral resistance is associated with structural changes in the resistance vessels.

Sodium cyanide antagonism of the vasodilator action of sodium nitroprusside in theisolated rabbit aortic strip.

Resistance to sodium nitroprusside (SNP) is uncommon, but its occurrence has led to massive overdoses of SNP and sometimes death. To examine the mechanism responsible for resistance, aortic smooth muscle strips were prepared and dose-response curves for norepinephrine (NE) obtained. SNP alone caused a shift of the dose-response curve for NE to the right. However, this shift was less when the strips were exposed to both SNP and sodium cyanide (CN-). When CN- alone was added to the aortic strips, the response to NE was unchanged. In a further group of aortic muscle strips first contracted with NE and then relaxed with SNP, the addition of CN- caused the muscles to contract again. It is concluded that CN- antagonizes the action of SNP in vitro, and that this antagonism is specific for SNP.

ALPHA-ADRENERGIC BLOCKING ACTION OF FENTANYL ON THE ISOLATED AORTA OF THE RABBIT

The contractile response of helically-cut strips of rabbit ascending aorta to transmural electrical stimulation was attenuated in a dose-dependent manner by treatment for 20 min with fentanyl, 10(-6) to 10(-5) M. Fentanyl also shifted the dose-response curve of the contractile response of aorta to norepinephrine to the right. The response to transmural stimulation was more resistant to fentanyl than was the response to an equipotent dose of norepinephrine. The inhibitory effect of fentanyl was neither prevented nor reversed by naloxone, but was partially reversed by repeated washing of the preparations. The contractile responses to histamine and serotonin were not significantly altered by fentanyl. Treatment with fentanyl as well as phentolamine protected alpha-adrenergic receptors from persistent blockade by phenoxybenzamine. Morphine to 10(-3) M failed to influence the dose-response curve of norepinephrine significantly. It may be concluded that fentanyl reversibly blocks alpha-adrenergic receptors in a competitive manner in vascular smooth muscle, and the potency of fentanyl is approximately 1/30 that of phentolamine.

Specific α-Adrenergic Receptor Desensitization in Vascular Smooth Muscle

Specific desensitization of vascular tissue to norepinephrine was studied using an open-ring-type strip preparation of rabbit aorta. Aortae were taken from untreated, reserpine-pretreated (0.1 mg/kg/day; 3, 5, or 7 days) and 6-hydroxydropamine-pretreated (50 mg/kg; 3 or 7 days before sacrifice) rabbits. Aortic strips from the pretreated rabbits were supersensitive to norepinephrine. Subsequent to obtaining the first dose-response curve for norepinephrine, aortic strips were less sensitive to norepinephrine. It was found that desensitization to norepinephrine occurred following 10 min preincubation with 10(-6)m norepinephrine, and 20 min preincubation with 10(-7)m norepinephrine. The desensitization of supersensitive strips was significantly greater than in controls. Dopamine and isoproterenol desensitized both untreated and pretreated strips to norepinephrine. Phenylephrine desensitized strips to norepinephrine from 6-hydroxydopamine-pretreated rabbits. Acetylcholine and potassium did not desensitize strips to norepinephrine; however, acetylcholine desensitized strips to acetylcholine. It is concluded from these studies that norepinephrine desensitization in a specific alpha-adrenergic-related event.

Influence of indapamide on isolated rabbit arteries (author's transl)

The effect of indapamide, a high ceiling diuretic, antihypertensive drug, was investigated in helically-cut strips of rabbit cerebral, coronary, renal, mesenteric and femoral arteries and aortae. The addition of indapamide in concentrations higher than 3 X 10(-5)M relaxed those arterial strips contracted with prostaglandin F2alpha; the relaxation being greater in cerebral, coronary and renal arteries. Atropine propranolol and aminophylline did not reduce the relaxing effect. Hydrochlorothiazide produced fewer incidences of relaxation. Treatment for 20 min with indapamide (3 X 10(-5) or higher) significantly attenuated the contractile response of mesentric arteries to nicotine and tyramine. Contractile responses to transmural electrical stimulation were not reduced but rather potentiated. The dose-response curve of norepinephrine was not significantly affected by indapamide. The contractile response of nictitating membranes of anesthetized cats to stimulation of preganglionic sympathetic nerves was not influenced by indapamide but was abolished by hexamethonium. It may be concluded that arterial relaxations induced by indapamide are due to a direct action on smooth muscles. Indapamide appears to attenuate the response to nicotine and tyramine by interfering with the mechanism of neuronal amine uptake. However, such does not involve a ganglionic blockade nor a reduction of the release of norepinephrine from adrenergic nerve terminals.

Relationship between Noradrenaline-Induced Depolarization and Contraction in Vascular Smooth Muscle

In strips of rabbit main pulmonary artery, full dose-response curves of noradrenaline for the mechanical and electrical responses of the vascular smooth muscle cells were obtained under normal conditions and after increasing and decreasing the membrane potential with strychnine and tetraethylammonium, respectively. The results suggest that the magnitude of the noradrenaline-induced contraction is related to the height of the membrane potential and not to the degree of depolarization produced by the adrenergic transmitter.

Adenylate cyclase activity in corpus striatum of rats with porto-caval anastomosis.

Altered catecholamine receptor sites within the striatum have been proposed to be an important pathogenetic factor in hepatic and porto-systemic encephalopathy and coma. The unstimulated, fluoride-, norepinephrine- and dopamine-stimulated adenylate cyclase activity were measured in the corpus striatum of rats with a four weeks old end-to-side porto-caval anastomosis. There was no difference in unstimulated, fluoride- or hormone-stimulated adenylate cyclase activity between porto-caval shunted and sham-operated rats. The in vitro dose-response curves of norepinephrine and dopamine were similar in both groups of animals. Half-maximum and maximum stimulation were achieved in shunted and sham-operated rats by identical concentrations of norepinephrine and dopamine, respectively. The results indicate that neither changes in unstimulated adenylate cyclase activity nor changes in the response of adenylate cyclase activity to fluoride, norepinephrine and dopamine had developed in the rats at the stage studied.

Selectivity of blocking agents for pre-and postsynaptic alpha-adrenoceptors.

1. Low frequency (0.1 Hz) electrical stimulation of the rat isolated vas deferens produced regular contractions that were inhibited by low concentrations of clonidine. 2. The inhibition of the vas deferens produced by clonidine was presynaptic in origin and involved alpha-adrenoceptors. 3. Presynaptic alpha-adrenoceptor antagonist activity was assessed by studying the effects of increasing concentrations of the antagonists on cumulative clonidine dose-response curves on the stimulated vas deferens. 4. Postsynaptic alpha-adrenoceptor antagonist activity was assessed by comparison of control cumulative noradrenaline dose-response curves with those in the presence of increasing concentrations of antagonists in the rat anococcygeus muscle. 5. The results indicate that yohimbine and phentolamine are more potent in blocking presynaptic than postsynaptic alpha-adrenoceptors. Phenoxybenzamine and prazosin block postsynaptic alpha-adrenoceptors preferentially. 6. The findings support the view that presynaptic and postsynaptic alpha-adrenoceptors differ in their sensitivity to alpha-adrenoceptor antagonists.

RECIPROCAL INHIBITION OF CARDIAC RESPONSES TO NORE-PINEPHRINE AND INSULIN

The effects of insulin (IN) and norepinephrine (NE) on left ventricular contractility (LVc) were studied in 20 piglets. Aortic pressure, cardiac output, heart rate and temperature were controlled, and LV dP/dt max was taken to reflect LVc. Hypoglycemia was prevented by giving glucose as necessary. LVc increased 28% (p<.02) 45 min. after IN (20U). Following β-blockade (practolol, 1 mg/kg), in caused a 55% increase in LVc. NE dose-response (DR) curves were obtained before and after IN. Maximal changes in LVc with each dose of NE (0.5-2.0μg) were significantly less in hearts exposed to IN. Duplicate DR curves were identical, indicating that prior exposure to NE did not inhibit subsequent responses. However, in those piglets initially subjected to a NE dose-response curve, IN elicited only an 11% increase of LVc, and this was not significant. Thus, the inotropic response was attenuated (p<.01) in animals previously exposed to NE, Conversely, in animals first subjected to β-blockade, the response to IN was greater (p<.005). These data indicate a reciprocal blocking action by these hormones on piglet myocardium. The cellular mechanisms responsible for these findings remain to be elucidated, but likely involve modulation of Ca++ fluxes.