Cardiac alpha- and beta-adrenoceptor sensitivity and binding characteristics after chronic reserpine pretreatment

Effects of an n-butanol extract from the stem of Tinospora crispa on blood pressure and heart rate in anesthetized rats

ETHNOPHARMACOLOGICAL RELEVANCE

Tinospora crispa has been used in folkloric medicine for control of blood pressure, as an antipyretic, for cooling down the body temperature and for maintaining good health.

AIM OF THE STUDY

To investigate the effects and mechanisms of action of an n-butanol extract from the stems of Tinospora crispa (T. crispa extract) on blood pressure and heart rate in anesthetized rats.

MATERIALS AND METHODS

Air-dried stems of T. crispa were extracted with water, followed by partitioned extract with chloroform, ethyl acetate, and finally by n-butanol. The n-butanol soluble part was evaporated under reduced pressure and lyophilization to obtain a crude dried powder (T. crispa extract). The effects and mechanisms of the T. crispa extract on blood pressure and heart rate were studied in anesthetized normal and reserpinized rats in vivo in the presence of different antagonists.

RESULTS

T. crispa extract (1-100 mg/kg, i.v.) caused a decrease in mean arterial blood pressure (MAP) and this effect was inhibited by propranolol, phentolamine, atenolol and/or the β(2)-antagonist ICI-118,551, but not by atropine or hexamethonium. In reserpinized rats, the T. crispa extract had a dual effect: reduction in hypotensive activity, followed by a small increase in blood pressure. The decrease in MAP in reserpinized rat was slightly potentiated by phentolamine, but inhibited by propranolol or ICI-118,551 only if atenolol and phentolamine were also present. The increase in MAP was potentiated by propranolol and ICI-118,551, but was inhibited by phentolamine. The T. crispa extract had a dual effect on heart rate in the normal rat: a small transient decrease, followed by an increase in heart rate. The positive chronotropic effect of T. crispa extract was inhibited by propranolol, phentolamine and atenolol, but not by ICI-118,551, atropine or hexamethonium. Reserpine potentiated the positive chronotropic effect of the T. crispa extract and this effect was inhibited by propranolol, atenolol and ICI-118,551, but not by phentolamine.

CONCLUSIONS

From these results we suggest that T. crispa extract possesses at least three different cardiovascular-active components that act directly via (1) β(2)-adrenergic receptors to cause a decrease in blood pressure, and β(1)- and β(2)-adrenergic receptors to cause an increase in heart rate, (2) α-adrenergic receptors to cause an increase in blood pressure and heart rate, and (3) a non-adrenergic and non-cholinergic pathway to cause a decrease in MAP and heart rate. These findings provide scientific support for the tradition of using this plant to modify the actions of the human cardiovascular system.

Different response of ANP secretion to adrenoceptor stimulation in renal hypertensive rat atria

Sympathetic nervous system and atrial natriuretic peptide (ANP) system play fundamental roles in the regulation of cardiovascular functions. Overactivity of sympathetic nervous system can lead into cardiovascular diseases such as heart failure and hypertension. The present study aimed to define which adrenergic receptors (ARs) affect atrial contractility and ANP release and to determine their modification in renal hypertensive rat atria. An alpha(1)-AR agonist, cirazoline increased ANP release with positive inotropism. These alpha(1)-AR agonist-mediated responses were attenuated by the alpha(1A)-AR antagonist, but not by the alpha(1B)- or alpha(1D)-AR antagonist. An alpha(2)-AR agonist, guanabenz and clonidine increased ANP release with negative inotropism and decreased cAMP level. The order of potency for the increased ANP release was cirazoline>>phenylephrine=guanabenz>>clonidine. In contrast, a beta-AR agonist, isoproterenol decreased ANP release with positive inotropism and these responses were blocked by the beta(1)-AR antagonist but not by the beta(2)-AR antagonist. The increased cAMP level by isoproterenol was suppressed by pretreatment with both beta(1)- and beta(2)-AR antagonists. In renal hypertensive rat atria, the effects of isoproterenol on atrial contractility, ANP release, and cAMP level were attenuated whereas the effect of cirazoline on ANP release was unaltered. Atrial beta(1)-AR mRNA level but not alpha(1A)-AR mRNA level was decreased in renal hypertensive rats. These findings suggest that alpha(1A)- and beta(1)-AR oppositely regulate atrial ANP release and that atrial beta(1)-AR expression/function is impaired in renal hypertensive rats.

Cardiovascular effects of an n-butanol extract from fresh fruits of Randia siamensis

Randia siamensis is used in Thai folklore medicine for inducing abortion and controlling blood pressure. The present study investigated the cardiovascular effects of an R. siamensis fruit extract, and mechanisms involved in anesthetized normal and reserpinized rats. R. siamensis (0.4-12 mg/kg) i.v. increased the mean arterial pressure (MAP) and heart rate. Both effects were significantly inhibited by phentolamine (2 mg/kg, i.v.) or propranolol (0.6 mg/kg, i.v.). The combination of phentolamine and propranolol, or reserpine pretreatment, inhibited the positive chronotropic effect with a slight decrease in the MAP. In vitro, R. siamensis (0.001-0.3 mg/ml) increased the rate of beating of the right atrium and the strength of the electrical field-stimulated contraction of the left atrium, both effects were inhibited by propranolol, or with reserpine pretreated rats. R. siamensis (0.01-3 mg/ml) produced a contraction of isolated thoracic aorta, which was potentiated by N(G)-nitro-L-arginine (LNA), or by removal of the vascular endothelium, but inhibited by phentolamine, or reserpine. R. siamensis (0.3-3 mg/ml) caused a relaxation of phenylephrine-preconstricted aortic rings, which was potentiated with reserpine pretreatment, and abolished after removal of the vascular endothelium, or in the presence of LNA. These results suggest that R. siamensis extract exerts both hypertensive and positive chronotropic effects via the alpha- and beta-adrenergic receptors of blood vessels and the heart, due to release of endogenous catecholamines, likely from nerve ending and adrenal medulla. The hypotensive activity results from the release of nitric oxide causing dilatation of the blood vessels. The present data support the folklore therapeutic uses of this plant.

Alpha-1 adrenoceptor up-regulation induced by prazosin but not KMD-3213 or reserpine in rats

1. We have investigated the effects of chronic administration of prazosin (a subtype-nonspecific alpha-1 AR antagonist), KMD-3213 (an alpha-1A AR subtype-specific antagonist) and reserpine (a catecholamine depletor) on the density of alpha-1 AR subtypes in various rat tissues (liver, kidney, submaxillary gland, heart and spleen). 2. Administration of prazosin (2 mg kg(-1) day(-1), i.p.) for 2 weeks did not affect K(D) values for [(3)H]-prazosin or [(3)H]-KMD-3213 of alpha-1 ARs in five rat tissues tested. However, it caused 52% up-regulation of alpha-1B AR in the spleen, and 84% and 107% up-regulation of alpha-1A- and alpha-1B ARs, respectively, in the heart. Although major subtypes of alpha-1 AR are alpha-1A AR in the submaxillary gland, alpha-1B AR in the liver, and alpha-1A and alpha-1B ARs in the kidney, these tissues showed no up-regulation. The mRNA levels of alpha-1 AR subtypes were not affected by prazosin administration in any tissue tested. 3. Neither administration of KMD-3213 (2 mg kg(-1) day(-1), i.p.) nor reserpine (0.5 - 1 mg kg(-1) day(-1), i.p.) for 2 weeks caused any change in either the binding affinity for [(3)H]-prazosin or [(3)H]-KMD-3213 or the density of the alpha-1 AR subtypes in the five rat tissues. 4. Neither prazosin nor KMD-3213 treatment reduced the noradrenaline content in the five rat tissues, in contrast to reserpine treatment, which markedly reduced it. 5. The findings of the present study demonstrated that up-regulation of alpha-1 AR is selectively caused by prazosin treatment in some tissues but neither by KMD-3213 treatment nor by chemical denervation with reserpine. These results suggest that up-regulation of alpha-1 ARs is not caused by a simple blockade of sympathetic tone.

Functional analysis of desensitization of the beta-adrenoceptor signalling pathway in rat cardiac tissues following chronic isoprenaline infusion

1. This study examined beta-adrenoceptor signalling in cardiac tissues following infusion of isoprenaline (400 microg kg(-1) h(-1)) or vehicle to rats for 14 days. 2. Isoprenaline infusion caused marked hypertrophy of atria and ventricles and reduced the resting rate of spontaneously beating right atria and the basal force of left atrial contraction. 3. In spontaneously beating right atria, concentration-response curves to isoprenaline and forskolin were shifted 7.9 and 3.2 fold to the right following treatment whereas responses to the cyclic AMP analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphorothioate were unchanged. 4. In electrically driven left atria, concentration-response curves to isoprenaline and forskolin were shifted 4 fold to the right and maximum responses reduced. Responses to dibutyryl cyclic AMP were shifted 3.2 fold to the right but those to Ca2+ were unchanged. 5. Inotropic responses of left and right ventricular papillary muscles to isoprenaline were abolished and markedly reduced respectively by isoprenaline treatment. Responses to forskolin were shifted 5 fold to the right. Responses to dibutyryl cyclic AMP were shifted to the right 3.2 and 2 fold in left and right ventricular papillary muscles. Responses to isobutyl methyl xanthine were shifted to the right 15.8 and 6.3 fold in left and right papillary muscles whereas those to Ca2+ were not significantly altered. 6. This study indicates differences in beta-adrenoceptor desensitization in different regions of the heart following chronic infusion of isoprenaline. Chronotropic responses showed impaired signalling between the receptor and adenylate cyclase whereas inotropic responses exhibited additional desensitization at the level of cyclic AMP dependent protein kinase.

Differential regulation of cardiac alpha- and beta-adrenoceptors by the sympathetic nervous system

1. The effects of depressed sympathetic function on the inotropic responses of the heart to sympathomimetic amines have been examined in rats immunized against nerve growth factor (NGF) 6 weeks prior to the isolation of cardiac tissues. 2. The activity of tyrosine hydroxylase in cervical sympathetic ganglia and the levels of noradrenaline in ventricular tissue were significantly reduced in NGF-immunized rats. 3. Left atria and papillary muscles from NGF-immunized rats were supersensitive to the beta-adrenoceptor agonist isoprenaline when compared with controls. 4. The responses of cardiac tissues to the alpha-adrenoceptor agonist phenylephrine were unaffected by immunization. 5. These results support the hypothesis that cardiac beta- but not alpha-adrenoceptor sensitivity is regulated by the sympathetic nervous system.

Potentiation of alpha-adrenoceptor-mediated responses following chronic beta-adrenoceptor stimulation in the rat heart

1. Noradrenaline and isoprenaline were infused subcutaneously in rats by use of Alzet osmotic minipumps. The effects of catecholamine infusion on ventricular alpha- and beta-adrenoceptor density and also the responses of isolated cardiac tissues were compared with saline infusion. 2. Noradrenaline (1 mg kg-1) or isoprenaline (40 micrograms kg-1) infused for 3 days resulted in a desensitization of beta-adrenoceptor-mediated responses of isolated left atria and papillary muscles. Concentration-response curves to isoprenaline were shifted to the right in left atria whilst maximum responses were reduced in papillary muscles. Right atrial rate responses were not affected by infusions of catecholamines. 3. Infusions of either noradrenaline or isoprenaline resulted in a supersensitivity of alpha-adrenoceptor-mediated responses in isolated papillary muscles with leftward displacements of concentration-response curves to phenylephrine. 4. The density of both ventricular [3H]-dihydroalprenolol and [3H]-prazosin binding sites was reduced following noradrenaline infusion. Isoprenaline infusion reduced only the density of [3H]-dihydroalprenolol binding sites. 5. Noradrenaline infusion therefore 'down-regulates' both alpha- and beta-adrenoceptors in the rat heart but at the same time ventricular alpha-adrenoceptor-mediated responses are enhanced. Isoprenaline similarly enhances responses to phenylephrine and possible mechanisms for this phenomenon are discussed.

Transient elevation of cardiac beta-adrenoceptor responsiveness and receptor number in the streptozotocin-diabetic rat

1. The effects on cardiac responsiveness of diabetes of up to 12 weeks duration has been examined in streptozotocin-pretreated rats. 2. Two weeks of diabetes resulted in a supersensitivity of isolated left atria and papillary muscles to isoprenaline, which was associated with an increase in the density of ventricular [3H]-dihydroalprenolol binding sites. 3. The beta-adrenoceptor supersensitivity was still evident in both tissues after 4 weeks of diabetes, but in left atria the supersensitivity was reduced compared with that observed at 2 weeks, while for papillary muscles it was greater than at 2 weeks. 4. Following 12 weeks of diabetes, responses of papillary muscles to isoprenaline were similar to controls, while beta-mediated responses of left atria were significantly depressed. 5. The alpha-adrenoceptor-mediated responses of cardiac tissues to phenylephrine were similar to controls following diabetes of 2 or 4 weeks duration. At 12 weeks, however, papillary muscle alpha-adrenoceptor-mediated responses were enhanced. The change in ventricular responsiveness to phenylephrine was not accompanied by any change in [3H]-prazosin binding to ventricular membranes. 6. The results demonstrate a transient elevation of cardiac beta-adrenoceptor sensitivity and receptor density during acute diabetes and illustrate the time- and tissue-dependence of diabetes-induced changes in cardiac adrenoceptor sensitivity.

Diabetes-induced changes in cardiac beta-adrenoceptor responsiveness: effects of aldose reductase inhibition with ponalrestat

1. The responses of isolated left atria and papillary muscles to isoprenaline, forskolin and calcium have been examined in 3 week streptozotocin-diabetic rats and the effects of oral ponalrestat administration (25 mg kg-1 daily) on diabetes-induced changes in cardiac responsiveness investigated. 2. Three weeks after animals were made diabetic, cardiac responses to isoprenaline were enhanced and this was accompanied by an increase in the density of ventricular [3H]dihydroalprenolol binding sites. Treatment of animals with ponalrestat prevented the increase in cardiac beta-adrenoceptor responsiveness and receptor number. 3. Diabetes also enhanced the sensitivity of cardiac tissues to forskolin, an effect that was not prevented by the treatment of animals with ponalrestat. 4. Ponalrestat treatment increased the resting and maximum tensions developed by cardiac tissues from diabetic animals and increased the maximum tensions developed by tissues from control animals. Diabetes alone had no effect on resting or maximum developed tensions. 5. Ponalrestat therefore prevents the changes in beta-adrenoceptor density and responsiveness induced by short-term diabetes in the rat and also increases the tension developed by cardiac muscle, an effect observed in diabetic and normal animals.

Evidence from lack of decentralization-induced supersensitivity that beta 2-adrenoceptors of the cat nictitating membrane are non-innervated

The right superior of cervical sympathetic trunk of cats was sectioned preganglionically under anaesthesia. Six days later the blood pressure, heart rate and contractions of the left (control) and right (decentralized) nictitating membranes were recorded under chloralose anaesthesia (80 mg kg-1). The alpha-adrenoceptor-mediated contractile responses of the nictitating membrane to intravenous adrenaline were greater on the decentralized side than the control side, with a significant shift of the dose-response curve to the left. After phentolamine (8 mg kg-1 i.v.), adrenaline administered intra-arterially exerted beta-adrenoceptor-mediated relaxation of the nictitating membranes. However, there was no difference in the sensitivity or magnitude of responses between decentralized and control sides. In a separate series of experiments, the alpha-adrenoceptor-mediated contractile responses of the nictitating membrane to intra-arterial noradrenaline displayed supersensitivity on the decentralized side, the dose-response curve being significantly shifted to the left. In the same animals, the beta-adrenoceptor-mediated relaxation responses to intra-arterial isoprenaline were non-significantly greater on the decentralized side, presumably because of raised tone. However, when expressed as a percentage of the maximum relaxation, there was no difference in sensitivity. This study shows that the alpha-adrenoceptor-mediated contractile response of the nictitating membrane displays supersensitivity after preganglionic section of the sympathetic innervation. This is presumably because of an up-regulation arising from loss of sympathetic traffic onto the receptor. The relaxation response is mediated via adrenoceptors of the beta 2-subtype and shows no supersensitivity. This suggests that these receptors are not under the influence of the sympathetic innervation.

Reserpine induces vascular alpha 2-adrenergic supersensitivity and platelet alpha 2-adrenoceptor up-regulation in dog

1. The aim of the present study was to investigate the influence of catecholamine levels on the regulation of alpha 2-adrenoceptor sensitivity in dogs. 2. Blood pressure and heart rate values at rest, plasma catecholamine levels, platelet and adipocyte alpha 2-adrenoceptors as well as the alpha 2-mediated cardiovascular responses to clonidine (10 micrograms kg-1 i.v., after alpha 1-, beta-adrenoceptor plus muscarinic blockade) or noradrenaline (0.5, 1, 2 and 4 micrograms kg-1 i.v. after alpha 1- and beta-adrenoceptor blockade) were measured before and after reserpine treatment (0.1 mg kg-1 day-1 s.c. over 15 days). 3. Reserpine induced a significant decrease in resting systolic and diastolic blood pressures (213 +/- 2/87 +/- 6 mmHg before vs 158 +/- 5/59 +/- 3 mmHg after treatment) as well as in heart rate (91 +/- 2 beats min-1 before vs 76 +/- 3 beats min-1 after treatment). 4. A 5 min tilt test performed under chloralose anesthesia, failed to modify blood pressure before treatment whereas it induced a significant fall in the same animals after the 15 day treatment. Plasma levels of noradrenaline significantly decreased (262 +/- 58 vs 66 +/- 31 pg ml-1) whereas plasma adrenaline levels were unchanged. 5. The alpha 2-mediated pressor responses to noradrenaline were significantly increased after reserpine. Clonidine induced a marked pressor effect (+72 and +45% in systolic and diastolic blood pressures respectively) after reserpine treatment. This effect was suppressed by administration of RX-821002, a new specific alpha 2-adrenoceptor antagonist. 6. Reserpine treatment significantly increased platelet alpha 2-adrenoceptor number (identified with [3H]- yohimbine or [3H]-RX821002) with no change in Kd values. alpha 2-Adrenoceptor number remained unchanged in adipocytes (identified with [3H]-RX821002). 7. These results show that a 15 day treatment with reserpine induces a vascular alpha 2-adrenergic supersensitivity and an up-regulation in platelet alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since adipocyte alpha 2-adrenoceptors escape the effect of reserpine. We suggest that the levels of plasma noradrenaline play an important role in the regulation of the platelet and vascular alpha 2-adrenoceptors. In contrast, adipocyte alpha 2-adrenoceptors are not affected by changes in plasma noradrenaline levels.

Regulation of beta 1- and beta 2-adrenoceptors following chronic treatment with beta-adrenoceptor antagonists

The effect of chronic pretreatment of guinea pigs with various beta-adrenoceptor antagonists on the binding characteristics of the radioligand 125I-cyanopindolol (ICYP) and responsiveness of adenylate cyclase to isoprenaline in the gastrocnemius muscle (beta 2-adrenoceptors) and the left ventricle (beta 1-adrenoceptors) were compared. Pretreatment of guinea pigs with propranolol or ICI 118,551 for one week significantly increased the density of the beta 2-adrenoceptors in the gastrocnemius muscle. Atenolol pretreatment for one week did not affect the density of the receptors. Pretreatment of the animals with pindolol for one week reduced the density of beta 2-adrenoceptors in skeletal muscle. In the left ventricle pretreatment of the guinea pigs with any of the antagonists did not alter either the density or the KD of beta 1-adrenoceptors. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in the left ventricle or skeletal muscle was not affected when the guinea pigs were pretreated with propranolol. Pretreatment of the guinea pigs with reserpine (0.5 mg.kg-1) intraperitoneally for one week, to deplete catecholamines did not affect beta-adrenoceptor density or KD in the left ventricle or skeletal muscle. We conclude that the regulation of beta-adrenoceptors by the antagonist may not be caused by the prevention of the access of endogenous agonists to beta-adrenoceptors and it is dependent on the selectivity of the antagonist and on the susceptibility of the receptors to regulation.

Effect of reserpine pretreatment on brown fat iodothyronine 5'-deiodinase of mouse

The effect of pretreatment with reserpine (1 mg kg-1 i.p. daily for 7 days) on the regulation of iodothyronine 5'-deiodinase (5'D) in mouse brown adipose tissue (BAT) has been examined. 5'D activity of BAT homogenate was assessed by the in-vitro formation of 3,5,3'-triiodothyronine from thyroxine and 3,3'-diiodothyronine from 3,3',5'-triiodothyronine in the presence of 20 mM dithiothreitol. Reserpine treatment decreased the stimulation of BAT 5'D induced by acute cold exposure (4 degrees C, 2 h) without a significant decrease in the basal 5'D activity, whereas stimulation of BAT 5'D elicited by noradrenaline (0.4 and 0.8 mg kg-1 s.c. 2 h previously) was not augmented after reserpine treatment. Although both noradrenaline and acute cold exposure increase BAT 5'D through alpha 1-adrenoceptors, our results show that chronic reserpine treatment prevents the effect of cold, but does not induce alpha 1-adrenoceptor supersensitivity in BAT.