Indian Journal of Pharmacology Home 

RESEARCH ARTICLE		 [Download PDF]
Year : 2013  |  Volume : 45  |  Issue : 4  |  Page : 365--370

A study to investigate capsaicin-induced pressure response in vagotomized rats

Abhaya Dutta, Aparna Akella, Shripad B Deshpande 
 Department of Physiology, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh, India

Correspondence Address:
Shripad B Deshpande
Department of Physiology, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh
India

Abstract

Objectives: Capsaicin is used to evoke pulmonary C reflexes and produces complex pressure responses along with apnea/tachypnea, and bradycardia. In the present study, the mechanisms involved in capsaicin-induced pressure responses were explored. Materials and Methods: Tracheal, jugular venous, and femoral artery cannulations were performed in anesthetized adult rats. Blood pressure, respiratory excursions, and electrocardiogram were recorded. Cardiorespiratory reflex changes evoked by jugular venous injection of capsaicin (10 μg/kg) were recorded in vagotomized and antagonist pretreated animals. Results: Capsaicin produced triphasic pressure response exhibiting immediate hypotension, intermediate recovery, and delayed hypotension. Time-matched respiratory changes showed apnea, bradypnea, and tachypnea, respectively. Bradycardia occurred at immediate and intermediate phases. After vagotomy, immediate hypotension was abolished; the intermediate recovery was potentiated as hypertensive response; and the delayed hypotension persisted. In case of respiration, the immediate bradypnea persisted and delayed tachypnea was abolished; while heart rate changes at immediate and intermediate phases were abolished. Antagonists of α1 -adrenoceptor (prazosin or terazosin, 0.5 mg/kg), β-adrenoceptor (propranolol, 1 mg/kg), AT 1 receptor (losartan, 10 mg/kg) and Ca 2+ channel (diltiazem, 1 mg/kg) failed to block the capsaicin-induced intermediate hypertensive response in vagotomized animals. Conclusions: These observations implicate the existence of mechanisms other than adrenergic, angiotensinergic, or Ca 2+ channel-dependent mechanisms for mediating the capsaicin-induced intermediate hypertensive response in vagotomized animals.


How to cite this article:
Dutta A, Akella A, Deshpande SB. A study to investigate capsaicin-induced pressure response in vagotomized rats.Indian J Pharmacol 2013;45:365-370


How to cite this URL:
Dutta A, Akella A, Deshpande SB. A study to investigate capsaicin-induced pressure response in vagotomized rats. Indian J Pharmacol [serial online] 2013 [cited 2023 Jun 10 ];45:365-370
Available from: https://www.ijp-online.com/text.asp?2013/45/4/365/115019


Full Text

 Introduction



Capsaicin is a nociceptive substance present in Capsicum annuum and is a potent stimulant of transient receptor potential vanilloid type 1 (TRPV1) receptors present on C fibers. [1],[2] Intravenous injection of capsaicin produces reflex changes in cardiorespiratory parameters manifesting as apnea, bradycardia, and triphasic pressure response in anesthetized animals. [2],[3],[4],[5],[6] Capsaicin-induced triphasic pressure response exhibits as immediate decrease, intermediate increase, and delayed prolonged decrease in blood pressure (BP). [7],[8],[9],[10] Vagotomy abolished the immediate fall in pressure (1 st phase of response) while potentiating the intermediate increase of pressure. [10] The immediate hypotensive response therefore appears to be vagally mediated; however, the mechanism underlying the capsaicin-induced intermediate increase of pressure is poorly understood. [7],[8],[9],[10] The increase in pressure can be due to the activation of adrenergic or angiotensinergic mechanisms. The actions of adrenoceptors either at α- or β-receptors are mediated through G-protein-coupled receptors (GPCR) which in turn increases the intracellular calcium level via Ca 2+ channels. [11] Angiotensin-II, another potent vasoconstrictor, also activates GPCR stimulating PIP 3 -mediated pathway for the opening of L-type of Ca 2+ channels. [12],[13],[14] Increased intracellular Ca 2+ in turn increases arteriolar muscle tone and also activates TRPV1 receptors. [13],[15],[16] Further, TRPV1 receptor is shown to regulate intracellular Ca 2+ levels independently. [16] Thus, Ca 2+ plays a vital role in TRPV1-mediated actions and its role in capsaicin-induced pressure response is not known. Therefore, the present study was undertaken to explore the involvement of Ca 2+ channel for the hypertensive response induced by capsaicin. Further, the involvement of adrenoceptors and angiotensinergic receptors for these responses was also delineated.

 Materials and Methods



Animals

The study protocol was approved by ethics committee of the Institute (Dean/2007-08/1442) for conducting animal experiments. Adult female rats of Charles Foster strain weighing 176 ± 14 g were used. The animals were housed in a temperature, humidity, and light (12 h:12 h light dark period) controlled room with ad libitum food and water.

Dissection and Recording

The methods for dissection and recording of cardiorespiratory parameters were as described earlier. [17],[18] Briefly, animals were anesthetized with urethane (1.5 g/kg i.p.). An additional dose of urethane (0.1–0.15 g/kg i.p.) was injected whenever required as assessed by the corneal and withdrawal reflexes. Trachea, jugular vein, and femoral artery were cannulated. Tracheal cannulation was used to keep the respiratory tract patent; jugular venous cannulation for capsaicin/antagonist administration, and femoral artery cannulation for recording BP via Statham transducer. Eletrocardiographic potentials were recorded by connecting the needle electrodes in standard limb lead-II configuration. Respiratory movements were recorded by securing a thread to the skin over the xiphisternum to a force-displacement transducer. All the recordings were taken on a computerized chart recorder.

Drugs and Solutions

Capsaicin, prazosin (α1 -adrenoceptor antagonist), propranolol (β-adrenoceptor antagonist), and losartan (AT 1 receptor antagonist) were obtained from Sigma Chemical Company St. Louis, MO, USA. Dilztiazem (L-type of Ca 2+ channel antagonist) was from WAKO pure Chemical Industries Ltd, Osaka, Japan. Terazosin (α1 -adrenoceptor antagonist) was from Abbott India Ltd, Mumbai. Stock solution of capsaicin (1 mg/mL) was prepared in ethanol. Stock solution of other drugs was made in distilled water and diluted with normal saline at the time of administration. The volume of the injections was kept at 0.1 mL.

Experimental Protocols

The animals were divided broadly into two groups: In Group-I (n = 12) after obtaining the initial recordings [respiration, electrocardiogram (ECG), and BP], capsaicin (10 μg/kg) was administered intravenously as bolus injection. The arterial pressure, respiration, and heart rate (HR) were recorded for 1 min. Subsequently, bilateral vagotomy was performed and 10 min later capsaicin (10 μg/kg) response was obtained as before.

In Group-II (n = 17), capsaicin (10 μg/kg)-induced reflex response was recorded initially and after bilateral vagotomy as described in Group-I. Subsequently, prazosin (0.5 mg/kg), terazosin (0.5 mg/kg), propranolol (1 mg/kg), losartan (10 mg/kg), or diltiazem (1 mg/kg) was injected and 15 min later capsaicin (10 μg/kg) reflex response was recorded. The dosage of various antagonists was selected from the earlier reports. [19],[20],[21]

Statistical Analysis

Peak changes in mean arterial pressure (MAP) (mm Hg) from initial at different phases after capsaicin were computed and expressed as percentage. The time-matched respiratory frequency (RF) and HR at immediate (0–10 s), intermediate (10–20 s), and delayed (45–60 s) phases of BP response were computed after capsaicin administration. The responses were normalized to the respective initial values (before capsaicin administration). The data were pooled to obtain mean ± standard error of the mean (SEM). One-way analysis of variance (ANOVA) was used to compare the significant difference between values before vagotomy, after vagotomy or after pretreatment with antagonists in vagotomized animals. Student-Newman-Keuls test was applied for multiple comparisons at each phase of response. In addition, Student's t test was used to compare the paired responses before and after vagotomy. P < 0.05 was considered significant.

 Results



A typical triphasic response pattern in arterial BP at 10 μg/kg of capsaicin was observed with preliminary experiments. Therefore, this concentration was used for further experiments.

Capsaicin-induced Pressure Response

Injection of capsaicin (10 μg/kg) produced changes in MAP in a time-dependent manner. There was immediate fall (by 50%) after a latency of 1.3 ± 0.1 s and lasted for 5–6 s. Subsequently, the pressure recovered and peaked slightly above the initial level [Figure 1]. The latency and duration of this response was 6.1 ± 0.4 s and 10 s, respectively. Thereafter, there was a delayed fall in pressure (by 30%) with a latency of 17.7 ± 1.4 s and persisted for longer time. Thus, the MAP response can be categorized as immediate hypotensive, intermediate recovery, and a delayed hypotensive response. The time-matched respiratory responses to the pressure responses manifested as bradypnea in immediate and intermediate phases and tachypnea in delayed phase, respectively [Figure 1]. Bradycardia was observed at immediate and intermediate phases [Figure 1].{Figure 1}

Capsaicin-induced Pressure Response Following Vagotomy

The initial values of MAP, RF, and HR before vagotomy were 97 mm Hg, 66 breaths/min and 352 beats/min, respectively [Table 1]. After bilateral vagotomy, resting RF was decreased by 50% (P < 0.05), while there was no alteration in MAP or HR. In vagotomized animals, capsaicin-induced intermediate hypertensive response was potentiated (44% increase; P < 0.05) [Figure 1]. However, the bradycardiac response was abolished but bradypneic response persisted at this phase. In these animals, capsaicin-induced immediate hypotensive and bradycardiac responses were not seen while bradypnea persisted. The delayed hypotensive response persisted even after vagotomy, while tachypnea and bradycardia were abolished [Figure 1].{Table 1}

Since, intermediate hypertensive response was potentiated after vagotomy; further, experiments were performed to understand the mechanisms involved in the potentiation of intermediate hypertensive response.

Effect of α1 -Adrenoceptor Antagonists on Capsaicin-induced Hypertensive Response in Vagotomized Animals

In vagotomized animals, prazosin (0.5 mg/kg) per se decreased the resting MAP significantly (P < 0.05) [Table 1], but HR and RF were not altered from the pre-prazosin values. However, prazosin pretreatment did not reverse the intermediate hypertensive response produced by capsaicin [Figure 2]. Similarly, the HR and RF changes at various phases after prazosin treatment remained similar to the responses in vagotomized animals [Figure 2].{Figure 2}

In terazosin-treated group, capsaicin-induced triphasic pressure response before and after vagotomy were similar to the earlier group [Figure 1]. In vagotomized animals, terazosin (0.5 mg/kg) per se decreased the resting MAP significantly (P < 0.05) [Table 1], but HR and RF were not different from the pre-terazosin values. After terazosin pretreatment, the intermediate hypertensive response produced by capsaicin was not blocked (P < 0.05) [Table 2].{Table 2}

Effect of β-Adrenoceptor Antagonist on Capsaicin-induced Hypertensive Response in Vagotomized Animals

In vagotomized animals, propranolol (1 mg/kg) per se did not produce alterations in the resting MAP, HR, and RF [Table 1]. Propranolol pretreatment did not block the intermediate hypertensive response produced by capsaicin [Figure 3]. The RF and HR changes at various phases after propranolol remained similar in vagotomized animals [Figure 3].{Figure 3}

Effect of AT 1 Receptor Antagonist on Capsaicin-induced Hypertensive Response in Vagotomized Animals

In vagotomized animals, losartan (10 mg/kg) per se decreased the resting MAP significantly [Table 1]. Losartan pretreatment did not block the intermediate hypertensive response produced by capsaicin [Figure 4]. The HR and RF changes at various phases after losartan remained similar to the responses in vagotomized animals [Figure 4].{Figure 4}

Effect of Ca2+ Channel Antagonist on Capsaicin-induced Hypertensive Response in Vagotomized Animals

In vagotomized animals, diltiazem (1 mg/kg) did not produce alterations in the resting MAP, HR, and RF [Table 1]. After diltiazem pretreatment, the intermediate hypertensive response produced by capsaicin was not reversed to the initial level [Figure 5]. The HR and RF changes at various phases after diltiazem remained similar to the responses in vagotomized animals [Figure 5].{Figure 5}

 Discussion



The present study while confirming the triphasic BP response produced by capsaicin furhter reveals that the immediate hypertensive response is vagally mediated but not the intermediate and delayed responses. In contrast, the intermediate hypertensive response was potentiated and manifested as hypertensive response after vagotomy which was not mediated through the Ca 2+ channel-dependent mechanism.

The immediate phase of the triphasic pressure response occurred with a short latency and was abolished after vagotomy. These observations support for the vagally mediated reflex response. The intermediate hypertensive response persisted even after vagotomy rather it got potentiated. This can be due to the increased cardiac activity or due to increased peripheral vascular tone. Intermediate response was associated with bradycardia (HR-response) before vagotomy and after vagotomy the HR response was abolished while the hypertensive response was potentiated. Thus, it indicates the noninvolvement of HR changes for the potentiation of hypertensive response [Figure 1]. In the absence of pacemaker involvement, hypertensive response can be due to the increased peripheral vasoconstriction produced by sympathetic overactivity. In a study elsewhere, the hypertensive response to capsaicin was not attenuated in spinalized rats. [8] Thus, indicating the noninvolvement of medullary sympathetic drive. Further, the noninvolvement of α-adrenoceptors and angiotensinergic mechanisms has been mentioned elsewhere with no supporting data. [10] The present results exclude the involvement of α1 -adrenoceptors and β-adrenoceptors. A study elsewhere also demonstrated that phentolamine failed to block the hypertensive response with capsaicin. [8] Thus, the intermediate hypertensive response induced by capsaicin is not due to α1 or β adrenoceptors. In the absence of sympathetic involvement, angiotensin-II, a potent vasoconstrictor, can be expected to produce capsaicin-induced hypertension. However, our results exclude the involvement of AT 1 -receptor for mediating the response. Similar to angiotensin-II, endothelin-1 is another potent vasoconstrictor. It has been reported that capsaicin induces the release of endothelin from nerve terminals. [22] Therefore, the possibility of endothelin involvement cannot be ruled out for mediating the capsaicin-induced pressure response.

Studies have shown that acute responses to capsaicin involve TRPV1 receptors via calcium-dependent or independent mechanisms. [16] Calcium-calmodulin modulates the TRPV1 activation by capsaicin and is also involved in smooth muscle contractions. [13],[16] Recently, it has been shown that TRPV1 channels are involved in regulating muscle tone largely through Ca 2+ influx. [15],[16] Therefore, we tested the involvement of calcium by blocking L-type of calcium channel. Our data with diltiazem excludes the involvement of L-type of Ca 2+ channels. TRPV1 receptor itself is a nonselective cation channel with high permeability for calcium. [23] Further, the capsaicin-induced activation of TRPV1 receptors is known to increase the influx of Ca 2+ into the endothelial cells. [23] Even in the absence of Ca 2+ influx, the lyophillic TRPV1 agonists may act directly on the endogenous TRPV1 receptors located on the endoplasmic reticulum to increase the intracellular calcium level which may be responsible for capsaicin-induced contractions (tonic activity). [24]

The hypertensive response can also be due to activation of peripheral chemoreceptors because of hypoxemia resulting from long-lasting apnea. Stimulation of chemoreceptors produces hyperventilation and hypertension. However in the intermediate phase, there was hypoventilation and hypertension, which is contradictory to chemoreceptor stimulation. This is supported by an earlier study where capsaicin-induced hypertension was potentiated after carotid sinus nerve transection. [25] Thus, the hypertensive response may not be mediated by chemoreceptors.

Capsaicin-induced hypertension can also be due to the decreased vasodilatation induced by the depletion of calcitonin gene-related peptide (CGRP) as reported. [26],[27] They demonstrated hypertension in rats pretreated (4 days before) with very high concentration of capsaicin (50 mg/kg). [24] In contrast to these observations, our data show that the latency for hypertensive response is too short (6 s) to produce the depletion of vasodilator substances. Further, the concentration of capsaicin used in the present study is too small (5000 times). In addition, the hypertensive response was transient and was followed by a delayed hypotensive response. Thus, depletion of CGRP for the capsaicin-induced intermediate hypertensive response is unlikely. However, in acute experiments similar to the present study, CGRP is shown to be released from the capsaicin-sensitive sensory nerves and produces transient hypotension, which may be accounted for delayed hypotensive response. [28]

Taken together, our study demonstrates that capsaicin produces initial hypotensive, intermediate recovery, and delayed hypotensive response as reported earlier. [10] The initial hypotensive response is vagally mediated, while the intermediate and delayed responses are not mediated through vagus. The capsaicin-induced hypertensive response is also not mediated through adrenergic, angiotensinergic, and Ca 2+ channel-dependent mechanisms. In the absence of vagal or sympathetic mechanisms, the direct action of capsaicin on smooth muscle may be a possibility. Supporting this speculation, it has been shown that capsaicin produces contraction of vascular smooth muscle in vitro. [29],[30]

 Acknowledgment



Financial assistance from University Grants Commission, New Delhi and ICMR, New Delhi is acknowledged.

References

1Pingle SC, Matta JA, Ahem GP. Capsaicin receptor: TRPV1 a promiscuous TRP channel. Handbook Exp Pharmacol 2007;179:155-71.
2Lee LY, Lundberg JM. Capsazepine abolishes pulmonary chemoreflexes induced by capsaicin in anesthetized rats. J Appl Physiol 1994;76:1848-55.
3Coleridge HM, Coleridge JCG, Kidd C. Role of the pulmonary arterial baroreceptors in the effects produced by capsaicin in the dog. J Physiol 1964; 170:272-85.
4Pórszász J, György L, Pórszász-Gibiszer K. Cardiovascular and respiratory effects of capsaicin. Acta Physiol Hung 1955;8:61-76.
5Ravi K, Singh M. Role of vagal lung C-fibres in the cardiorespiratory effects of capsaicin in monkeys. Resp Physiol 1996;106:137-51.
6Toh CC, Lee TS, Kiang AK. The pharmacological actions of capsaicin and analogues. Br J Pharmacol 1955;10:175-82.
7Chen IJ, Lo YC, Lo WJ, Yeh JL, Wu BN. Capsazocaine: A capsaicin-sensitive functional antagonist displays an argument on sensory capsaicin receptor. Gen Pharmacol 1997;29:387-95.
8Donnerer J, Lembeck F. Analysis of the effects of intravenously injected capsaicin in the rat. Naunyn-Schmiedberg's Arch Pharmacol 1982; 320: 54-7.
9Skofitsch G, Saria A, Lembeck F. Phenyldiguanide and capsaicin stimulate functionally different populations of afferent C-fibres. Neurosci Lett 1983;42: 89-94.
10Dutta A, Deshpande SB. Mechanisms underlying the hypertensive response induced by capsaicin. Int J Cardiol 2010;145:358-9.
11Bühler FR, Bolli P, Erne P, Kiowski W, Müller FB, Hulthén UL, et al. Adrenoceptors, calcium, and vasoconstriction in normal and hypertensive humans. J Cardiovasc Pharmacol 1985;7 Suppl 6:S130-6.
12Hilgers RH, Webb RC. Molecular aspects of arterial smooth muscle contraction: Focus on rho. Exp Biol Med 2005;230:829-35.
13Moosmang S, Schulla V, Welling A, Feil R, Feil S, Wegener JW, et al. Dominant role of smooth muscle L-type calcium channel Cav1.2 for blood pressure regulation. EMBO J 2003;22:6027-34.
14Tanaka Y, Hata S, Ishiro H, Ishii K, Nakayama K. Stretching releases Ca 2+ from intracellular storage sites in canine cerebral arteries. Can J Physiol Pharmacol 1994;72:19-24.
15Ohanyan VA, Guarini G, Thodeti CK, Talasila PK, Raman P, Haney RM, et al. Endothelin-mediated in vivo pressor responses following TRPV1 activation. Am J Physiol Heart Circ Physiol 2011;301:H1135-42.
16Rosenbaun T, Gordon-Shaag A, Munari M, Gordon SE. Ca 2+ /calmodulin modulates TRPV1 activation by capsaicin. J Gen Physiol 2004;123:53-62.
17Deshpande SB, Bagchi S, Rai OP, Arya NC. Pulmonary oedema produced by scorpion venom augments phenyldiguanide-induced reflex response in anesthetized rats. J Physiol 1999;521:537-44.
18Kanoo S, Alex AB, Tiwari AK, Deshpande SB. B 2 kinin receptors mediate the Indian red scorpion venom-induced potentiation of visceral reflexes via the nitric oxide cyclic guanosine monophosphate pathway. Acta Physiol (Oxf) 2009;196:365-73.
19Bagchi S, Deshpande SB. Phenyldiguanide activates cardiac receptors to produce responses by involving three different efferent pathways in anaesthetized rats. Indian J Exp Biol 2000;38:881-6.
20Bertera FM, Mayer MA, Opezzo JA, Taira CA, Bramuglia GF, Höcht C. Pharmacokinetic-pharmacodynamic modeling of diltiazem in spontaneously hypertensive rats: A microdialysis study. J Pharmacol Toxicol Methods 2007;56:290-9.
21Sarkis A, Liu KL, Lo M, Benzoni D. Angiotensin II and renal medullary blood flow in Lyon rats. Am J Physiol Renal Physiol 2003;284:F365-72.
22Szolcsanyi J, Oroszi G, Nameth J, Szilvassy Z, Blasig IE, Tosaki A: Functional and biochemical evidence for capsaicin-induced neural endothelin release in isolated working rat heart. Eur J Pharmacol 2001;419:215-21.
23Yao X, Garland CJ. Recent developments in vascular endothelial transient receptor potential channels. Circ Res 2005;97:853-63.
24Reilly CA, Johansen ME, Lanza DL, Lee J, Lim J, Yost GS. Calcium-dependent and independent mechanisms of capsaicin receptor (TRPV1)-mediated cytokine production and cell death in human bronchial epithelial cells. J Biochem Mol Toxicol 2005;19:266-75.
25Wang R, Xu F, Zhuang J, Zhang C. Carotid sinus nerve is involved in cardiorespiratory responses to intracarotid injection of capsaicin in the rat. J Appl Physiol 2006;100:60-6.
26Deng PY, Li YJ. Calcitonin gene-related peptide and hypertension. Peptides 2005;26:1676-85.
27Peng J, Lu R, Deng HW, Lee YJ. Involvement of alpha-calcitonin gene-related peptide in monophosphoryl lipid A-induced delayed preconditioning in rat hearts. Eur J Pharmacol 2002;436:89-96.
28Peng J, Lee YJ. The vanilloid receptor TRPV1: Role in cardiovascular and gastrointestinal protection. Eur J Pharmacol 2010;627:1-7.
29Duckles SP. Effects of capsaicin on vascular smooth muscle. Naunyn-Schmiedeberg's Arch Pharmacol 1986;333:59-64.
30Toda N, Usui H, Nishino N, Fujiwara M. Cardiovascular effects of capsaicin in dogs and rabbits. J Pharmacol Exp Ther 1972;181:512-21.

 

 
Saturday, June 10, 2023
 Site Map | Home | Contact Us | Feedback | Copyright and Disclaimer