Intra-arterial injection of bradykinin produces reflex cardiorespiratory changes involving histamine receptors in anesthetized rats

INTRODUCTION

It is well known that the baroreceptors and chemoreceptors present in great blood vessels originating from heart, play an important role in regulation of cardiorespiratory parameters but the role of peripheral blood vessels in this regard has been a matter of discussion.^[1,2] Sensory nerves innervating the blood vessels have been associated in the pain of vascular origin such as migraine, angina, embolism, intermittent claudication, and myocardial infarction.^[3,4] It has been suggested that the autonomic changes and cardiorespiratory changes seen in vascular disorders are mediated reflexly by the activation of sensors located around the peripheral blood vessels.^[5,6] The involvement of vanilloid receptor 1 (VR1) has been associated for the immediate nociceptive responses evoked after intra-arterial (i.a.) injection of capsaicin.^[7,8] Further, it is also known that capsaicin-induced responses are short lived (15 s) in comparison to the venom-induced vasosensory reflex responses.^[9] The shorter duration of action of capsaicin is attributed to the phenomenon of rapid desensitization produced by it. Intra-arterial injection of Mesobuthus tumulus venom produced long-lasting sequential alterations in cardiorespiratory parameters.^[9-11] Involvement of prostaglandins has also been demonstrated for mediating venom-induced vasosensory responses.^[10]

Bradykinin (BK) is a potent nociceptive component of the venom; therefore, it was planned to use the BK for the elicitation of the vasosensory reflex responses in this study. It has been shown elsewhere that i.a. injection of BK produced immediate hyperventilatory and hypotensive responses.^[12] However, the effect of BK on heart rate (HR) was not assessed in their study. Previously, the role of peripheral blood vessels in regulation of cardiorespiratory parameters has been demonstrated by some workers.^[7,8,10] It has also been shown that BK receptors are involved in producing these responses.^[12] Since, all the inflammatory mediators (e.g., BK, histamine, and prostaglandins) are well known nociceptive agent; therefore, the role of histamine receptors in mediating the BK-induced responses cannot be ruled out. Therefore, the present study was undertaken to evaluate the role of histamine receptors (H1 and H2 receptors) in producing the BK-induced vasosensory reflex responses.

MATERIALS AND METHODS

All the chemicals (saline/drugs) were instilled in the femoral artery retrogradely and six animals were used in each group. The volume of injectables was kept minimum and constant (0.10 ml) for all chemicals and temperature of the lab was maintained at 25 ± 2°C.

Drugs and solutions

Urethane was procured from Merck, Germany, and was freshly prepared (0.5 g/ml) in double distilled water before each experiment. BK acetate salt was procured from the Sigma Chemicals Company, St Louis, USA. Stock solution of BK (1 mg/ml) was prepared in double distilled water and was refrigerated. Subsequent dilutions were made in normal saline at the time of experimentation. Volume of all the drugs was kept minimum and constant (0.10 ml) to minimize the stretch-induced responses and systemic spillage. Pheniramine Maleate injection (20 mg/ml) was obtained from the Sanofi India Limited, Vadodara, Gujarat, India. Heparin (1000 IU/ml) was obtained from Biological Evans Ltd., Hyderabad.

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Figure 1:

Diagrammatic presentation of the rat showing experimental setting for recording of various cardiorespiratory parameters. TC: Tracheal cannulation, N: Needle secured to skin, FT: Force transducer, FA: Femoral artery, C: Cannula (24G, double ported), I: Instillation port for drugs/saline, BPT: Blood pressure transducer; DAS: Data acquisition system (AD Instruments), LLE: Left leg electrode, RAE: Right arm electrode, ECGE: ECG electrodes, GE: Ground electrode.

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RESULTS

It has been observed that i.a. injection of 1 μM concentration of BK (0.10 ml) produced optimal cardiorespiratory responses on all parameters. Therefore, this concentration was used for the elicitation of vasosensory reflex responses.

Pheniramine maleate pre-treatment blocked BK-induced changes

Injection of pheniramine maleate (0.11 mg or 0.45 mg/kg) per se did not produce any change in the resting RF, RMV, MAP, and HR up to 20 min. However, pheniramine maleate pre treatment attenuated the tachypneic changes (RF from 90.5 ± 3.63 to 99.8 ± 4.22/min), hyperventilatory changes (RMV from 171.8 ± 7.59 ml/min to 189.5 ± 8.51 ml/min), hypotensive changes (MAP from 90.7 ± 1.94 mmHg to 82.8 ± 2.57 mmHg), and bradycardiac changes (HR from 323.5 ± 6.49 to 315.8 ± 8.3) significantly, produced by BK [Figures 2 and 3]. The changes in these parameters were significantly greater than the time-matched BK only group (P < 0.05, post hoc Dunnett’s t-test [two sided]) or from the initial responses (P < 0.05, Student’s t-test for paired observations).

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Figure 2:

Original recordings showing the effect of injecting 1 µM concentration of bradykinin (BK) (0.10 ml) in a segment of femoral artery on respiration (Resp), blood pressure (BP), and electrocardiogram (ECG) in the naive and in pheniramine maleate pretreated rats. The responses after BK are shown at different time intervals as indicated in the lower panel. The point of injection is shown by dotted line. The horizontal line between Resp and BP indicates 15 s for all parameters.

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DISCUSSION

Intra-arterial administration of BK (BK, a nociceptive agent) produced immediate hyperventilatory, hypotensive, and bradycardiac responses within a latency of 5–8 s. The shorter latencies of responses indicate the neuronal mechanisms in producing the responses. Since the BK is injected in a segment of femoral artery retrogradely; therefore, it is likely that it would remain intravascularly for some time. The femoral artery was also made an end artery by tying the cannula along with the artery which further prevented the spillage of the BK. Moreover, the volumes of all the injectables were kept constant at 0.10 ml to minimize the systemic spillage. Similar results with shorter latency in ventilatory changes are also observed with known nociceptive agents (capsaicin/anandamide) in similar experimental settings elsewhere.^[8,13] In another study, it has been shown that intra-arterial injection of BK produced immediate hyperventilatory and hypotensive responses but the HR changes were not observed in their study.^[12]

In the present study, the responses were recorded for extended time up to 30 min to visualize the delayed changes in cardiorespiratory parameters. The experimental design was also improvised using the cannulation of single femoral artery only. This cannulation was used for the BP recording as well as for the administration of chemicals (saline/agonist/ antagonist) without stoppage of BP recording as pediatric iv cannula (24 G, double port) with injection valve was used. The interruption in BP recording during drug injection marks the stimulus artifact which interrupts the BP recording hardly for the fraction of a second.

It is well known that intra-arterial injection of a nociceptive agonist causes hyperventilatory and hypotensive responses.^[8,10] As B/L vagotomy attenuated the venom-induced bradycardiac responses;^[14] therefore, it is possible that the present bradycardiac responses are parasympathetically mediated. However, the precise mechanisms and the involvement of central and peripheral sympathetic reflexes need to be explored.

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Figure 3:

Pheniramine maleate (+Phenira) pre-treatment blocked the bradykinin (BK)-induced responses. The time-matched responses relationship in pheniramine maleate pretreated animals and BK only group on respiratory frequency (RF), respiratory minute ventilation (RMV), mean arterial pressure (MAP), and heart rate (HR) is shown. The RF, RMV, MAP, and HR responses are significantly different from BK only group (P < 0.05, post hoc Dunnett’s t-test [two sided]). Dotted line indicates the point of injection of saline (s)/BK. An asterisk (*) indicates P < 0.05 as compared to “BK only” group from pheniramine maleate pre-treated group.

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Involvement of kinin and prostaglandins to noxious stimulation has already been shown by some workers.^[15] BK is an inflammatory mediator and produces their actions involving BK 1 (B1) and BK-2 (B2) receptors.^[16] The B1 receptors are normally dormant and are expressed after exposure to toxic chemicals/tissue injury products such as cytokines, substance P, neuropeptides, capsaicin, or after repeated exposure to BK.^[16] The inflammatory mediators (serotonin, BK, prostaglandins, histamines, etc.) mediate the nociceptive vasosensory reflex responses via the VR1 which is activated by capsaicin and other vanilloid compounds.^[17] The VR1 is a ligand gated, non-selective cation channel, expressed predominantly on sensory neurons. VR1 responds to noxious stimuli including capsaicin, heat, and extracellular acidification and it is able to integrate simultaneous exposure to these stimuli.^[17-20] BK is known to stimulate the release of histamine from mast cells.^[21] Mast cells are also found to be located in the perivascular areas.^[22] Thus, it is possible that locally injected BK might have augmented the release of histamine from the perivascular mast cells to mediate the responses through histamine receptors. Our results are consistent with the above findings as pheniramine maleate (H1 and H2 receptor blocker) pre-treatment completely blocked the BK-induced hypotensive responses. In addition, it also blocked the hyperventilatory and bradycardiac responses indicating the role of histamine receptors in modulating the vasosensory reflex responses. These findings also indicate that there is some linkage between the histamine receptors and the BK-receptors in producing the reflex responses. It would have been better to include histamine treated group, that is, histamine + BK antagonist group to demonstrate the cross reactivity, but we could not perform those experiments because of the limitations of our resources.

CONCLUSION

The present results demonstrate that the intra-arterial injection of 1 μM concentration of BK produced immediate hyperventilatory, hypotensive, and bradycardiac responses which were abolished completely in the pheniramine maleate pre-treated group. Our data provide the evidences for the role of H1 and H2 receptors in producing the vasosensory reflex responses. Therefore, it is concluded that BK-induced reflex responses can be modulated by histamine receptor antagonist. However, the precise interaction between bradykinin and histamine receptors requires further investigation.