Structural Versus Functional Modulation of the Arterial Baroreflex
Abstract Structural changes in large arteries are often considered the predominant mechanism responsible for decreased baroreflex sensitivity and baroreceptor resetting in hypertension, atherosclerosis, and aging. Recent work has demonstrated that “functional” mechanisms, both at the level of the peripheral sensory endings and within the central nervous system, contribute significantly to altered baroreflex responses. We have conducted both reductive studies of mechanoelectrical transduction in cultured baroreceptor neurons and integrative studies with in vivo recordings of the activity of baroreceptor afferent fibers and efferent sympathetic nerves. Results suggest that the primary mechanism of mechanical activation of baroreceptor neurons involves opening of stretch-activated ion channels susceptible to blockade by gadolinium. Baroreceptor nerve activity is modulated by the activity of potassium channels and the sodium-potassium pump and by paracrine factors, including prostacyclin, oxygen free radicals, and factors released from aggregating platelets. Endothelial dysfunction and altered release of these paracrine factors contribute significantly to the decreased baroreceptor sensitivity in hypertension and atherosclerosis. The central mediation of the baroreflex depends on the pulse phasic pattern of afferent baroreceptor discharge. Baroreflex-mediated inhibition of sympathetic nerve activity is well maintained during pulse phasic afferent activity. Continuous, nonphasic baroreceptor discharge or a rapid (>1.5 Hz) pulse phasic discharge results in disinhibition of sympathetic activity. This disinhibition during continuous baroreceptor input is exaggerated with aging. Thus, a defect in central mediation of the baroreflex may be a major cause of the impaired baroreflex and sympathoexcitation in the elderly. In summary, functional neural mechanisms, in addition to structural vascular changes, contribute importantly to altered baroreflex responses in normal and pathophysiological states. Therefore, it may be possible to implement therapies that reverse functional changes and restore baroreflex sensitivity long before the reversal of structural vascular changes.
- carotid sinus
- ion channels
- sympathetic nerve activity
The baroreceptive endings of the carotid sinus nerve and aortic depressor nerve are the peripheral terminals of a group of sensory neurons with their soma located in the petrosal and nodose ganglia. The endings terminate primarily in the adventitia of the carotid sinus and aortic arch. When stretched, they depolarize. Action potentials are then triggered from a spike initiating zone on the axon near the terminal. The action potentials travel centrally to the nucleus tractus solitarius in the medulla. There, the sensory neurons synapse with a second group of “central neurons,” which in turn transmit impulses to a third group of “efferent neurons” that provide the parasympathetic and sympathetic innervation of the cardiovascular system.
The vascular structure of the carotid sinus and aortic arch determines the deformation and strain of the baroreceptor endings during changes in arterial pressure.1 2 For this reason, structural changes in the large arteries and decreased vascular distensibility are often considered the predominant mechanisms responsible for the decreased baroreflex sensitivity and resetting of baroreceptors in hypertension, atherosclerosis, and aging.
The recent explosion in our knowledge of the functional properties of the vessel wall, in particular the endothelium,3 4 led us to reexamine systematically the premise that baroreceptor activity is merely a reflection of arterial pressure and associated vascular strain. Therefore, we aimed to identify factors, unrelated to vascular structure and its mechanical consequences, that influence the activation of peripheral sensory baroreceptive neurons and the central mediation of the baroreflex.
This article reviews recent work published or in progress from our laboratory. The studies allowed us to identify several factors and neural mechanisms involved in the modulation of the arterial baroreflex. These are referred to as functional factors to distinguish their influence from that of structural vascular changes. They are described below under two headings depending on their site of action: (1) peripheral sensory mechanisms, which occur at the baroreceptor nerve ending or the sensory neuron, and (2) central mechanisms, which relate to central neurons and the coupling of afferent baroreceptor activity to efferent autonomic outflow.
Peripheral Sensory Mechanisms (Determinants of Activation of Baroreceptors)
The process of mechanoelectrical transduction in the baroreceptors depends on two components: (1) The mechanical component is determined by the viscoelastic characteristics of the coupling elements between the vessel wall and the nerve endings.1 2 It will not be considered in this review. (2) The functional component, which will be the focus of this review, is related to (a) ionic factors resulting from activation of channels or pumps in the neuronal membrane of the baroreceptor region, altering current flow and causing depolarization and the generation of action potentials, and (b) paracrine factors released from tissues and cells in proximity to the nerve endings during physiological or pathological states. These include the endothelial and vascular muscle cells; monocytes and macrophages; and platelets that release prostacyclin, nitric oxide, oxygen radicals, endothelin, platelet-derived factors, and other yet unknown compounds.
To demonstrate the contribution of ionic and paracrine factors, we have used reductive and integrative approaches.
Because of the technical inaccessibility of the nerve terminals in vivo, studies were done in isolated mechanosensitive neurons from the nodose ganglia of rats. This approach allowed us to evaluate directly the membrane properties of the soma of baroreceptor neurons. By injecting 1,1′-dioleyl-3,3,3′,3′-tetramethylindocarbocyanine methanesulfonate (DiI), a fluorescent dye, into the adventitia of the aortic arch we specifically labeled the soma of aortic baroreceptor neurons in the nodose ganglion.5 6 7 8 The nodose neurons were acutely dissociated and maintained in culture with methods7 adapted from Ikeda et al9 and DeKoninck et al.10
Whole-cell ionic currents were measured from cultured baroreceptor neurons with patch-clamp techniques.7 Baroreceptor neurons were mechanically stimulated either by cell swelling induced by hyposmotic extracellular fluid7 or by puffs of saline ejected from a micropipette connected to a pneumatic picopump11 (Fig 1⇓). Mechanical stimulation triggered an inward current that was inhibited by gadolinium (Gd3+),7 11 a relatively selective antagonist of stretch-activated ion channels12 13 14 (Fig 1⇓). The stretch-induced current was not inhibited by blockers of voltage-gated sodium, potassium, or calcium channels.7
A second approach was to measure the changes in cytosolic Ca2+ concentration ([Ca2+]i) that occurred after mechanical stimulation of cultured baroreceptor neurons with puffs of saline ejected from the micropipette (5, 10, and 15 psi). [Ca2+]i was measured with a fluorescence microscope digital imaging system and fura 2.8 15 Mechanical stimulation increased [Ca2+]i in proportion to the intensity of stimulation.8 The stretch-induced increase in [Ca2+]i was blocked by Gd3+ (20 μmol/L) but not blocked by another trivalent lanthanide, lanthanum (20 μmol/L).8 Although both lanthanides may block voltage-gated Ca2+ channels, only Gd3+ at this concentration is an effective blocker of stretch-activated channels. Therefore, we could characterize the channel that was activated by mechanical stimulation as a stretch- activated channel that is permeable to Ca2+ rather than as a voltage-gated Ca2+ channel.
A third approach used to study mechanoelectrical transduction in cultured baroreceptor neurons was to measure the opening of single stretch-activated ion channels with the use of the patch-clamp technique in the cell-attached patch configuration.12 13 14 In preliminary experiments we have found that application of graded suction through the patch-clamp pipette increased the opening probability of stretch-activated channels in the patch.
Taken together, these experiments support the concept that the mechanoelectrical transduction in baroreceptor neurons occurs through a stretch-activated channel. Current and future experiments will define the behavior of this channel in the presence of putative paracrine factors and in animal models of hypertension and atherosclerosis.
Effect of Carbacyclin on Outward Current
In preliminary experiments we have tested the effect of the putative paracrine factor carbacyclin, a prostacyclin analogue, on the outward whole-cell current measured during depolarization of baroreceptor neurons in culture. The reversible blockade of the outward current by carbacyclin suggests that prostacyclin may increase the activity of baroreceptor neurons by inhibiting K+ channels.
Spike Frequency Adaptation (Role of the Transient K+ Current)
Depolarization of sensory neurons with progressive current injections causes an increase in action potential frequency. The neurons may adapt and lose the action potentials that are triggered with the beginning of current injection. This adaptation is reversed and spike frequency is enhanced with the addition of 4-aminopyridine, which blocks the transient K+ channel.16 17 18 In preliminary experiments we have found that spike frequency adaptation is also reversed by 4-aminopyridine in cultured baroreceptor neurons.
(1) The mechanical deformation of isolated baroreceptor neurons in culture triggers mechanoelectrical transduction by opening stretch-activated ion channels. (2) Prostaglandins may sensitize baroreceptor neurons by blocking outward K+ currents. (3) The triggered action potentials during current injection in these neurons may not be sustained because of 4-aminopyridine–sensitive outward K+ currents.
The roles of paracrine and ionic factors in the modulation of peripheral sensory mechanisms of baroreceptor neurons were examined in isolated carotid sinus preparations.19 20 21 22 23 24 25 26 Experiments were carried out in anesthetized, healthy dogs and rabbits and in chronically hypertensive rabbits as well as in hypercholesterolemic and atherosclerotic rabbits.
Carotid sinus nerve activity was measured in multiple or single baroreceptor fibers during increases in carotid sinus pressure. The method for isolating the carotid sinus and recording baroreceptor nerve activity has been described frequently in the literature.20 21 22 25 The increases in pressure were either nonpulsatile or pulsatile and delivered through a pressure reservoir. Carotid sinus diameter was measured with two sonomicrometer crystals placed across the sinus and determined from the transit time of acoustic signals between the crystals or with a videomicrometer.20 22 25
Paracrine Modulation of Baroreceptor Activity
Endothelial dysfunction in hypertension and atherosclerosis is associated with impaired release of prostacyclin (PGI2), enhanced formation of oxygen free radicals, and platelet aggregation.
Impaired PGI2 formation. The inhibition of endogenous formation of PGI2 in the isolated carotid sinus with indomethacin reduces baroreceptor activity significantly in healthy rabbits but not in rabbits with renal hypertension (one-kidney, one wrap) of 3 months’ duration20 21 22 (Fig 2⇓). Endogenous production of PGI2 from arachidonic acid is also reduced in the isolated carotid sinus of these hypertensive rabbits.22 The administration of exogenous PGI2 increases baroreceptor activity in both healthy and hypertensive rabbits, indicating that the defect is one of impairment of PGI2 formation rather than decreased neuronal responsiveness to endogenous PGI220 21 22 (Fig 3⇓).
Similar findings with respect to impairment of PGI2 formation, lack of an effect of indomethacin, and the positive responsiveness to exogenous PGI2 were seen in dietary hypercholesterolemia of 3 to 4 months’ duration in rabbits.23 Thus, decreased endogenous PGI2 contributes to decreased baroreceptor activity in both chronic hypertension and hypercholesterolemia.
Free radicals. Atherosclerotic lesions were present in the carotid sinus of rabbits fed a high-cholesterol diet for a longer duration (6 to 8 months).24 Exposure of the atherosclerotic sinuses to scavengers of free radicals (superoxide dismutase and catalase) increased baroreceptor activity.24 Conversely, the free radical generation by the chemical reaction of xanthine and xanthine oxidase suppressed baroreceptor activity in normal carotid sinus.24 These results suggest that free radicals contribute to the reduced baroreceptor activity in atherosclerosis.
Platelet aggregation. Isolated platelets activated with thrombin profoundly suppress baroreceptor activity recorded from the whole carotid sinus nerve25 (Fig 4⇓). This inhibitory activity remains in cell-free filtrates, indicating that a diffusible mediator is involved.26 Thus, platelet factors may contribute to decreased baroreceptor activity in thrombotic and atherosclerotic states in which platelet aggregation may occur at sites of endothelial damage in the carotid sinuses.
To summarize, a defective endogenous production of PGI2 contributes to a decline in baroreceptor activity in hypertension and atherosclerosis. In addition, in the atherosclerotic state free radicals and platelet aggregation further contribute to the impaired baroreceptor activity. The cumulative effect of these factors may exceed the influence of structural changes in atherosclerosis.
Ionic Modulation of Baroreceptor Activity
Experiments using the isolated carotid sinus of healthy dogs and rabbits were performed to test the effect of blockade of ionic currents and of the Na+-K+ pump on carotid sinus nerve activity.
Stretch-activated channels. The increase in baroreceptor activity in rabbits during ramp increases in carotid sinus pressure is reversibly blocked by Gd3+ without a change in the distensibility of the sinus27 (Fig 5⇓). This finding implicates the stretch-activated channels as the mechanoelectrical transducer. Whether the stretch-activated channel plays a role in the decreased baroreceptor activity in pathological states is yet to be determined.
Na+-K+ pump and baroreceptor resetting. Na+ entering the baroreceptive terminal during a rise in distending pressure may activate the Na+-K+ pump, causing subsequent hyperpolarization. A similar phenomenon was described to explain the posttetanic hyperpolarization in crayfish stretch receptors and the postexcitatory depression of rat aortic baroreceptors after the period of elevation of distending pressure.28 29
Exposure of the isolated carotid sinus of the dog to ouabain (to inhibit the Na+ pump) prevented the acute resetting of the baroreceptor pressure-activity curve to a higher pressure in response to a 15-minute period of elevated carotid sinus pressure.30 Thus, the higher pressure threshold with acute hypertension is the result of increased activity of the Na+-K+ pump. Conversely, acute carotid sinus hypotension may decrease the pressure threshold as a result of inactivation of the Na+ pump. The pathophysiological importance of the Na+-K+ pump in reflex circulatory control has become evident: inhibition of the pump by ouabain in the isolated carotid sinus restores baroreceptor sensitivity in dogs with heart failure,31 and administration of digitalis to humans with heart failure inhibits efferent sympathetic nerve activity (SNA), thereby reversing the sympathoexcitatory state.32
Rapid reversal of “chronic baroreceptor resetting” by acute hypotension. In chronically hypertensive rabbits, the baroreceptor pressure-activity curve is already shifted to a higher level of pressure.2 33 Acute lowering of carotid distending pressure for 10 to 15 minutes in those animals that had been chronically hypertensive for 3 to 4 months restored baroreceptor activity at lower pressures; ie, chronic resetting of the baroreceptors was reversed rather acutely33 (Fig 6⇓). Such a rapid restoration of the baroreceptor pressure-activity relationship to normal could not be ascribed to a reversibility of structural change; rather, it may reflect a functional inhibition of the Na+-K+ pump with the fall in pressure. The resulting increase in baroreceptor activity may help restore arterial pressure to normal levels during antihypertensive treatment.
Activation of an outward K+ current causes baroreceptor adaptation. A progressive decline in baroreceptor activity is seen over a period of seconds to several minutes despite a sustained increase in carotid sinus pressure.34 35 We have reported that this adaptation can be significantly reversed with 4-aminopyridine, which is known to block the transient K+ channel, referred to as A-current, in a relatively selective manner35 (Fig 7⇓). Ouabain, which may enhance depolarization by blocking the Na+ pump, does not prevent adaptation.29 35 The adaptation phenomenon is not seen to the same extent during elevated pulsatile pressure when the spike frequency in systole remains constant over time with a decline only in diastolic spike frequency.
Thus, activation of 4-aminopyridine–sensitive K+ currents may impair the ability of baroreceptors to buffer a sustained increase in pressure, particularly in a noncompliant vascular system. Blockade of this K+ current would be beneficial under those circumstances.
Summary and Therapeutic Implications
The changes in baroreceptor sensitivity mediated by paracrine and ionic factors are significant and can occur independent of any change in vascular distensibility. Therefore, it may be possible to implement therapies that restore baroreceptor sensitivity rapidly and without awaiting the reversal of structural changes in disease states such as chronic hypertension and atherosclerosis. For example, drugs that enhance PGI2 formation, antioxidants, and antithrombotic agents may have beneficial effects on cardiovascular reflex control in such patients.
The ionic factors that cause resetting and adaptation may become more clinically relevant with the advent of specific K+ channel blockers. Furthermore, the reversal of chronic baroreceptor resetting, even after relatively brief normalization of arterial pressure with treatment of hypertension, may reverse the baroreceptor resetting. Once reset to a lower pressure level, the baroreceptors would be functional within a normal pressure range and would enhance blood pressure lowering.
Central Mechanisms (Determinants of Afferent-Efferent Coupling)
The coupling of the afferent baroreceptor activity with the central group of neurons leads to inhibition of SNA. This coupling was examined by determining the relationship between afferent baroreceptor activity and efferent SNA measured simultaneously.
Importance of Phasicity and Burst Frequency of Baroreceptor Input
We have found that sustained inhibition of SNA is not simply a function of baroreceptor spike frequency but depends on a phasic burst pattern with on and off periods during systole and diastole, respectively.36 SNA is disinhibited (because of what may be viewed as a “central adaptation”) during nonpulsatile, nonphasic baroreceptor activity. It is not actually the pulse pressure that is important in sustaining sympathetic inhibition but rather the magnitude of pulsatile distension of the sinus and the corresponding phasic baroreceptor discharge. Based on these results, one would predict that a decrease in large artery compliance, as might occur in chronic hypertension or atherosclerosis, could result in a decrease in pulsatile distension of the carotid sinus and a blunting of the phasicity of baroreceptor input with a progressive loss of the buffering capacity of the baroreflex because of central adaptation.
The reflex inhibition of SNA was also most pronounced at lower frequencies of pulsatile pressure and bursts of baroreceptor activity (between 1 and 2 Hz)36 (Fig 8⇓). When the burst or pulse frequency exceeded 3 Hz, there was a significant disinhibition of SNA despite a maintained high level of total baroreceptor spike frequency per unit time.36 Thus, at very rapid pulse rates the efficiency of afferent-efferent coupling is reduced.
Central Baroreflex Impairment With Aging
We studied young (1 year old) and old (10 years old) beagle dogs37 38 and found that the reflex inhibition of SNA after a rise in carotid sinus pressure was maintained in the young but was very transient in the old dogs38 (Fig 9⇓). The “escape” of SNA from baroreflex inhibition occurred in the old dogs despite a maintained increase in afferent baroreceptor activity.38 Thus, the major defect in the baroreflex with aging may not be a structural vascular defect or an impaired baroreceptive process but rather a central neural defect in the afferent-efferent coupling.
Important alterations in the arterial baroreflex occur in physiological and pathological states, such as hypertension, atherosclerosis, and aging. These alterations are not simply the result of changes in vascular structure and distensibility. The roles of ionic and paracrine factors were examined in a series of reductive studies on cultured baroreceptor nodose neurons and in integrative studies using the isolated carotid sinus preparation of dogs and rabbits.
Peripheral sensory mechanisms include the following: (1) Endogenous PGI2 has an important excitatory effect on baroreceptors that is lacking in chronic hypertension and atherosclerosis. Free radicals and aggregating platelets contribute significantly to the decreased baroreceptor activity in atherosclerosis. (2) A stretch-activated channel may be the mechanoelectrical transducer on the baroreceptor neuron, and a transient outward K+ current causes baroreceptor adaptation and a decline in spike frequency during sustained elevation in carotid sinus pressure. (3) Chronic resetting of baroreceptors in chronic hypertensive rabbits was rapidly reversed after a 10- to 15-minute exposure of the isolated carotid sinus to low arterial pressure. An ionic factor (inhibition of the Na+-K+ pump during the fall in arterial pressure) rather than a structural change may mediate this rapid improvement in baroreceptor activity.
Central mechanisms include the following: (1) Phasicity and a low burst frequency (<3 Hz) of the baroreceptor input rather than spike frequency per unit time sustains the reflex inhibition of SNA. Decreased compliance of large arteries and a rapid pulse rate may result in ineffective afferent-efferent coupling and disinhibition of SNA. (2) A defect in central mediation of the baroreflex may be the major cause of the impaired baroreflex with aging rather than decreased vascular distensibility or a defect in the generation of baroreceptor activity.
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