George Hajduczok

Mechanisms of Resetting of Arterial Baroreceptors: An Overview

Arterial baroreceptors are reset when their afferent nerve activity is reduced at an equivalent arterial pressure and vascular strain. Resetting occurs as a result of stretch of the baroreceptors, usually during an acute or chronic rise in arterial pressure. It may be seen during the diastolic phase of a cardiac cycle (instantaneous resetting), after brief exposure to a sustained elevation of pressure (acute resetting), and after chronic elevation of pressure or in physiologic or pathologic states associated with structural changes in the vascular regions of baroreceptors (chronic resetting). The mechanisms reviewed here include mechanical, ionic and chemical factors. Viscoelastic properties of the carotid sinus and aortic arch may explain the instantaneous resetting that occurs with each cardiac cycle when activity begins in early systole and stops in early diastole. Viscoelastic properties and ionic mechanisms may play a role in acute resetting. Inhibition of Na+K+ ATPase reduces the magnitude of acute resetting. The release of chemicals from the endothelium may modulate baroreceptor activity. Exogenous prostacyclin suppresses and indomethacin augments acute resetting in the rabbit, suggesting that the release of endogenous prostacyclin during a rise in arterial pressure attenuates resetting. Changes in pulsatility and blood flow also may modulate baroreceptor activity. The addition of pulsatile pressure at an increased mean pressure attenuates resetting.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical stimulation increases intracellular calcium concentration in nodose sensory neurons.

The cellular mechanisms involved in activation of mechanosensitive visceral sensory nerves are poorly understood. The major goal of this study was to determine the effect of mechanical stimulation on intracellular calcium concentration ([Ca2+]i) using nodose sensory neurons grown in culture. Primary cultures of nodose sensory neurons were prepared by enzymatic dispersion from nodose ganglia of 4-8 week old Sprague-Dawley rats. Whole cell [Ca2+]i was measured by a microscopic digital image analysis system in fura-2 loaded single neurons. Brief mechanical stimulation of individual nodose sensory neurons was achieved by deformation of the cell surface with a glass micropipette. In 31 of 50 neurons (62%), mechanical stimulation increased [Ca2+]i from 125 +/- 8 to 763 +/- 89 nM measured approximately 10 s after stimulation. [Ca2+]i then declined gradually, returning to near basal levels over a period of minutes. [Ca2+]i failed to increase after mechanical stimulation in the remaining 19 neurons. The mechanically-induced rise in [Ca2+]i was essentially abolished after the neurons were incubated for 5-10 min in zero Ca2+ buffer (n = 7) or after addition of gadolinium (10 microM), a blocker of stretch-activated ion channels (n = 5). The effect of gadolinium was reversed after removal of gadolinium. The results indicate that: (1) mechanical stretch increases [Ca2+]i in a subpopulation of nodose sensory neurons in culture, and (2) the stretch-induced increase in [Ca2+]i is dependent on influx of Ca2+ from extracellular fluid and is reversibly blocked by gadolinium. The findings suggest that opening of stretch-activated ion channels in response to mechanical deformation leads to an increase in Ca2+ concentration in visceral sensory neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Gadolinium inhibits mechanoelectrical transduction in rabbit carotid baroreceptors. Implication of stretch-activated channels.

Gadolinium (Gd3+) has been shown to prevent mechanoelectrical transduction believed to be mediated through stretch-activated channels. We investigated the possible role of Gd(3+)-sensitive channels in mediating baroreceptor activity in the carotid sinus of rabbits. Baroreceptor activity induced by a ramp increase of carotid sinus pressure was reduced significantly during exposure to Gd3+. The inhibition was dose-related and reversible, and was not associated with alteration of carotid sinus wall mechanics as the pressure-strain relationship was unaffected. Veratrine triggered action potentials from single- and multiple-baroreceptor fibers when their response to pressure was inhibited by Gd3+. This suggests that the effect of Gd3+ on baroreceptors in the isolated carotid sinus was specific to their mechanical activation. The results suggest that stretch-activated ion channels sensitive to Gd3+ may be the mechanoelectrical transducers of rabbit carotid sinus baroreceptors.

PERIPHERAL CENTRAL MECHANISMS OF BAROREFLEX RESETTING

1 A change in the arterial pressure‐sympathetic activity or heart rate relation (baroreflex resetting) can result from resetting of baroreceptors (‘peripheral’resetting) or from an altered coupling within the central nervous system of afferent baroreceptor to efferent nerve activities (‘central’resetting). 2.‘Peripheral’resetting involves a shift in the pressure‐baroreceptor activity curve in the direction of the prevailing level of arterial pressure, e.g. after elevations in pressure, the baroreceptor pressure threshold (Pth) is increased and activity reduced at equivalent pressures and vascular strains. 3.‘Peripheral’resetting occurs during the diastolic phase of a cardiac cycle (instantaneous resetting), after brief exposure to elevated pressure (acute resetting), and during chronic hypertension or when chronic structural changes in the vasculature have occurred (chronic resetting). 4 Mechanisms include: (i) changes in the mechanical properties of the vessel wall that may alter the tension on the receptors; (ii) ionic mechanisms operating at the neuronal membrane such as activation of Na+, K+‐ATPase; and (iii) release of endothelial factors that may modulate baroreceptor sensitivity. 5 Acute resetting of baroreceptors can be prevented or attenuated when the sustained elevations in pressure are pulsatile rather than static. Increases in flow increase carotid sinus nerve activity at constant pressure and strain and decrease the Pth of baroreceptors. 6.‘Central’resetting can involve neural‐humoral interactions or an altered responsiveness of central neurons mediating the baroreflex to changes in afferent baroreceptor activity. 7 During static pressure, the continuous baroreceptor discharge causes significant‘central’resetting, i.e. sympathetic activity escapes from baroreflex inhibition. In contrast, during pulsatile pressure, the pulse phasic baroreceptor discharge minimizes‘central’resetting causing sustained sympathetic inhibition.

Mechanosensitive ion channels in putative aortic baroreceptor neurons

Cell-attached patch-clamp experiments were performed on dissociated neurons from nodose ganglia of adult rats. Putative aortic baroreceptor neurons were identified by labeling nerve endings in the adventitia of the aortic arch with the carbocyanine dye DiI. Whereas previous experiments demonstrated the presence of mechanosensitive (MS) whole cell currents, these experiments studied single MS ion channels and examined the influence of culture conditions on their expression. Single MS channels were activated by applying negative pressure through the recording pipette. Channel openings became more frequent as the negative pressure was increased, with open probability increasing significantly above 30 mmHg. MS channels had a slope conductance of 114 pS and a reversal potential of approximately 0 mV, consistent with a nonspecific cation conductance. Channels were not affected by antagonists of voltage-gated conductances but were blocked by 20 microM gadolinium, a known blocker of MS ion channels. When nodose neurons were cocultured with aortic endothelial cells, but not aortic smooth muscle cells, the percentage of patches exhibiting MS ion channels increased significantly, suggesting that aortic endothelial cells secrete a diffusible factor that increases channel expression.Cell-attached patch-clamp experiments were performed on dissociated neurons from nodose ganglia of adult rats. Putative aortic baroreceptor neurons were identified by labeling nerve endings in the adventitia of the aortic arch with the carbocyanine dye DiI. Whereas previous experiments demonstrated the presence of mechanosensitive (MS) whole cell currents, these experiments studied single MS ion channels and examined the influence of culture conditions on their expression. Single MS channels were activated by applying negative pressure through the recording pipette. Channel openings became more frequent as the negative pressure was increased, with open probability increasing significantly above 30 mmHg. MS channels had a slope conductance of 114 pS and a reversal potential of ∼0 mV, consistent with a nonspecific cation conductance. Channels were not affected by antagonists of voltage-gated conductances but were blocked by 20 μM gadolinium, a known blocker of MS ion channels. When nodose neurons were cocultured with aortic endothelial cells, but not aortic smooth muscle cells, the percentage of patches exhibiting MS ion channels increased significantly, suggesting that aortic endothelial cells secrete a diffusible factor that increases channel expression.

Mechanism of baroreceptor adaptation in dogs: attenuation of adaptation by the K+ channel blocker 4-aminopyridine.

1. Increased arterial pressure increases baroreceptor activity but activity declines (i.e. baroreceptors adapt) as the pressure is maintained at the higher level. The purpose of this study was to investigate the role of a 4‐aminopyridine (4‐AP)‐sensitive K+ current in causing baroreceptor adaptation. 2. Multi‐ and single fibre recordings of baroreceptor activity were obtained from the vascularly isolated carotid sinus in anaesthetized dogs during step increases in carotid sinus pressure sustained for periods up to 5 min. 3. Baroreceptor activity increased with the rise in pressure, declined markedly over the first minute, and continued to decline at a slower rate during the remainder of the 5 min period of elevated pressure. Exposure of the isolated carotid sinus to 4‐AP (10(‐5) and 10(‐4) M) attenuated adaptation in a dose‐dependent and reversible manner (P < 0.05). 4‐AP attenuated the gradual decline in single fibre activity and also prevented derecruitment or dropout of fibres that occurred over time. 4‐AP did not alter peak nerve activity measured within the first 2 s of the pressure step. 4. Ouabain (5 x 10(‐7)‐10(‐6) M), an inhibitor of Na+,K(+)‐ATPase, increased baroreceptor activity but did not attenuate baroreceptor adaptation. 5. Neither 4‐AP nor ouabain altered the distensibility of the carotid sinus as measured with sonomicrometer crystals suggesting that the agents act directly on the nerve endings. 6. The results suggest that activation of a 4‐AP‐sensitive K+ current contributes significantly to baroreceptor adaptation with little or no contribution of Na+,K(+)‐ATPase.

Rapid baroreceptor resetting in chronic hypertension. Implications for normalization of arterial pressure.

The purpose of this study was to examine the ability of baroreceptors of renal hypertensive rabbits to reset rapidly during acute changes in arterial pressure. The carotid sinus (CS) was vascularly isolated and baroreceptor activity was recorded during slow ramp increases in CS pressure in hypertensive (one-kidney, one wrap; 127 +/- 3 mm Hg) and normotensive (one-kidney, no wrap; 85 +/- 3 mm Hg) rabbits anesthetized with chloralose. Control measurements were made after holding pressure for 10-15 minutes at the level of arterial pressure recorded before each experiment. Baroreceptor threshold pressure (Pth) was higher in hypertensives (78 +/- 4 mm Hg) compared with normotensives (55 +/- 3 mm Hg, p less than 0.05), and nerve activity was less in hypertensives over a wide range of pressure. CS distensibility (sonomicrometers) was not significantly different in the two groups. After increasing holding pressure from control by 30 and 60 mm Hg for 10-15 minutes, the extent of baroreceptor resetting (delta Pth/delta holding pressure x 100%) in normotensives was 39 +/- 6% and 33 +/- 2%, respectively, but only 14 +/- 5% and 9 +/- 3% in hypertensives (p less than 0.05). After decreasing holding pressure by 30 and 60 mm Hg, resetting was similar in normotensives (32 +/- 6% and 28 +/- 3%) and hypertensives (34 +/- 3% and 30 +/- 4%). In hypertensive rabbits, acute (10-15 minutes) exposure of baroreceptors to normotension (71 +/- 4 mm Hg) decreased Pth to 62 +/- 4 mm Hg and increased nerve activity to levels not significantly different from those of normotensive animals without altering CS distensibility.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid adaptation of central pathways explains the suppressed baroreflex with aging.

Aging is associated with suppressed baroreflex function. Renal sympathetic nerve activity was recorded from young (1 year) and old beagles (11 years) during a step rise in isolated carotid sinus pressure. An abrupt increase in pressure resulted in a significant and similar inhibition of efferent nerve activity in both groups, but the inhibition was not sustained in the old as compared with the young animals. The escape from sympathetic inhibition in the old could not be explained by a decline of input from sensory baroreceptor neurons. Thus the defect in the aged animals is caused by a rapid adaptation of central baroreflex neurons to the baroreceptor input instead of a lack of responsiveness of these neurons, suggesting a functional rather than a structural impairment.

Intercellular communication between renin expressing As4.1 cells, endothelial cells and smooth muscle cells.

Angiotensin II (AII) regulation of renin production by the juxtaglomerular (JG) cells of the kidney is commonly thought to occur through a direct feedback mechanism. However, recent evidence suggests that other cells in the vicinity may indirectly mediate AIIs effect on renin production. Therefore we investigated whether an in vitro model of JG cells (As4.1) could have intercellular communication with endothelial or smooth muscle cells, which are in proximity to JG cells in vivo. 6-carboxyfluorescein was introduced to individual bovine aortic endothelial cells in co-culture with As4.1 cells. Coupling was observed 84% of the time at resting membrane potential and was attenuated by membrane depolarization or octanol (1 mM). Calcein green transfer between human aortic smooth muscle and As4.1 cells occurred 82% of the time and was inhibited by octanol. Expression of connexin 37, 40, 43, and 45 were detected in As4.1 cells using RT-PCR. Stimulation of As4.1 cells by AII failed to alter [Ca(2+)](i) or renin mRNA levels. These findings support the existence of gap junctions between renin producing cells and other cell types of the JG region. Moreover the lack of effect by AII suggest that feedback regulation of renin by AII may be due in part to intercellular communication with cells in proximity to JG cells.

Activated endothelial cells in culture suppress baroreceptors in the carotid sinus of dog.

Vascular stretch increases the activity of arterial baroreceptors along with the production and release of substances from the endothelium. We hypothesized that endothelial factors modulate the sensitivity of baroreceptors during increases in arterial pressure. Baroreceptor activity was recorded from single fibers innervating the isolated carotid sinus of dogs anesthetized with chloralose after removal of the endothelium (balloon denudation) and after replacing into the denuded sinus bovine aortic endothelial cells cultured on microcarrier beads. The endothelial cells were activated with either the calcium ionophore A23187 (2 fxM) or bradykinin (10 fxM). The threshold pressure (n=7) determined with a slow ramp increase in static pressure averaged 73 ± 7 (SEM) mm Hg during exposure to naked beads and was increased significantly (96 ± 18 mm Hg; p < 0.05) during exposure to endothelial cell cultures. During stepwise increases in pressure, activity (n=6) averaged 14 ± 5, 40 ± 8, and 54 ± 8 spikes/sec at 75, 125, and 175 mm Hg during exposure to naked beads and decreased significantly to 2 ± 2, 30 ± 11, and 35 ± 12 spikes/sec at equivalent pressures during exposure to the cell cultures. The activity was restored after replacement of the cell cultures with naked beads. The suppressed activity was not caused by changes in carotid sinus diameter or strain (sonomicrometers) or by the chemical activators that were also added to the naked beads. The results indicate that chemically activated endothelial cells release an inhibitory factor that suppresses baroreceptor activity.