Michael C. Andresen

Localization and retention in vitro of fluorescently labeled aortic baroreceptor terminals on neurons from the nucleus tractus solitarius

The anterograde fluorescent tracer DiA was used to visualize baroreceptor fibers and synaptic terminals both in living and fixed tissue. Baroreceptor fibers labeled with DiA terminated as a dense synaptic field in the medial nucleus tractus solitarius (NTS), making synaptic contact on the soma, as well as processes of neurons that they innervated. A similar distribution and morphology was observed in baroreceptor fibers and terminals labeled with horseradish peroxidase. DiA also identified baroreceptor terminals and the neurons receiving these synaptic contacts in vitro. NTS neurons were dissociated from their surrounding tissue and identified by attached baroreceptor terminals that retained the fluorescent dye. These results will enable us to study the electrophysiological properties of dispersed neurons that receive identified baroreceptor synaptic terminals.

Baroreceptor function and changes in strain sensitivity in normotensive and spontaneously hypertensive rats.

Baroreceptor resetting during hypertension has been attributed to a reduction in the distensibility of the vessel wall in which the receptors are located. According to this hypothesis, a simple increase in pressure is all that is required to overcome the increase in vessel wall stiffness. However, previous work from our laboratory suggested a more complicated situation. Measurements of both vessel wall mechanical properties and baroreceptor discharge characteristics in spontaneously hypertensive rats (SHR) showed that distortion thresholds for the receptors also undergo changes. Here we have examined the time course of resetting, measuring both aortic distensibility and barore-ceptor properties in SHR and mormotensive Wistar-Kyoto rats (WKY) from 5 to 30 weeks in age. An in vitro aortic arch-aortic nerve preparation was used. We found that for a given pressure the aortic radii of WKY were increasing much more rapidly than the aortic radii for SHR and, beyond 5 weeks of age, were much more distensible. The lower distensibility in SHR was accompanied by increased wall thickness. The discharge characteristics of single baroreceptors were expressed in terms of both pressure and distortion or circumferential wall strain. The change in distensibility of WKY aortas from 5 to 30 weeks was suitably matched by an increase in the strain threshold for discharge of WKY baroreceptors resulting in a constant pressure threshold for discharge. The lower distensibility of SHR aortas was accompanied by lower threshold strains in SHR baroreceptors, but the changes were not suitably matched, and progressive resetting of SHR baroreceptors to higher threshold pressures occurred. The two sets of receptors appear to be different as early as 5 weeks of age when blood pressures are similar and, futhermore, these differences are accentuated by age and hypertension. CircRes 47: 821-828, 1980

Arterial baroreceptor resetting: contributions of chronic and acute processes.

1 Pressure threshold (Pth) and suprathreshold pressure sensitivity (Sth) are important measures of the pressure‐discharge characteristics of arterial baroreceptors. An in vitro preparation of the rat aortic arch‐aortic nerve has been used to assess the influence of extracellular ion concentration, distensibility, smooth muscle activation and rapid resetting on single fibre baroreceptor discharge. 2 Changes in extracellular cations alter Pth and Sth in a reciprocal manner, suggesting that these two properties share common excitatory mechanisms probably at the level of membrane ion conductance channels. 3 During normal development and ageing in normotensive rats, Pth and blood pressure are fairly constant even during periods of greatly changing aortic distensibility. Sth increases progressively to maturity and then decreases somewhat with advanced age. 4 During hypertension, changes in distensibility in spontaneously hypertensive rats do not account for changes in Pth and Sth. 5 The capacity of arterial baroreceptors to rapidly reset during acute changes in the conditioning mean arterial pressure is not altered by chronic resetting, decreases in distensibility or by differences in the initial Pth or Sth of individual baroreceptors. 6 Within the maximal physiological or pathophysiological range, the prevailing or conditioning mean arterial pressure appears to be the most potent modulator of arterial baroreceptor discharge.

Short- and long-term determinants of baroreceptor function in aged normotensive and spontaneously hypertensive rats.

In a variety of animal models, baroreceptor resetting during chronic hypertension has been correlated to vessel wall hypertrophy and decreased distensibility. In one possible mechanism of chronic resetting, termed the splinting hypothesis here, it has been suggested that a stiffer vessel wall might increase the minimum pressure required for activation of these mecha-noreceptors (pressure threshold) and decrease suprathreshold pressure sensitivity. Lower vessel distensibility would alter baroreceptor function by preventing equivalent pressures from producing equivalent vessel distensions and, thus, receptor distortions. Recent studies have also suggested that the pressure threshold is strongly influenced by the most recent (minutes) history of blood pressure exposure during a process termed rapid resetting. Hypertension and advanced aging are associated with distensibility changes. The present study examines pressure and equivalent mechanical response characteristics of aortic baroreceptors from aged normotensive Wistar-Kyoto and spontaneously hypertensive rats. An in vitro aortic arch-aortic nerve preparation was used to assess the discharge properties from a number of baroreceptors and the pressure-diameter relationship of each aorta. Both control and rapid resetting protocols were used to study the baroreceptor characteristics. Aged Wistar-Kyoto rats were normotensive and averaged 115 weeks of age. Aged spontaneously hypertensive rats had systolic tail pressures of 187 mm Hg and averaged 76 weeks of age. Although aortic distensibility of aged WKYs was much lower than previously found in younger animals, the pressure threshold was unchanged. Aged spontaneously hypertensive rat receptors were chronically reset in proportion to their blood pressure. Decreased distensibility did not alter the rapid resetting process. It is concluded that baroreceptor pressure sensitivity is more closely related to aortic distensibility under several conditions altering vessel stiffness, whereas, the pressure threshold may be regulated additionally by mechanisms inde pendent of distensibility. The results are inconsistent with the splinting hypothesis.

Individual and combined actions of calcium, sodium, and potassium ions on baroreceptors in the rat.

We examined the effects of changes in the extracellular concentrations of Na, K, and Ca ions on the steady state pressure-discharge relationships of individual baroreceptors, using an in vitro aortic arch-aortic nerve preparation. We determined the concentration-response relationships for each of the three cations, as well as for the combined effects of changes in Ca2%-K% and Ca2%-Na%. The concentrations tested ranged from normal values to values well above and below normal. For each ion and each combination, effects on the pressure-discharge curves appeared as continuous functions of concentration. Lowering [Na%]0 or [K%]0 or increasing [Ca2%]o shifted pressure-response curves to the right. Increasing [K+]o or decreasing [Ca2+]0 shifted the curves to the left. The steady state effects of small increases in [Na+]0 were inconsistent. Combinations of changes in ion concentration enhanced the effects in a way that was consistent with the results of individual changes, although quantitative comparisons were not made. Thus, the Ca2+ curves for threshold pressure were shifted to the left by lowering [Na+]0 or to the right by increasing [K+]0- Measurements of static volume, diameter, and distensibility showed no significant changes in the vessel wall during these changes so that, for equivalent strains, nervous discharge was clearly altered. Hence, the main actions are likely to be on the receptor and its axon rather than on the smooth muscle of the vessel wall, although local mechanical effects cannot be excluded completely. The results were interpreted by applying the electrophyeiological theory of nerve membranes. These effects of extracellular ions on baroreceptors may have particular significance due to the special role of the baroreceptors in regulatory reflexes. Cirt Res 435:757-763, 1979

Baroreceptor function in spontaneously hypertensive rats. Effect of preventing hypertension.

In the previous paper we showed that baroreceptor resetting in spontaneously hyperten-sive rats (SHR) resulted from an incomplete matching of increased receptor strain sensitivity with reduced vessel wall distensibility. Here SHR and Wistar-Kyoto controls were treated with antihyper-tensive drugs in their drinking water (reserpine, hydrochlorothiazide, and hydralazine) from 5 weeks of age so that we might examine the role of blood pressure in the SHR vessel wall-receptor mismatch. Radius and receptor properties were measured using the in vitro aortic arch-aortic nerve preparation at 8, 14, 20, and 30 weeks. We found that baroreceptors from SHR with normal blood pressures had normal pressure thresholds and suprathreshold pressure sensitivities. However, the treated SHR receptors still had lower strain thresholds and the aortas still were less distensible than were the normal. Now, however, the increased receptor sensitivity was sufficient to normalize the relationship between baroreceptor function and arterial blood pressure. We conclude that, although blood pressure may be restored to normotensive levels by therapy, vascular abnormalities may continue to develop. CircRes 47: 829-834, 1980

Peptidergic modulation of mechanotransduction in rat arterial baroreceptors.

Regularly discharging baroreceptors in a rat in vitro aortic arch preparation were exposed to increasing concentrations of one of four vasoactive peptides: angiotensin II, arginine vasopressin, atrial natriuretic factor, or substance P. Slow ramps of pressure evoked discharge responses in single-fiber baroreceptors. Instantaneous discharge frequency was measured simultaneously with aortic diameter and pressure. During constriction induced by angiotensin II or arginine vasopressin, baroreceptor diameter threshold (Dth) decreased and pressure threshold (Pth) tended to increase; these effects were reduced or eliminated by nitroprusside. Atrial natriuretic factor and substance P by themselves were without effect on vessel diameter or on baroreceptor discharge. In preparations preconstricted with a moderate concentration of phenylephrine (10(-8) M), atrial natriuretic factor reduced the phenylephrine-induced constriction and increased Dth and decreased Pth. Substance P, even at high concentrations, was less effective than atrial natriuretic factor in reducing phenylephrine constriction and in altering baroreceptor discharge. Baroreceptor gain was unaffected by any of these peptides. Thus, changes in smooth muscle tone altered mechanotransduction by shifts in 1) the vessel pressure-diameter relation and 2) baroreceptor threshold requirements (Pth and Dth). Changes in the baroreceptor mechanical threshold (Dth) reduced the effects on Pth expected from changes in vessel wall mechanics. Pth reflects the net effects of vessel wall and Dth changes. Pth generally increased during constrictions and decreased during dilations. The changes in Dth and their selectivity (no changes in gain) during vasoactive peptide action closely resemble rapid resetting of baroreceptors. We propose that vascular smooth muscle lies in a parallel arrangement with aortic baroreceptors and that a common compensatory mechanism regulates Dth during sustained changes in vessel diameter. Activation of smooth muscle and reductions in transmural pressure would reduce loading of baroreceptors, and the proposed compensatory mechanism would tend to keep discharge constant by decreasing Dth. Our experiments, however, cannot distinguish between hypotheses for local micromechanical changes in coupling or for changes modulating excitability within the baroreceptor neuron itself as the basis for Dth adjustments.

High-salt diet elevates baroreceptor pressure thresholds in normal and Dahl rats.

Dahl Salt Sensitive (DS) rats rapidly develop high blood pressure when exposed to a high-salt diet. Recent studies suggest that DS rats have poorly functioning baroreceptor afferents and baroreflexes even when salt intake is restricted. This study examines baroreceptor pressure- and mechano-transduction in DS, Dahl Resistant (DR), and Sprague-Dawley (SD) rats during low- and high-salt conditions. Single unit, regularly discharging baroreceptors were studied using an in vitro aortic arch-aortic nerve preparation. Pressure thresholds and suprathreshold pressure sensitivities were determined from responses to slow ramps of pressure. Pressure-diameter relations measured in each rat were used to transform pressure threshold and pressure sensitivity values to their mechanical equivalents in terms of aortic wall strain. A total of 407 unit baroreceptors were studied from 49 rats. Tail systolic blood pressures were significantly higher only in DS during high salt. Pressure threshold was similar for all groups on low salt. Exposure to a high-salt diet increased the mean pressure threshold for all three groups. Pressure threshold for high-salt diet was highest in DS and lowest in DR. Pressure sensitivities were lowest in DS and highest in DR on low salt. High salt had no significant effect on pressure sensitivity. The differences in threshold apparent when expressed in terms of pressure were eliminated by conversion to their mechanical equivalents (strain threshold and strain sensitivity). The results suggest that baroreceptors in the two Dahl rat strains represent two extremes from normal baroreceptor function. DS tend to be less pressure responsive than normal (SD), and DR tend to be somewhat more responsive to pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of changes in extracellular ionic concentrations on aortic baroreceptors with nonmyelinated afferent fibers.

The effects of ionic changes upon one of the two classes of aortic baroreceptors, those having unmyelinated axons or C-fibers, have not been examined heretofore. Differences from results in aortic baroreceptors with myelinated axons might be expected because of differences in accessible surface area-volume relationships. Recordings were obtained using an in vitro aortic arch-aortic nerve preparation from 21 aortic C-fibers in 15 normotensive Wistar-Kyoto rats (WKR) 13–17 weeks old. During a slow pressure ramp, the endings responded with an irregular discharge. However, if the discharge was averaged over 10–20 seconds, the average discharge correlated well with the pressure level in the arch. The maximal discharge increased with faster ramp rates but the threshold was not shifted over the range of ramp rates employed. The relationship between pressure and discharge was tested during alterations in the extracellular concentrations of Na, K, and Ca ([Na]o, [K]o, and [Ca]o). A 12% decrease in [Na]o clearly decreased the firing in the aortic C-fibers during a ramp stimulus. During a pressure step, the initial discharge was not decreased but the steady state discharge was clearly depressed. When the aortic arch was perfused continuously at constant pressure, a decrease in Na of only 6% depressed the activity. Doubling [K]o increased firing for a few minutes and was followed by a reduction of discharge. Previous reports have shown no effect on myelinated baroreceptor discharge by a 12% decrease in [Na]o, a change which significantly depressed firing in all C-fiber baroreceptors in this study. Thus, C-fiber baroreceptors are more sensitive to changes in extracellular ionic composition than are baroreceptors with myelinated axons.

Dynamic and static conditioning pressures evoke equivalent rapid resetting in rat aortic baroreceptors.

A recent study suggested that exposure of carotid sinus baroreceptors to pulsatile pressures for a period of minutes can decrease the static threshold pressure at which they begin to discharge; that is, the exposure can sensitize baroreceptors. Another study found that the rapid resetting of carotid sinus baroreceptors to elevated conditioning pressures was reduced or eliminated by pulsatile conditioning. In the present study, we tested the responses of aortic baroreceptors in an in vitro preparation to a range of static and dynamic conditioning pressures lasting 15 minutes. Slow test ramps of increasing pressure were used to assess static discharge properties (threshold and gain). To be accepted for analysis (n = 12), each baroreceptor had to successfully complete static and dynamic test sequences for at least three different conditioning mean arterial pressure levels. We measured aortic diameter simultaneously with pressure and baroreceptor discharge. Generally, we found no significant difference between the static pressure threshold measured before and after dynamic conditioning. The static pressure threshold was linearly related to the mean level of the conditioning pressure, and no differences in the slopes of these relations (a measure of the ability of a baroreceptor to rapidly reset) were found between static and dynamic conditioning. After dynamic conditioning, discharge rates returned to near control levels in all cases within a few seconds of the return to static conditioning. Two baroreceptors exposed to the vasodilator nitroprusside throughout the experiment showed similar results. Diameter measurements indicated no role of vessel diameter changes during dynamic or static conditioning. In conclusion, we found no evidence of a long-lasting sensitization of aortic baroreceptors by dynamic pressure inputs.(ABSTRACT TRUNCATED AT 250 WORDS)