Renal nerve responses to somatic nerve activation in stroke-prone spontaneously hypertensive rats

Baroreflex and somato-reflex control of blood pressure, heart rate and renal sympathetic nerve activity in the obese Zucker rat

It has been reported that the baroreflex control of heart rate (HR) and sympathetic nerve activity (SNA) is attenuated in obese Zucker rats (OZRs) compared with age-matched lean animals (LZRs). What is not known, however, is the extent to which the baroreflex control of mean arterial blood pressure (MAP) is altered in the OZR. In addition, it is not known whether the interactions of other sensory nerve inputs on autonomic control are altered in the OZR compared with the LZR. The aim of this study was to determine the baroreflex control of MAP, HR and renal SNA (RSNA) in the OZR and LZR using an open-loop baroreflex approach. In addition, the effect of brachial nerve stimulation (BNS) on the baroreflex control was determined in these animals. Age-matched, male LZRs and OZRs were anaesthetized, and the carotid baroreceptors were vascularly isolated, bilaterally. The carotid sinus pressure was increased in 20 mmHg increments from 60 to 180 mmHg using an oscillating pressure stimulus. Baroreflex function curves were constructed using a four-parameter logistic equation, and gain was calculated from the first derivative, which gave a measure of baroreceptor sensitivity, before and during BNS. The range over which the baroreflex could change MAP (28 ± 6 versus 87 ± 5 mmHg; mean ± SEM), HR (17 ± 4 versus 62 ± 11 beats min(-1)) and normalized RSNA (NormNA; 22 ± 4 versus 76 ± 11%) was significantly decreased in the OZR compared with the LZR. Likewise, the maximal gain was lower in the OZR, as follows: MAP -0.88 ± 0.22 versus -2.26 ± 0.17; HR -0.42 ± 0.18 versus -1.44 ± 0.22 beats min(-1); and NormNA -0.54 ± 0.14 versus -1.65 ± 0.30% mmHg(-1). There was no difference in the mid-point of the baroreflex curve for each variable between the OZR and LZR. However, the minimal values obtained when the baroreceptors were maximally loaded were higher in the OZR (MAP 68 ± 5 versus 53 ± 4 mmHg; HR 455 ± 7 versus 390 ± 13 beats min(-1); and NormNA -19 ± 4 versus -48 ± 8%). Brachial nerve stimulation in the LZR resulted in an upward and rightward resetting of the baroreflex control of MAP and RSNA, and abolished baroreflex control of HR. The baroreflex control of RSNA in the OZR during BNS was further attenuated and reset upwards and to the right, while the HR response was abolished. With respect to MAP, the baroreflex curve reset upwards and to the right to a point comparable with the LZR during BNS. These data show that there is an attenuated baroreflex control in the OZR and that the ability to reset to higher arterial pressure during somatic afferent nerve stimulation is similar to that in the LZR.

The influence of acute hypothermia on renal function of anaesthetized euthermic and acclimatized rats

Acute hypothermia has a major impact on cardiovascular control and renal function, but the extent to which these can be correlated with and influenced by changes in the altered pattern of sympathetic outflow to the kidneys is unclear. Moreover, it is unknown whether these responses to acute hypothermia are altered by chronic cold exposure and this study aimed to examine these factors. Renal function and renal sympathetic nerve activity (RSNA) were measured in male Wistar rats, euthermic (control) or acclimatized (exposed to progressively lower environmental temperature and photoperiod over 8 weeks), anaesthetized with chloralose/urethane. Reduction of core temperature (Tc) to 25 degrees C caused approximately 40% reduction in heart rate (HR), approximately 10% fall in mean arterial blood pressure (MABP), and decreased glomerular filtration rate (GFR) by approximately 50% and approximately 5% in euthermic and acclimatized rats, respectively. At 25 degrees C, urine flow increased some two-fold and absolute and fractional sodium excretions by 4- to 6-fold in the euthermic rats and to a lesser extent in the cold acclimatized rats, while basal levels of fluid excretion were higher in the acclimatized rats. A loss of pulsatility in the RSNA signal with cooling was seen in both groups. One of the factors contributing to modest hypotension during acute hypothermia is a reduction in RSNA. There was a progressive fall in the proportion of RSNA power at HR frequency with cooling of 20% in euthermic and 80% in acclimatized rats. All variables were restored to basal levels on rewarming in both groups of rats. We conclude that natriuresis and diuresis in euthermic rats during hypothermia is a consequence of a reduction in nephron reabsorption, reduced urine osmolality and possibly altered patterning of RSNA. In acclimatized rats, the response was modified by altered renal haemodynamics and/or hormonal influences induced by chronic cold exposure to minimize the hypothermic stress on renal function.

Chaotic characteristics of renal nerve peak interval sequence in normotensive and hypertensive rats

1. The use of non-linear dynamic analysis for the measurement of control processes in low-dimensional signals, for example, blood pressure and heart rate variability, are well established and accepted. However, the application of these analytical techniques to a high-dimensional signal, such as renal sympathetic nerve activity (RSNA), has not been validated. 2. The present study set out to develop an approach whereby the high-dimensional signal of RSNA was reduced to a low-dimensional one by extracting the peak interval sequence (PIS), using Cluster analysis, in order to allow the use of non-linear dynamics analysis. Brachial nerves were electrically stimulated (1.6 Hz, 0.2 ms, 15 V) to elicit a sympatho-excitation in groups of anaesthetized normotensive Wistar and stroke-sprone spontaneously hypertensive rats (SHRSP). 3. It was found that, under basal conditions, the correlation dimension, D2, was stable over a range of embedding dimensions from 12 to 25. Moreover, the largest Lyapunov exponent had a small positive value that was also stable over these embedding dimensions. These values showed that the signal was of low dimensionality and that chaos was present. 4. In Wistar rats, brachial nerve stimulation significantly (P < 0.05-0.001) increased blood pressure (by 25%), heart rate (by 5%) and RSNA (by 200%), which was associated with significant (P < 0.05) reductions in the correlation dimension D2 and the largest Lyapunov exponent of the PIS generated from the renal nerve signal. In contrast, in SHRSP, there were similar increases in blood pressure, heart rate and RSNA in response to brachial nerve stimulation, but neither the correlation dimension nor largest Lyapunov exponent was altered. 5. These findings demonstrate that by extracting the PIS from the renal sympathetic nerve signal, the application of non-linear chaos analysis makes it possible to distinguish differences in the pattern of reflexly induced excitation in sympathetic traffic to the kidney in the pathophysiological state of hypertension. Whether this applies to sympathetic outflow to other organs and tissues remains to be investigated.

Neural regulation of kidney function by the somatosensory system in normotensive and hypertensive rats

This investigation examined the renal sympathetic nerve and renal excretory responses to somatosensory stimulation in normotensive and stroke-prone spontaneously hypertensive rats (SHRSP). Somatosensory activation was achieved by either subcutaneous capsaicin administration or exposure of the airways tract to irritant fumes from acetic acid in chloralose-urethan-anesthetized animals. In Wistar rats, blood pressure increased between 10 and 20% (P < 0.001-0.01), renal perfusion pressure was maintained unchanged, renal hemodynamics were unaltered, and urine flow and sodium excretion were decreased by 25 to 50% (P < 0.001-0.05). In the SHRSP, the somatosensory-induced increases in blood pressure were slightly larger (approximately 15-20% P < 0.05) than those of the Wistar rats, whereas the excretory responses were one-half those of the normotensive animals (P < 0.05). The somatosensory challenges reflexly increased integrated renal sympathetic nerve activity in both normotensive and hypertensive rats. The power spectral analysis demonstrated that the increases in percentage power at heart rate frequency and total power were two to three times more (P < 0.05) in the Wistar rats compared with the SHRSP. The reduced ability of the SHRSP to modulate the energy in the renal sympathetic nerve signal at heart rate frequency might explain in part the attenuated functional responses to the somatosensory challenges.