Sympathetic nerve regulation to heating is altered in senescent rats

Abstract

Kenney, Michael J., and Richard J. Fels. Sympathetic nerve regulation to heating is altered in senescent rats. Am J Physiol Regulatory Integrative Comp Physiol 283: R513-R520, 2002. First published May 6, 2002 10.1152/ ajpregu.00683.2001.-Renal and splanchnic sympathetic nerve discharge (SND) responses to increased (38-41°C) internal temperature were determined in anesthetized young (3-6 mo old), mature (12 mo old), and senescent (24 mo old) Fischer 344 (F344) rats. We hypothesized that SND responses would be altered in senescent and mature rats as demonstrated by attenuated sympathoexcitatory responses to heating and by the absence of hyperthermia-induced SND pattern changes. The following observations were made. 1) Renal and splanchnic SND responses were significantly increased during heating in young and mature but not in senescent rats. 2) At 41°C, renal and splanchnic SND responses were higher in young compared with senescent rats, and renal SND was higher in mature than in senescent rats.3) Heating changed the SND bursting pattern in young, but not in mature or senescent, rats. 4) SND responses to heating did not differ between baroreceptor-innervated (BRI) and sinoaortic-denervated (SAD) senescent rats but were higher in SAD compared with BRI young rats. These results demonstrate an attenuated responsiveness of sympathetic neural circuits to heating in senescent F344 rats.Fischer 344 rats; sympathetic nerve discharge CHANGING THE LEVEL OF ACTIVITY in sympathetic nerves in response to acute environmental stress is a primary means by which mammals maintain physiological homeostasis. In young Sprague-Dawley rats, elevated internal body temperature (Tc) produced by acute heat stress increases efferent sympathetic nerve activity (9,12,13,15) and alters the pattern of sympathetic nerve discharge (SND) bursts (12,13,15). With respect to the SND bursting pattern, increased Tc transforms cardiac-related SND bursts to low-frequency bursts that contain more power than cardiac-related bursts, demonstrating an important role for pattern transformation in mediating heating-induced sympathoexcitatory responses (13).Aging is known to alter sympathetic nerve responses to various acute stressors, including hypoxia (30, 31), footshock (23), and 2-deoxy-D-glucose administration (23). In addition, senescent rats demonstrate altered norepinephrine turnover rates in selected organs during heating (18), suggesting age-related changes in sympathetic nerve responses to heating. However, our understanding of the effect of age on directly recorded SND responses to heating is limited in at least three ways. First, the effect of age on renal sympathetic nerve responses to increased Tc is not well established. Stauss et al. (32) reported that the level of renal SND remained unchanged in response to hyperthermia in 12-mo-old (mature) and 24-mo-old (senescent) Fischer 344 (F344) rats. However, renal SND responses to heating in young F344 rats were not reported. Therefore, it is not clear whether the lack of renal sympathoexcitation to...

Hyperthermia-enhanced splenic cytokine gene expression is mediated by the sympathetic nervous system

“…Experiments were performed on Sprague-Dawley rats (381 Ϯ 6 g, n ϭ 32). Anesthesia was induced by isoflurane (3%) and maintained during surgical procedures using isoflurane (1.5%), ␣-chloralose (80 mg/kg ip), and urethane (800 mg/kg ip) (31,32,43). During the experimental protocol, maintenance doses of ␣-chloralose were infused intravenously (femoral vein, 35-45 mg⅐kg Ϫ1 ⅐h Ϫ1 ).…”

“…The filtered neurograms were routed to an oscilloscope and a nerve traffic analyzer for monitoring during the experiment and for subsequent data analysis. Sympathetic nerve potentials were full-wave rectified, integrated (time constant 10 ms), and quantified as volts ϫ seconds (V⅐s) (29)(30)(31)33). The level of activity in sympathetic nerves was corrected for background noise after administration of the ganglionic blocker trimethaphan camsylate (10-15 mg/kg iv).…”

“…At the end of the control period, heating experiments were initiated in splenic-intact and splenic-denervated rats by increasing Tc at a rate of 0.1°C/min from 38 to 41°C (30 min), followed by a maintenance phase in which Tc was maintained at 41°C for an additional 90 min. Increases in Tc were produced using a heat lamp (29)(30)(31)(32)(33). Nonheated experiments were completed in splenic-intact and splenic-denervated rats by maintaining Tc at 38°C for an additional 120 min beyond the initial 30 min control period.…”

“…Increased internal body temperature produced by whole body heating increases muscle sympathetic nerve discharge (SND) in conscious humans (10)(11)(12)50), splanchnic SND in conscious rats (36), and renal, splanchnic, and splenic sympathetic nerve activity in anesthetized rats (22,(29)(30)(31)33) and decreases SND directed to the caudal ventral artery in the rat (27). Moreover, moderate body warming decreases skin sympathetic nerve activity in human subjects (13).…”

Heat stroke: Role of the systemic inflammatory response

“…Heat strain imposes large cardiovascular demands on the body as blood flow is shunted from core organs to the skin to dissipate excess heat to the environment. Physiological decrements with aging may include impairment of baroreceptor reflex modulation (80), lower sweating rate and longer onset to sweating (43), and diminished renal and splanchnic sympathetic nerve discharge (44). Minson et al (62) demonstrated that older men relied on a higher percentage of their cardiac chronotropic reserve compared with young men during heat exposure.…”